Lopez

Hey - so, no one sells these since they are so simple (that's what the threads all say) so I went and built one. For the benefit of others I thought I'd post a quick write up...

So, it was easy. I used a 2.5" stainless pre-bent 45-degree mandrel bent tube from Magnaflow - these are available all over the web for about $20. The tube was 4" on one side of the bend and 14" on the other. It just so happens that if you go about 3 inches from the bend center you get the pipe exiting precisely in the center of the stock muffler cutout in the bumper.

I paired this with a 2.5" single inlet->dual 2.5" slash-cut outlet resonator tip made by Hooker Headers. Again, easy to find on the web for around $35. This part has a slip-fit which you can either clamp or weld.

Finally, I made a flange out of 3/8" stainless steel (fortunately for me I work in a lab which has a fabrication shop, and I made this on a CNC mill). If you wanted to, you could re-use the flange on the stock muffler - at least on my muffler it is just tack welded onto the inlet tube, and I could have ground off the welds and hammered it off there.

The 45-degree pipe was cut down to around 3" on the inlet end and 4" on the outlet. This put the exhaust tips just poking out about the same as my stock pipes:


Here's a shot from the bottom (don't flame me, I know I am not the world's greatest welder):


I still have to tack on the attachment so it can utilize the factory rubber donut and hangar - this will be easy though as the pipes run directly under the existing hangar.

It sounds fantastic - my wife actually thinks that the car is quieter than before when driving at low rpm or freeway cruising with the windows up (this is clearly not true. but the fact that she finds it at all tolerable proves it's not outrageously loud). At WOT, well, you can imagine how it sounds. It's good, though...

FYI this car is a stock '79, it's exhaust is now stock manifolds, a Y-pipe with high-flow cat, the stock mid-pipe (with the resonator), and this RMB.

pcar928fan

Excellent! Almost exactly what the shop I take my cars to did to my '81!! Sounds pretty dang good too!

911tracker85

I am about to buy a Y pipe w/out catalysts from 928 Motorsport.

I was considering something like what you did, but ...

on my 84's exhaust, the components are welded from the current catalyst back. no flanges connecting the rear muffler to the mid-pipe with the resonator.

it sounds like there is a flange on your system where the rear muffler bolted to the mid-pipe section, correct?

Lopez

hi, yes. On the early cars the muffler bolts on.
If you can do all the other steps though, then you could make a second flange and convert it to work that way (or a third and add it to your muffler too).

blandis

Any noticable throttle response or low torque difference.

Im thinking of doing the same on my 84.

Lopez

please keep in mind these are totally "***-o-meter" readings:

It feels like some of the low-end torque is gone. It also feels like it simply relocated above 4500 r.p.m. The engine feels more "peaky" without the rear muffler - it is much happier revving into the 5-6000 range than it used to be. Personally, I like it this way - it feels more like a sports car. An '84 is significantly different inside (different camshaft profiles & obviously bigger displacement) so the result might be somewhat different.

The nice thing about this is that if I wanted to I could easily bolt the stock muffler back on - the swap takes about 10 minutes tops. If I was going to drive the car to Vegas or San Francisco (long 2-lane freeway trips) I would seriously consider doing that...

bronto

???

How could removing a muffler reduce torque anywhere? The other side of the coin would be adding a muffler to add torque(?).

SharkSkin

Bronto, the back pressure can help low RPMs. It won't always, just as opening up the pipes won't always help the high end. It depends on what you're starting with. For example, starting with a 4" pipe, going to a 5" would probably be detrimental, killing all scavenging effects.

I have a Borla cat-back, so I can't try this. My mid-resonator was blown out anyway, if it had been just the rear muffler that was corroded I might have tried something like this. I like the way it turned out -- can't really hear it from inside the car, except in a tunnel with the windows down.

Lopez

Right - Bronto, when you have too little backpressure at a given RPM the scavenging effect can be too great during valve overlap (when both intake & exhaust are partially open) and pull unburned fuel and air out into your exhaust. This reduces the amount to burn and consequently robs torque. It also makes the car run leaner at that RPM range too.

In a car like this which has CIS and no programmable mixture / timing curves then I don't think you can do all that much about it. If you had a modern EFI/solid state ignition system you could probably tune some of the loss back in on a dyno by adding more fuel/air at the rev range where its being excessively scavenged.

Lopez

So... I finally got around to doing sound clips for this. I couldn't figure out a good way to host them so they are on youtube (although there's no video). The recordings were made with the microphone in the car (wedged between the passenger seat and the center console) and with both windows open. Some wind noise, and of course the sound is reflected from outside.

They are here (starting the car and idling out of the garage)

http://www.youtube.com/watch?v=htZ4bG8Hgqo

and here (going from a stoplight onto a freeway onramp, wot from about 15 - 65 m.p.h.)

http://www.youtube.com/watch?v=JcK0zAgJRo0

anyway, for those of you wondering what a single-exhaust 16v 928 sounds like without mufflers, you can hear it doesn't sound like an American V8...