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Posted by: sideways Oct 31 2004, 02:13 PM

QUOTE

EXHAUST

for most engines you need some sort of exhaust... well, literally, some length of pipe coming from the exhaust ports. it's not for back pressure, that's pretty much always bad. instead, it's for velocity. think of an intake with a velocity stack, it lines up the air going into the engine so that the velocity will be higher and the engine will ingest more air. it's the same for an exhaust, just more complex because of things like cam timing.

most high revving engines for example will use a bit of intake/exhaust cam overlap so that the leaving exhaust gas with create a vacuum and pull in the fresh A/F mixture. (of course you run into less low end power but that's another topic)

in relation to this "vacuum/momentum" idea, you want a pipe that will be just the right diameter attached to the exhaust port. this way, the exhaust gas has a much better path to maintain its velocity than if it just dumped right to atmosphere from the port. this velocity, and then the little bit of vacuum it makes in it's wake, helps pull out the next cycle of exhaust gas.

issues like this are why it's never correct to say "true dual exhaust is better". the FC does have a dual exhaust, it just uses a 2-to-1 header, into a 1-to-2 exhaust. now, you're probably thinking "ah-HAH!", that's makes back pressure and costs hp. well, it probably does, but mostly because of the old ass design, NOT the theory. the theory is just the same as running an "X" pipe on a multi-bank engine. it's a literal X where the gas from bank 1 intersects with the gas from bank 2. you can pretty easily see why it would make the exhaust a little longer and therefor, not save the manufacturer any money. the reason it's there is because it smooths out the exhaust pulses and helps share the velocity between banks.

now, an X pipe like this could become a restriction when exhaust flow reaches a certain point, but that's almost another issue, because exhaust tuning is almost as complex as cam tuning.

All information was posted by VrFd, so thanks goes to him on this one.

Posted by: VRr1FD Nov 1 2004, 11:23 AM

just too add, something i ran across today and just read over. Subaru centric specifics but very solid basics.

http://www.cobbtuning.com/tech/exhaustdesign/index.html

Posted by: takahiro1985 Jun 28 2005, 02:26 PM

Yeah. That all may be true on engines that are just for racing and nothing else. IN reality, the engines that are designed for regular cars, that are EFI, require some ammount of back pressure. The reason is that the timing on most cars overlap some. When the exhust valve opens hte piston isn't at BDC(Bottom Dead Center) it is only about half way down when the exhust valve opens. If you have no backpressure on that all the gasses that are still expanding go out and no more force on the piston to make you go.

So, thus, you do need some back pressure. It is true that on most enginest that have a carb setup, instead of EFI, almost zero back pressure is good. ust not on fuel injected engines.

I'd suggest for a street racing car, to use a high flow cat and a flow-thru muffler. The cat gives you just enough back pressure, so does the piping, and the muffler will make it all legal.

Posted by: MidnightViper88 Jun 28 2005, 02:51 PM

Wouldn't some backpressure be OK for specific engines, such as small engines (Physically with displacement or cylinder count) that are naturally aspirated? I heard that anything from a dramatic decrease to no backpressure in such engines could seriously reduce torque outputs...

Posted by: sideways Jun 28 2005, 04:25 PM

Back pressure can help with low end torque, reason is pretty simple. As mentioned before cars have valve overlap, ie both intake and exhaust valves are open at the same time. In the lower rpms the actual amount of time both are open is longer compared to what youd find at the higher rpms- for more than obvious reasons.

Thing is this amount of time, while both valves are open- can let the incoming air/fuel mixture enter, and leave the cylidner- without even being burned (in the cylinder at least). Back pressure will help keep the new air/fuel mixture in the cylinders, also means you can keep some mixture thats already burned in there, which will be burned again (good for emissions). Also means the engine has to waste a little of its power "pushing" the exhaust out, takes power to gain power thing.

At higher rpms (this is what VR was talking about- drag was never on our minds i dont believe, apologies for that) the mixture doesnt have enough physical time to enter, and leave the cylidner with the valve overlap. So back pressure is far from needed, and a vacuum in the exhaust can actually help (as pointed out before) pull the exhaust out of the cylinder, and bring in the new mixture- freeing up power that wouldve been used otherwise to move exhaust gases out, putting more power to the wheels.

(Doesnt matter how much your engine makes at the fly- its all about whats going to wheels. Something to keep in mind- Your engine can make 100 hp, but it might take some hp spinning a clutch powered fan, and may take some hp spinning an ac pump, and pushing the exhaust gases, power steering pump, drivetrain- you can end up with less then 80 at the wheels.)

