What is back pressure

W

wyze

Tuner
Points
70
Location
Kent UK
Car
Polo 1.1
Is back pressure a good thing? I've been told that a 4 inch exhaust will lose too much back pressure but no one seems to be able to explain what they mean.

Is there a way to calculate the optimum exhaust size for an engine based on say the CC or BHP rating?
 
errr i think back pressure is the gas that returns back into the engine or somthing like that i did read about this a while ago basically if you lose back pressure you will lose bhp or power.
 
Back pressure is, I think, the build up of gases in the exhaust because of not being able to escape. I'm unsure though as to whether or not you can calculate the correct size. But i'm sure someone will be able to tell you....:D
 
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Very Interesting Article. It's missing one vital bit of info lol, Turbo's! They work better with less back pressure due to the seals on the turbo being able to create more pressure without goin pop hehe
 
Back Pressure,

Usually refers to exhaust systems. It is the amount of restriction in the whole system.
It can be measured usually by tapping an air pressure gauge into the collector or using the O2 sensor bung. The higher the reading the more back pressure or restriction in the system.

Less is better, none is best, but loudest. So it's a game of compromise.
 
Dont narrow pipes give a faster flow rate? If you had no back pressure, ie a massive pipe then surely the gases would be moving slowly out of the exhaust and this would not be good for power? The speed helps create a initial pull on each cylinder? Or am I understanding it wrong?
 
i think back pressure stops sucking out the air fuel mix in the cylinder when valve over lap occures. there is a formular to work out how long the pipe is the bore ect based on bore x stroke . there is also a pulse that shoots back up the exuast once the gases leave the tail pipe the timimg of this is important so as to keep the new charge of air/feul from being sucked out the cyclinders I THINK thats near it
 
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nig said:
i think back pressure stops sucking out the air fuel mix in the cylinder when valve over lap occures. there is a formular to work out how long the pipe is the bore ect based on bore x stroke . there is also a pulse that shoots back up the exuast once the gases leave the tail pipe the timimg of this is important so as to keep the new charge of air/feul from being sucked out the cyclinders I THINK thats near it
Click to expand...

Pretty much, You only get this scavengine effect in the manifold really, once you get to where the cat is you are best off going as large as you can (with diminishing returns) This is fairly important on a NA

For a turbo you want to maximise the pressure difference between both sides of the turbo, scavengine takes a back seat to making the turbo spool as quickly as possible as thats where you get your power. The runners on turbo manifold are usually about 3/4 the Internal diameter of an equivalent NA engine to keep velocity up
 
Sid447 said:
Useful link,

For people to self-learn:-

http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm
Click to expand...


All questions answered here, I think. Only gripe I have is with the statement:

After all, this was the explosion of an air/fuel mixture, right?

If you are getting explosions in your engine your are getting detonation! Fuel and air should combine to produce a controlled 'burn' with the flame front swishing around the combustion chamber.
 
Hello OG,

I think it's more a case of Transatlantic terminology, I think.
 
I think back pressure would be diff of up stream to down stream press. in between is the
muffeler. Reduce back pressure as much as possible, no matter what CC.
 
I think that the problem here is that by increasing the pipe diameter you will reduce the gas speed which will INCREASE the back pressure. What you need is high exhaust gas speeds so they help to extract burnt gasses and suck in fresh. That is one reason why the standard exhaust is the size it is. On a small engine the exhaust system design is more critical and sensitive than on a larger more powerful one.
 
another thing to take note about on back pressure is that reducing the back pressure on a Turbo Diesel is not a good idea, given the high pressures diesel engines are designed to run at, reducing this is a bad idea.

If you have ever herd a Turbo Diesel car and a loud whoosh similar to the sound of a BOV its a bad thing, especially for the turbo. might sound good but your turbo wont last any decent lenght of time.

But getting back to the car at hand, 1.1 Polo id recomend against having an exhaust with a 4" bore. i think 3" would be fine and if you wanted, a 4" back box.
 
is the standard exhaist not about 2"

i don't know if it is or not as my polo didn't have an exhaust (fell off driving round the field)
 
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Polo exhaust is approx 1 1/2"

If you fitted a 3" pipe it wouild probably stop working :)

This is one of the most popular tuning myths. Most tuned cars, and every modifiied Saxo, Corsa and Ka, have exhaust pipes that are far too big.

My project car has a 3" diameter exhaust. The RS200 built for last, and this, year's Pikes Peak hillclimb in Colorado has around 900bhp and has a 3 1/2" exhaust system!!!

Big is not always beautiful :)
 
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