Pre-muffler vs bypass pipe

[MotoSook]

I inquired about the construction of the Fabspeed pre-muf some time ago, and I came away (that's what my memory tells me) with the idea that it is just the perforated inner tube surounded by a larger tube. The Dansk is built the same way from what I can tell. No packing in either the fabspeed or Dansk. I have the Dansk pre-muf, but have not handled a Fabspeed. A Rennlister sent me some photos and it looked as I described above.

How the pre-muffler is better than the straight pipe of equivalent ID?

Not sure that they are the same id, but my inpression of the pre-mufflers are that they are essentially a low pass filter with the intend (or maybe unintended) benefit of a slight flow advantage. The perforations (my guess) reduces the boundary layer thickness and allows for a faster flow rate (larger effective flow diameter). The expansion chamber...more a noise controlling device. The low pass filter aspect of the pre-muf's is what makes the engine sound throatier.


I think the pre-mauffler was originally designed to lower the pitch of the exhaust, cancel some of the high frequency noise. An unexpected benefit was the HP gain, slightly better flow with the perforated surface.

[anh911]

I have to go back and review BAs book but I don't believe he was talking about the difference between a straight bypass and premuffler but was in fact comparing premuff to excavated cats. Again, I have to check.
The dyno tests in the thread posted by Rich do not address the question. No one is saying that premuffs don't produce more HP than cats. The question is does a straight through pipe = premuff for power. I think the answer is more complex than just yes or no. For a stock motor I don't see that there could be appreciable difference (except in noise level) The benefit of an engineered premuff is that it is designed to maintain gas verlocity at varying volumes. Velocity of a gas through a straight cylinder varies with volume. So the straight pipe is not as efficient at certain RPMs as the premuff. Both are better than a cleaned-out cat. If you just clear out the cat you are creating a huge chamber in which gas flow stalls. I believe that's what BA was talking about.

Cheers~
Anthony

[RickM]

How does the Euro Pre muffler compare to the Dansk or Fab? I'm sure BA was speaking of the Factory piece and not an aftermarket unit that most likely differs.

And shouldn't the proper back pressure that a pre muff offers over a straight pipe be considered?

[dickster]

rick

i think the euro is similar. the gains appear to be the same - per BA.

BA doesnt mention using a straight thru' pipe so i assume its not going to improve matters - surely he tried it?!!

[MotoSook]

Originally posted by RickM
And shouldn't the proper back pressure that a pre muff offers over a straight pipe be considered?

Any back pressure created by the pipe or pre-muf is going to be nil compared to that in the muffler. IMO, not a factor.

[D Hanson]

RE: Fabspeed service

I have made three separate purchases from them recently and my products arrived within a few days and less than a week. Pleased with their products also.

[RickM]

I can tell you with a decent degree of confidence that when going to a straight pipe on other cars that I had lost HP. I believe that back pressure was more of a factor than out of tune/sync exhaust pulses.

This observation is between a Cat and straight pipe. Perhaps more obvious than Cat to Pre-muff.

[beepbeep]

Hmm...dunno about N/A cars but 930's don't have pre-muffler , just OEM straight pipe.

[JackT]

Before installing the SSI's on my 911SC, I had a test pipe and then a pre-muffler. I could not tell a difference between the test pipe and pre-muffler, except for sound. I did not dyno the car. I then put SSI's on the car and could notice a difference in throttle response. The SSI's feel like more horsepower and torque. It sound great too. It has that clean hollow headers sound.

[RickM]

Just for the record I still have the Cat on my '85.

Out of curiousity does anyone recall when Porsche announced that they developed a Cat that would not rob any HP from there cars? Was it on the 944 and what year? Does this apply to the 911?

[Noel]

Gentlemen,

I searched my archives and found no reference in the "911 Performance Handbook." However, I found two tech questions in my old Up-Fixin der Porsche. Here they are. No mention that a straight pipe costs power.
BTW, the answers to these are both from Bruce Anderson.
BTW, I have a cat by-pass on mine.

Noel

[Wil Ferch]

It was a similar response then , from BA, either in Pano or Excellence...that mentions the hp issue...if it can't be found in his Performance Handbook. I'm certain of him saying that.
---Wil Ferch

[juanbenae]

with my 78 three more years, three more years!!!!! then it wont matter, ssi's and a daul in muffler