Posted by: VRr1FD Jun 29 2005, 10:37 AM

2nd try, accidentally lost the 1st. possibly numerous formatting errors as i rewrote in msw3rd.---

the first, biggest problem that jumps to mind with running a smaller exhaust to make back pressure, is that (duh) back pressure is RPM dependent.

what most people think of as back pressure creating "performance" exhausts, don't make any back pressure in the low rev range. it's not till you start revving the engine high that it starts pumping enough air per second to be clogged by the exhaust.

so if you are running an exhaust that chokes the engine on the low end, so that you wont bleed off your combustion cycle while you run a cam that is way too big, you will be seriously be strangling the engine higher in the rev range. where that big duration cam is supposed to be paying off.

low power economy cars don't get that way efficiently (economically) by running massive cams and tiny exhausts. they run small cams with exhausts designed to promote velocity in that cams sweet spot, the low end, not choke
it off with back pressure, because of point 2.

point 2, back pressure means that the engine needs to pump harder to get the gas to flow. this is pumping loss, and this costs power. the smaller exhaust on the economy car will tend to choke the engine off (costing HP)... but only higher in the rev range. where it's small cam just isn't made to flow, because the engine isn't made to rev up there anyway.

point 3 is that the exhaust valve opens early for the point of bleeding off gas before the piston has to push it out. it cuts into your combustion stroke of course, but it helps a lot on the exhaust stroke. so the effect is positive as long as you aren't opening too early for your engine speed. if the valves could just instantly open and shut, you'd see a lot less of this.

QUOTE (MidnightViper88)

Wouldn't some back pressure be OK for specific engines, such as small engines (Physically with displacement or cylinder count) that are naturally aspirated? I heard that anything from a dramatic decrease to no back pressure in such engines could seriously reduce torque outputs...

well, you are confusing a few things. this first is just a pet peeve of mine: torque does not mean low end. you mean to say low end. i know, it's just a stick up my ass because of all the HPvsTQ shit. people confuse them as different parts of the same thing(high/low), which they aren't...

the second is the big lie you've been lead to believe by people who don't fully grasp exhaust systems. that's why i originally posted what i did.

the lie is mainly that back pressure equals low end power. and it's confusing two things: back pressure and optimum flow. back pressure, as i stated earlier in the post, is bad. optimum flow (velocity) is good. and while optimum low end flow typically means high end back pressure, the entire reason that it isn't optimum on the high end is because it makes too much back pressure up there. it chokes the engine.

as long as the exhaust gas maintains velocity, the engine doesn't need to pump it out as hard, and the gas will help to suck out the next combustions gas.

take a big diameter exhaust and the exhaust wont have enough air pumped through it to be perfectly officiant on the low end. but back pressure wont help, it never does. take your huge diameter race exhaust and plug up the end down to half it's diameter, it won't flow any better on the low end, and therefor wont make any more power, because the inner diameter is too big to maintain the velocity the gas has leaving the combustion chamber.

Posted by: ChennyZ Apr 24 2006, 05:55 AM

so in order to have turbo...you need a turbo based' exhaust?

Posted by: VRr1FD Apr 24 2006, 07:20 AM

well, the turbo sits in the exhaust path, so naturally you are going to run a turbo manifold. but turbos naturally provide annoying back pressure, they aren't as free a lunch as some people would have you think. the smaller the turbo's exhaust turbine/housing (for early spooling), the more back pressure it'll give you as you rev up. but if you run too big of a "hot side" (turbine/housing) for your application, you will have trouble spooling it at all (but you'll have no back pressure

).

turbo exhaust systems can be broken up into 2 parts. pre-turbo (the manifold), and post turbo (the turbo-back exhaust).

-pre-turbo has 2 considerations that i can think of right away. the first is the obvious, inner pipe diameter. this will affect how much exhaust velocity you can feed your turbo, and how early you can meet this velocity. and whether or not it will choke off the exhaust flow at higher rpm's/boost levels. the second is turbo placement. that is, how much room you have to mount the turbo. this affects how smooth your pipe intersections will be, which has an obvious affect on velocity/back pressure, just like a normal header.

i guess it's a lot like the factors that go into building a nice header for an NA car, but the big difference is how fast the exhaust gas volume will go up when the boost hits.

-post turbo, well, shit, i guess it's a lot like what happens behind a NA header. you basically want to get the exhaust away from the turbo as fast and as smooth (they tend to go together) as possible. you do not want ANY back pressure post turbo, as this will cut into how much boost you can run into the higher rpm range, which will have a big effect on power. but of course, if you're running a small turbo, it might not be able to keep up your peak boost into the high rpms anyway (it would be over spinning and burning up, bad for reliability).

i guess it's all basically the same. just using a different scale for the escalation in back pressure due to the boost.

typically though, if you are going for big power, you'll be running a turbo back that is much bigger than what you'd need if you were NA, because of the boost. that's basically the biggest distinction.

Posted by: Akira Feb 10 2007, 04:59 PM

You know, even in auto class exhaust was always a bit tough to understand for me.. Just when I think I'm getting it I read something or see something that just completely makes me confused again.

Back pressure is bad, but good for low end torque, got it, but how?

Particularly I was looking at exhaust systems on 3SX.com(for a 3000GT) specifically for a twin turbo car with dual exhaust, what I don't get is what a dual exhaust actually is because most I see are coming from teh same point at one point, and then I see two intakes to that pipe.

What I'm not getting is what a TRUE dual exhaust exactly is, and when is it better in tersm of power, but mostly efficiency to convert that to a single exhaust? If it's coming from one pipe what motive do you have to make it two? releasing back pressure?

For example under an FC, what would something like that look like? and is that second intake to the pipe on anything? I just really never touched on exhaust guys, help finally understanding this would be awesome.

