[enkeivette]

I like Ben's definition, except for where he said the pressure in the exhaust is greater than that in the cylinder. If this were true, your exhaust wouldn't flow out.

I would say, as the exhaust gas flows into the exhaust chamber, and the chamber is unable to accomodate the full amount of gas without offering resistance (as the gas is being compressed in the exhaust) that is back pressure.

In other words, a 4 year old kid doesn't exert more force than Stone Cold Steve Austin... but he does exert some force on him.

[Vettezuki]

Originally Posted by enkeivette

I like Ben's definition, except for where he said the pressure in the exhaust is greater than that in the cylinder. If this were true, your exhaust wouldn't flow out. . . .

Sure it would. It flows out to the atmosphere where the pressure is lower and some back into the cylinder where the pressure is lower. Imagine pressurizing a tube of air. Now open a vent on both ends. Pressure "normalizes" from both ends.

[BRUTAL64]

Originally Posted by enkeivette

I like Ben's definition, except for where he said the pressure in the exhaust is greater than that in the cylinder. If this were true, your exhaust wouldn't flow out.

I would say, as the exhaust gas flows into the exhaust chamber, and the chamber is unable to accomodate the full amount of gas without offering resistance (as the gas is being compressed in the exhaust) that is back pressure.

In other words, a 4 year old kid doesn't exert more force than Stone Cold Steve Austin... but he does exert some force on him.

Very good Adam!!
You have it. People are mistaking Back Pressure with resistance to flow.

Scavenging is something that has to be addressed- that is the final factor.


A balancing act is what is needed with scavenging. Too much and you lose low and mid range. Not enough and the top end suffers.

[BRUTAL64]

Originally Posted by Vettezuki

Sure it would. It flows out to the atmosphere where the pressure is lower and some back into the cylinder where the pressure is lower. Imagine pressurizing a tube of air. Now open a vent on both ends. Pressure "normalizes" from both ends.

What you have is hot expanding gas and a piston trying to push the spent gases out a little hole- resistance to flow. So you use scavenging to PULL the exhaust out of the cylinder. Very simple.

[BRUTAL64]

Ben, do you have your answer now?

[Throttle Crazy]

I am going to input my $.02 on this one. Any guy who has ever been BLUE BALLED knows exactly what back pressure is!
On a more serious note. Back pressure is created when the volume of exhaust gas leaving the combustion chambers,exceeds the capability of the exhaust system to release exhaust gas. This lack of ability to release exhaust gas can take place at any point in the exhaust system (normally cat.). If the exhaust system is not capable of releasing that volume of air before the next exhaust valve opening, then the exhaust system will continue to bulid "back pressure". Most production exhaust systems will maintain around 2 lbs to 3 lbs of pressure on acceleration. Anything over about 4 lbs is excessive. If there is excessive restriction in the exhaust system then the "back pressure" will continue to bulid until it equalizes with the pressure in the cylinder putting back pressure against the piston. When the intake valve opens this pressure will release against the volume of fresh air trying to be drawn (normally aspirated) or forced (super charger, turbo) into the cylinder, reducing the engines ability to pull fresh air into the cylinder and affecting engine vaccum. This is why a restricted exhaust can cause a mass air flow sensor code to set. The computer sees throttle angle and engine load but not enough air being drawn into the engine so it thinks the maf is not working properly.

Scavenging takes place when an exhaust system is tuned to create a negative pressure back pulse in the exhaust system. this negetive pressure creates a vaccum to draw the inert gas from the combustion chamber,while at the same time drawing fresh air in through the intake valve on overlap for a few deg. before top dead center. I believe that a very slight exhaust back pressure in the exhaust system (tuned exhaust) between exhaust valve openings creates a vaccum in the exhaust sytem which helps draw exhaust out of the combustion chamber. As the exhaust pressure is leaving the exhaust system it creates a venturi vaccum effect which draws exhaust gas from the combustion chamber.


Thats my $.02.

[Vettezuki]

Originally Posted by Throttle Crazy

I am going to input my $.02 on this one. Any guy who has ever been BLUE BALLED knows exactly what back pressure is!
. . .

Most production exhaust systems will maintain around 2 lbs to 3 lbs of pressure on acceleration.

A ha! That's why I get blue balls while trying to accelerate in a production car.

This has been instructive. Is there a way to measure back pressure for the common man or is that only a big fancy OEM task?

With a ported TB, LS6 intake moderate cam, high flowing heads, and stock manifolds and cats, I must have quite a bit of BP.

[Throttle Crazy]

Testing exhaust back pressure is simple. remove the pre cat. oxygen sensor from the pipe or exhaust manifold and adapt in a vaccum/pressure guage. Start the engine and give it a couple of good revs while watching the guage. Look to see if the guage goes over 4psi. I have seen a plugged cat go as high as 30psi. normally a restricted cat. ends up around 10 to 15 psi. Twice in the 17 years as a tech I have seen an exhaust so
plugged the car would not start. Most tool trucks sell an exhaust back pressure tester kit with all the adapters needed to perform the test. Your car may build some back pressure but if it was excessive it would be low on power. Of course with a properly tuned exhaust I am sure there is a lot of power to be gained.

[Vettezuki]

Originally Posted by Throttle Crazy

Testing exhaust back pressure is simple. remove the pre cat. oxygen sensor from the pipe or exhaust manifold and adapt in a vaccum/pressure guage. Start the engine and give it a couple of good revs while watching the guage. Look to see if the guage goes over 4psi. I have seen a plugged cat go as high as 30psi. normally a restricted cat. ends up around 10 to 15 psi. Twice in the 17 years as a tech I have seen an exhaust so
plugged the car would not start. Most tool trucks sell an exhaust back pressure tester kit with all the adapters needed to perform the test. Your car may build some back pressure but if it was excessive it would be low on power. Of course with a properly tuned exhaust I am sure there is a lot of power to be gained.

Thanks for the advice. I'm galdurned curious.

[BRUTAL64]

People are mistaking Back Pressure with resistance to flow.
If you want to use that term, not a problem. I guess it is just easier to say "Back Pressure" than resistance to flow.



Back pressure (resistance to flow) is bad. Yery simple.

Maybe the "Back Pressure" people are talking about is the 14.7 lbs at sea level.