back pressure??? - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars

Luckys94-GTR · 12-23-2001, 14:18

Im currently running pace setter headers with catco high flow catalytic and a greddy stlye muffler. i was wondering what would be more beneficial 3inch cat back madrel bent or 2 1/2. Also considering doing side exhaust like rx7s and eclipses.

Fantomas · 12-23-2001, 17:08

Your engine isn't big enough to need a 3 inch, 2 1/2 will do just fine. And it's not about the backpressure, it's about the flow. If the gases are moving as fast at the end of the exaust as they are when they come out of the engine, this is the ideal combination. If you have big piping the gases will just expand and start to slow down and this goes all the way to the engine making it loose power. Sorta like a traffic jam, except it's the opposite in term of size.

Trey · 12-29-2001, 00:12

That is the *perfect* defintion of how the exhaust works, and what happens when people put too large pipe on their cars.

The faster the exhaust flows out the rear the better because as it's flowing out of the car, it creates a vaccuum behind it that sucks more exhaust out of the combustion chambers, making a mroe effective and efficent air pump...errr...motor.

12-29-2001, 22:27

indeed that is why you have that H connection on your exhaust.
when certain valves are closed the exhaust gasses are just sitting there.. but when the two pipes are connected there is always a vacuume going.... the exhaust sucks itself out, its not pushed.

edit> the 2.5 naturally aspirated engines shouldnt go any more that 2.5inch pipe
turbo, no more than 3inch

if you do the exhaust will lose its flow and wander around in there.. and mess the flow yo!

gxt89 · 01-15-2002, 20:50

Finally!! people that understand exhaust. You guys need to go into 1gen forums, esp. on PT to explain this, they don't get it there...
Also another trick, from what I understand, you want the exhaust to go out the side of the car, or just angled in the back?
Angling in the back is an other trick to improve flow. When they come out straight, the turbulent area behind the car slows the whole thing down, too. Angling it allows the exhaust to I guess go "between" the turbulence. Instead of hitting it dead on. And out the sides of the car would be pretty cool too, I mean in looks. Side exits are always good in performance anyways

Armon · 12-03-2002, 09:00

The 70 shot of nitrous you are using would benefit from a 3 inch exhaust. However if you did this, any time you weren't spraying you would have a decrease in power, especially in the low end.

Gil Starr · 02-11-2003, 22:17

I'd like to clarify this a bit. Backpressure is needed. I found that in a modded PGT with a 2.5 catback, headers and highflow, I lost lowend but gained massive amounts of high end especcially with N20. Since then I have changed back to the stock 2.25 catback and have seen better launch time and a better lowend. Of course when reaching that transition to mid/highend, without N20, I've noticed the dreaded stock bog return.

You need to know what you want from your car before you start modifying it.

Does this make sense?? I'm buzzed.

Vulcan Power · 02-22-2003, 02:47

Yeah man it makes sense...

Another thing to mention that alot of people forget. If you have your stock exhaust manifold(s), and a large exhaust the stock exhaust manifolds will provide enough backpressure for a good low end, and the cat-back will provide a decent upper end boost. I have a 2.5" mandrel bent which is probably equal to a 3" regular bent exhaust, but it's a cat-back w/stock headers, etc and the low end response is still very good..

06-05-2003, 08:04

I have a 3" exhaust on my probe GT.

I ran a 14.4@98mph all motor with I/H/E ze Intake manifold and modified y-pipe