Contour Enthusiasts Group Archives Forums Modifications Forced Induction Bosal cat-back looks to be a FI winner...

Originally posted by ssmumich00:

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you have to remember, if this guy is working with NA applications, it's a lot different. . .

FI is forcing air in, you don't even really have to change headers out on an FI s/c since the flow is that much the better, so I think this dude's theory may stand on certain NA app's, but as far as tc, the wider the better since it RUNS on exhaust gases velocity exiting in order to turn the turbine to pump the air in. . ., it'll exit faster if it's BIGGER. OF course headers come with most tc app's, but I'm sure DemonSVT or someone else mentioned you really don't need headers with a sc. . .

just my .02, I may be wrong, everybody has their theories, hell we could banter about this for 200+ posts. . .

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I mentioned before that you don't need headers with FI. However that isn't entirely true. The cost/performance ratio is not good enough for most people to justify it and there are other factors that make header design with turbos a very complicated process.
The part about what you said bigger = faster, is not really true as well as it may not be the whole story. Think of an analogy where you are washing your car on a concrete driveway. You want to push some dirt off the drive with the hose and put your thumb over the end. The water shoots further now than it did before....What's changed? It has a higher velocity now, but the opening is smaller because your thumb is pressed over it. Well, that is the first rule, smaller diameter hole = higher velocity, and also allows the jet of water to shoot further than it normally would. The next factor is volume. If you hold your thumb over the end you are restricting the flow and reducing the volume. Now you may be shooting out a jet of water real far, but you aren't letting out enough water volume to push the dirt off the driveway now. The result is you are letting off the pressure at the thumb and allowing a little more water to come out and provide enough force to push the dirt the rest of the way off the driveway.

This analogy makes sense to me, but then it is friday night and I am buzzing off of a good French wine!
The moral of the story is that you must restrict the flow to speed up velocity, but you also need to compromise between velocity and volume. There is a 'sweet spot' for exhaust in either turbo or Forced induction where the diameter and the length of the pipes come together to provide the right combination to yield the best power...AT A SPECIFIC RPM! It doesn't apply to the whole rpm range either. Sometimes big power numbers don't tell the whole story. Just compare the vortech kits with 275HP to the streetflight kit at similar HP measurements, 240-290 depending on state of tune, the average being 265HP. The area under the torque curve is hugely in the turbo kits favor and the car should out-accelerate the average vortech kit significantly. (I say average because some people out there have above average kits!!).

warmonger