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

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

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.

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...

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.)

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.

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.

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

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.

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.

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

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.

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

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?

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

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)

This post has been edited by atlantian on Mar 20 2008, 03:42 PM

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.

pretty sure having no cat is illegal

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.

This post has been edited by rockyz on Jun 11 2008, 10:42 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.

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.

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

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.

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!

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.