just wondering if i could go from 3" downpipe to 4" inch straight to 3" duals (with stainless flex pipe over the axle) ? Will that cause too much restriction?

You know, I wondered about that when I got mine done... I asked the tech "should I get duals?"...

He looked at me like I'd lost my mind and said "Why? - It's all gotta go through the same skinny little turbo, anyway."

So far, nobody has been able to explain the reason for putting duals on a TurboDiesel truck... other than that it looks cool (which is hard to argue with).

Anybody got a better answer?

Well, to me that is like saying "Why go with a full 4" exhaust if all the gasses go through that little turbo anyway?" IMO, it cant hurt, it would be kinda like going from 4" to a 5" tail pipe. So long as the transition is smooth, and doesn't neck down, or interupt flow, where is the harm?

Besides, it will look Bad A$$! I personally want to do that with my truck, but I want dual 4" tips exiting on either side of my reciever hitch.

Bill

thats what i was going to do ...beside my receiver hitch with big downspouts.

Like I said, it's hard to argue with 'Looks Cool'

:-)

Mos Def looks cool. I saw a duramax running duals stop outside my house at the stopsign. Smoke blowing out both sides is definately cool!

Simple math:

Area of a circle would give a reasonable comparison of the single 4" and the two 3" tubes. Area=pi x radius squared.

Single 4" -> 3.14 x (2 x 2) = 12.56 square inches.

Dual 3" -> 2 (two tubes) x 3.14 x (1.5 x 1.5) = 14.13 square inches

The dual 3" should flow about 12.5% more air than the 4" tube. No restriction back there.

Just this last weekend I tore off my 3inch to 2-1/2 inch duals. The Y pipes in the back were always in the way of the spare - rubbing squeaking PIA to get the thing in and out to rotate. I now have a simple 4inch sewer pipe system and the sound is a bit "deeper" than the previous set!
Kind Regards
Rick

Exhaust flow seems to be an ever changing theory.

Once upon a time (still is in the factories minds) it was thought that exhaust should be big at the engine, smaller at the tailpipe.

Why? Scavenging.

As the exhaust gasses cool, they slow down. When they slow down, they don't flow well. You end up with a high pressure "plug" of slow moving air at the tailpipe which slows down the whole system.

By reducing the diameter of the tailpipe, velocity is increased and a "scavenging" effect begins to take hold. In physics terms, it means a low pressure area is created at the end of the exhaust pipe to actually "pull" gasses out of the engine.

Now...it's big pipes all the way.

Which way actually works better? Dunno. Look at the username.

In "theory" you may actually reduce the flow capacity if the area in the tailpipe is greater than the pipe in front of it (it creates that high pressure area when the gasses expand into the larger pipe).

OK...I'm going back under the rock I crawled from.

One thing to look at: The racing world. You want to be able to move the gasses out. This equals less pressure on the engine and more efficency. The better an engine can breathe in and out, the better of it is. Can you go to big, Yes. Exhaust can become too big in which vavles start sucking exhaust back, not so good. Look at the big rigs, some of those guys are running 8 inch stacks with no problem, so you should be okay.:grd:

iirc if your engine is turbocharged, there is no such thing as too big.

Ain't no pipe too big for a turbo?

Well...I know physics is a bit of an art but...if the pipe is big enough, velocity COULD be lower through the pipe. The turbo is run by that velocity. The lower the velocity, the slower the turbo spins...the less boost you get. Or vice versa, the lower the velocity and the greater amount of gas in the exhaust must be overcome by turbo.

Just as a column of air has a weight...so does the air in the pipe...and the turbo must "move" that larger column of air with a larger pipe.

I know I'm being technical but...it does make one wonder.

That's why I asked earlier if the addition of a larger crossover pipe is really the right thing to do. If its large enough...the velocity will be lower and hence (again in pure theory) the turbo will be slower to react and less likely to spin fast.

