muffled performance

[lowact]

Chopped open the rear muffler/resonator, discovered that the exhaust flow area is reduced to 80 x 1/4" diameter holes:


Ignoring the orifice effect, net gas flow area is therefore 2534 mm2.
I was expecting it to be closer to the exhaust pipe rough rule-of-thumb which is 1" diameter for every 100 hp; suggests that for 300 hp, 4536 mm2 would be more appropriate.

Also shows why all the exhaust is from the center two outlets of the fan-tail

[Bob.]

Hi Colin,
Would be interesting to know what brand that silencer is as the position of the welds on this original box appear to suggest a different internal construction on the inlet side.
Bob

[Series1 Stu]

Chopped open the rear muffler/resonator, discovered that the exhaust flow area is reduced to 80 x 1/4" diameter holes

Ignoring the orifice effect, net gas flow area is therefore 2534 mm2.
I was expecting it to be closer to the exhaust pipe rough rule-of-thumb which is 1" diameter for every 100 hp; suggests that for 300 hp, 4536 mm2 would be more appropriate.

Hi Colin,

That's interesting, but would you mind explaining the maths to us, please? I agree with your workings on the area of the 80 inlet holes but I'm struggling to see how you have arrived at the 4536 mm2 figure.

Regards

[MarekH]

This is quite a restriction and it all comes down to the diameter change.

1/ Flow speed is proportional to diameter squared, so lots of small holes adding up to the same area as one big hole forms a big restriction in its own right. Doubling the area isn't enough to compensate for the huge drop in diameter - this is the dominant factor in play here by a huge margin.

2/ Eighty 1/4" diameter holes means pi x r² x 80 x 25.4 x 25.4 is 2532mm² so nothing wrong with the maths there. By comparison, two 2" diameter pipes are 4051mm², so nothing there either as I assume the pipes are just over 2" diameter.

kind regards
Marek

[Series1 Stu]

Ok. I kind of get it. Colin's figure is for a 3" diameter pipe and I assume he is suggesting that there should be more than 143 holes of diameter 1/4" to conform with his 'rule of thumb '. Obviously, this doesn't account for the throttling effect of the smaller holes.

Regards

[lowact]

When I bought this car (from US a decade ago) it had an almost new, fully welded exhaust system. No brand or any markings at all on the silencer or on any other part.

I suspect my “rule of thumb” is more applicable to exhaust pipes for competition rather than street cars; not something that should be used to assess mufflers. A better comparison would have been with the oem pipe area. Mine are 1-7/8“ OD, same as the engine pipes so I guess that is the oem size? ID of these is 44 mm so total gas flow area of the two pipes is 3041 mm2.

I’m not trying to calculate a “correct” size. With my engine removed and replaced and different exhaust headers, the fully welded exhaust doesn’t fit like it used to, my solution is to do away with the rear muffler, make the exhausts from each bank completely independent. Is why I can be so cavalier about chopping up an otherwise perfectly good muffler, I was just curious …

[Bob.]

Norman Dewis doing max speed runs in Italy in July 1970 and chasing the magic 150mph, noticing that only the centre two outlets of the four were effective, had the rear silencer and fantail removed and replaced with two straight through pipes. He reported a 3mph improvement in top speed.

[malcolm]

I'm sure that's right, but I don't think I'd arse around for 3 mph!

[lowact]

Sans muffler

[Woolfi]

For the problem with the 80 too small holes in the back muffler, there is a super simple solution. I took a drilling machine and made 2 one inch holes through the left and right outlet of the muffler into the wall of the two pipe ends. The muffler is not much louder. You can start with a hole with 1/2" diameter, und listen to the sound. Then you can decide to continue with drilling bigger or stopp.

Exactly the same you can do with the entrance of the two front mufflers. This will add also few 'ponies'.

But if the back-pressure is lower, you need to change the diameter of the 4 carb-needles. Otherwise the mixture is becomming leaner and you loose ponies,
I have done this with my EV12 20 years ago. Without a lambda-measuring system like an Innovate LC1 or something else,it is difficult to find a good 'fitting' carb needle profile. Without this needle-change I had flat spots.
A friend has cut the two pipes entering the back muffler. He saw, that the two holes into the muffler have been smaler than the inner diameter of the pipes.

Regards Wolfgang Gatza

[chrisfell]

I recall one UK owner had removed the centre silencers from his S3, leaving only the rear box and fantail tips. It wasn't hugely loud, but needed rubber mounting for the straight through pipes, rather than solid mounts he'd used, to eliminate drumming in the cabin at certain RPMs. I don't recall whether he'd claimed a performance or economy improvement.

[paydase]

That's the issue when you modify the exhaust line.
The car builder in principle optimises the line to ensure not only limited sound level but also parasitic issues such as drumming at some rpms.