3.6 exhaust brainstorming... need help.

Moses · 11-06-2002, 01:02 PM

first, I'm using the stock 993 headers. When routing exhaust pipe from the headers, what diameter exhaust tubing should I use? I am space restricted, so whats the reasonable minimum?

What type of flow (CFM) should I be looking for in muffler and cats?

It looks like I'm going to need a dual inlet muffler with the inlets on the side like the B&B or GHL models. Are there any other muffler types you would suggest?

Again, thanks for your patience with this endless project.

JonT · 11-06-2002, 01:20 PM

So is the 993 cat not going to work? Too close to the fiberglass bumper? If you do go the B & B muffler route, you will need some custom work done to mate up to the 993 headers. I have a B & B muffler for sale (dual in/dual out) I bought prior to deciding to go the 993 header/cat route if you are interested I will sell cheap. You may be able to rig some mini-cats in between the muffler and the headers. Hope your exhaust guy is good! The smog/decibel issues sound like a major PIA.

Joe Bob · Joe Bob's Garage

The way I did it was to match the diameter of the collector, 2.5 in my case, get twin high flow single cats and weld them together.

Piece the piping to the cats, sit them where the old muffler was and weld supertrapps on the end. I had to pay for some pre-bent 2.5 tubing, but everything else cost me 150 labor, 300 for the cats.

Moses · 11-06-2002, 02:16 PM

Jon, I was hoping to route curved tubing away from each header outlet toward the sides of the car. Random Technologies makes a high-flow cat only 9 inches long and 4 inches in diameter. These cats would run in opposite directions toward the sides of the car, then curved tubing into the sides of a dual-in muffler. What do you think? Is there adequate ground clearance for a 4 inch cat running slightly lower than the heat exchangers?

Moses · 11-06-2002, 02:18 PM

Mike,
If my setup doesn't work, I'm copying yours!

Bill Verburg · Bill Verburg's Garage

Option 1: modify this type of muffler, shouldn't take much for a muffler man to plumb from the 993 header flange to modified inlets on this muffler

Option2: plumb as Mike did to 2 bulllit type mufflers placed side by side(vertical stack) but with reversed flows ie 1 flows L to R the other R to L. Weld their cases together, strap into place in the stock location, then plumb to the outlets. If cats won't be used you won't run into the problem that Mike did w/ the cat followed by the long muffler leaving no room for outlet plumbing.

Option 3: (the one I think I will reuse)

Option 4: Steve Weiners solution very similar to option 1 and probably better than option 1.

Joe Bob · Joe Bob's Garage

Moses....using cats that turn out from the collector sends them near the oil tank on the passenger side.

Also if you do a 180 from the collector and run them along side the valve covers, the heat could melt the plug wires ..... remember....dual plugs NOW....and make valve adjustments a nightmare......

Trust me, we scratched our heads for over an hour trying to figure it out.....you owe me a beer if ya "borrow" my design.....

Joe Bob · Joe Bob's Garage

The final look....

Bill Verburg · Bill Verburg's Garage

I sort of prefer this look myself, but thats just me

Moses · 11-06-2002, 03:01 PM

Bill,

That is EXACTLY the look I'm going for! Here's the real question: If I'm going to run exhaust tubing from the header collector outlet to the end of a dual-inlet muffler (with inlets on the outside), why can't I just plumb in a pair of 9" cats along the way? I'm sure it's more than 9" from you collector outlet to the outside edge of your muffler, right?

Bill Verburg · Bill Verburg's Garage

Compare the angles and distance between my header collector outlet and the 993 header collector outlet. The close spacing of the 993 means that an entry into the bottom forward wall of the muffler is relatively easy(needs no major bends like Steve W. did). Entry at any other point is difficult because the tubing has to make such drastic bends hich takes room. There just isn't room for another can and the muffler. Take a look at Mikes, there just isn't room for the cat, the muffler and the final bend. So if you need cats I would stick w/ the stock ones and plumb an outlet sort of like Mikes, or I have seem mufflers with various angled outlets, Coast has one with a 20&deg angle on the outlet.

The wide spacing of the B&B headers leaves plenty of room for the inlet bends and the outlet bends(that part was still tricky w/ 2.5" sss tube). Using smaller diameter tubing makes the installation easier. But I wouldn't want go smaller than 2"

JonT · 11-06-2002, 07:37 PM

It looks like neither of Bill's proposed setups have the cats necessary to help you meet smog emmissions but Mike's does however aesthetically it does not look as clean as Bill's. I have heard of the 993 header outlet flanges being cut off and repositioned to improve the plumb angles necessary to attach a muffler and in your case cats then a muffler. I think until you get the engine in your car and see for yourself along with your exhaust man what clearance issues you are up against it will be difficult to propose the correct solution to your problem. You are treading in unchartered waters with this 3.6 conversion in CA and all the smog/sound issues involved. You'll figure it out!

Moses · 11-06-2002, 07:58 PM

When Mike adds a valance, that solution will look great, too. The only thing holding me back from Mikes approach is the noise level. I hear those Supertrapps are LOUD!

(Mike, my valance has an exhaust slot in the left rear that doesn't match yours. I don't know if it will work for you.)

Bill Verburg · Bill Verburg's Garage

Some more food for thought from over on Rennlist(courtesy of Jack Ennuste). This is a 993 w/ the cat removed, it may help visualize the necessary angles/plumbing.

Joe Bob · Joe Bob's Garage

Yeah....but is that NOT where the a/c on an SC has one of the condensors?

Moses · 11-10-2002, 10:48 AM

This is what I've come up with. The diagram is roughly to scale, but the distance from the manifold collector to the muffler is very small. It requires the use of a Random Technology High Flow 8100 series catalytic converter. They are 4" in diameter and only 8" long. (11" with ends.) I will use a Dynomax Super Turbo muffler. I will put heat shields above the cats.

I think this setup (if I can get it to fit) will give me an equal-length high flow exhaust that might satisfy the smog gods.

Am I missing something obvious?

Sorry, image to follow!

Moses · 11-10-2002, 10:53 AM

Moses · 11-11-2002, 08:00 AM

The diagram above fails to illustrate that the cats will actually be partly UNDER the exhaust manifold collector. (limited space). Will the heat from the cats present a problem in that location?

I really have very few choices. I can put them on after the muffler where they won't work properly, put them to the sides where they can burn a hole in my oil tank, put them in back where they can melt my fiberglass bumper, or put them underneath the manifold collector.

pbs911 · pbs911's Garage

Just curious, when did you get your car last smogged. Is the issue of the smog inspection imminent? If not, and it were me, I would just bolt it up like Steve W's pic above and worry about it later. The way these 3.6 conversions are increasing in popularity, someone may come up with a solution other than MikeZ with the supertrapps.

Also, why did Thom not have to have the cat? I checked his site and he said the cat is considered part of the car and not the emissions. Didn't the 77's have a cat? PP lists a cat -by pass for the 75-80 911's.

Or how about relocating the oil tank to the front to allow more of a 993 exhaust set up in the rear fender well areas?

JWest · 11-11-2002, 09:00 AM

How about real small cats with real small tubing and maybe a Y into a single smalll tailpipe (no mufflers). This will choke it up and hopefully make it quiet enough. Mufflers are not a requirement AFAIK, just noise level.

Get you testing done with this setup, then unbolt it and put one like Bill's on until your next emission check.