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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.

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


This post has been edited by Cactus on Aug 18 2012, 03:14 PM

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.

This post has been edited by elemein on Aug 18 2012, 03:19 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.

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?

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.)

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

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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.

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

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.

This post has been edited by Cactus on Aug 18 2012, 04:49 PM

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.

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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This post has been edited by Mazda ina Ford guy on Aug 18 2012, 08:11 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?

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!

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.


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.

This post has been edited by Cactus on Aug 18 2012, 10:46 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...

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.

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:

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...)

This post has been edited by elemein on Aug 19 2012, 07:04 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."