billd17

I'm considering bypassing one of the mufflers on my RSA. Does anyone on the board have any advice on whether its better to bypass the primary or secondary? Any real (measured by dyno) data on horsepower gain?

Thomas Owen

Bill,
I have no dyno info, but here is what I have:
Cat bypass and primary bypass = too loud for me
changed to Fabspeed premuffler in place of the cat with a primary bypass = sounds good
I MAY try the premuffler with a stock primary muffler and a secondary bypass to see how it sounds.
Good luck -

ltc

No HP #'s either (some say 10-15hp FWIW).

My RSA and Cup run with cat and secondary muffler, just the primary bypassed.
I was happy with the sound and both were able to pass noise restrictions at Lime Rock Park.

joey bagadonuts

I like the secondary bypass and found it faster from 0-60 according to my G-Tech. I played with different setups and here are my results:

Stock chip, stock exhaust and airbox w/ holes
Avg 0-60: 5.4

Stock chip, primary bypass and airbox w/ holes
Avg 0-60: 5.8

GIAC chip, primary bypass and airbox w/ holes
Avg 0-60: 5.8

GIAC chip, secondary bypass and airbox w/ holes
Avg 0-60: 5.5

GIAC chip, secondary bypass and cone filter
Avg 0-60: 5.2

As for hp gains, only a dyno could confirm which setup produces the best numbers. These test measured only one thing, 0-60 times.

JEJ 149

Bill,

I wasn't sure either, so went for the easiest, and reversible option. I have swopped the secondary for a G pipe. Cheap to buy, 1 hr tops to fit, and 1 hr to put the secondary back on if you want. Having said that, just pop off the secondary and take the car round the block to see if you like it before splashing out on a G pipe.

I love the sound.

Best wishes

Julian

springer3

I have toyed with the idea too. I see the secondary muffler holds a shield to keep the grit from the rear tire out of the tail lamp cavity. That is one consideration for keeping the secondary. The primary muffler is heavier, and more to the rear of the car. If I proceed, I will probably do the primary bypass. I believe both give some weight, performance and sound advantages, but the primary seems to have more side benefits, fewer negatives.

Bill Verburg

Here are the flow rates for 964 exhaust broken down by component,
Section Flow Rate (CFM)
Catalytic Converter 442
Primary Muffler 396
Secondary Muffler 462

billd17

Its hard to factor out the GIAC chip, but from your data it looks like the stock exhaust is faster than bypassing either muffler. 5.4 sec for the stock exhaust vs 5.8 for the primary and 5.5 for the secondary.

Are these numbers with or without the cat bypass? Also, where did you get your bypass pipes?

billd17

Can you explain what these numbers mean? How can three components in series have different airflow rates? Is this a test with a particular pressure head on the component input?

Bill Verburg

Of course, how else?

Greg_L

Bill, i very recently heard a primary bypassed car next to my own cat and secondary bypassed car. They both sound great but the secondary is a little harsher and louder. I prefer it I can email you the videos we took if you would like to hear them. Actually i should host them for everyone. . .

Cupcar

FWIW the most weight is lost bypassing the secondary, but the weight is not at the end of the car as in the saving the primary effects and mimimal in difference.

Primary Muffler Weight 23.5# minus Primary Bypass 7.75 pounds= 15.75 pounds saved
Secondary Muffler Weight 23.25# Minus Secondary Bypass 3.75 pounds= 19.5 pounds saved
So bypassing the secondary saves 3.75 pounds more.
This is weighing factory Cup bypass parts.

Given all of above I went with the better flow of the secondary and bypassed the primary.

joey bagadonuts

Yes, the stock exhaust is fast. I guess those Porsche guys knew what they were doing. And, yes, the effect of the GIAC chip is hard to discern. I'd have to do back-to-back tests with all of the different configurations to see if it does anything at all.

The cat bypass was added after all of these changes and was never tested (broke my G-Tech somehow). The butt dyno suggested that it was good for another tenth or so--the whole car felt noticeably lighter, if that makes any sense. The tradeoff is that this last tweak definitely made the car louder. Generally, the exhaust sounds the same as it does with just the secondary bypass but it's now accompanied by a deep, booming rumble.

I picked up a B&B cat bypass off ebay ... some cab owner just couldn't take it anymore. Here's the thread I started describing the mod.

Dunasso

I think what you guys are missing is that a restricted exhaust system produces more torque and moves the hp to the lower end of the RPM range. This is true for any engine includind your lawn-mower. When you open up the exhaust for a less restricted flow then you are in essence reducing the power at lower revs, ie 0-60 times will suffer. However the engine will prove to be efficient and produce more power at the top of RPMs, but this is not a direct transfer from one end of the spectrum to the other. The power at the upper end is not as readily usable as at the lower end.

The idea of a free flowing exhaust therefore must be compensated by adding more HP (displacement) to the engine in order to increase base hp. Someone else can ad on I'm out of time; igoing out for the wifes B-day and she's walking out the door.

Be back in a few hrs,

Duncan

billd17

Can you explain why this is so?
A simple analysis suggests that backpressure on the exhaust puts a load on the engine during each exhaust stroke that robs power from the crankshaft at any engine speed.