Diesel backpressure

[TFerring]

Alright, we all know that with a gas motor a less restrictive exhaust with less backpressure is a good thing. I was talking to a freind who owns a Duramax and told him I was going to straight pipe my 04. 5. He told me to becareful because unlike a gas motor where a less restrictive exhaust is a good thing a diesel motor needs backpressure from the exhuast to help with combustion... ... . What are you opinons?

 

[RRoton]

My opinon.

With a gasser if you go to big, you lose the scavenge effect.

There is so much flow with the turbo diesel that you dont need the scavenge effect. Therefore I think, with the diesel, bigger is better, and backpressure is BAD.



We dont need no stinkin back pressure

 

[FDavid]

Actually if you go to big on a gasser you lose some of the bottom end. On a turbodiesel back pressure is useless. However if you str8 pipe a 03+ a computer box with a boost fooler is a good idea as we have learned the computer defuels w/o one.



dave

 

[Udaloy]

Goes to show you what a Dmax owner knows. He's dead wrong. Since we have a turbo pushing air for us, bigger is better! With gas, naturally aspirated engines, it is possible to get too big on the exhaust, and reduce the scavenging effect. Which, by the way, helps to accelerate the exhaust velocity.

 

[SRadke]

Am I wrong here? I would expect much higher backpressure pre turbo than post. Opening up the exhaust should have little effect on anything that could change combustion efficiency.



-Scott

 

[TFerring]

Hmmmm

That's what I thought... ..... Stupid Duramax owner!

 

[Udaloy]

Originally posted by SRadke

Am I wrong here? I would expect much higher backpressure pre turbo than post. Opening up the exhaust should have little effect on anything that could change combustion efficiency.



-Scott

I don't know about that. That would depend on how well the exhaust manifold flows, wouldn't it? That's besides the point anyhow, as the question really revolves around the scavenging effect you get with the right size exhaust. Naturally aspirated engines will benefit from scavening, because it will cause the spent gases to be "sucked" from the combustion chamber faster than if too big of an exhaust system were used. This also helps suck in fresh air and fuel for the few degrees of duration that both intake and exhaust valves are open (cam overlap). Since we have a turbo, it doesn't act the same. