Moss928

I have yet to see a RMB (Rear Muffler Bypass) for earlier 928 models. My understanding is that they require welding versus the bolt on method for later models.

My 84 has a very nice rumble to it. How much does that rumble increase with a RMB and what is the advantage (performance wise)?

Joe R

Chris Lockhart

Moss, the performance gains are minimal. Probably around 5 HP, but the sound difference is great. Deleting the rear muffler allows these cars to sound like the fantastic european sports/GT cars that they are. Definitely worth the effort to cut/weld the pipes IMHO.

DoubleNutz

5 HP! If that,,,after dynoing with my RMB on my 85S I got ZERO,,, DRECK,,,NIL,,,NOTHING in HP gains from my baseline.

But my car car sounded absolutely AWESOME! After RMBing my 85S I never listened to my car stereo again (seriously)!

Moss928

Pat, My next question is: Could you hear your car stereo?

How did you go about this? Did you buy a RMB and modify/weld it to fit your 85S. I'm assuming that your 85S has the same exhaust system as my 84S.

DoubleNutz

HMMMMMMMM,,,good question. I suppose I could but I completly stopped listening to it because i just wanted to hear that engine roar. GOD, it was so frigg'n SWEET! Not like my 993 (even with the drilled out air box) just sounds so wimpy. Think I am going to change the muffler on that thing (B&B or Fab Speed both sound great).

Anyway, for the 84 simply go to your local muffler shop (any Midas will do) have them remove the muffler from the rear of the car, bend and weld on some straight pipes at the point where they cut them off (weld on some chrome, or black subdued tips to add a nice detail).

PS- it is important that at the along the length of the two straight pipes have them weld small bars between both pipes (on top if possible)to keep them together and from bending when they get hot. The extreme heat of the 928's exhaust will warp the straight pipes out of shape. More importantly, DON'T FORGET TO DISCONNECT THE ECU BEFORE LETTING THEM WELD ON YOUR CAR IF THEY ARE USING TIG,MIG, or any ELECTICAL ARC WELDER. No problem and no need to do this if they are using a Gas Welder (i.e. Oxy/Acetylyne).

Total cost for this wonderful enhancement: NO MORE THAN $75 with the tips.

John Struthers

Moss/Prdatr,
Alas, no RMB for my 1982 either, although, Porsche 928 Specialties does offer a cat back system. The offered MSD headers are where you should start seeing a few horsepower, though at $500.00 a pop the value /return remains to be seen. And tweaking the fuel/air mix is probably a given. Might try both later this year- if I do I'll post results. PRDATR, disconnect, or unbolt and remove the ECU before welding?

Moss928

Will disconnecting the ground on the battery do the same as disconnecting the ECU. Or is it just a bigger task. I have never disconnected my ECU (the battery I have). How does one do it?

928 Maniac

Moss, I'm more than happy with FAT BULLY's RMB and there was definately a significant performance gain, although I cannot supply dyno graphs to substantiate my claim. (flame suit on.) Actually, I can kick my own butt for not doing before & after dyno runs, because I'm sure an overlay of the graphs would've prove the following:

I've experienced a definite power-surge above 4000rpm. The engine is crispier and revs easier. I cannot say there is any gain in overall HP, but I'm dead certain FAT BULLY is now reaching peak (6300) rpm a lot sooner. My tuner also agrees with me. I have to point out though - FAT BULLY's heads are ported & polished and that could also account for it somewhat.

This is how I've done my RMB. First, I cut the pipes in the centre of the car, about 1ft in front of the water-collector. I reamed the new pipe to slide over the old and clamped it, instead of welding it and I suggest you do the same. Next, I installed 2 resonators where the water collector used to be, routed the pipes underneath the highest point of the rear cross-member, then over the half-shaft and added another two 6" resonators. Resonators act like bike silencers - they tone the sound down a tad and supply back-compression that is crucial to performance.

I also put 3 balancers in. One before the first two resonators, another just after them, and another one just before the short resonators. Uh, `balancer' could be local slang, so let me explain. You cut 2 holes, one in each pipe, facing each other about 1 & half inch diameter and you join the pipes together with another short piece of pipe. Joining the gas flow of the 2 pipes will prevent the crackling sound likened to Detroit V8's.

You've got to hear it to believe it. It's loud, but smooth. Something in-between a F1 car and NASCAR, keeping in mind 928's don't rev as high. The resonators give just enuff back-compression without compromising performance or sound and the balancers smoothen out the roughness of the sound.

Also, you're gonna have to make up strong hangers in the back, or the pipe will swing excessively. Pay attention because this is a small but important issue.

A final word of advice: Some nifty cutting and welding is required to do what I just told you, so make sure you go to someone who knows his stuff and who's patient enuff to do it right.

Hope this helps...

Ps, I can't help but bragging a bit: FAT BULLY's got 367 ponies on tap!!! He-he-he...

pd'83SC

...just to add to the fun, I deleted the middle muffler from my '80 928 and kept the rear muffler....sound is big V8 rumble but not too loud...but of course there is still 30 pounds of steel hanging on the rear of my car...
PD

John Struthers

Moss, back to the RMB;
When I was stationed in Germany-both times-
when someone scratched up the money to purchase a new car it HAD TO have the conversion kit. They weren't burning unleaded over there at the time. What I remember was that it had to have the 'kit'which I'm sure contained other things but definitely safety glass and unleaded gas/catalytic converter certification paperwork. Obviously, the Converter would have been toast after swilling unleaded for extended periods. When you got the car there was a catalytic converter with a short pipe on one end and a long one on the other. Each end had one of those triangle shaped, heavy-duty, coned, muffler plates. This pipe with new hardware was located in the trunk. Mounted on the car was a straight pipe, sans converter, with identical mounting flanges, when you got back to the States you swapped pipes and got emmision certifed at the Port of Entry.
What I'm getting at is since the RMB is unavailable to earlier 928 models I might cobble together a similar pipe. The idea being come State Inspection time I can simply swap the pipes till after inspection Only problem I see is finding
the right sized flangesso they don't knock the bottom pan or hang to low.
I think it was Maniac or Prdatr who advised a stiffner or two, I'll use angle iron. The cross pipe balancers work fine on dual exhausts but less than helpful on my single exhaust. I'll spec' it out and get back to the board by mid August. Resonators...Hmm.
John S. 1982 Weissach, Auto. "Pattycakes"

Moss928

Thanks guys! Another important question for those of you with RMB's. Does it pass inspection?