Performance Exhaust + and -?

[Craig944]

I've heard that a new 'performance' exhaust system adds power at higher end RPM's and loses some at lower. Wouldn't this be a total defeat? It's like it's robbing the lower end to provide the higherend w/ more. Only reason I was looking into them was to increase Horses & acceleration speed. My 0-60 into the 6's.

[mideastmafia]

you are thinking about adding INTAKE to your car.. that would rob low end but help high.. with exhaust, it should give power MORE at high RPM's but be similar at low RPMs.


SHAUN

[Craig944]

So the exhaust system shouldn't hurt my lower end RPM's just add more power at higher end?

[ribs]

On a naturally apsirated car, exhaust velocity is very important to low end torque. Its very hard to explain, but once you figure it out you will understand. I can't fully explain it, but it made sense to me one time somebody told me. You also need a certain amount of backpressure at low RPM's to maintain the pulsing of air through your exhaust to keep the engine pulling tight. This is why exhaust velocity is important, but I am sure this makes sense to no one but me in my own sick, twisted, nonsense way.

But anyways, think about it this way: you are blowing (with your mouth...an no, nothing perverted) into a straw, and then you are blowing the same volume of air into a 2" pvc pipe. Which is going to give you the faster air speed exiting? Which is going to provide you with the most volume to get rid of if you could blow hundreds of times harder than a human nomally could?

This is low end versus high end. The high end gains will usually offset the low end losses by a fairly large margin, or else people wouldn't put larger higherflowing exhaust systems on their cars.

For instance, my friend has a honda civic ex. This thing has no low end what so ever to begin with. When he upgraded the stock exhaust to a 2.25" cat back unit, his car got noticably faster (.3 second drop in his 0-60 with a g-tech averaged over several runs). He had to sell the exhaust and put the stock one back on because he was hard pressed for money. His car became dog ass slow again. I had driven his car several times, with and without the exhaust, and while the low end seemed ever so slighly crippled, over 4 grand when the motor started to pump some serious air out, the exhaust system showed where it shined and made the car feel quicker.

I imagine the same would be applicable to a 944 n/a, but I am not sure. It was hella well worth it on his car. But 944's come with relatively well tuned exhaust from the factory, and it is high nickle steel (almost like stainless) so it may corrode a bit on the surface but will never rust through...but oddly enough just the tip of the whole exhaust system is mild steel, so that usually will rust out.

BTW, on a turbocharged car, the turbo provides all the backpressure you could ever need, so removing the exhaust system from the back of the turbo is ideal, but too loud, not practical, and would burn the bottom of your car. When I recently got my cat back exhaust put on, the car felt seat of the pants definitely quicker. It may have given a 10 HP or greater boost in power. Next comes the test pipe....this time, I am really going to bed. I mean it.


edit

BTW...a higher flowing intake (like a k&n filter) should only add HP and TQ at all RPM's, never take any away...albeit it may only be a 2-5 HP, but still...no negatives really about an intake if it is a cold air intake...otherwise, fashion a cold air system for your cone filter.
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ribs, 86' 951

[This message has been edited by ribs (edited 07-25-2001).]

[Macabre]

Re: exhaust diameter on NA cars..

Think of it like blowing through a wide straw vs. blowing through a narrow one. You get higher velocity through the narrow one and in the wide one the air just stalls. However, if you could blow a lot of air the small straw would be restrictive.

The higher the exhaust gas velocity the more intake air the exhaust gass pulses will help suck into the cylinder during valve overlap and the less exhaust gas will be left in the cylinder when the exhaust valves close.

At least that's how I understand it. There are a lot of opinions flying around on this subject. Your best bet is really just to buy a system that has been dyno tested to confirm the benefits.

Re: intakes
A higher flowing air filter drop-in should never hurt HP.. but replacing your airbox with an open element filter can either hurt your or help you depending on a lot of different factors. There are benefits to an airbox with some setups, but don't ask me to explain why.

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'86 951 Graphite Metallic/Tan