I wanna know which creates enough backpressure for an na FC for better performance. I currently have an all in one Magnaflow Cat/Resonator combination on my car to pass the emissions test, which it did. My friend who also drove my car a while back said he noticed the car accelerated faster with only the Racing Beat res than with the current cat. Id like to know true gains of either, the cat/res which causes some backpressure from all the things in between it, or the resonator alone that causes an airflow turbulance for backpressure.

 

I'm a real novice when it comes to exhaust theory, so I'll keep this brief. There are camps that believe: the less back pressure the better and others that say the n/a rotary needs some (which I believe, if nothing else to operate the aux. ports on the s4).

What I know to be true is that a res. cat-less pipe will always yield more power than a cat, being the #1 restriction in the system. I think the question of back pressure comes into play more when the entire exhaust is being designed from scratch.

Like I said, I'm a novice though, wait for the more experienced tuners to weigh in..

 

As mentioned above, the S4 engine needs exhaust backpressure to activate its ports. This is not a factor on your S5 engine, so backpressure is just going to make it run a little worse. If you want to know the true gains of either of your configurations, then you will need to schedule some dyno time at your local auto shop. I have a feeling that there will be less than 3hp difference between the two if they are both in good shape. If your friend can tell the difference then either he is the Stig, or there is a major problem with the weaker one.

 

I can tell the difference. Unfortunately it is the drastic noise level increase on the na rotary.

 

More noise = less retriction on a rotary, period.

Let me qualify that. In my 1st 2 years of racing in 1980/'81 I ran an RX2 in the I.M.S.A RS series. We ran straight exhaust the length of the car into a megaphone. My bridgeported 12A dynoed at 213 hp. At the end of the '81 season it was anounced that the rotaries would have to run a muffler mandated by I.M.S.A. in all classes. The muffler was installed in place of the megaphone. We lost 12-15 hp on a bridgeport engine, lost only 6-8 hp on a stock intake port engine(which they made us run in'82).

 

this is harder to do on an S5, but on an S4 you can tee a vacuum/boost/pressure gauge into the 6 port line, and actually see if backpressure is the difference between the two cars.

on an S4 hp goes up when backpressure goes down, until the 6 ports don't open anymore

 

Leave the exhaust alone. If you remove the cat/resonator you'll lose mid-range and the noise will be obnoxious.

 

I put the resonator back on, and it is louder again and I definitely feel the slight stronger pull while accelerating opposed to the cat. Im sticking with it until emissions time comes around again.

 

i have rb headers to straight pipe to stock mufflers on a s4 n/a and i can tell a positive difference in power