Too much exhaust flow?

[Ferb]

We have all heard the argument of "too big of exhaust pipes reduces low-end torque". But does anyone have dyno sheets to prove this?

And not just you personally, but online have you stumbled across these.

Also want to make sure i know why this "might" happen. During the overlap of exhaust and intake, if the exhaust is too 'free flowing' the intake charge can be whisked away. That it? Same reason cams with large LSAs "produce" more low end torque but tend to fall on their faces at high rpm.

ferb!

 

[SteveD2]

Why it is, I don't know. But I do know it is a fact. No dyno sheets, but my last cherokee temporarily had a 3" exhaust pipe replacing the muffler and cat. (long story) Definately waaaay less power. (sounded alot faster though

)

 

[sbc350gearhead]

Too big of an exhaust....reduces exhaust flow velocity and cylinder scavenging, and raises the torque peak of an engine.

 

[Last_Z]

Too big a pipe and you lose the bottom torque and many times gain horsepower. Extremely big pipes, you lose everything.

 

[Gary Allan]

Let me try this.

The reason that you lose low end torque is because you lose "entrainment". Ever have a garden hose with a pistol grip nozzle and cycle the handle real fast (probably not something you've done since a young child) and see the shockwave of the moving water slamming against the closed vavle? Well the same thing happens if you have the valve on the other end of the hose ..except that instead of trying to expand the hose by disapating its pressure against the walls of the hose ...it trys to collapse the hose by "wanting" to continue forward by its momentum.

That same "suction" is created by every exhaust pulse as the exhaust valve closes behind it ..the flow "entrains" the next exhaust pulse. Going to too big a pipe gives your low rpm flow NO VELOCITY and you lose this tendancy. This effect is neutralized at a certain rpm when any backpressure is created.

Most exhaust systems are a compromise. NONE are all things at all rpms.

 

[Pablo]

There is some level of urban myth to this one...what you guys are writing is basically true, but this does NOT apply to turbocharger engines. The reason I bring this up, I had an exhaust shop not install the system I wanted (not even that big of diameter) because the guy said; "you'll lose bottom end power"...I went elsewhere. (It's kinda like some mechanics and oil lore). Turbocharger don't want, don't like, don't need ANY slowing of exhaust gases after the turbo and before, the turbine itself slows the flow.

Sorry no dyno links

 

[Jelly]

http://www.n2performance.com/d?function=download&lecture=exhaustsystems.pdf

 

[Ferb]

Zang! Thanks Jelly! And thanks to the rest of you. This topic is something i've only mildly grasped in the past, but now it makes more sense.

ferb!