Newbie question about exhaust and "back pressure"

[David.Seth]

OK, I know very little about engine mechanics. The little I do know came from me working on my 450 and my Ninja. Anyway, as I've been reading the threads here on the gs forum, I keep coming across the fact that if you change your stock exhaust, you will almost always have to adjust your carbs to compensate (re-jet, adjust idle screws, etc). My question is... why? I don't know the mechanics and physics of carburetored engines that well, and it puzzles me as to why changes in the "back pressure" would have huge effects on engine performance (i do understand why changing the air box/intake would have effects). I tried googling it, but most answers were mixed, and really only pertained to car engines. Here is an easier way of me asking the question... if you were to replace stock exhaust with straight pipe (no baffles), why does this change performance? What is happening in the engine due to different exhaust? Not saying that I will go with straight pipe, I'm just using it as an example...

 

[Guest]

More air in and/or out and more fuel to compensate.

 

[David.Seth]

More air in and/or out and more fuel to compensate.

I understand the "in" part, but I just don't get why the "out" would have an effect... I mean, isn't it just the piston "pushing" out the burnt gasoline/fumes through a valve and into the exhaust? So what if theres more or less restriction on the exit? Why would this have an effect on the fuel? I'm not trying to be a smart ass or sarcastic, I really just don't understand

 

[GSX550]

If you have more back pressure, you could assume that less than total evacuation of each cylinder would occur during the exhaust stroke. A free-er flowing exhaust would allow more exhaust to evacuate, allowing more good fuel/air to enter on the intake stroke.

All of a sudden the amount of fuel/air you were flowing when there was say 10% of the cylinder full of exhausted air/fuel (ie all burnt up from the last bang) becomes a completely different ratio when only 2% of the cylinder is full of exhausted fuel/air.

It all comes down the the stiometric-ness of the mix once in the cylinder.

Hope this kind of convoluted example/explanation helps a little.

cheers
Scott

 

[modsoundmind]

so would changing from stock exhaust to something more modern improve gas mileage once everything is adjusted right?
MY exhaust on my bike has some holes on the underside towards the end, I figure this is hindering performance.

 

[David.Seth]

If you have more back pressure, you could assume that less than total evacuation of each cylinder would occur during the exhaust stroke. A free-er flowing exhaust would allow more exhaust to evacuate, allowing more good fuel/air to enter on the intake stroke.

All of a sudden the amount of fuel/air you were flowing when there was say 10% of the cylinder full of exhausted air/fuel (ie all burnt up from the last bang) becomes a completely different ratio when only 2% of the cylinder is full of exhausted fuel/air.

It all comes down the the stiometric-ness of the mix once in the cylinder.

Hope this kind of convoluted example/explanation helps a little.

cheers
Scott

thanks... this actually cleared it up a bit. Like I said, I am very technically challenged when it comes to engine physics/mechanics

 

[gpd211]

OK, I know very little about engine mechanics. The little I do know came from me working on my 450 and my Ninja. Anyway, as I've been reading the threads here on the gs forum, I keep coming across the fact that if you change your stock exhaust, you will almost always have to adjust your carbs to compensate (re-jet, adjust idle screws, etc). My question is... why? I don't know the mechanics and physics of carburetored engines that well, and it puzzles me as to why changes in the "back pressure" would have huge effects on engine performance (i do understand why changing the air box/intake would have effects). I tried googling it, but most answers were mixed, and really only pertained to car engines. Here is an easier way of me asking the question... if you were to replace stock exhaust with straight pipe (no baffles), why does this change performance? What is happening in the engine due to different exhaust? Not saying that I will go with straight pipe, I'm just using it as an example...

Think of an engine as an air pump. The more air you can gen in and then out the faster it will go. Baffles in the exhaust cause back pressure. Same as blowing through a straw with you finger covering part of the other end. That is how the back pressure effects the engine. The less restriction the better it can exhale