MTBer

To my understanding, higher exhaust gas velocity is critical for low-end torque. As pipe diameter increases, exhaust velocity must decrease. Is a 2.5" exhaust with a catted x-pipe and Loudmouth 2 mufflers too free-flowing for a stock engine, causing reduced low-end torque? I have a K&N drop in filter and removed the air silencer.

Planned future mods include a JLT ram air intake, Accufab or BBK throttle body, and SCT or Diablo tune, which should all help get more air into the engine.

Pohatu18

Shouldnt hurt low end torque enough to even feel it since you still have cars its not as free flowing as catless.

socalwrench

OOOOOOooooo, very good question. The simple answer is- yes. Well, actually anything that decreases backpressure will result in less torque. This relates to how having a small amount of left over exhaust actually helps push the pistons back down (during the engine cycle). However, there's a limit to this as having too much backpressure prevents the engine speed from increasing (it slows the engine down).

I will send you a PM with some other information.

DreamerGT

With my set up its as free flowing as it gets, but the car make 263hp/305tq. I forget at what rpms. But It will help your performance more than It will hurt it and you have a 4v.

DreamerGT

I agree with that like putting a 3" exhaust on a stock car will hurt your performance.

Jfsram

Nope.

Bman2000

Ok not to totally burst your bubble, but thats not entirely true...there are advantages to big exhaust, but having exhaust gasses pushing your pistons down is false...the main advantage is called exhaust scavenging, which relates to pressure waves and exhaust gas velocity pulling exhuast out of the cylinder allowing more good oxygen and fuel in, via cam over lap and what not. In anycase it can get kinda complicated, so i dont really know enough to totally enlighten everyone but that is the advantage, not pushing the piston down. A more free flowing exhaust helps with exhaust scavenging, but there is such a thing as too much for what you got, altho i find it unlikly that most people will go to big on anything but a honda...I am running a 2.5 inch through resonators(like slp) and a bassani x pipe, with stock headers. My car does just fine, probebly help your car out alot more though.

OnyxCobra

It's nothing you're going to notice while driving it...

mrtstang

A 2.5 cat back on our cars should increase both torque and horsepower. It may shift that torque a little higher in the rpm's (not sure though) but nothing you'll ever feel.

67mustang302

It's a combination of exhaust gas velocity and pressure pulse wave tuning. Higher velocity results in lower pressure and can help draw more gasses out of the cylinder, but higher velocity comes from decreased diameter, so too small and you lose flow capabilty. You want ZERO backpressure. Backpressure is resistance to flow and is not good ANYWHERE in an exhaust system(and those of you who think that you need some backpressure are totally wrong, mainly due to false information on exhaust system tuning that's been propagated for years).

The other portion is highly important, pressure wave tuning. Energy waves being not composed of matter, but pure energy can change direction and rebound off objects with little to no loss of intensity. Pressure wave tuning, or pulse wave tuning, is the reason that headers have certain length and diameter piping. By controlling the length and diameter properly you can cause the energy wave that's generated during the initial opening of the exhaust valve to travel the length of the header and when it reaches the termination of the primary, rebound back UP the primary tube as gasses are still flowing down it(it also creates a pulse wave that helps to draw gasses into the collector)...once that energy wave reaches the cylinder head port it rebounds back and when it does so the energy creates a small pulse that can help pull the last bit of exhaust gas out of the cylinder. The trick is to time it so the rebounding pulse reaches the exhaust valve at the end of the exhaust stroke, and use the energy pulses to generate real world pressure gradients to help move exhaust gasses. That's the reason big race teams in Cup and F1 spend hundreds of hours on the dyno just tuning the exhaust system.

Catalytic converters have their own oddity though. Because the reaction that takes place in a modern cat is exothermic, it adds heat to the exhaust gasses as they pass through the cat, causing the volume to try to increase. To prevent the outflowing gasses that are hotter from generating higher pressures past the cat, modern cat-back systems typically have piping sized larger than that which is entering the cat, to try to keep resistance to flow down.

Performance in an rpm range will primarily be dictated by the header design, but too large of an exhaust system can reduce system velocity and increase pressure, also reducing the amount of pull on the collector gasses and slowing down total exhaust system flow.

Most people tend to size their exhaust much too large, a single 2.5" pipe is free flowing enough to support upwards of 275hp, so a dual 2.5" system can support over 500hp, depending on how restrictive the muffler is, a typical street system of 2.5" with good flowing mufflers can easily support 400-450hp before it becomes anything worth worrying about. A dual 3" system can support more flow than most street cars will ever produce. Smaller systems are quieter as well.