Dude, what are you trying to get at? I'm not denying your point that increasing the diameter of the exhaust will slow the flow. You're hurting your own line of reason by citing boyle's law though - as that volume expands the pressure will drop across the surface - it has to, it's conservation of energy. This is not physics 101 though and there will undoubtedly be more factors at play - CFD is stupid complicated. Flow patterns (laminar vs turbulent flow), boundary layers, vortices, standing waves and resonance patterns, all of this stuff will play into what actually happens in the real world. You clearly understand general physical principles so resistance should not be a foreign concept here. Resistance is the "thin line" so to speak of when it comes to flow. Too much and you choke the system, too little and you lose patterned (laminar) flow which ultimately will siginificantly disrupt the system.

If you don't believe this, take a tour of a major engineering complex (like GE for example). They have an entire team of engineers with impressive credentials whose job it is to model flow systems on computers that rival deep blue.

This part of my project started with the fact that the stock exhaust system is undersized for the hp rating of the vehicle at baseline, and the VE of the stock setup is nothing to write home about. This is my attempt at getting out of the motor what stuttgart left behind 20 years ago. You clearly understand physical principles, so don't you think it's a bit shortsighted to look at one aspect of this entire system from the viewpoint of one principle and then call me dumb for it?

Nevermind, please don't even bother answering that question. I am truly over this thread and I apologize to everyone who has read it.


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I didn't call you dumb, a cardiovascular surgeon? clearly you are not. I was referring to the flippant joke you made to BYP's comment about a fact.I am not prone to using emojis and LOLS. So carry on I still say you'd be better off not disrupting the flow especially slowing and disrupting the scavenging effect.

Time for the update - it took a while to sort out some coolant issues but I've been running the straight pipes for about 50 miles... It's too loud... Way too loud... Like permanent hearing loss by the end of summer too loud. Not only that, but dirty too. At 100mph you don't notice any fumes, but you will reek of fumes every time you drive. This may be because the exhaust terminated under the car and not behind the car, but never the less the whole setup wasn't a good fit for a DD.

So, here's what I've done. I added high flow universal cats at the end of the 2.5" pipes. Quieted things down but my wife still refused to get in the car lol. So I stuffed some bored out internal baffles (db killers) into the pipe before the cat. And now my wife (and I) love it! It's not quiet by any means, still on the loud side (probably will get your name brought up in an HOA meeting) but it's got a phenomenal deep growl and you can have a conversation while standing behind the car.

On an interesting side note, while the car was straight piped, there was still a drone! Exactly where it ought to be - 2-2.8k rpm. Unbelievable! It's clear to me now that this a feature of the engine harmonics and not the exhaust system, and the only way Porsche was able to kill it was with a choked off set of pipes and a gas tank sized muffler. Ahhh the m96 motor, what a marvel of engineering this machine it. If anyone has ever ported their head, I would love to know what effect that had on the drone.


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Update 2

The idea of the internal baffles bothered me, so I removed them and added shorty glass packs following the cats. Good thing I did! The damn things had dislodged and were eating through the cat matrix (I should've seen that one coming. Oh well, ya live and ya learn).

Interesting side note - the heat wrap works. I had a bit of trouble lining up the pipes perfectly when I installed the glass packs so I left it and figured I would straighten out another day. The top 1" or so of the passengers side muffler overlapped the bottom of the bumper with about an inch between the two. Well, that was close enough that it melted the bumper! The melting point of fiberglass is 2050 deg F so, like I said, the heat wrap is clearly working lol. This gives me a good excuse to now make the "aero mods" to the rear bumper I've been scheming for a while now.


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