Posted by: sideways Feb 10 2007, 05:15 PM

as far as i know is can help due to the overlap of cams, at low engine speeds with ovr lap (both valves open) the fresh air/fuel on a free flowing exhaust will get pulled right out before it has a chance to actually fire- some back pressure will help keep the air/fuel in, so it can be ignited. Problem is at higher engine speeds the actual time of overlap is so small it wont get pulled through, so with the backpressure an engine has to "push" the exhaust out, where as with a free flowing (and once again to my knowledge race exhaust actually use the exhasut pulses to create some vacuum to help suck the next pulse along) exhaust its not a problem

Posted by: VRr1FD Feb 12 2007, 11:06 PM

QUOTE (Akira @ Feb 10 2007, 04:59 PM)

1. Back pressure is bad, but good for low end torque, got it, but how?

2. Particularly I was looking at exhaust systems on 3SX.com(for a 3000GT) specifically for a twin turbo car with dual exhaust, what I don't get is what a dual exhaust actually is because most I see are coming from teh same point at one point, and then I see two intakes to that pipe.

3.What I'm not getting is what a TRUE dual exhaust exactly is, and when is it better in tersm of power, but mostly efficiency to convert that to a single exhaust? If it's coming from one pipe what motive do you have to make it two? releasing back pressure?

4.For example under an FC, what would something like that look like? and is that second intake to the pipe on anything? I just really never touched on exhaust guys, help finally understanding this would be awesome.

if i never had to talk about exhaust again, it would be too soon.

1. no. reread my posts.

2. i have never gotten to ask an OEM engineer why they would rejoin exhaust paths on a multi turbo car after the turbo's. but there are several possible reasons:
-share a cat
-packaging regarding going under the seats
-sound engineering

why have them THEN seperate after being joined?
-packaging regarding pipe size doing by the rear axles
-sound engineering
-looks, honestly. (s2000 anyone? etc)

3. true dual exhaust is generally fully devided exhaust paths. could be better on multi-turbo cars after the turbos. could be weaker on most NA cars because you lose the ability to tune a cross bank scavaging effect.

4. on an FC, it would look just like the stock setup, accept there would be 2, seperate pipes, one fed by each rotor, they would never join in the "downpipe" area.

Posted by: backalleyracer May 6 2007, 02:18 PM

I have a nice comment to add to this, but alas, it is for rotary however the same theory works for all cars, and should be very helpful

http://www.yawpower.com/techindx.html

Posted by: SR5Sedan Jun 22 2007, 05:28 AM

I'm going to complicate things:

1) You simply cannot think of an exhaust system as something that is static. Just as in an intake system, RPMs, atmospheric pressure, temperature, gas composition, leftover combustion, ect. all affect an exhaust system.
2) Think of an exhaust system like you would an intake system with one difference: With the exception of bypass gates, you can't currently tune an exhaust system dynamically.
3) So, what's so hard? Isn't an engine just an air pump? Yes. It is, but I'll make things simpler and then harder.
4) Let's say you have a 1 cylinder engine and we want it to make the most HP at 1000 RPM. Simple, right?
5) Actually not.
6) Let's say we get rid of "all backpressure" by not having an exhaust manifold? Let's let the cylinder dump exhaust gas straight into the ambient air! Shouldn't this be the "highest horsepower?"
7) Why wouldn't this work?

Posted by: zariki Jan 26 2008, 07:05 AM

It will work....question is can you stand that noise??

Posted by: atlantian Mar 20 2008, 03:36 PM

off topic...just wondering... if i am going to have a turbo kit with antilag... it's best to remove the cat... and have flow through everything, the turbocharger should be enough back pressure...

EDIT: i think that back pressure with the twisting and bending of the exhaust is for the sake of muffling the sound(street use). but i am a young hooligan and i don't want to use my horn, i am just going to say my exhaust note is sufficiant for a siren(i am going to have an antilagged turbo)

Posted by: vash169 May 28 2008, 11:13 AM

It was once explained to me that:

An NA engine requires an amount of backpressure in order to maintain engine vacuum. So probably an ideal performance setup (depending on the car) would be a pipe diameter about half an inch wider than stock and a single catalytic converter in the secondary location (along the midpipe).

A super/turbocharged engine has air being forced into it, so it's important to have appropriately high exhaust velocity to produce smooth acceleration. My own turbo car has a 3 inch exhaust and no cats, and it runs like a dream.

Posted by: EA99 May 28 2008, 12:49 PM

pretty sure having no cat is illegal

Posted by: Tessou May 28 2008, 01:10 PM

QUOTE (atlantian @ Mar 20 2008, 07:36 PM)

(i am going to have an antilagged turbo)

Have fun with your dead turbo after a month or so.

Posted by: rockyz Jun 11 2008, 10:39 PM

QUOTE (Tessō @ May 28 2008, 01:10 PM)

Have fun with your dead turbo after a month or so.

???

I can see why you might think it would cause damage because freeing up the exhaust means letting the turbo spool easier and boost higher. Say a FC TII converts to a 3inch downpipe/resonator/cat-back, the turbo will be able to boost past the stock 8PSI limit. A fuel-cut defender would fix this problem letting the turbo boost past that limit, but there is also other factors to worry about(overboosting, running to lean, etc). Overboosting can easily be solved by a boost controller. But if you own a turbo car, you would probably know what you're doing before you start messing with your car.

