924Board.org

[Oz Owen]

On my recently purchased naturally aspirated 77' 924, the previous owner fitted a "sports" exhaust - which has a 5 inch diameter with a small amount of honeycomb baffling. Its quite a nice muffler as its stainless steel and isn't as bulky as the NA rear muffler.

Apart from the rude sound, the car runs a rough in lower revs (presumably as there's not enough back pressure - as there is on the NA tunes exhaust system).

Has anyone mucked around with the tail pipe to restrict the flow of exhaust?

I was thinking of maybe putting on a sleeve which basically lessens the diameter of the end of the tail pipe.
Anyone think this may help to improve the situation, or is it dumb?
Or should I just get another NA rear muffler?

Cheers,
Owen
_________________
1977 924
1989 Pugot 405 Mi16

[ZODIAC]

im not incrediblyfamiliar with these cars, as im still peeling off the first layer of the onion that is the 924. but 5 inch sounds way to big for a 2.0L N/A engine. on my kia (2.0L) the max recomended size is 3 inch, and that has 135HP(supposedly).
_________________
"Kraut cars, they know how to build 'em".
1977 924 N/A - Red with waterdamage interior
yes...waterdamage is now a color...

[Oz Owen]

yeah 5 inches its definitely way too big for the engine.
I'd like to use the currently mounted muffler, as it has been mounted professionally and sits nicely.
But if I can't modify it, I will replace it with a muffler similar to the original.
_________________
1977 924
1989 Pugot 405 Mi16

[StienbargerR]

I could be wrong, but I recall someone on here saying that these engines don't need exhaust back pressure to run well.

Have you checked the timing and checked for vacuum leaks? Seems like those are common culprits for running rough, especially at idle.

Ryan
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1978 924 NA
-250lb lowering springs, Euro Pistons

[Oz Owen]

Hey StienbargerR

I read whatever I could about the exhaust system by sifting through forum posts.

Some people suggested that the exhaust is tuned, therefore it needs a certain level of back pressure.

I'm going to mount some sort of ring that impedes the outflow of exhaust - and see whether that works.

I'll post back with the outcome of my trials.

but if anyone has tried this and has some tips, please let me know.
_________________
1977 924
1989 Pugot 405 Mi16

[Rich H]

The primaries are very short but the secondaties are very long, they had space to tune so I think they have to soem extent. 5" is massive, that would be more suited for a turbo
_________________
1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

[RC]

Please clarify. Is it 5" pipe or 5" muffler?

Presume you mean a 5" SS muffler, AKA a fart cannon?

Theoretically, no back pressure is best for power. Tuning the length & diameter of primary pipes permits extraction of gasses at certain RPM. In practice though with the inefficient head, cam & manifolds, some extra baffling will not only quieten the noise but lower the power band. This principle is used on Supertrap mufflers. Only played with one on a bike but does make a difference fitting more or less plates.

More info from google. SuperTrapp.
http://www.supertrapp.com/technology/index.asp

[Vince Ponz]

There is a tunable muffler??? called a Suppertrapp that you can find online. By adding and subtracting round plates to the end of the pipe you can tune sort of the power band. It also makes the exhaust sound louder or softer.
I had one on my car and didn't like it.
_________________
"Never let them see you sweat"
77.5 924 modified track car
79 931 Euro stock
88 924S SE
87 911 Targa stock

[ideola]

Pretty sure he means a 5" muffler, not a 5" exhaust pipe.

Less back pressure means more power, but higher up in the power band. More back pressure means less power, but more torque lower in the power band. This is probably why you feel like the car doesn't run well at low RPM.

A street-driven NA will benefit from a driveability perspective by retaining stock back pressure because it will provide more torque from idle (at the expense of high RPM horsepower). For a car that's driven hard and constantly in the upper RPM range, you would definitely want a more open exhaust.

None of this applies to the 931, which does just fine with a stock 3" exhaust all the way back.
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erstwhile owner of just about every 924 variant ever made

[Mike9311]

I think he means only the muffler is 5 inches. Normally those who install fart cannons don't do anything with the rest of exhaust

Back pressure, less or more, matters most from the head on back. Its effect lessening as the exhaust system is farther away from the engine.

I also wonder if its a perception that the car is running rough because of the "cannon" farting away. Personally I would just put a smaller muffler on and see what happens
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

[Martijnus]

it has nothing to do with back pressure imho... it's not the pressure, it's the velocity of the exhaust gasses, which is drastically reduced by going big on the exhaust's diameter.
you want the exhaust gasses to escape as fast as possible from the exhaust tract, and with a large diameter that won't happen.
Can anyone elaborate the backpressure theory? I can see no reason for pressure improving the performance. If unburnt mixture is sucked past the exhaust valve because 'there is too little back pressure', the engine would have the same cylindrical filling, unless the mixture is pushed back like on a two stroke. If that happens, you have some weird resonance in your exhaust system Plus that it can't happen because the exhaust valve is closed already.

When the exhaust gasses flow very fast out of the system, the mixture is sucked in even more when there's a vacuum, created by the flow of exhaust gasses. Then the exhaust valve closes, and even more mixture is pushed in the cylinder due to the inertia of the mixture and possibly a descending vacuum.
intake valve closes... compress the stuff...etc.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[ideola]

Nobody said back pressure improves performance.
_________________
erstwhile owner of just about every 924 variant ever made

[Mike9311]

ok, now we opened up a can of worms and sorta hijacked the thread. I will provide a link to one of my favorite suppliers and exhaust engineering pioneers. Its a good place to start ...

[/url]http://www.burnsstainless.com/theory.aspx[url]

[/url]
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

[ideola]

[ideola]