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

[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

[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

[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

[gary_tholl]

kneegrow said:

You could have a variable choke in the exhaust but this would be a bit silly really, considering a compromise build would give you a very tractable engine.

Why would this be silly?? You could gain the slighlt backpressure needed to combat overlap at low rpms, and still open the exhaust to flow freely at high rpms. I'd say that building a compromise into the engine would be silly.

Mr2Mike said:

Not if the lost charge has already been replaced with more. It would certainly hurt the fuel efficiency of the engine, but as long as you can still push an arbitrary amount of mixture into the cylinder once the exhaust valve has closed then power will not suffer.

The trouble with this is that you don't have a never ending supply of intake charge at a certain boost. As well, pumping fuel into the exhaust can harm cats/sensors/etc.

I believe the difference of opinions is simply down to relative amounts. While blow through at low rpm won't rob top end power, it does rob low to mid-range torque. This may not be of any concern to some, but I like low to mid-range torque. You can eliminate/reduce blow-through by reducing overlap to near nothing, but this DOES affect top-end power, even on a SC engine. So, why build that compromise into the engine? Why not tune (not restrict) the exhaust to reduce the blow through at low rpm while still flowing freely at high rpm, even with an overlap cam? Wouldn't that be the best solution?

Any ideas on how that is achievable?

Gary

>> Edited by gary_tholl on Wednesday 22 September 16:23

[gary_tholl]

jwb said:

Just out of interest for a no loss exhaust system it must flow 2.2 cfm per bhp. John

Not sure where this came from, but I'm willing to bet that it depends on a few variables (say, rpm), and is far from being a constant.

Gary

[gary_tholl]

stevieturbo said:

Do you blame the exhaust on a n/a car with a 300deg cam, that only produces power above 4000rpm, for causing a lack of low down torque It basically boils down to, Forced induction engines do NOT need wild cams, so why would any idiot try and use them ?????

You wouldn't blame the exhaust, but you can tune the exhaust to reduce the lack of low-down torque.

Who mentioned anything about wild cams? Even regular cams have overlap. So, if by changing(tuning) the exhaust you were able to use the 286 degree cam in your mini, with it's 8000rpm top end, and still have useability below 3000, that's a bad thing?? I don't understand why you feel you need to compromise on the cam (either top end or low end, not both), when there are other factors that can accomplish a similar outcome.

Pigeon said:

There are plenty of possibilities... one that suggests itself to me as a two-stroke-head is pulse tuning. On a four-cylinder four-stroke you could have a 4 into 1 exhaust with the pipe lengths from head to collector are such that the exhaust pulse from the next cylinder to exhaust causes a positive pressure pulse to arrive at the cylinder which is completing its exhaust stroke during the overlap period, thus effectively "closing" its exhaust valve sooner, and pushing any spilled charge back into the cylinder. At higher revs this pulse will arrive after the exhaust valve has already closed, so will have no effect. An Exup-valve arrangement, considered as a switchable reflector rather than a switchable restriction, is another possibility.

Doesn't a 4-1 collector create a negative pulse travelling back down the other 3 primaries though? That can be used for scavenging purposes, but wouldn't create a 'wall' to stop blow through. You need something for the exhaust pulse to reflect off of (such as the bike's exup valve) to create a positive wave back towards the exhaust valve.

Gary

[gary_tholl]

The funny thing about all this is, fundamentally we agree. The difference is that we are talking about degrees of change. You say that blowthrough due to overlap is miniscule and should be discounted, and that a low overlap cam will work at higher rpms.

Peter (and now I) say that, while blowthrough at low rpm won't make or break an engine, why put up with the losses it creates when you can tune the exhaust to reduce the blow through? By doing this, you can run the cam with some (not a race cam) overlap, which is going to work better at higher rpms.

Now, if we could come up with something that would reduce the blow through at low rpm, but flow freely at higher rpm, why wouldn't that be the best of both worlds?

As for the 2.2 cfm bit, if that's the case, why do most diesels run drainpipe exhausts for maybe 200-250 hp, while a gas engine that produces the same can get by with a much smaller exhaust? I've read some of Vizards stuff, and he does seem to know it, but that just seems way to simplistic.

Gary