Why does an over-sized exhaust kill bottom end power?

[deXtrous]

Hey was reading up on extractors and exhausts and it was said an exhaust over 2.5" will kill bottom end power, but help top end.

Anyone know why?

[Brett_jjj]

It is to do with the gas flow,when exhaust gas is dumped out of the motor into a big pipe at low revs, theres no back pressure and hardly any flow.The inertia of the exhaust gasses going out of a good proper flowing exhaust pipe, helps pull along the next lot of exhaust gas thats coming out of the engine ,extractors work on the same principal,hence the name extractors.The back pressure and flow is lost in a big pipe. An over exhausted engine will be worse on fuel too.

[deXtrous]

You're a champ brett

[Brett_jjj]

I learned this the hard way,I had an XB Falcon V8 that had a single exhaust system.I fitted a duel exhaust system and it absolutely killed the engines bottom end power.It went better up near the redline,but who drives around near the redline all the time.I went back to the single system real quick.

[Not_An_Abba_Fan]

It's not back pressure that you need, it's exhaust velocity. A bigger system will slow exhaust velocity down and create back pressure just like a system that is too small. If you can keep the exhaust gas moving as fast as possible, this makes room for the clean air/fuel mix so you gain more bang which equates to better torque. The faster you can get the spent gases out, the faster you get clean air in.

There is a misconception that back pressure equals better bottom end. A smaller eaxhust system moves the gas faster at lower revs than a big system does, this is what retains bottom end.

[Scido]

I learned this the hard way,I had an XB Falcon V8 that had a single exhaust system.I fitted a duel exhaust system and it absolutely killed the engines bottom end power.It went better up near the redline,but who drives around near the redline all the time.I went back to the single system real quick.

Semi related reply

I bloody hate those ads in mags and tv where they say the new hsv or new ford has 320kw at 7000rpm, Like who the **** is goin to rev there car to 7000rpm haha

[Brett_jjj]

It's not back pressure that you need, it's exhaust velocity. A bigger system will slow exhaust velocity down and create back pressure just like a system that is too small. If you can keep the exhaust gas moving as fast as possible, this makes room for the clean air/fuel mix so you gain more bang which equates to better torque. The faster you can get the spent gases out, the faster you get clean air in.

There is a misconception that back pressure equals better bottom end. A smaller eaxhust system moves the gas faster at lower revs than a big system does, this is what retains bottom end.

Ive got a good story on exhaust systems in a magazine somewhere and theres a bit in it where some F1 mechanics talk about different exhaust systems on F1 cars and they talk about the engines needing some residual back pressure and high gas flow speeds.

[boonga]

now as all he needs is an f1 haha. always woundered the same knew it was the same with bikes

[woteva]

It's like farting in a small space has more effect than farting in an open area.

[moff_man]

some newer cars now have a flap that opens at x rpm and allows extra flow in the exhaust.

[Reaper]

some newer cars now have a flap that opens at x rpm and allows extra flow in the exhaust.

I think that's more to do with EPA noise regs over anything else.

Reaper

[Jesterarts]

Just one small input from me...

Good read:
Exhaust System Theory 101

Regards.

[Not_An_Abba_Fan]

There ya go....quite a good read and explained rather well. Saved me a lot of typing.

[moff_man]

I think that's more to do with EPA noise regs over anything else.

Reaper

alloytech captiva's are not louid, they have the seccont exhaust that opens when you get up em. at idle only one side is working till you get up it for the exact reasion as this thread is about, flow up higher in the rpm's