Exhaust backpressure

[gary_tholl]

Ok, do ANY engines actually need backpressure in the exhaust to make power???

I've heard this many times on everything from high revving 4 cylinders to big V8s...

I've played with engines alot, and have read quite a bit on the theory behind them, so I have to say that, IMHO, it's bollocks, but I don't have a good explanation for why.

So, any of you engine experts out there care to enlighten me on this one?

Gary

[GreenV8S]

Turbo and supercharged ones certainly do, you need adequate backpressure to prevent blow-through at low rpm.

On NA engines I'm not so sure. I guess back pressure could extend the exhaust pulse duration and change the tuning characteristics, which could change the behaviour during the overlap period, or could change the interaction between cylinders. Don't know whether these changes are significant though.

[gary_tholl]

Ok, but wouldn't the 'proper' way to fix blow through be to minimize overlap between the intake and exhaust valves? With forced induction the overlap shouldn't be required to get a good intake charge like it is on N/A.

I'm fairly sure that backpressure would change the tuning characteristics, but I still fail to see how, in general, it can improve anything.

It's just one of those things that always annoys me when I hear someone say an engine needs backpressure to make power.

Gary

[deltaf]

GreenV8S said:

Turbo and supercharged ones certainly do, you need adequate backpressure to prevent blow-through at low rpm. On NA engines I'm not so sure. I guess back pressure could extend the exhaust pulse duration and change the tuning characteristics, which could change the behaviour during the overlap period, or could change the interaction between cylinders. Don't know whether these changes are significant though.

I dont subscribe to the theory.

Put your hand over your mouth then attempt to breathe out through it. Hard isnt it.
Backpressure is a killer of power whether in a turbo or N/a application.
Backpressure raises exhaust temperatures through restriction, restriction is a bad thing all round for power.
Exhaust valve opens, backpressure pushes burnt gasses back into the exhaust port and contaminates the charge.
Burnt gasses dont burn, so power drops. I understand your point about drawing thru, but all that will happen there is that unburnt fuel gets into the exhaust increasing emissions and lowering economy, but theres better ways to get by that problem rather than using what is basically a restricted exhaust; restricted by backpressure that is, using a properly matched turbo for instance, correct cam and ignition timing also.
I cannot see there EVER being a NEED for backpressure of any levels being desirable as there is NEVER a rise in power on an engine when backpressure is introduced.
None is the best you can aim for, and smarter people than me would attest to it. Im sure the designers of F1 engines would agree with what ive said...(well i hope they would)

[Pies]

2 Sheds

[HarryW]

Certainly one aspect of engines I've never fully understood myself. Very similar to the effect of having no exhaust headers fitted, AFAIK its no good for power .
Perhaps this is where the back pressure idea comes from confusing the need to extract the exhaust gasses with a well tuned exhaust. Which through a high exhaust gas speed it creates a minor suck at the end of the exhuast stroke helping to pull through the mixture charge (I realise this is also part of overlap as well), or maybe I'm talking out of my arse again. Not sure anyone really know

Harry

[deltaf]

HarryW said:

Certainly one aspect of engines I've never fully understood myself. Very similar to the effect of having no exhaust headers fitted, AFAIK its no good for power . Perhaps this is where the back pressure idea comes from confusing the need to extract the exhaust gasses with a well tuned exhaust. Which through a high exhaust gas speed it creates a minor suck at the end of the exhuast stroke helping to pull through the mixture charge (I realise this is also part of overlap as well), or maybe I'm talking out of my arse again. Not sure anyone really know Harry

The no headers thing Harry. The power loss is measured because ill bet a pound to a penny that the engine wasnt re-optimised for the "new" conditions.
Also lets not forget about "tuned lengths" that make use of extraction effects due to those lengths.
One thing is for certain, optimise an engine with no backpressure in its exhaust and itll always make more power than one optimised WITH backpressure.

[Pigeon]

Piston-ported two-strokes have the problem that because the exhaust port opens before the transfer ports, it closes after them. With a bare exhaust port this would result in losing charge out of the exhaust port, so the exhaust pipe is designed to reflect a positive pressure pulse back to the cylinder during the period when the exhaust is still open but the transfers have closed, to prevent charge loss. This is often called a "plugging pulse".