I don't see why people make such a big deal about cars running without cats. Before the accident, my FC ran a racingbeat headers->presilencer->catback setup. Yah it's illegal not to have a catalytic converter, but so is ANY modification to the emissions system in a car in California. I'm not telling people to break laws here, but if you have a cone style intake or an aftermarket catback system, you're on the same boat as us.(for Californians of course) I'm just trying to make a point that it's not black/white, especially when it comes to tuning older sports cars. I understand that the matter of emissions is a very touchy subject, but this IS the technical discussion, so TECHNICALLY, running a freer flowing exhaust (no cat) is better than running an exhaust with a cat.

ah, I forgot the address the issue with sound levels. A cat isn't the only way to muffle sound, that's what "mufflers" are for. I mentioned that I replaced my cat with a racingbeat resonator that was packed with wool. I had the same exact setup with a high-flow cat and the resonator and the sound difference was barely noticeable in the low end.

Posted by: Meteor Jun 11 2008, 11:05 PM

^I believe Tesso was referring to atlantian's desire to install an ALS on his turbo.

You know, that thing that completely gets rid of turbo lag, makes bang-bang noises, is absolutely useless in normal everyday driving and shortens the life of the turbo.

Posted by: OverDrift7 Jun 12 2008, 05:34 AM

ALS = you need a new turbo sonny. *cry* again? LOL wondering about a true E-charger placed infron of a turbo though oops exhast section. sorry i leave now.

Posted by: The Stig Jun 12 2008, 12:18 PM

QUOTE (Meteor @ Yesterday at 11:05 PM)

shortens the life of the turbo.

I think they already got that it whoever puts ALS onto a car will need a turbo soon after... Reading comprehension is your friend?

Posted by: vash169 Jun 13 2008, 03:31 PM

Having an ALS is meaningless anyway, all you have to do is shift high and you won't fall into the lag. Having a foreign system force your turbo to spin when it shouldn't instead of just pushing on the gas a little harder is definitely not how to win teh driftrace.

Posted by: Jardim Jun 13 2008, 04:56 PM

ya'll need to run a full 3 inch pipe from the headers to the last muffler no cat , resinator or nothing +5 hp MaD JdM TiTe Y0!

Posted by: elemein Aug 18 2012, 08:30 AM

Sorry for posting in a very "dead" thread, though seeing as it is a sticky, I think it is okay.

First, backpressure is bad. 100% Bad. Is it "necessary" for an engine to perform? Not at all. Will every engine have backpressure periods? Yes.

Let me explain; I will then give a PDF that gives examples and studies to support.

Firstly, what is backpressure?
It is pressure that impedes 100% efficient flow through the exhaust system. This can be cause one of two ways, and both ways happen EVERY single time you rev your engine through the RPM range (given that your exhaust pipe is not 1/25" thick or not 25" thick... Exaggeration, but you get the point.) What exactly is are the causes?

Overpressurizing and underpressurizing. The first one is easier to understand and explain, so I will go over that first.

Overpressurizing is pressurizing a gas (or liquid) beyond the container's ability to dispose of the gas (or liquid). Want to test this? Get a small straw, then get some chocolate milk (or any other tasty beverage!), try to suck in the liquid as fast as possible. At some point, the rate of the liquid's acceleration will stop, and it will go into a steady speed into your mouth; the force that is preventing the liquid from going any faster is called "overpressurizing". To put this to a car; your mouth (the sucking power of your mouth, specifically) is the car's engine, the straw is the exhaust, and the chocolate milk (or other tasty beverage) is the exhaust gasses. Overpressurizing prevents the acceleration of gasses through a chamber.

Now, the second type of backpressure is one that is harder to understand. Underpressurizing. This is simply the resistance that the gasses put up to being pushed through a chamber (the exhaust system) due to it's properties. Let's look at it chemically; when a gas slows down, it expands and cools. If the gasses get a chance to slow down and cool (due to not being enough pressure in the exhaust system), it will create resistance (colder air is dense; and therefore heavier-- therefore harder to accelerate!). Want another example? Sure!

Go fill up your chocolate milk glass again, and go get a garden hose... Or an obsenely thick straw... Or something that has a far bigger diameter than the previous straw. Now try to suck in the chocolate milk slowly. Hard eh? That's because the amount of pressure put on the milk is very low, therefore it is putting a lot of resistance up and it requires more power to suck in. Why is this? Simple. You already know it. Pressure. The bigger a chamber is, the more power is required to pressurize the chamber to the same psi (or bar) of a smaller chamber with the same amount of liquid (or gas). Less pressure = less velocity = power is lost !

So, if revving your engine too high creates overpressurization, and revving the engine too low causes under pressurization, isnt backpressure always present no matter what I try to do?

No. It's called balance. On a teeter-totter, there are two ends. Let's say the left side is "underpressurization" and the right side is "overpressurization". The further you rev the engine, the further the load is placed on the right. The lower you rev it, the more left the load is placed. But what if the load (your butt.) is placed right directly in the middle of the teeter-totter over the fulcrum? The teeter-totter doesnt move. It is balanced. It is not over, nor under pressurized. Exhaust gasses and pressure works the same way; try to find the "sweet spot" where your exhaust system dispenses of your exhaust gasses 100% efficiently. The bigger you go, the higher the RPM will be where that "sweet spot" is, the smaller you go?The lower in the RPM range the sweet spot is. Though remember, RPM is NOT the only factor; throttle position is also a major contributor to the volume of flow that will create 100% efficiency at a certain RPM and % Throttle.

