Not much concern but no optimum either, It's quite common for even the primaries to be off by an inch or so.

The bigger concern is the relatively small size of the muffler cavity, I could barely get a 3.0 SC to pass Lime Rock sound(on most days) w/ off the shelf mufflers that size.

This IMO is probably the best way to go, if it's too noisy open it up and add f/g matt

These also flow quite well

there are little things that can add to the flow efficiency and power
one thing would be to extend the inlets into the cavity an inch or so(see what the factory does) and use a bell shaped transition on the end. Once the initial expansion has occurred it's all about absorbing reflected sound waves and efficiently moving the cooling and contracting gasses to the outlets
The 993 GT2 Evos added a reverse cone to the outlets i don't know that it did any good after the muffler but the factory thought so, It certainly is very helpful for open exhausts

How much does fiberglass or other mat on a straight through muffler help vs an empty but larger muffler cavity? We could make a muffler chamber from 6” tubing x 36” long with the inlet and outlet pipes curving into the chamber so they’re not directly across from each other like a modified banana muffler is. I’ll draw it up tomorrow.

Bill, you have often extolled the virtues of the Fabspeed muffler. I thought you posted a cut away view of one at some point. Or maybe I saw it elsewhere where. Do you happen to have that internal design? Since you like it (so does Steve Wong and others) it might help Dave with his?

An empty cavity is very loud, Flowmasters are empty cavity designs they use a series of coffer dam like diverters, I used one for decades but it would never pass sound at LRP, fine a WGI w/ no sound restrictions.

The thing is you do want as large a cavity as possible iff you have cams w/ decent overlap, The biggest and best cavity is an open exhaust, the bigger the cavity the bigger the return acoustics that the cams can use.

To suppress noise you need f/s or ss wrap or ss scrubble and the more of of this the more noise suppression, most efficient flow comes from solid , w/ perforated pipe a close second. You have to integrate al of this into the available volume.

I often though that ex[anding a bannana type into the aft ends of the rear wheel wheels, sort of like a semi side muffler extension ala 993 , 996, 997 etc.

A 993RS 3.8 w/ separated cat bypass and only 993 n/a or 993 tt side mufflers is perfectly ok at LRP w/ turndowns at the exit tips. Even modified 993 sides like the Fister 2s are ok in that environment.

yes the Fabspeed that i have is by far the best sounding muffler i have ever used. I have no dyno #s on it yet but it sure feels better than even the Flowmaster i used to use. and the noise level is very well controlled, I can even hear the cooling fan again.

I did ask Joe about the interior design and was assured that there is no head on impingement to flow, but beyond that he wanted to keep details close.

I suspect that there is no expansion cavity as w/ the bananas or 993 type side mounts but the stock 993 or even an RS doesn't really have a lot of TDC compression overlap any way.

A couple more designs.

First one is two chamber (per side) with no packing/fiberglass. Overall muffler is 36" long x 6" diameter so each chamber is 9" long.

http://forums.pelicanparts.com/uploa...1754503225.jpg


The second one is three chamber (per side) with packing/fiberglass in the central chamber. Each chamber is 6" long.

http://forums.pelicanparts.com/uploa...1754503286.jpg

Thoughts?

EDIT: I just checked the weight of these two designs and it's not good. They're fabbed from 18ga or 0.065" 304SS and both weigh about 45 lbs vs about 17 lbs for the dual in/out off the shelf muffler including the inlet/outlet pipes!

w/o packing the first will be too loud

the second is better but a third chamber isn't going to do much, i'd like to see long perforated /wrapped pipe, exiting into a larger central cavity and ovalized case
http://forums.pelicanparts.com/uploa...1754507945.jpg

I wonder if the WK headers spacing might be compatible w/ the Fabspeed muffler

It really is a great muffler
http://forums.pelicanparts.com/uploa...1754508209.JPG
http://forums.pelicanparts.com/uploa...1754508209.JPG

and Skinnerd's modified banana is also a great design

some other designs
http://forums.pelicanparts.com/uploa...1754508570.jpg
http://forums.pelicanparts.com/uploa...1754508604.gif

Found Fabspeed internals, bothe sides dump into a central cavity w/ angled ends
http://forums.pelicanparts.com/uploa...1754508604.jpg
http://forums.pelicanparts.com/uploa...1754508604.jpg

Some Burns implementations
http://forums.pelicanparts.com/uploa...1754508604.jpg
http://forums.pelicanparts.com/uploa...1754508604.jpg

964 RS/LW
http://forums.pelicanparts.com/uploa...1754508604.jpg

A few more
 

SC/RS rally
http://forums.pelicanparts.com/uploa...1754508604.jpg

Singer
http://forums.pelicanparts.com/uploa...1754508604.jpg

Again I always liked these
http://forums.pelicanparts.com/uploa...1754508604.jpg

I made something simple when I installed my 3.6 engine.
A few 90 degree bends and 4 cheap small simple straight thru mufflers.

Its not loud and according to the dyno its not a disaster.

Please don't look at the welds! But they have held up.

http://forums.pelicanparts.com/uploa...1754514487.jpg
http://forums.pelicanparts.com/uploa...1754514499.jpg

I'm not a fan of the internal cross over. Over the years I've tried a few but they all exhibited less than desirable resonanance at certain frequencies.
Almost as if the the opposing exhaust pulses are 'crashing' into each other.

I don't know the best way to go but it sure looks like you are more than capable of solving the problem. Looking forward to seeing the outcome!

Does the bottom one have some kind of mechanical valve?

Do you have any sound clips? Video?

yes, you can see the tool insertion points to manually turn the valves

Nah, nothing good. It sounds deep, not sharp at all. I manage 95db flyby on trackdays.

Only some crappy in car audio from track days.
https://www.youtube.com/watch?v=sMmwq6w8mwU

Lots of good ideas here but the problem statement is to have IROC RSR spacing on the rear outlets which severely limits the options.

I could not find the Fabspeed muffler on their website that Bill likes but I have emailed them to check if it's still available. I would like to find out the overall length of the muffler to see how it fits.

Here's my final design if Jay's onboard. It would allow coming back and installing a larger 22" long muffler if it's too loud and if a larger muffler is found or fabricated:

http://forums.pelicanparts.com/uploa...1754578312.jpg

I like that and feel the pain on the RSR width challenge. I would mimic the fabspeed internals, and maybe make teh box as wide as possible before encroaching on your mandrel bends for teh existing. Based on your drawings that could be as wide as 22" on the bottom. You could even go wider possible on the top half, using a tighter bend similar to the outlets, and gain some chamber size and sound deadening. Just a thought.

This type has the RSR outlet spacing, all you need to do is modify the inlets and if too loud open it up to add f/g packing
http://forums.pelicanparts.com/uploa...1754508604.jpg

That last drawing is very similar externally to what many of us had from Fabspeed, Borla etc.

This is my old Fabspeed


http://forums.pelicanparts.com/uploa...1754581845.JPG
http://forums.pelicanparts.com/uploa...1754581845.JPG

I spoke with someone named Andrew at Fabspeed. He told me they discontinued the exhaust Bill likes in favor of their x-pipe design. He said it was both quieter and better on the dyno.

He did tell me he would check if they had any stock left of Bill's type muffler and was supposed to send me dimensions and/or drawings. So far I have only received photos of what it looks like installed from him. I'll update the thread if I hear back.