I'm writing this to explain why backpressure is bad. If you want to argue, please read up on it first, then come back. If you want to just continue on with your life, please skip this.

This has always irked me. People are always saying how engines need exhaust backpressure, or all these mysterious bad things happen. I know it's kindof just semantics, but I just want to clear it up.

I'm here to tell you that pressure in the exhaust is 100% always a bad thing, turbo or not.

I'm going to focus on a non-turbo engine, because putting a turbo into the mix makes a bit of pressure in the exhaust manifold/headers no matter what, and I just want to explain the concepts.

A long time ago, back in the caveman days of carburetors, changing anything on the engine meant adjusting the carbs, because they're designed to work for that very particular engine flow rate, ve, blah and blah. That is why when people would install larger exhausts, they might burn valves, because the car was actually running lean, because they wouldn't adjust their carbs to compensate for the exhaust. Not because of the exhaust itself.

To explain the loss of torque in the lower rpm ranges that does happen: exhausts, if well designed, use the principle of scavenging to help pull exhaust out of cylinders. How this works is, imagine you have a manifold with equal length runners, and they all meet up in a collection area. One exhaust pulse races down its runner, into the collection area, and down the exhaust. It has forward momentum, but is eventually slowed, because the valve has closed behind it, so the pulse will create a slight vacuum behind it, then the next valve opens, and it's pulse is helped slightly by that vacuum. This is scavenging. Scavenging only works at one particular frequency/temperature/volume level though. Move out of that frequency, and you have exhaust pulses that actually run into eachother, creating a restriction. The frequency all depends on the size of the exhaust, runners, and many other factors. In general, a smaller exhaust (read stock) is most efficient at a lower frequency/rpm range. Car manufacturers do this, because most people drive around at 1-3k rpms all the time, and making the exhaust efficient there increases gas mileage. Larger, aftermarket exhausts are more efficient at scavenging at a higher rpm. Thus the apparent loss of low rpm torque. The exhaust is like a musical instrument that can only play one note. Try to play another note through it, and it's not going to work as well. However, if you were to remove the exhaust system altogether, you would pretty much see gains all across the board (unless you just removed a very well constructed/designed/tuned exhaust, and then you would only lose torque where the scavenging effect was the greatest). This is because there is _no_ exhaust back pressure anymore at all. There is no scavenging effect, but you wouldn't have to worry about it. However, you would have to worry about your car burning down.

The easier the exhaust can get out, the better, no matter what. Back pressure in the exhaust is always bad, and just causes the engine to have to work harder to push the exhaust out, losing torque. Saying that the engine needs back pressure in the exhaust is like saying that you need vacuum in the intake. The intake and exhaust are very similar, and all the scavenging applies to the intake too, that's why runner length/diameter/shape on both the intake and exhaust are important.

A vehicle with a turbocharger, you can still worry about all of these things. However, the gains you get from making the turbo more efficient far outweigh the gains you get from making the exhaust more efficient. Turbos make the engine have to push the exhaust out into a manifold with lots of pressure in it, but the gains you get from the turbo allow you to say screw it. It's like how a supercharger may take 20 ft-lbs of torque to run, but you gain another 60 ft-lbs back, so you ignore the fact that the supercharger is sapping energy, and just look at the overall. Turbos work well with high manifold pressure, and low exhaust pressure, engines do not work well with high manifold pressure. You ignore one for the benefits of the other.

So, exhaust backpressure is always bad. Good exhaust design can move the efficiency range to a higher rpm, where it's more useful for racing, but decrease the low end torque, and bad exhaust design just sucks (not in a good way). It's not backpressure you need, it's a well designed exhaust suitable for your needs.

-Jesse

"Always bad?" Have you ever heard of toss control?

FINALLY someone who knows what they are talking about. I have been arguing with this noob in the newbie forums who "says" he knows what hes tlaking about. Just would like to thank you.

Look into 2-stroke engines for exhaust tuning information. On The Pipe.

Good write-up, Enigma.

Some EGR systems use exhaust backpressure to open/close the pintle and I would think cams with alot of overlap would benefit from some backpressure.

Originally posted by LightningGSX
Some EGR systems use exhaust backpressure to open/close the pintle and I would think cams with alot of overlap would benefit from some backpressure.

Click to expand...

One of the benefits of overlap on long duration cams is the scavenging that it promotes. The effectiveness of the scavenging is tied to the pressure differential of the intake and exhaust manifolds, and the inertia of the air/fuel mixture shooting into the cylinder.