Want proof? Here you are (it is in .pdf format, make sure you can open it): http://www.mediafire.com/view/?lf7bm4n5f6n4y8b

There is also another term called "scavenging" in there that is important to exhaust flow. I can explain that also if requested.

Also, I hate when people refer to "low end" as "torque". That is not correct. I can also post a VERY thorough explanation I wrote up awhile ago that explains the exact differences between HP and Torque. Let me know if it's okay to post it.

Posted by: Nomake Wan Aug 18 2012, 02:12 PM

Please do explain scavenging.

Also, I don't think it's as much people saying that low-end is torque and more it's people saying 'low-end torque' as a single phrase. As in, having a significant amount of torque at the low-end of the rev range. Like driving an '80s Corolla up a steep incline versus driving a '70s Ford. The Corolla makes more horsepower...but the Ford has no problem accelerating at the start. It's got torque at the bottom of its rev-range. The Corolla's won't kick in until above 4k RPMs or so.

Posted by: MetalMan777 Aug 18 2012, 03:05 PM

Exhaust scavenging is one of those magical things that you really can't visualize very well without a flow bench or a computer that'll do flow analysis. In short, it's a fancy application of Bernoulli's principle. A well designed exhaust system will bring the headers' runners together in such a way that they create a partial vacuum on each other. This is done by computing the timing of exhaust pulses and velocity of the exhaust gas.Like intake tuning, the diameter of the tubes and how far from the port they meet will be more effective at certain RPM levels than others. A fancy side effect of this "sucking" is that if you have any cam overlap (as high revving performance engines should), the suck on the exhaust port should help to pull in the intake charge while both are open.

Further reading: http://honda-tech.com/showthread.php?t=2614040

Posted by: elemein Aug 18 2012, 03:12 PM

QUOTE (Nomake Wan @ 1 hour, 0 minutes ago)

Sure, I don't mind explaining scavenging. Though your Ford and Corolla statement is correct (well, assuming that the Ford engine follows the American principle that "torque is king" like most of them do, and not these new Ford engines that are some kind of weird mix...). Torque is king after all!

Scavenging? Well this would be very hard to make an example for, so instead, just try to follow me without many familiar examples.

Scavenging is the term used to describe the effect that takes place in the exhaust system when the exhaust system works to "suck" gasses out of the engine cylinder, making power lost to engine plumping very little. Scavenging works on the exact same "efficiency curve" as backpressure-- meaning, that, when your exhaust system is working at it's full efficiency and NO backpressure is present, scavenging is also at it's most efficient point, and the scavenging affect is at 100% efficiency.

But what is scavenging? Well, hopefully you've done some physics homework before... Or if you're still in the early years of highschool, it might be a good idea to go to the bathroom sink, plug it, fill it with water, and observe how the water acts to certain interactions with your hand. Scavenging is a term to describe an event of fluid dynamics.

Anyway. What is scavenging on a "fluid dynamic" level? It is fluid filling a low-pressure area once filled with a fluid. Some of you may be reading this and thinking: "What?" Don't worry, I got an example.

Back to the bathroom sink. Run your finger over the surface of the water, touching the finger to the very surface. As you move your finger, the fluid in front of your finger will disperse and get out of the way, this is because you created a "high pressure area" in front of your finger. While, behind your finger, the opposite is happening. Some of the water is actually being pulled WITH your finger, because behind your finger there is a "low pressure area." The same applies for cars and their "coefficient of drag." The more low pressure is behind a car, the higher it's drag coefficient is, and the more power is needed to overcome this drag.

So now we understand that a low pressure area sucks fluids in. Alright, but what does this have to do with our exhaust system? Well let's assume we have a single-cylinder engine (the same principle applies exactly to multi-cylinder engines, but the affect is actually multiplied by the number of cylinders due to the number of exhaust pulses). This single cylinder is running at full throttle, and it is hooked up to an exhaust system.

The engine will go through two revolutions and dispose of a single unit of exhaust gasses, let's call this "exhaust unit #1" (quick! Imagine this unit/discharge of exhaust gasses as your finger in the bathroom sink!), then two more RPMs pass, and it discharges of another unit of exhaust gasses, let's call this "exhaust unit :2". Quick! What happened to the fluid behind your finger when you moved your finger through the water? The fluid (or gas!) got sucked behind the finger and followed it! The same is true for our exhaust system! As exhaust unit #1 flows through the exhaust system, it will PULL exhaust unit #2 along with it! Therefore releasing the burden of plumping exhaust unit #2 from the engine! Yay! This is scavenging! It is the vacuum created in the exhaust system that pulls exhaust gasses out of the cylinder to aid with plumping!

Yay for fluid dynamics!

(Note: If you still do not understand, substitute examples. Use the car and the drag coefficient as the example. Replace car with finger and the dragged air with the fluid behind the finger.)

Edit: Cactus is also correct about the statement about scavenging pulling in intake gasses through valve overlap. I seemed to have been completely focused on the exhaust side of things and forgot about overlap. This is another good affect about scavenging.

Edit #2: Aha, Cactus' link is also correct. Either explanation will work, but the link is a little bit more technical and shys away from "user friendliness", so it may be a bit easier to understand my explanation.

