Exhaust Backpressure: Fact or Fiction

[Guitar Man]

Engineers use formulas and equations when designing and building engines. They have way more resources than we can ever hope to have. I do trust they knew what they were doing when they designed our engines. Someone would really have to convince me that an exhaust mod would actually be worth it.

 

[Motogiro]

This is not to argue fact or fiction of back pressure.

Regardless of equipment and manufacturing capability an exhaust system is also built to meet other factors such as economics, decibels, EPA and emissions. Depending on application and tuning there can be performance gains from an other than factory exhaust.
This science is one that is a constant variable. As volume of exhaust gas, temperature of gas and frequency of pulses are introduced, performance characteristics of the exhaust system will have different effects on efficiency of the engine.

The old 2003 SAE horsepower dyno results for my SV1000 were 3.5 H.P. gain at 4,000 RPM from just Two Brothers Racing slip on exhaust. After 2003 the H.P. was greater on the SV1000 so the performance gain may have been a little better. This is without retuning and just from swapping the cans.

OEM Exhaust System......................... 17.45 lbs
Two Brothers Exhaust System.......... 8.9 lbs
Horse Power Gain.............................. +3.5 @ 4000 RPM

And the sound........

 

[Water Bear]

I have read that two stroke exhausts are "tuned" so that exhaust gas pressure can be used to keep the cylinder from leaking air / fuel mixture. Anyone can Google this if curious.

The equations used for fluid flow through a pipe (or blood vessel) are the Navier-Stokes equations. They are a series of partial differential equations that are very difficult to study from a theoretical perspective. Mechanical engineers that specialize in fluids probably see more of them than they care to ever discuss.

Edit: I guess I will add that to some degree the question seems silly to me; If you have an exhaust pipe full of high pressure exhaust gas and you open a valve that leads into the cylinder, the rate of flow of gas from the chamber will probably depend on the difference in pressure of the exhaust gas and the interior chamber gas. I have no idea how any exhaust is designed, but if you make it possible for exhaust gas to come into contact with the interior of a cylinder at an inappropriate time, that could very easily have some kind of consequence.

 

[Neal]

weird thread bump..

Even stranger looking over the thread, not sure what anyone is trying to talk about.


However, if you wanted to calculate back pressure for an exhaust it seems very straightforward equation.

http://www.cat.com/cda/files/3375367/7/Exhaust+Systems+LEBW4970-05.pdf

page. 18.

 

[Water Bear]

weird thread bump..

Even stranger looking over the thread, not sure what anyone is trying to talk about.


However, if you wanted to calculate back pressure for an exhaust it seems very straightforward equation.

http://www.cat.com/cda/files/3375367/7/Exhaust+Systems+LEBW4970-05.pdf

page. 18.

I guess what I was trying to carefully say was this: If "backpressure" is the notion that exhaust gas can exert pressure on things, the answer is 'yes' according to the laws of physics.

I am forced to assume people mean something else, and I am (and probably everyone else is) carefully trying to figure out what the question is.

 

[iSteve]

I am forced to assume people mean something else, and I am (and probably everyone else is) carefully trying to figure out what the question is.

The right amount back pressure can stop the flow of fresh air/fuel mixture escaping along with the burnt gasses. There is a slight time between the exhaust and intake stroke when exhaust valve is closing and intake valve is opening. At low rpm's this is important to not waste any potential HP (fresh mixture) out the exhaust.

With drag racing you just want to pump as much fuel through as possible to make the maximum amount of HP so any loss of low end is unimportant.

This is more important in 2 cycle engines where they have no exhaust valves

I know this is a over simplified explanation but thats the basics.

 

[Water Bear]

The right amount back pressure can stop the flow of fresh air/fuel mixture escaping along with the burnt gasses. There is a slight time between the exhaust and intake stroke when exhaust valve is closing and intake valve is opening. At low rpm's this is important to not waste any potential HP (fresh mixture) out the exhaust.

With drag racing you just want to pump as much fuel through as possible to make the maximum amount of HP so any loss of low end is unimportant.

This is more important in 2 cycle engines where they have no exhaust valves

I know this is a over simplified explanation but thats the basics.

I've heard something like this before about two strokes. Thanks for filling in the details!

 

[Marthy]

Back pressure tuning is very important on a 2 stroke but not something I will worry too much on a 4 stroke engine. The whole scavenging is done from a mixture of overlap, primary pipe length. Once pass the collector the tuning is pretty much over. Going from a stock exhaust to a full open exhaust do required some tuning of some sort (PCV or any other fuel manager widget) Other than that it all a matter of how much noise you or your neighbor can handle.

On the FZ6R the slip-on is pretty much bolted on the collector. I did the same on the FZ8. Once all tune right, there a gain in the mid range all the way to the top. So from a side mounted to a mid ship exhaust set up there's a gain... even with about 2 ft shorter exhaust. Back pressure IMO... fiction, thats a fact! LOL

 

[Water Bear]

I have been convinced to vote "huh?"

 

[Neal]

I have been convinced to vote "huh?"

haha...

The thread is all over the place.


In summary, I would say this.

BackPressure with respect to exhaust systems is the measure of resistance to the flow of air exiting the exhaust.

Backpressure can be increased or decreased by lengthening or shortening the exhaust pipe.

Backpressure is meaningful because it affects the rate at which old air/burnt fuel leaves the combustion chamber which can change the performance characteristics of the engine.