Look at it this way. If there is more room in the crossover pipe, there is more gas to compress. Since the turbo does cause resistance in the exhaust prior to it...it must cause that gas to go into compression. The more gas in the crossover, the more space to compress before enough pressure builds to "spin" the turbo.

Blast...I wish I had the tools and the time to prove or disprove the theory once and for all. I know just enough about physics and auto mechanics to make be go nuts thinking about it.

It's probably only miniscule either way but. Some things work but make now sense (like injecting water into an engine...which by the was way done for years on farm tractors long before the automotive crowd latched onto it.)

Truly optimum or not...big pipes are cool...no doubt!

I didn't say crossover. I meant AFTER the turbo. Your arguement about the crossover makes perfect sense but when we talk about exhaust for the 6.5, I usually take that to mean turbo back, unless the crossover is specifically mentioned.

I knew what you said and I still stand behind what I said. There are valid physics questions that arise when you add a big pipe.

Read this: http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

Pay special attention to the section on exhaust pulsing.

Big pipes make big power at high rpm (as on a racing vehicle). Do our diesels run at high rpm?

I'm not trying to honk anyone off. I'm just saying there are two sides to the exhaust coin when it comes to performance and one size does not fit all (pun intended).

Knowing the two sides will lead us to better decisions (which in my case will probably lead to a bigger pipe no matter what the theory is!)

DD we can agree to disagree. I read the whole thing and haven't changed my mind. They are, generally, talking about ricers and definitely not talking diesels. When I say "can't get too big", obviously you can only fit 4" inch max in our trucks so that would be the "biggest pipe possible".

No disagreement here big guy. I was just pointing out that there were two camps. I'm just ridin' the ol' fence trying to figure out where to land.

I wouldn't DARE recommend an exhaust for someones turbo diesel. All I've got is a bag full of theories and no experience (i.e. I'm a DUFUS!)

Many thanks for the all the thoughts. I am filling the void of ignorance betwixt my ears.

Gay driving!
No...that's not it, uh...
Happy driving!
No...no...no...what was it that steakgoop feller says...dang my memory...

Gay driving!
No...that's not it, uh...
Happy driving!
No...no...no...what was it that steakgoop feller says...dang my memory...

:muahaha: :lol: :funnypost

iirc if your engine is turbocharged, there is no such thing as too big.
Bingo!!
Exhaust sizing is much more complicated when an engine is NA. Too large a diameter will kill low end power.
With forced induction, the more flow post turbo, the better. The simple fact that EGT is reduced by the addition of larger exhuast is a testament to this.

Could be.

EGT's could also be TOO low. Remember...cold gasses move like a slug in the exhaust because they are of higher density. Since turbo's are NOT mechanically driven...they will be effected by resistance in the tail pipe...be it from a pipe that's too small...or plug of cold, expanded gas in a pipe that's too big.

Then again...what do I know! WOOHOO!!!!

EGT's could also be TOO low.
I'd love to have that problem...

All i have to say is for maximum power you want the shortest possible route (without hurting the engine) for exhaust gasses to flow. The more efficiently you can do this the more power your make. Im not positive on this but I have heard that for exhaust (maybe it was something else) but for it to have to go through a 90 deg. elbow would create the same amount of loss as 20ft of straight pipe....Thats my $.02, anyone wanna gimmie change?

Sprint cars run long, equal length, headers to maximize flow. "Shorty" headers are known power robbers.

Too long, too short, too big, too small...AHHHHHHHHH!!!!!!!!!!!!!!!!!!!

Exhaust scavenging is severely reduced when a restiction such as a turbocharger is mounted in the exhaust flow. Pretty much to the point that exhaust pulse physics are irrelavant - unless you have a 8000rpm ricer. Exhaust pulses are destroyed by the turbocharger turbine and therefore scavenging post turbine wheel is non-existant (however velocity does affect the turbine speed). Post turbine - generally the bigger, shorter and least restriction the better.

It is not reasonable to compare the exhaust scavenging of a n/a engine to a turbocharged engine.