Posted by: Nomake Wan Aug 18 2012, 03:28 PM

So...when the exhaust pressure is at 0 kPa...and the atmosphere is at 14.7 kPa... then I am scavenging gases at maximum efficiency...from the atmosphere?

Perhaps I misunderstood your explanation of what exactly 'backpressure' is. It seems that an exhaust would certainly need to be pressurized in order for these effects to work.

Posted by: elemein Aug 18 2012, 03:40 PM

QUOTE (Nomake Wan @ 12 minutes, 1 seconds ago)

No, when exhaust BACKPRESSURE is at 0psi, you're exhaust system is 100% efficient. Perhaps this will clear things up:

Backpressure is the RESISTANCE pressure that is being built in the exhaust system that makes it HARDER for the engine to evacuate gasses.

Let me try with a different explanation now.

Let say your exhaust system requires 10 psi of pressure for your exhaust gasses to be evacuated 100% efficiently. Let's say this is reached at 100% throttle at 4000 RPM.

Now, run the car at 2000 RPM at 100% throttle. About HALF the amount of exhaust gasses are going through the exhaust gasses, therefore halfing the pressure.

So, if 10 psi is required to make 100% efficiency at 4000 RPM (and 100% throttle), and 2000 RPM is half of 4000 RPM, what is the pressure? 5 psi. This psi is underpressurized (familiar term from my previous post?) therefore, the engine must MAKE 5 psi to push the exhaust gasses out. This makes the engine work harder than is has to, therefore robbing power!

Now, run the engine at 6000 RPM (and 100% throttle... Do I have to repeat this everytime?), now there is 150% of the exhaust gasses being pushed into the exhaust gasses than the amount of gasses being dispelled at 4000 RPM (because 6000 RPM is 150% of 4000 RPM). So, naturally, pressure is 15 psi (150% of 10psi), and this is overpressurized! (another familiar term) The engine will now have to work to EXPEL 5psi from the exhaust system to flow the next unit of exhaust gasses.

Now, run the engine at 4000 RPM (and... 100% throttle... Redundant today arent we?...). Pressure is now 10 psi, the perfect balance between overpressure and underpressure to flow the gasses 100% efficiently! Your engine will not have to work harder than it has to in order to push exhaust gasses out!

(Note: 10 psi is a made up number. The psi required for full efficiency will vary from exhaust system to exhaust system)

Understand now?

Posted by: Nomake Wan Aug 18 2012, 03:46 PM

QUOTE (elemein @ 6 minutes, 46 seconds ago)

No, when exhaust BACKPRESSURE is at 0psi, you're exhaust system is 100% efficient. Perhaps this will clear things up:

...Let say your exhaust system requires 10 psi of pressure for your exhaust gasses to be evacuated 100% efficiently.

It appears we are having a disagreement of definition.

Is not backpressure the pressure of gases in the exhaust system?

Posted by: elemein Aug 18 2012, 03:57 PM

QUOTE (Nomake Wan @ 10 minutes, 44 seconds ago)

It appears we are having a disagreement of definition.

Is not backpressure the pressure of gases in the exhaust system?

No. Backpressure is the resistance. Take apart the word: Back - Pressure.

Backpressure is simply the pressure pushing BACK and making resistance in the exhaust system. Resistance =/= pressure, resistance = strain on the engine to push exhaust gasses out. Resistance = lost HP

Exhaust system will NEVER operate at 0 psi overall pressure, or else there would be ZERO movement of gasses in the exhaust gasses. This can only ever be true when the engine is off; if it is on to ANY degree (even idle), there is pressure in the exhaust system.

Now, can an engine operate at 0 backpressure? Yes. 0 backpressure = 0 resistance = 100% scavenging = 100% efficiency = 0 HP lost from engine not having to push out the exhaust gasses.

To summarize; backpressure is simply the amount of excessive, or lack of overall pressure in the exhaust system. (Remember my example; overpressure is excessive (or too much) exhaust gasses. If overall exhaust pressure is 15 psi, in a system that requires 10 psi to flow 100% efficiency; 5 psi of that 15 psi is backpressure, because it is 5 psi excessive over the 10 psi required.)

Posted by: Nomake Wan Aug 18 2012, 04:06 PM

How is pressure generated? Is pressure not generated by resistance in a system?

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Posted by: elemein Aug 18 2012, 04:18 PM

QUOTE (Nomake Wan @ 12 minutes, 39 seconds ago)

How is pressure generated? Is pressure not generated by resistance in a system?

No.

Start the car.

Pressure is caused by the piston moving up, pushing the exhaust gasses out of the cylinder, pushing it into the exhaust system, having that gas collide with atmospheric air in the exhaust system. Some pressure is generated because there is now more air in the exhaust system than there would be at atmospheric pressure.

Now have this happen thousands of times per minute.

After a few seconds from starting the car, the engine would have dispensed so much exhaust gasses that scavenging would come into effect, therefore relieving some of the pressure itself.

