Myth or Reality, low backpressure=low torque

[doradox]

Funny thing is, the pressure will always be highest right at the exhaust port if the exhaust system is properly designed.

Except when the low pressure part of the the pressure waves are at the exhaust port.

Steve

[ozconnection]

5 verses 8 so far have said that backpressure in the exhaust is important to maintain low speed engine torque.

Any thoughts from those people?

[John Coffey]

Except when the low pressure part of the the pressure waves are at the exhaust port

Yes, at very specific rpm points in wave tuning. If you hook up pressure gauges (which I saw done on an engine I had built for me) the readings show higher pressures at the primaries and lower pressures at the tailpipe mostly because the gauges are not fast acting enough to show wave action. Ideally you also see a low pressure point just past the header merge collector or the secondary pipe merge collector. A pressure difference of 1 to 2 psi is ideal.

[d240zx2]

:dead:Ok, I'm guilty. But after reading the excellent dissertation, I know why I was wrong.

Years ago (60?) my dad was overhauling a lawnmower. Stick with me, now.

Since the contraption was noisy as all get out anyway, I asked why he needed to put the muffler back on it. His reply is muddled in my memory now but it had something to do with burning/damaging the exhaust valve if cold air(relatively speaking) hit the hot valve. Therefore, the exhaust required a muffler to keep cold air from hitting the valve.

That bit of info translated into "backpressure" as I matured and, until now, never put the noggin into gear to examine my thinking.

Thanks for the intellectual stimuli.

Frank

[doradox]

Yes, at very specific rpm points in wave tuning. If you hook up pressure gauges (which I saw done on an engine I had built for me) the readings show higher pressures at the primaries and lower pressures at the tailpipe mostly because the gauges are not fast acting enough to show wave action. Ideally you also see a low pressure point just past the header merge collector or the secondary pipe merge collector. A pressure difference of 1 to 2 psi is ideal.

Yes I agree, not always.

Steve

[ozconnection]

I'm wondering how many of those people who responded on the poll with a 'engine needs backpressure' response actually have stock exhaust systems on their cars?

Would that fact alone invalidate their responses in the poll?? "I don't like cheese" and "Have you ever eaten it?" would be my response. Equally, the question could be applied to our exhaust systems too, I think.

Are we getting it all wrong when there is still such a high percentage of people out there how refute the notion of reducing backpressure loses engine torque?

What will it take to convince those who still aren't convinced? Suggested reading and dyno results aren't having the anticipated effect.

[m4xwellmurd3r]

:dead:Ok, I'm guilty. But after reading the excellent dissertation, I know why I was wrong.

Years ago (60?) my dad was overhauling a lawnmower. Stick with me, now.

Since the contraption was noisy as all get out anyway, I asked why he needed to put the muffler back on it. His reply is muddled in my memory now but it had something to do with burning/damaging the exhaust valve if cold air(relatively speaking) hit the hot valve. Therefore, the exhaust required a muffler to keep cold air from hitting the valve.

That bit of info translated into "backpressure" as I matured and, until now, never put the noggin into gear to examine my thinking.

Thanks for the intellectual stimuli.

Frank

ah, yeah, we did that to a friends go-kart and my dad said that if we don't put the muffler back on we'd burn the valve up. the reason being is that the exhaust itself works as a sort of heatsink, directing all the heat away from the valves and such, or something like that.

[WingZr0]

Just a thought here but which would be "better" then at the

points dicussed out of these four set ups and why?

1.- A 6 into 1 header with single exhaust pipe out back.

2.- A 6 into 2 header going to a sinlge exhaust pipe out back.

3.- A 6 into 1 header with dual exhaust out back.

4.- A 6 into 2 header with dual exhaust back.

~Z~

[doradox]

Just a thought here but which would be "better" then at the

points dicussed out of these four set ups and why?

1.- A 6 into 1 header with single exhaust pipe out back.

2.- A 6 into 2 header going to a sinlge exhaust pipe out back.

3.- A 6 into 1 header with dual exhaust out back.

4.- A 6 into 2 header with dual exhaust back.

~Z~

Any one could be better depending on the specifics of the exhaust design ( tubing size and length to name two) and what your definition of better is. More top end power? More low end? Works best with cam X, Y, or Z. I think the whole point is that it's not black and white. It's the combination of lots of factors, many of which are not well defined. There's a science to it but there's some dark art to it as well.

Steve

Steve

[WingZr0]

Any one could be better depending on the specifics of the exhaust design ( tubing size and length to name two) and what your definition of better is. More top end power? More low end? Works best with cam X, Y, or Z. I think the whole point is that it's not black and white. It's the combination of lots of factors, many of which are not well defined. There's a science to it but there's some dark art to it as well.

Steve

Steve

Ok, lets say cam, everything being stock.

Will one set up yeild a neccesary bad result over all?

I'm more of a Drag Strip guy myself just because I see more stop light to

stop light action than a racetrack and prefer low end torqe over

top end.

But at a Road Course I have to say top and speed over anything.

~Z~

[doradox]

Ok, lets say cam, everything being stock.

Will one set up yeild a neccesary bad result over all?

I'm more of a Drag Strip guy myself just because I see more stop light to

stop light action than a racetrack and prefer low end torqe over

top end.

But at a Road Course I have to say top and speed over anything.

~Z~

If you want someone to design an exhaust system for you try...

http://www.burnsstainless.com/TechArticles/Theory/theory.html

Look at this..

http://www.burnsstainless.com/Xdesign/Race_Engine_Spec__Form/race_engine_spec__form.htm

to get an idea of the complexity of designing a system for a particular engine.

Or just go with what other racers are using. At least you won't be any worse off than they are.

Steve

[Oiluj]

Being one of the "back pressure guys" in the poll, I'll give my thoughts...

1) As has been stated above, I think it's more an issue of semantics, not interpretation or analysis of technical data.

2) If "zero" back pressure is ideal, why have exhaust manifolds and exhaust systems at all? In fact, why don't all race cars have no exhaust systems?

My understanding is that you can "tune" the exhaust system using a combination of variables to maintain flow to help "scavage" the exhaust gases to achieve optimal performance within a specific power band. Like most things, one size does not fit all.

3) Then there is the more "aesthetic" nature of the sound produced and personal preferences. For example, I want a more restrained exhaust note, and don't want my car to sound like a high school kid's car... That will affect the design and effieciency of my exhaust system.

4) Lastly, there's the local noise law issues...Moving violations anyone?

From what I've seen, I believe there is as almost as much art as science to exhaust system design. Having performed fluid flow analysis in the past, I found that sometimes there are almost as many "assumptions" as known or predictive variables in the calculations.

Just my 2 cents...