Jakes

As DanCOBRA said, the object is to reduce restriction while keeping the velocity up. Wide open exhaust is good if you're running high up in the rpm band and only care about maxing your HP (like when racing). The high engine rpms will take care of the exhaust gas velocity all by itself. The problem with wide open exhaust is that low in the rpm range the ehaust gases loose velocity. A narrower exhaust pipe (more restrictive) helps keep velocity up at lower rpms and boosts the torque output. High velocity is good because it helps scavaging. Scavaging is when the out flow of exhaust from the cylinder helps draw in the fresh charge of air/fuel.

So what does all this mean? Designing an exhaust system is a balancing act between low restriction and high velocity. How you use the car will determine to which side of the equation you want to be. For street driven (stop light to stop light) where low end torque is important, a smaller diameter pipe that keeps velocity up is what you want. But, be careful not to get to restrictive. For race applications, where you spend the majority of the time in the higher rpm's, a more wide open flow is the answer.

Makes everything clear as mud, right.[sm=headbang.gif]

Js 99 GT

That, my friend, is the answer i was looking for! Thanks. I just didnt understand why you would want backpressure.

rainydayracing

You lose hp with dumps cause it'll be so loud you can't hear yourself shift...lol.

ZW99GT

code did say that, and its true. He says so, and so does my muffler guy. You will lose TQ plain and simple. Ask him. Sooo. What are yall talking about?

Loss of backpressure=good
Loss of too much backpressure = bad. Just keep your tailpipes on there cheif

Mark99GT

How does removing a couple feet of piping AFTER the mufflers result in an exhaust that is so open you lose a measurable ammount of torque from decreased backpressure? Pre cat cut outs, yes I can see that, but dumps or turn downs (which ever term you prefer) after the muffler...I don't see it. I just installed a dumped exhaust on my car. No difference as far as I can tell.

ZW99GT

I didnt say it was 20ft/lbs. You cant feel 2 or 3. But I guess thats not enough to keep you from doing it if you like it.

oxfordgt

I knew someone would bring up the "you need back preasure" argument. Well you are wrong!!!!!!! You don't need backpreasure because all it does is cause your engine to work harder inorder to push the exhaust gasses out. Like stated above you want to maintain high velocity flow with minimal backpreassure for a good performance exhaust system. You do not lose power or torque from a high flow exhaust system but it moves them in the powerband which makes uneducated folks think they lost torque.

To answer the original posters question: The only downside to turn downs is the smell of exhaust in the cabin.

sleeping stallion

ok this is for turn downs or dumps but what about straight pipe no mufflers just straight i heard that looses tq tru or what r ur thoughts on that

Dan04COBRA

If it loosens, you just need to tighten it.

If you are talking about 'losing' hp, people running straight pipes (straight pipes actually means straight piping from the manifolds back with no type of cross-over pipe) are hampering scavenging and may have uneven exhaust pulses.

Colorado_Mustang

Depends on the size of the pipe. If it's too big of a pipe, you'll lose power (but sound good). Too small, you'll lose power (and sound like crap).

Same goes for any exhaust you buy. You want to match the system to the engine size and build up. Stock engines need backpressure to operate properly. Race engines don't. Most factory systems, however, are more detrimental than open headers.