The pressure would not continuously build up to infinite because air would be flowing out the end of the exhaust system (though this single cause is not enough to regulate THOUSANDS of litres of air going through an exhaust system per minute), scavenging would kick in and help regulate the pressure. Rev too high, and scavenging will be dappened due to the EXCESS pressure in the exhaust system (again. 100% efficiency is at 10 psi. 15 psi overall in the exhaust system? 5 psi of that is BACKPRESSURE, it is resistance pushing back and resisting flow), meaning that overall exhaust pressure would build up past the optimal 10 psi. Rev too low, and scavenging is again dappened due to the LACK of pressure in the exhaust system, meaning overall pressure is low. (Again. 100% efficiency is at 10psi. 5 psi overall in the exhaust system? The MISSING 5 psi is backpressure! The resistance to flow! It may sound silly saying lack of pressure is backpressure, but backpressure is simply resistance to flow; and not enough pressure is indeed a resistance to flow.)

In summary; the engine pushes out exhaust gasses into the exhaust system to create pressure, this pressure builds up and is relieved by the exhaust system (and scavenging) in order to maintain a happy overall pressure in the system to keep the gasses moving.

Posted by: MetalMan777 Aug 18 2012, 04:30 PM

QUOTE (Nomake Wan @ 24 minutes, 49 seconds ago)

How is pressure generated? Is pressure not generated by resistance in a system?

Sorta kinda not really. Even if your exhaust is blowing into a perfect vacuum (opposed to the ~100 kPa that exists at sea level), you're still going to have pressure in the system. Fluids (and in fact everything does, if you want to be pedantic) move in waves. The gas wave is pushed through the exhaust manifold because it has a higher pressure than atmospheric, which is compounded by the piston action decreasing the volume of the cylinder (increasing pressure). As the wave travels through the header pipe, it leaves a low pressure area behind it. The goal of an effectively scavenging exhaust system is to have the pipes meet such that a wave passes by the pipe of another cylinder right as the exhaust opens. The pipe will briefly see the partial vacuum that followed the wave, helping the next wave flow out of the cylinder.

2 strokes are even more fun, because you actually want the pressure wave to bounce back on the cylinder, due to the design of the ports. See: http://en.wikipedia.org/wiki/2_stroke

QUOTE (Elemein)

Rev too high, and scavenging will be dappened due to the EXCESS pressure in the exhaust system (again. 100% efficiency is at 10 psi. 15 psi overall in the exhaust system? 5 psi of that is BACKPRESSURE, it is resistance pushing back and resisting flow), meaning that overall exhaust pressure would build up past the optimal 10 psi. Rev too low, and scavenging is again dappened due to the LACK of pressure in the exhaust system, meaning overall pressure is low. (Again. 100% efficiency is at 10psi. 5 psi overall in the exhaust system? The MISSING 5 psi is backpressure! The resistance to flow! It may sound silly saying lack of pressure is backpressure, but backpressure is simply resistance to flow; and not enough pressure is indeed a resistance to flow.)

This is a pretty accurate description of what unwanted backpressure is. Backpressure is typically caused post-header in the cats, resonators, mufflers and other post-collector plumbing. Backpressure isn't going to ruin your scavenging effect, but as the backpressure increases, the less the waves are going to line up with the appropriate pipes.

Except that last part is a bit wonky. Insufficient pressure doesn't cause a resistance to flow, it just represents a lack of flow, because it can't push as hard against atmospheric pressure.

Posted by: elemein Aug 18 2012, 05:00 PM

QUOTE (Cactus @ 30 minutes, 0 seconds ago)

Except that last part is a bit wonky. Insufficient pressure doesn't cause a resistance to flow, it just represents a lack of flow, because it can't push as hard against atmospheric pressure.

Ah yes, this is more technically correct. I always saw it as a hinderance to flow, so I kind of filed it under "resistance" by mistake.

Posted by: Mazda ina Ford guy Aug 18 2012, 08:05 PM

Jeeezzz, elemein what are you, an engineering student? I was sort of, but I found I had the EYE OF THE ENGINEER but not the math skills.

Don't forget most production engines are designed to run with some back pressure to cushion the closing of the exhaust valve, used to be with leaded fuel a layer of lead would build up to cushion and seal the port and valve, but that's not the case these days. In WW2 the P-51 Mustangs exhaust produced as much thrust as it did drag, for a wash , engineeringly speaking.

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Posted by: MetalMan777 Aug 18 2012, 09:41 PM

I'm having trouble believing that. The cam profile is curved, so it's not like the valve is crashing into the seat. If that's the case, why don't desmodromic engines just explode?

Posted by: Nomake Wan Aug 18 2012, 10:13 PM

If having resistance is bad... then why don't F1 cars just run without headers? No resistance if you just dump exhaust out the exhaust ports!

Posted by: MetalMan777 Aug 18 2012, 10:44 PM

They've got stepped headers in F1. The smaller diameter just off the port maximizes velocity of the wave close to the valve, where the wider diameter(s) increase total flow.
user posted image

And it's not true that there's no restriction. Having to push against 100kPa of air is much harder than the partial vacuum (let's just assume 50kPa for the sake of argument) you'll get with a properly scavenging exhaust.

Posted by: Nomake Wan Aug 18 2012, 10:48 PM

But that makes no sense. A restriction that close to the exhaust ports, engineered on purpose? Why would they create resistance in the exhaust system on purpose? After all, backpressure = resistance, and backpressure = 100% bad! They should have just had the whole header be a uniform diameter, or even better, just vent the ports to atmosphere!

I suppose I just don't get it...

Posted by: MetalMan777 Aug 18 2012, 10:59 PM

It doesn't help that I'm an awful teacher, and that I have to sort through everything else in the thread to expose the truth.

