Tom Cloutier

Hi Rob!

I'll be happy to make an exhaust system for you! What's wrong with your car and why aren't we having a group attack to fix it? Let me know when you're ready to roll.

Tom Cloutier

Tom Cloutier

Mark,

Either my memory is failing or you're making a joke, right? Or are you talking about the X-pipe? I remember buying some Bullet mufflers and an installation sketch back then. Was that you? So you're the one who started all this X craze!

Tom Cloutier

Tom Cloutier

Maybe I just missed it, but it appears that not one single reader has responded to my CFD data. Not even one, "Pretty pictures, Tom" comment. Come on you guys! This is very fun and interesting stuff!

Tom Cloutier

Joe F

Tom,
Let me be the first then to congratulate you on those pretty pictures. To be frank, that stuff is so far over my head, I would have to look up to see the bottom of it.

sportscarclassics

I would like to answer several of the email questions:
Tom sent me the CFD's charts for his X pipe and Y pipe. The Y pipe does look better with cleaner flow characteristics than the X pipe. A single 3.5” pipe flows better than dual 2.5” pipes because the 2.5” pipe has more surface area in comparison to cross sectional area, here is some math.
Surface contact: Two 2.5” pipes=15.7" Single 3.5”=10.99” (3.142 x D)
Area: Two 2.5” pipes=9.82 in*2 Single 3.5”=9.6 in*2 (3.142 x R*2)

The comparison in cross sectional area of the .3.5” to 2.5” is very close while the surface contact of the gases is far greater with a dual exhaust. The gas has to rub against this extra contact surface and the flow along the surface will be reduced. While the velocity pressure is reduced a little the stagnant pressure is greatly reduced. Velocity pressure flows along with the flow of the gas (inner flow), however the stagnant pressure acts normal (90°) to the flow and always has a zero velocity (outer flow). The total pressure in a pipe comprises of velocity and stagnant pressure. Stagnant pressure pushes against the pipe wall and tries to bulge the pipe out.

When you look at Tom’s CFD for the U bend you can see the flow trying to go straight but it is being pushed around the bend and the velocity along the outer bend has slowed down a lot because the pressure has gone up. When the pressure drops then the high pressure region will want to go to the low pressure area and this will even things out, which is evident after the bend where the pressure drops off again. The straight pipe has better distribution along the flow layers within the pipe.

Someone asked about densities:
Densities at 25C.
Mild steel 7.72 to 7.9 kg/m*2
Stainless steel 7.75 to 8.1 kg/m*2

Someone asked mass flow:
Mass flow in equals mass flow out…100 ***** in per second, 100 ***** out per second. If you have a dual then the two pipes will split the total and it may not be even because of the pressure in the system.

Carl

sportscarclassics

If anyone has questions regarding CFD's please ask. I will try to explain what is happening with the flow which is based on velocities, pressure, enthalpy, temperatures. I noticed that some of the velocities were just sub-supersonic which will give off a boom, that disrupts laminar flows in a pipe. The highest velocity was 1072 ft/s which is 730 mph or Mach .98. Stay under mach 1.

Carl

Tom Cloutier

Thanks, Joe! That means a lot to me. Really. It makes it all worthwhile just to know somebody cares!

Think of gas flow like honey do's, painting a deck more specifically. Gas tends to flow to a low pressure region or take the path of least resistance. So, like gas, when your wife says, "It's time to paint the deck this weekend," you naturally flow to the refrigerator, get a beer, then into the garage, polish a couple wrenches, check the oil, fantasize about the 600hp motor you want to put in the 928, etc. Now when your wife says, "Joe, get your lazy *** out there and paint my deck," a large local high pressure region is created, thus causing a low pressure zone in the garage which further hastens your trek out there! Quite considerably I might add.

Best regards,

Tom Cloutier

Joe F

Tom, have you been reading my mail? I didn't realize you had met the "queen of everything" otherwise known as She who must be obeyed. This is, unfortuantly, something I can relate to. "wife says, 'Joe, get your lazy *** out there and paint my deck.'"
Kind of like asking how high? while on the way up, in response to the command to "Jump" given by the same source.

cheers

MarkRobinson

I never had a resonance, it was that the pulse from each manifold were intersecting at times not intended by the factory on our "pulse-tuned exhausts systems". so, but putting the X where (in distance from the flanges of the manifolds) Porsche would have placed it keeping the pulse theory alive, I removed the odd "thump thump thump" sound that resulted in the gases mixing at the wrong time...yes, 6.5" made all the difference. Restored torque the the point that my system is now starved for fuel during peak torque..will have to add and AFC to correct.

mark

Glen McCartney

Tom, sent you a PM with some questions on exhaust system.

Tom Cloutier

Uh Joe,

Didn't I tell you my real job is working for the IRS and FBI? Of course we've been reading your mail--since May of 2002. It seems there were some significant discrepancies in what you reported as your income and our findings. We are looking at your records dating back 10 years.

BTW, your phones are tapped too and we have transmitters hidden on all your vehicles.

Have a nice week! You'll be hearing from us.

Tom Cloutier

Louie928

Tom,
I did single vs. dual pipe comparison several years ago. Also 2.25" vs 2.5" comparison. Results are here.
http://www.performance928.com/cgi-bi...ss_parent=1125

Peak power was very close to the same on single vs. dual. There's a bit more torque with the duals.

Replacing the stock rear exhaust with either will get you about 12-15 more hp.

Louie

Tom Cloutier

Louie,

I can't recall for sure, but weren't those tests with straight pipes (either dual 2.5 and single 3.5) with mufflers to the rear of the car? I want to compare a dual 2.5" system with pipes following the stock paths to a 3.5" system with the pipe located on the left side out to the rear. Probably do this without mufflers as I want to try and evaluate the loss in the two pipe design as compared to a single pipe design. The right side pipe has several pretty torturous bends that will inhibit gas flow. The bends coupled with the additional wall surface area of the two pipes have to cause a performance hit and I'd like to know how much.

I pulled out a pair of stock pipes yesterday to measure lengths and angles for some CFD tests. I expect Gordon Blair or one of those guys has a correlation between gas flow and power. I'll have to root around to find it. It would be curious to see if the actual dyno data and the flow predictions correlate.

See you in a few days!

Tom

gruffalo

I'm building a Y-system similar to the Avenger for my 4,5L 1981 euro.

So, how far back should I let the pipes meet? Does it matter at all with the stock headers?

MM

MarkRobinson

Hi Tom,

You, Louie Ott, and a few others were a few gentlemen I assisted with a very early exhaust design & muffler system. Both of you two have elevated exhaust to an appropriate level among enthusiasts: well done. I didn't nor have I ever claimed to start any X or Y exhaust craze...the sketches to you two were with a basic H-pipe design & even w/o proper positioning. This is my first X configuration on my car, and the only thing I'm stating is that just recently I've verified that the position is very important to sound/power/vibration as I tried 3 configurations on my car before going to stock location for my stock motor. I think Louie proved about 5 years ago that the X was the way to go, but that's just what I remember.