Hey Keith,
I know you addressed this to Cpires, but I may be able to offer some help too. The optimal included angle for a V or a Y collector is about 20 deg (10 to 15 for each leg). This has been proven to be the best angle to transfer kinetic energy from the gases flowing down one leg (let's call it the active leg) to the gases remaining in the other leg (the one that doesn't attach to a port with its exh valve open). This transfer, affected by molecules bumping into one another like pool *****, along with entrainment, increases the flow rate in the inactive leg. This is called inertial scavenging. Inertia or momentum is transferred from the fast moving molecules of the active leg to the slower moving ones in the inactive leg.

Additionally, as the slower molecules accelerate, they move farther away from those nearer the port. This is often described as a plug of gases moving down a pipe, but this is not very accurate since the flow is more nearly continuous than discreet. The result is a low pressure zone moving down the pipe into which the stagnant gases near the exh port are drawn.

A 3" collector outlet is really BIG. A 2.5" diameter and maybe even 2.25" if yours is a stock 4.5L engine would be a good choice. If this smaller diameter leads into your Y or V, the gas velocity will be higher, more kinetic energy will be transferred, and scavenging will be increased. More power and fun will result!

For best performance the volume in a merge collector (a 4 into 1 such as you have on your headers) should be kept small. This will maintain a high gas speed and maximize scavenging. If the volume is too large, gas speed is lost never to be fully regained. So performance is lost at the first collector and the loss snowballs right on down the line.

Hope this helps. Like i said a couple years ago, "It ain't just pipes, it's science!"
Cpires, Rich?

Tom Cloutier

 

I have a X over from Dr. gas that has 3" inlets into a very wide X. The 3" collector is for the 3" header collectors from the MSDS - I'm not sure how I would change that.

It then goes through the X and into a swage down to 2.5 for meeting with the 2.5 in dual exhaust.

Is that bad?

For best velocity in a Y from the 3" collectors on the headers, what would be suggested to get into a 2.5 single into a rear turbo?

 

George I just got my foot out of my mouth and sent off an apology to Tom offline. Thanks for your post.

 

Heinrich and I are happy again. Sorry, but no WWW this weekend!

Tom Cloutier

 

 

Tom,
If I'm not treading on propriety info, could you please indicate what sort of cross sectional area must be preserved where the two pipes of a 'X' are siamesed? ie., if the cross section area of one pipe is "A" (or the diameter 'D'), then at the minimum width of the ovalized intersection must be somewhat less than 2xD ( or <2A) - but how much less?
Yes, I'm on the DIY kick: to import one will double the #@*%$ cost thanks to UPS - so I have a pal, owner of a muffler shop, who wants to experiment on one with me. As the rest of my S4 system is all GT ( resonators & muffler), I was thinking of staying with the same 2.25" pipe, doing the 'X' angles @ 30 deg total: this leaves the question of how much to shave off each in the weld zone. TIA

 

Hey thanks Tom,

so if I read correctly.... from the 3" header collector flanges, reduce down to a 2.5" "V" (given the distance of C to C on the headers, 10 degrees or so maybe tough in the amount of "V" space given), merge to 2.5" and attach to the 2.5" Borla out to daylight.

Worth a shot and sounds cheap

 

I got some pricing.
$350 for the x-pipe (aluminized steel?)
$50 for the ceramic coating
$450 for stainless steel plus coating.

Doesn't the porsche have stainless on it already? If so, why would anyone want to put something substandard on theirs? Maybe the ceramic coating eliminates the need for stainless for rust prevention. Not sure on that.

 

Still waiting for x-pipe with cat quotes. I thought that a stainless summit or magnaflow dual unit would be fine, but others think different brands are better. I would like to know why they think this is true, just for research purposes.

 

Warren till now, there was only one Ott X ... and it wasn't stainless, but it was the best, and not just the best because it was the only one either

 

Uh, I think Warren has moved on to mufflers now Heinrich. I have the stock rear muffler on Heidi, and Helmet has a RMB and in need of a x-pipe.

 

Brendan,
Just for kicks have a look at www.burnsstainless.com to see some of the finest merge collectors and other exhaust components available. These guys set the standard that I try to attain in my work.

One could replace the MSDS collectors with some from Burns and reduce the down tube diameter to 2.5", but it wouldn't be cheap and you won't notice any performance change in the seat of your pants. My own personal inclination is to make things the way they "should" be whether I can detect the difference or not. Since I do all of my own work and because I work for free, this doesn't cost me much!

I think the larger potential problem here is your X design. If I understood your description correctly, the inlet pipes to the X have a large included angle, right? Depending on the angle, this could reduce flow and certainly will diminish any scavenging effects. If you consider the extreme example of a "T" you will see that the incoming gases are not going to be overly inclined to flow out the bottom of the T. Neither will these gases transfer any momentum to those in the other leg; in fact, they will tend to pressurize that leg.

In addition to helping to empty the other exhaust pipes, scavenging can also "empty" the cylinder, even create a pressure drop, and draw in the fresh charge of air and fuel. And with ram effect, pressures above atmospheric can be attained--sort of like a mini supercharger.

As for the turbo, you should probably stick with 3" pipe.

So I guess the question you have to ask yourself is, "Is it worth it?" I suppose you might see 10hp which isn't much and you certainly won't feel it.

Hope this helps.

Tom Cloutier

 

Garth,
That dimension is about 3.75" on the outside. Good luck and don't burn your fingers!

Tom Cloutier

 

Keith,
See my reply to Brendan. Do you have 2 Borlas or a single 2 in and 2 out?

Tom Cloutier

 

But the legs have been changed to suite the exhaust I had made. The transissions are horrible because they were cobbled back together.