There will always be restriction to the exhaust flow, even if it went straight from the port to a perfect vacuum. You have to think about the exhaust in terms of pulses or pressure waves. When the exhaust port opens, you get a pulse. You want a header tube that's relatively small in diameter, to maximize the velocity of the pulse. You want the pulse to pass from the immediate header through the collector/merger, so that you have the low pressure spot behind the pulse acting as a partial vacuum on the adjacent tube(s). This partial vacuum lowers the pressure (and thus resistance) in the port that is opening next, effectively sucking the exhaust pulse out.

The term backpressure is one of the most misleading I can think of.

Posted by: elemein Aug 19 2012, 05:32 AM

QUOTE (Mazda ina Ford guy @ 9 hours, 26 minutes ago)

Let me give it a shot.

@ Mazda in a ford

This may appear so, but this is untrue. Cactus has explained 1/2 the reason as to why the valves do not damage themselves upon closing. Closing is not a big slam, it is a gentle and gradual drop on most cars. Now, for the cars that have extremely aggressive cams, the other 1/2 of the reason as to why the cams do not damage themselves; is because the valves actually ROTATE on their follower as they open and close. The rotate open x degrees, and the then close those x degrees on the axis of their own followers. This rotation on the close causes the force caused when hitting the seat to disperse around the entire valve. This causes all the energy to be taken by the entire valve, and NOT just the part that hits the seat. This also gurantees even and circular wear so that weakpoints in the engine are not made.

If anything, backpressure makes more wear on the valve and rob more power. It creates a high pressure area behind the valve, making it harder to close, and robbing power. Also, read the .pdf I supplied; excessive backpressure can cause detonation because it slows gasses in a high pressure area, heating everything up and making detonation much more likely to occur. Which is bad.

@ Cactus' first post
Aye. Correct.

@ Nomake's first post
This is true. There is no resistance if you just dump out into the atmosphere. The thing is, this also KILLS all scavenging. Remember, scavenging needs to be able to carry the exhaust wave behind it through a median. What you are suggesting is like if I were to go back to my bathroom sink analogy, if I were to dip my finger, lift it, move it to the other side of the sink, and expect water to follow it. It is not possible. You have removed the median in which the water will follow it by raising into the air. The same is true for the exhaust; no exhaust? No scavenging.

@Cactus' second post
Precisely. Scavenging.

@Nomake's third post
Remember what I said; they do not engineer a resistance into the exhaust on purpose; it is a necessary evil. They engineer pressure on purpose; remember what I said. If I have 10 psi overall pressure in my exhaust, and 10 psi is it's maximum efficient pressure, then I have ZERO resistance. If I had 12 psi, then I have 2 psi of resistance. It is inevitable that no exhaust system will suit any car for it's entire rev range to flow 100% efficiently.

Also, venting the exhaust straight to the atmosphere completely eliminates all scavenging. Like I said before; some pressure is needed for scavenging to work (some PRESSURE is needed, NOT resistance. While having some resistance in the exhaust will not kill scavenging, it will dampen it and it is not required.) Here, let me try to whip up an illustration in paint

Here we go, took a minute: user posted image

Now, in the top illustration, you will see the engine has an exhaust pipe. The green blobs are exhaust units. Now, these exhaust units are under constant pressure to move out of the exhaust, so naturally, they create a low pressure area behind them. Now, Science 101 says that fluids will always travel the path of least pressure (or resistance, but we will not use that word because it may get mixed up with backpressure, and that is not intended), right? So the orange area behind the green blobs are there to represent the low pressure area; this low pressure area SUCKS the exhaust blob behind it! This makes the exhausting much easier because the first exhaust (far left) pulls the exhaust unit behind it, which pulls the one behind it, which pulls the one behind it, which pulls the one behind it, and so on. You will also notice there is no high pressure area in front of the exhaust blobs. Why is this? Because the exhaust unit moving infront of the previous exhaust unit relieves this area of pressure, and even takes pressure away from the area, and even creates a low pressure area. Remember Bernouille's principle; pressure is always less in moving fluids than in stationary ones, therefore, the low pressure area is created.

Now ditch the exhaust pipe. Clearly, as you can see, the exhaust blobs are thrown every which way, slowing down the second they hit the atmosphere! This completely disposes of the low pressure area behind them, and these exhaust blobs are now stationary (represented by a yellow + sign on them), because they had gotten slowed down by the atmosphere (the atmosphere is a STATIONARY gas, while the exhaust unit moving in an exhaust system under pressure is a MOVING gas, and therefore creates a low pressure area behind it.)

Understand now?

@ Cactus' last post
Correct. There should ALWAYS be restriction. Do not confuse this with resistance! The two are similar, but different in this context! The difference between growing a pair, and growing a pear!

Edit: By the way, this is going to sound weird, but I like writing about cars (clearly...), would it be okay to post informative threads in the Technical section? Moreso just to educate (and burn my time... I have a lot of that...)

Posted by: JEV Oct 16 2012, 05:49 PM

my mind is blown... too much good and bad info,lol.

But i know nothing so... I'll just say +1 for the last guy.

I just like how it all got broken down to the little details, i just like what one guy wrote over this topic... on a mag, which pretty much was... "It's technically always a compromised design."