If you have less backpressure, then the pressure in the exhaust manifold is lower. If the pressure in the exhaust manifold is lower, then the pressure differential between the intake and exhaust manifolds is greater, so better scavenging.

Anything wrong with my reasoning?
Cheers,
Adrian

Very well done Jesse. I have been telling people this for years, but I have never taken the time to write up a nice article like that.

Toss control sounds like something you want at a ### bar. Are you going to provide us with any more information, or do you just like saying that because it makes you feel like you know what you are talking about?

People do not understand that airflow into the cylinders is dependant upon the pressure differential between the intake and exhaust ports. Less backpressure is like running more boost, because it will increase this differential.

With more overlap, you certainly want LESS backpressure.


....Kyle T.

What I meant was, during periods of overlap, scavenging of the intake charge would also occur.So some backpressure would keep this at a minimum while still scavenging exhaust gasses.Anyway it was just a thought

Or the more pressure differential between int and exh valves would,during periods of overlap, scavenge more of the intake charge.So with more overlap you certainly DON'T want less backpressure.

Originally posted by LightningGSX
Or the more pressure differential between int and exh valves would,during periods of overlap, scavenge more of the intake charge.So with more overlap you certainly DON'T want less backpressure.

Click to expand...

You've got me confused on that one.
Scavenging is good. Why would you want to decrease it?
The only reason I know of to reduce scavenging is for emissions.. and I assume this is not what you are talking about. (Scavenging gets more unburnt fuel into the exhaust)

When both int and exh valves are open at the same time you can't scavenge exhaust gas without taking some intake gas with it.I would assume this isn't the case in general but It can happen.I was just making a point,which is, there is almost always other variables in the equation.Extreme amounts of a "good" thing overall isn't always good, there is always a tradeoff of some kind.So you can't just say something is absolutely true, while it may be in most cases.

Originally posted by LightningGSX
When both int and exh valves are open at the same time you can't scavenge exhaust gas without taking some intake gas with it.I would assume this isn't the case in general but It can happen.I was just making a point,which is, there is almost always other variables in the equation.Extreme amounts of a "good" thing overall isn't always good, there is always a tradeoff of some kind.So you can't just say something is absolutely true, while it may be in most cases.

Click to expand...

What is wrong with scavenging "too much?" It is very doubtful that you are ever going to get to the point of a large amount of the intake charge escaping out the exhaust ports during overlap, but even if that does happen, how is it bad? That would almost guarantee a pure air/fuel mixture in the chamber, thus leading to more power.

...Kyle T.

Some of that pure A/F mixture would be scavenged with the exhaust gas.Not likely, but possible.Anyways like I said, it was just a thought.

Originally posted by Enigma_Man
That is why when people would install larger exhausts, they might burn valves, because the car was actually running lean, because they wouldn't adjust their carbs to compensate for the exhaust. Not because of the exhaust itself.

Click to expand...

Some new fuel injection vehicles have that problem too.I remember back in 97-98 we had alot of brand new Chevy pickups burn up valves from putting dual exhausts on them.The ones with the 454 if I remember...And what is toss control?

I'll cut and paste that into a thread where someone was saying that a turbo "Needed Backpressure to spool"

I still don't agree that backpressure is the enemy in N/T cars... Take the exhaust of a turbo car and it'll fly, take the exhaust off an N/T car and it will just be louder and lose power. Don't believe me, convince a friend with a Honda to disconnect his exhaust and go to the track, see if their times aren't slower than they were when they had an exhaust bolted up. I'm not saying they need a restrictive exhaust. I'm just saying that they need SOME backpressure.

Originally posted by AlecW81
I'll cut and paste that into a thread where someone was saying that a turbo "Needed Backpressure to spool"

I still don't agree that backpressure is the enemy in N/T cars... Take the exhaust of a turbo car and it'll fly, take the exhaust off an N/T car and it will just be louder and lose power. Don't believe me, convince a friend with a Honda to disconnect his exhaust and go to the track, see if their times aren't slower than they were when they had an exhaust bolted up. I'm not saying they need a restrictive exhaust. I'm just saying that they need SOME backpressure.

Click to expand...

I don't believe this is correct, I think what they really need is velocity.
Back pressure is just resistance to getting the exhaust out. This in itself is bad. But if you have a narrow tube for the exhaust to go through, it will have a very high velocity and therefore a high inertia. This high inertia can help to pull out more exhaust gasses than would otherwise exit the cylinder.

