Too much back pressure (too small of exhaust outlet). What size output is took small?

[octanemaestro]

On a 911SC with headers and a "non-disruptive" muffler (just a case muffler, without baffles disrupting the flow of exhaust like a maze) and no catalytic converter, how small of an exhaust output is the smallest one can have without damaging the engine, such as by causing too high of a temp at high rev levels, perhaps causing it to burn exhaust valves?

[Trackrash]

Interesting question. I would think the least back pressure caused by the outlet of the muffler would be best. Assuming that all the tuning and scavenging by the exhaust system had taken place before the muffler. Usually the muffler will result in some restriction itself.

Now it sounds like you want some back pressure? Why?

The thing is, you want the exhaust, including the muffler, to work in concert with the rest of the motor. So the muffler outlet size is only one piece of the puzzle.

[octanemaestro]

In my region I don't have easy access to parts like in the U.S. I can't easily obtain either a used original muffler or good aftermarket muffler. I currently have installed on the car a $2,000 aftermarket muffler and header set up which I inherited from the previous owner which I absolutely hate because it is so loud. It makes the car nearly undriveable and in my non-medical non-expert informal opinion maybe could cause hearing damage to the driver (me) over time. It has dual outputs (two tailpipes, one on each corner of the rear of the car). It looks like a beautifully designed stainless system but it's terrible from a practical usage viewpoint because it's so overly loud. It's probably great for a racetrack when the driver has a helmet and ear protection, but I do not use my car on a racetrack. I had a welder restrict one tailpipe completely by capping one side internally inside the tailpipe. That output side has been 100% restricted. Capping off that one side completely reduced the loudness by 30%. I wasn't satisfied. So I had the welder restrict the other side narrowing 80% of the pipe's inner diameter. The car still has plenty of power now and acceptable sound volume, but I've been warned that with only 1" square inch of total tailpipe output diameter it will destroy the exhaust valves or other internal components due to overheating at higher RPM.

Is that true?

[Joe Bob]

Cap one....the muffler should have a common chamber and the diverted gas should exit the remaining open port. If more needed, a baffle can be welded onto the end like a disc type SuperTrapp from a motorcycle.

[octanemaestro]

Originally Posted by Joe Bob

Cap one....the muffler should have a common chamber and the diverted gas should exit the remaining open port. If more needed, a baffle can be welded onto the end like a disc type SuperTrapp from a motorcycle.

That's basically what I now have per above description, right? But the mechanic is saying the increased resultant back pressure will burn the exhaust valves by not letting enough hot fumes escape fast enough. True?

[Joe Bob]

You burn a valve if you go too lean on your gas mixture. Do you or your mechanic have access to an exhaust gas analyzer? IF your SC still is using CIS fuel injection, fuel gauges and an analyzer is how you determine engine tune.

If you go too lean, you CAN fatten up the mixture with a hex key under the air box at the throttle body. You might want to purchase a few books on CIS, and the Bentley manual. Keep up with your "mechanic" such as he is....not rocket science.

BTW, how is the gas quality? Are you running straight ethanol/sugar based fuel?

[scary driving]

You're in Brazil! There's like thousands of VW gurus over there (like more per sq mile than most places). Those guys might be able to help you out with your issue and might even find a buyer for your old exhaust. On the other hand, if you have a respectable tig welder, you might want to consider opening up to add absorbing material to the muffler. Although the "damage to valves" might be a stretch, you don't want that much heat accumulated on the engine. Overheating will be a problem with restrictions on the exhaust. If it's too restricted, it could be the equivalent of having a clogged catalytic converter.

[octanemaestro]

Originally Posted by scary driving

You're in Brazil! There's like thousands of VW gurus over there (like more per sq mile than most places). Those guys might be able to help you out with your issue and might even find a buyer for your old exhaust. On the other hand, if you have a respectable tig welder, you might want to consider opening up to add absorbing material to the muffler. Although the "damage to valves" might be a stretch, you don't want that much heat accumulated on the engine. Overheating will be a problem with restrictions on the exhaust. If it's too restricted, it could be the equivalent of having a clogged catalytic converter.

Brazil is a vast country with 200 million people, without geographic or demographic homogeneity and I am not near such VW experts mechanics that I would trust. I believe I can find someone with a Tig welder but what absorbing material could be added to the muffler case? You're talking about glass packs?

With a clogged cat converter, I thought there would be more restriction than the equivalent of a a 1" diameter pipe passthrough, wouldn't there be?

It seems like you disagree with Joe Bob from Knott's Berry Farm about being able to compensate for a the 1" opening by richening the mixture. I don't understand how you can say damaging valves is an exaggeration while also say overheating is an issue...you can do much worse than damage valves with general overheating.

Wouldn't I see the temp rise on the engine temp gauge if it were overheating the exhaust ports or generally overheating?

[Joe Bob]

Backdate the exhaust, use early heat exchangers with no cat. A muffler shop can fabricate a cheap domestic dual in, single outlet muffler. Tune mixture to factory specs. You're golden.....except for fuel, alcohol fuel has a high moisture content that prematurely rusts exhaust systems.

[trond]

1" sounds awfully small. Can find a conventional car muffler and stick it in there under the bumper ? More difficult to find I know with a 2-in system as you have, than with 1-in. Use the forum search and look at how alternative budget exhaust systems can be built.

[scary driving]

Ok, so you had me looking at maps to find out where in Brasil you are located (I only know the obvious places, since I have friends and family over there), until I figured out ruralis is rural in portugués! I know that Florianopolis has some of the hottest women on the planet, which is need to know stuff! Anyway, no disrespect meant. Here where I live, I know of 3 people who are somewhat versed on porsches, and I don't trust any of them. Lucky for me, I'm not illiterate on the subject. The particularities of back pressure, I don't know HOW it will affect YOUR engine, but I can tell you this: You guys have mercedes benz torinos over there, so you'll know what I'm talking about. I had a case whith a 904 engined one just 2 months ago in our fleet in which the muffler became clogged with sooth after it blew a turbo. It would run fine, and after about 30 to 40 minutes it would start losing oil pressure and idle rough. Of course, the engine would shut down due to the safety programming on the ecu when it went below threshold oil pressure, but you get my point. It took us a while to figure it out. In most cases it will not be obvious. You can get lucky and it "just" overheats, or in a very extreme case, it wears your valve guides, or as is the case with jaguar v12s, it drops a valve seat, which is common on those engines when they overheat. I know engines in general, not porsche engines in particular, if you know what I mean.

The absorbing material is usually some kind of steel wool type of stuff, but again, someone who works with mufflers and is willing to take a crack at it might even recycle the material from other old mufflers. The reason for suggesting a tig welder is that it will create minimum deformation and will look presentable. Other than that, it's just a case of open it up, add absorbing material and weld it back closed.

P.S.: Which part of Brasil are you at?