Uniflow-scavenged two-strokes, and four-strokes, benefit from an exhaust which is designed to reflect a negative pressure pulse back to the cylinder during the period that the exhaust valve is open, to help suck the exhaust out of the cylinder. Not so much back-pressure as back-suction.

[gary_tholl]

Pigeon said:

Uniflow-scavenged two-strokes, and four-strokes, benefit from an exhaust which is designed to reflect a negative pressure pulse back to the cylinder during the period that the exhaust valve is open, to help suck the exhaust out of the cylinder. Not so much back-pressure as back-suction.

Yeah, that stuff I understand. Reverse pressure waves from primary-collectors connections, etc. That is based on tuning the exhaust size and length, not by adding a restriction anywhere.

Gary

>> Edited by gary_tholl on Monday 13th September 23:52

[GreenV8S]

deltaf said:

I cannot see there EVER being a NEED for backpressure of any levels being desirable as there is NEVER a rise in power on an engine when backpressure is introduced.

I'm open to pursuasion in the NA case, although I can see effects that *might* be useful I don't have any evidence to show that they actually are. Some of the factors which affect power (exhaust section, length, shape etc) also affect back pressure and gut feeling (always a sound basis for engineering decisions eh? ) is that the optimal compromise is not always the one which has least back pressure.

On the forced induction case however, I know that they don't work well without adequate back pressure to prevent blow-through. I accept that there are other ways to address the blow-through problem, and in an ideal world with infinitely variable cam timing and so on the need for back pressure could be engineered out, but we don't live in an ideal world . Back pressure is a good practical solution which works and is used.

[stevieturbo]

on a forced induction car, no backpressure is what you want.
Ever see a top fuel car with restrictive exhausts ?

Why on earth would you possibly want to restrict an engines breathing ???

Trying to say that backpressure on a n/a is required is also misleading. Using tuned lengths has nothing to do with backpressure, these systems may happen to introduce a small amount, but that is not what allows them to create the power.

On any exhaust system, you want the minimum possible backpressure, whilst also having as much scavenging effects as possible, using various manifold designs. Some may work better at particular rpm's, but its not because of restriction they work better

[Pigeon]

gary_tholl said:

Pigeon said: Uniflow-scavenged two-strokes, and four-strokes, benefit from an exhaust which is designed to reflect a negative pressure pulse back to the cylinder during the period that the exhaust valve is open, to help suck the exhaust out of the cylinder. Not so much back-pressure as back-suction. Yeah, that stuff I understand. Reverse pressure waves from primary-collectors connections, etc. That is based on tuning the exhaust size and length, not by adding a restriction anywhere.

So is the two-stroke case. The positive pressure pulse is created not by a restriction, but by a converging cone, in the same way as the negative pulse is created by a diverging cone (not generally a four-stroke technique, they use the stuff you mentioned). You don't want any restrictions in either case.

[grahamw48]

stevieturbo said:

on a forced induction car, no backpressure is what you want. Ever see a top fuel car with restrictive exhausts ? Why on earth would you possibly want to restrict an engines breathing ??? Trying to say that backpressure on a n/a is required is also misleading. Using tuned lengths has nothing to do with backpressure, these systems may happen to introduce a small amount, but that is not what allows them to create the power. On any exhaust system, you want the minimum possible backpressure, whilst also having as much scavenging effects as possible, using various manifold designs. Some may work better at particular rpm's, but its not because of restriction they work better

Or 'restricted' on F1's ?


>> Edited by grahamw48 on Wednesday 15th September 00:06

[GreenV8S]

stevieturbo said:

on a forced induction car, no backpressure is what you want.

I disagree. On a forced induction car, balanced (inlet and exhaust) pressure is what you want. If you don't have balanced pressure, all your expensive boost disappears down the exhaust pipe and you LOSE power at low RPM (as well as lighting up the exhaust). Turbo systems give you balanced pressure inherently. Supercharged systems don't, and are vulnerable to low RPM blow-through. Raising the backpressure at low RPM to give you balanced pressure prevents blow-through and hence increases power. Conventionally, supercharger cams are run with very low overlap to reduce the effect, but this compromises the top end performance and increases the risk of thermal runaway and the consequent grenading engine. Balancing the back pressure at low RPM solves the blow-through problem without compromising the cam.

