Topic: Exhaust

[Mudder]

With all the talk lately about exhaust and my research for the next exhaust system for the truck I'm currently working on I ran across this article. Very good read and makes complete sense.

http://www.gomog.com/allmorgan/exhaustbackpressure.html

[Plainview]

With all the talk lately about exhaust and my research for the next exhaust system for the truck I'm currently working on I ran across this article. Very good read and makes complete sense.

http://www.gomog.com/allmorgan/exhaustbackpressure.html

Damn, that should be required reading for anyone who wants to understand exhaust flow.  I get so tired of hearing "engines need backpressure" and then trying to explain how ridiculous that is and that it's all about flow velocity and pulse tuning.

Definitely bookmarking that one, thanks!

[Gnarly4X]

Damn, that should be required reading for anyone who wants to understand exhaust flow.  I get so tired of hearing "engines need backpressure" and then trying to explain how ridiculous that is and that it's all about flow velocity and pulse tuning.

Definitely bookmarking that one, thanks!

It really doesn't matter what word you want to use, or NOT use, to describe the "pressure" in an exhaust system, but without controlling flow restriction, please explain how you can control velocity.

Gnarls.

[emsvitil]

Diameter of the pipes controls the exhaust velocity

[Gnarly4X]

Damn, that should be required reading for anyone who wants to understand exhaust flow.  I get so tired of hearing "engines need backpressure" and then trying to explain how ridiculous that is and that it's all about flow velocity and pulse tuning.

Definitely bookmarking that one, thanks!

Hey Plainview,

Regarding exhaust systems technology, I believe it is generally considered to be very complex in terms of physics and even more complex in terms of measuring.  That is why the testing is often done on a dyno, and typically done as a trial and error process.

To really "understand exhaust flow" you would need to be EXTREMELY familiar with fluid dynamics, and at a somewhat simpler level, you'd need to understand the laws of physics as described by Bernoulli's Equation.

When tuning an exhaust system, "back pressure" (flow resistance) exists and is part of the calculated physics.

For me, I just have to go by my own experience, the feedback from the guys that have tried different combinations, and the "experts" who sell the headers, pipes, mufflers, and cats - who typically do not provide real comparison or test data.  For example... one supplier says their 4-into-1 is the hot header for a 22, but another header supplier says their Tri-Y design is better?  One engine expert says the stock Toyota 22 manifold and 2" piping works better than a header a 2.5" piping?

That's just my opinion - it may be worthless.

Gnarls.

[Plainview]

I'm not actually sure what point you were trying to make in that post.

Backpressure is NEVER desirable or something an engine "needs."  The article Mudder linked to explains very well how pipe diameters affect flow velocity but this sentence sums up the idea succinctly:

While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you also want make sure the pipe is wide enough so that there is as little back pressure as possible while maintaining suitable exhaust gas velocity.   

Backpressure may be an inevitable byproduct of a system designed to achieve a certain goal but that doesn't make it desirable.  As with anything in internal combustion engine design, there will be compromises.  There is no perfect engine that can run at 100% VE throughout it's RPM range.

So, the best we can do is learn the basics and that can give us good guidance on how to evaluate what the aftermarket has to offer and whether it will meet our particular goals.

One need not be a physicist or have a book full of equations to understand that smaller, longer pipes enhance low speed torque at the expense of high RPM capability and likewise larger, shorter pipes work better for high RPM engines at the expense of low speed power.

If one wanted to design a custom header specifically for their engine then I'm sure there would be some serious reading and computations involved but the vast majority of people are going to go with what the aftermarket has to offer so understanding some basic concepts is all that's necessary to make a semi-educated choice among the available offerings.

[Gnarly4X]

I'm not actually sure what point you were trying to make in that post.

Backpressure is NEVER desirable or something an engine "needs."  The article Mudder linked to explains very well how pipe diameters affect flow velocity but this sentence sums up the idea succinctly:

Backpressure may be an inevitable byproduct of a system designed to achieve a certain goal but that doesn't make it desirable.  As with anything in internal combustion engine design, there will be compromises.  There is no perfect engine that can run at 100% VE throughout it's RPM range.

So, the best we can do is learn the basics and that can give us good guidance on how to evaluate what the aftermarket has to offer and whether it will meet our particular goals.

One need not be a physicist or have a book full of equations to understand that smaller, longer pipes enhance low speed torque at the expense of high RPM capability and likewise larger, shorter pipes work better for high RPM engines at the expense of low speed power.

If one wanted to design a custom header specifically for their engine then I'm sure there would be some serious reading and computations involved but the vast majority of people are going to go with what the aftermarket has to offer so understanding some basic concepts is all that's necessary to make a semi-educated choice among the available offerings.

Well, I didn’t intend to get into a protracted discussion about exhaust systems and flow science.

I think in the context of typical forum discussions on headers, piping diameters, mufflers and cats, the term backpressure is a commonly thrown around.  I agree, we don’t need to understand Bernoulli’s Equation to figure out from the discussion what information you are looking for, or what you believe is factual or not, and then do what you need to do to tailor your exhaust system to fit your application.

I attempted to make two points…. Fact: backpressure is present in an exhaust system.  Is backpressure “needed” or “desired”.  In theory, I could say no. But when we have these discussions it is NOT a theoretical discussion.  Is air resistance needed or desired while driving down the freeway at 80 MPH?  No. Theoretically it would be nice if we didn’t have it.

Now…. is backpressure needed in my 22RE?  It is my unscientific opinion that YES, some amount is needed.  How do I know, because when I have reduced the backpressure (larger piping) in the exhaust system, I losed a very noticeable amount of torque….caused by a reduced velocity or loss of the scavenging effect?

The 2nd point was to simply say that it is obvious that exhaust systems designs are controversial.

Gnarls.

[H8PVMNT]

I like exhaust systems, because if you don't have them, flames shoot out of your head and burn things.  Also the exhaust gas makes you get sleepy, which is bad.  Having no exhaust is also very loud and can make you loose hearing so you have to make everybody shout into one of those cone things you hold up to your ear.

Exhaust pipe is good to have.

[emsvitil]

Exhaust is another compromise......

I don't believe it's backpressure you need, but it's flow velocity and scavenging.     An exhaust designed for lower rpm will strangle the engine at high rpm due to too much backpressure.    Conversely, an exhaust designed for high rpm will not be good at low rpm due to low velocity, and negative scavenging effects (reversion)

Is there a 4 valve head with 2 exhaust ports?   That way you could have a staged exhaust...........

[H8PVMNT]

Mt stock 22r with stock carb in stock trim liked smaller exhaust and a restrictive muffler. This surprised me.

After swapping a 20r head and carb/intake mods it seems to be more happy with a free flowing exhaust.  I think what is best really depends on a great many different things, based on my experiences.

[Gnarly4X]

.... I think what is best really depends on a great many different things, based on my experiences.

"To acquire knowledge, one must study; but to acquire wisdom, one must observe."

  ~ Marilyn vos Savant

Gnarls.