aggravation

I'm trying to find the perfect (for me) level of volume from my exhaust. Right now I have the stock manifold into Motorsports X pipe into Flowmaster Super 40's (installed in place of resonators) into RMB.

I'd like to add a bit more 'muffling' without hurting performance, either by replacing the Flowmasters with something a touch more quiet or adding to the system, maybe high flow cats.
Is there a rule of thumb I should be aware of regarding exhaust flow since I'm pushing approx. 10 psi into the intake? I don't want to restrict the flow in search of the right level of BWaaaaaa.

I guess I should also ask, if I remove the RMB and re-install the stock muffler, will I be losing power? I want to keep some of that beautiful sound I get now but on a scale of 1 to 10 I'd like to turn it down about 3 or 4 notches.

123quattro

The stock muffler will be restrictive. Maybe try switching over to Borla mufflers. I like Magnaflows better, but they aren't the quietest. Borlas seem to be built similarly but are quieter.

hacker-pschorr

Unless you measure the backpressure you are just guessing.

It's very easy to add a vacuum port to the exhaust. Z drilled a hole into an O2 bung plug, works like a charm.

aggravation

I could add a port easily enough or copy Z's idea and temporarily use one of the two O2 sensor bungs but what kind of measurement am I looking for there? Just the absence of positive pressure?
what does Tim run on his or his wifes car, he said he doesn't like them loud but I forget what he said he was using and his cell # is not working....

Tampa 928s

This may help I did this to mine also and it sounds great: S/C, X-pipe, no cats and mid muffler.
https://rennlist.com/forums/928-foru...214-277rw.html

hacker-pschorr

Someone can quote the shop manual, I thought 1psi or less is what the factory wanted. I do not recall seeing anyone report back after testing a stock exhaust. Just one of those easy items most over look. Then again, most who care strip off the CATS for an X-Pipe anyway.

IIRC X-pipe with mid-mufflers, stock rear muffler. He's had an RMB on there, not sure if he left it on.

We've never measured the exhaust pressure in his car. It's on the spring "to-do" list.

ddarnell

As a rule of thumb, the exhaust system is going be handling significantly more flow from forced induction. More intake charge = more gaseous volume. The stock manifolds and pipes are almost certainly no longer ideal.

Some generally accepted rules of thumb are:
Exhaust gas velocity should not exceed 250 ft/sec
Back pressure should not exceed 1-2psi
A 10% increase in exhaust tube diameter will yeild approx. a 20% increase in flow

There are many who will say that lowering exhaust gas back pressure will either lower torque or move it up in the RPM range. Clearly as a motor's tuning is moved toward the upper limit, zero back pressure is the goal.
The muffler will most likely be the biggest obstruction in the flow path. I would think that a person would probably want to work from that point back to the exhaust valve.
Given our engine's power producing characteristics I would also think it unlikely that a moderate oversizing of the exhaust components would rob usable power............under boost that is.

Hacker nailed it on the head. One needs to KNOW at what pressure the exhaust system is operating.

That being said, we have a Borla system on the 79, M28/11, Euro and it's a much more subdued tone than FM 40s. I have no idea how they flow.
David

123quattro

I have a Borla on my Range Rover. It's a straight through muffler with a perforated core like the Magnaflows on my Audi and 84S. I don't see how it could be restrictive at all.

Jim R.

I am running stock manifolds, a motorsports x with cats, stock small resonators, and a RMB. It is a bit loud, and sounds pretty wicked at WOT. There has to be some room for flow and power improvement there. I do wish someone would make bolt on a 2.5" cat-back system to match the x-pipe with 2 high flow mufflers. Local muffler shops tend to run the other way on this kind of stuff when they see it is for a Porsche.

Just one more thing on the wish list!

Jim

toofast928

I think replacing the Flowmasters is worth a few HP. I love Borla but too $$. I'm using Magnaflows, straight thru design. Two ovals and two rounds where the rear muffler use to be.
10# boost lot of air to move. Are you considering headers?
Tony

Z

If you make one of the fittings to go in place of an O2 sensor, you'll probably have a lot better luck at most places if you ask for an oil drain plug than if you ask for an O2 sensor bung plug. Even some hardware stores sell oil drain plugs in the regular nuts and bolts section. The thread size is M18x1.5.

You may get some different opinions on what backpressure you want, but I'd say the lower the better for making horsepower.

The exhaust on Tim's wife's car is the one that he had on his car. It consists of the stock exhaust manifolds, X-pipe, stock pipes with the smaller size stock center mufflers, and then two small, round, straight through type mufflers in the location that the stock rear muffler was. He's had an RMB on that exhaust setup, but thought it was too loud, and went with those two small mufflers back there instead.

Actually, the cats are the single biggest restriction of the stock parts that are located behind the exhaust manifolds. I'd say that it might be better to work from the front back. As the exhaust travels back along the system, it cools and the gasses contract when they do. Their volume goes down due to that, so the pressure and possibly speed goes down the further back in the system you go. With the pressure and maybe speed being lower back there, any restriction will be less significant than it would have been further forward, where the gasses were hotter, had greater volume, were at a higher pressure, and were possibly moving faster too. In any case, it's better to work from the most restrictive part to the least restrictive one, whether it's at the front or the back. Measuring the pressure drop across each individual part will tell you which is most restrictive.

RKD in OKC

I seem to remember back when I had my 90GT, back before the x-pipe, that gutting the cats added about 12 hp with an RMB and putting the stock muffler back on cost 3 hp. I've always understood that while stock Porsche headers and cats can be improved on, the mufflers are pretty good.

dprantl

+1. Weren't there tests done on RMB's and the conclusion was that the power gain from one was negligible? I am running stock manifolds with a motorsport X and high-flow cats into the stock GT resonators and stock rear muffler. It's the perfect sound level for a daily driver but will still sound great above 3k RPM. I bet if I had an RMB on my setup it would be too loud for me.

Dan
'91 928GT S/C 475hp/460lb.ft

Darien

Louie would be the one to answer your questions...he made hundreds of dyno runs with different set-ups (i.e., different pipes and mufflers)

I think he mentioned the best runs were with 2.5" pipes all the way back from the x-pipe with bullet mufflers in place of the resonators. That will be my set up once I find the right mechanic.

Vlocity

I'm running the stock manifolds, Motorsports X pipe, no Cats and the into Magnaflows. This is the 3rd or 4th exhaust that I've put together. This one is a little loud, but quieter than the flowmasters. IIRC the Magnaflowws had better flow than the Flowmasters. Tom Coultier had also done a lot of testing at the time and steered me away from the Flowmasters.

YMMV

Ken