Presumably you don't consider that low-rpm blow-through is a problem on supercharged engines, or you have solved the problem some other way?

[Mr2Mike]

Back pressure is most certainly not desireable on turbo charged installations (or N/A come to that). Back pressure reduces the performance of the turbo, limiting the maximum boost it can produce at high RPM and even worse makes the turbine and exhaust valves run hotter, promoting pre-ignition. Back pressure also increases pumping losses.

I don't understand why you think the boost disappears down the exhaust pipe? This can only happen during the overlap period, which is much lower on a turbo'd engine than a highly tuned N/A engine anyway.

[pdV6]

Pigeon said:

gary_tholl said: Pigeon said: Uniflow-scavenged two-strokes, and four-strokes, benefit from an exhaust which is designed to reflect a negative pressure pulse back to the cylinder during the period that the exhaust valve is open, to help suck the exhaust out of the cylinder. Not so much back-pressure as back-suction. Yeah, that stuff I understand. Reverse pressure waves from primary-collectors connections, etc. That is based on tuning the exhaust size and length, not by adding a restriction anywhere. So is the two-stroke case. The positive pressure pulse is created not by a restriction, but by a converging cone, in the same way as the negative pulse is created by a diverging cone (not generally a four-stroke technique, they use the stuff you mentioned). You don't want any restrictions in either case.

Furthermore, the positive pressure wave can be tuned to bounce back after there has already been some blow-through, forcing the excess charge back into the cylinder under pressure thus providing more bang per stroke.

[HarryW]

Mr2Mike said:

.....I don't understand why you think the boost disappears down the exhaust pipe? This can only happen during the overlap period, which is much lower on a turbo'd engine than a highly tuned N/A engine anyway.

I thought thats what Peter said overlap is determined by the cam type, in his case for superchargers . i.e. a cam suited with a small overlap. IMHO most NA performance cams have a fairly high overlap, hence a lot of poping and banging on the overrun. I have a 218 in mine which whilst a pretty good performance cam has a realtivley small overlap which gives virtually no popping and banging.
Maybe a good cam for the RV8 for forced induction too .

Harry

[stevieturbo]

Generally Turbo cams are best with small amounts of overlap. The reason being, is that exhaust gas backpressure is usually higher then inlet manifold pressure, and during overlap, the exhaust gasses, can very easily contaminate the inlet charge. Thats bad.
Unless you have an incrediably efficient manifold design, and good turbo sizing, then if you can manage to get EGBP lower than boost pressure, then you have a recipe for huge power. aka old F1 turbo engines.
If you have back pressure on the exhaust side of the turbine, all you are doing is making things very innefficient, and the turbo has to work even harder to try and overcome it. If there is 30psi before the turbine and 10psi after, then really you are only making use of 20psi to actually drive the turbine. 10psi is being wasted.
If there is 0psi after the turbine, then you can make use of the full 30psi that is available to it.
Anyone who things backpressure on a turbo car is good, is very badly misinformed.

A SC installation may be slightly different, but either way, zero backpressure would be the order of the day. If you are worried about blowing the inlet charge out the exhaust, then that problem is camshaft related, not because there is no backpressure. It is that cam that is making the system wasteful. Restricting the exhaust would only make another part of the system innefficient.

>> Edited by stevieturbo on Wednesday 15th September 20:30

[Julian64]

My R1 has an exup valve which is little more than a varying restriction in the exhaust pipe.

I think that a rotating butterfly valve in the exhaust is more likely to be providing back pressure than tuning the exhaust?

Maybe a google search on exup valves?

[gary_tholl]

Esprit's have the rotating butteryfly valve in the exhaust as well. That is there to allow the cat to warm up quicker.

Peter, I undrestand what you mean by not allowing intake charge to blow through, but I still contend that changing the cam to one with less overlap is a better solution than introducing and exhaust restriction.

I'm not sure how much of a detriment that low overlap would be at higher RPM, due to the fact that the intake is at a significantly higher pressure, being forced into the cylinder.

I've heard of engines being described, simply, as air pumps, you need to get it in, and get it out. Any restriction to that isn't helping.

But, I'm not a big turbo guru, so I can be swayed on this one.

Oh, and thanks to everyone for confirming my suspicions.

Gary