Besides this, you can have a tuned exhaust so that at your target RPM the exhaust pulse pressure wave bounces back and creates a low pressure area at the back of the exhaust valve, right as it is opening (or is open). This helps to suck out more exhaust gas as well.

It's all about pressure differentials. If you can get reduce the pressure at the back of the exhaust valve, then you have increased the pressure differential between it and the cylinder pressure. When you increase the pressure differential, the exhaust will exit the cylinder more quickly and more completely.

So backpressure is bad, but is a necessary side effect of increasing velocity by using a somewhat narrow passage.

-Adrian

Originally posted by ahains

So backpressure is bad, but is a necessary side effect of increasing velocity by using a somewhat narrow passage.

-Adrian

Click to expand...

Upon highest heights in heaven, the choir douth sing bawdy 70's porn music and thy angels did humpeth each other in mass quantity and it was good. The lord, almighty who art in heaven, looketh down from thine rosey throne, whom in thine infinity wisdom, installed a integrated toilet and remote control system, and rested on the 7th day for thy message had been passed: Velocity is the key and backpressure a mere side evil.

Here endeth the lessen. Amen.

Originally posted by AlecW81


I still don't agree that backpressure is the enemy in N/T cars... Take the exhaust of a turbo car and it'll fly, take the exhaust off an N/T car and it will just be louder and lose power. Don't believe me, convince a friend with a Honda to disconnect his exhaust and go to the track, see if their times aren't slower than they were when they had an exhaust bolted up. I'm not saying they need a restrictive exhaust. I'm just saying that they need SOME backpressure.

Click to expand...

Did you even read the original post?

That Honda does not need backpressure, it needs SCAVENGING, the velocity that ahains talked about. It DOES NOT need backpressure.

....Kyle T.

While I agree that backpressure is higly overrated, I have seen back to back dynoruns of all motor high revving cars like Hondas. Open header or even open HEAD the lose tq all the way until over 6k. And I would think of a header as a high velocity exhaust. Im not making any points here, just a statement of fact about how Hondas react here.

Sean

Yes, I agree that the Hondas, or most motors for that matter, will lose low end power when the exhaust is removed. However, this has NOTHING TO DO WITH BACKPRESSURE. This loss is due to a lack of scavenging, and a much slower exhaust velocity.



....Kyle T.

Originally posted by DV8tion
"Always bad?" Have you ever heard of toss control?

Click to expand...

What is toss control???? anyone????

Originally posted by LightningGSX



What is toss control???? anyone????

Click to expand...

I searched online but couldnt find anything about this.

Engines tend to "toss" connecting rods during valve float or at the end of the exhaust stroke or the beginning of the intake stroke due to the fact that there is little to no force on the top of the piston. This is when the tensile load on the rod is the highest. In a turbo application the back pressure in the turbine housing/manifold produce force on the piston to offset some of this tensile load. The intake pressure also imparts force on the piston during the intake stroke. These offsetting loads are relatively small but in the scale of the number of cycles being produced these small forces are a big help in keeping a turbo engine together.

I've never heard mention of that before, though it makes some sense. But, a search on google gives no mention of anything about it, so it sounds like another myth, like the backpressure myth.

Also:

I'm 100% sure that a connecting rod is more than adequate to withstand the tensile load of changing the piston's direction from up to down... even at extremely high rpms. A piston weighs maybe 700 grams max (a quick lookup on the 'net says that this one company's 4g63 pistons with the connecting pin weigh less than 500 grams, so I'm being conservative). I don't want to do all the math out, but it's well within the bounds of what the connecting rod and pin can handle, especially considering that the piston doesn't change direction suddenly, it's decelerated for half it's upward journey, then accelerated for half its downward journey. There's no jarring motions at all.

The pressure in a turbo manifold at full-bore I've heard is somewhere around 10-ish psi or so (just ballpark estimate, I could be wrong). Even with that pressure on top of the piston, it's not going to make that much of a difference in the tensile load of the rod anyway.

I disbelieve toss.

-Jesse

If you can't find it on google it cant be real.
I only said it helps, I did'nt say it was responsible for keeping the rods together.
Exhaust manifold pressure should stay fairly close to intake manifold pressure, but in reality exhaust pressure stays higher because of the use of restrictive small AR turbine housings.