axle-back vs cat-back

[Onnipotente]

is there really any difference in performance between an axle-back and cat-back if you were planning to keep the rest of the exhaust stock? since the price difference is a lil over 200 for just the extra piping to the cat, is it really worth it?

5zigen axle-back - $246
5zigen cat-back - $479

 

[Deadpool]

I would say 3 inch would be a waste on a NA car and might even kill HP, As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.

For turbo cars, you throw all that out the window.you want the least backpressure possible. No ifs, ands, or buts. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”

 

[crimsondragon]

As pressure increases, velocity decreases.

 

[BowWow]

DP, thanks for the reply. The car comes home tomorrow, so I'll do a quick measurement of the OD of the pipe from exhaust manifold to tailpipe and post my findings.

It sounds like I'll be doing the header/dp/hfc/catback thing soon! I plan on keeping this car normally aspirated, but will still be looking for HP bumps with bolt-ons...

 

[BowWow]

Well, just finished crawling around under the car ('99 RS N/T). From the exhaust manifold to the tail pipe the exhaust pipe OD is slightly over 2". In a couple of places (just after the cat, for instance) it goes to under 2". The stock exhaust is heat shielded for nearly the entire length of the run. There's also a resonator after the cat. The stock exhaust manifold is really a brick, ain't it!

I think a cat-back solution is only a half-way measure, since the plumbing from the manifold to the cat is 2". Isn't it rather pointless to have the front half 2" and the rest 2.5"?

So I'm shopping for a header, downpipe, hi-flow converter, and cat-back with one resonator, all 2.5". I'd like a nice sound, but NOT loud.

Next question: What use is a 4" (or larger) tailpipe? 2.5" in, 2.5" out is bad? Oh, and does anybody make a good muffler with an oval tail pipe? I'd like to fill the bumper cutout, but not necessarily with twin 2.5" pipes.