how big is too big?

[craig]

im redoing my exhaust this weekend and am planning on moving to a 3 inche pipe. is that too big? i know after a certain size you start losing power, or atleast thats what i was told.

[witeshark]

I would think 3 in is pushing it but idk

[xsperf]

what it is, is you need some kind of back pressure in your exhaust in order for the engine to function smoothly(atleast thats with cars that have cat converters) i dont know exactly why you need back pressure, not sure if its because the 02 sensor needs to get a reading, or so the air coming out of the pistons is sucked out b/c of a vacuum, i have no idea

[RSF5]

Yeah.. I also wish to know more on this topic. If it wasn't 3 AM I would look stuff up... maybe later

[Dudesky]

Actually, the need for back pressure is a misconception of the theory. Back pressure is inherent in properly designed exhausts systems, so people equate it to being a requirement. Back pressure occurs because exhaust gases move through in pulses or waves, a result of the valves opening and closing. Each high pressure wave, or pulse, has a corresponding low pressure volume right behind it, creating a push/pull scenario within the pipes.

The key to a good exhaust is exhaust gas velocity, and you want the smallest diameter pipe that will push gases out the fastest, with the least amount of back pressure. Case in point: a large diameter pipe = lessened or no back pressure, right? Right, but that's not why you lose power. In that bigger volume, exhaust gases expand & begin to cool, which reduces the exit velocity. Now you can see in that example that it's not the lack of back pressure that screws things up, but rather, the internal volume of the pipe that actually slows down the flow. Effects on the catalytic converter has more to do with air/fuel mixture than flow velocity or direction. Lean mixtures don't allow the cats to operate at the higher temps (500-750 degrees) they require to be efficient, and richer mixtures contaminate, overheat, and break down their internal chemistry

[elsabor67]

Wow, I never knew that. Always something new that's learned

[xsperf]

Originally Posted by Dudesky

Actually, the need for back pressure is a misconception of the theory. Back pressure is inherent in properly designed exhausts systems, so people equate it to being a requirement. Back pressure occurs because exhaust gases move through in pulses or waves, a result of the valves opening and closing. Each high pressure wave, or pulse, has a corresponding low pressure volume right behind it, creating a push/pull scenario within the pipes.

The key to a good exhaust is exhaust gas velocity, and you want the smallest diameter pipe that will push gases out the fastest, with the least amount of back pressure. Case in point: a large diameter pipe = lessened or no back pressure, right? Right, but that's not why you lose power. In that bigger volume, exhaust gases expand & begin to cool, which reduces the exit velocity. Now you can see in that example that it's not the lack of back pressure that screws things up, but rather, the internal volume of the pipe that actually slows down the flow. Effects on the catalytic converter has more to do with air/fuel mixture than flow velocity or direction. Lean mixtures don't allow the cats to operate at the higher temps (500-750 degrees) they require to be efficient, and richer mixtures contaminate, overheat, and break down their internal chemistry

heh, well there ya go!

good post btw

[denyo77]

thanks...
I've been a member of this forum for no more than two weeks, I think, and I've already learned a lot...
thank all of you guys!

[mullethunter3]

Craig, I've seen pipes up to 5" in diameter. That's not saying that they were practical, but I've seen it done on diesel trucks before.

[craig]

but mine is a gas truck. if i had a diesel i would probably have a 5 inch pipe run into 8 inch stacks out the top

[RSF5]

Originally Posted by craig

if i had a diesel i would probably have a 5 inch pipe run into 8 inch stacks out the top

That would be teh awesomeness.

[TerryY]

Your system restriction is the exhaust manifolds inside diameter.

We put 3" pipes/stacks on a "55 Dodge pickup converted into a tow truck that had a transplanted Hemi .

It was so loud that I watched glass falling from an apartment building next to us one day.

That may be a bit loud.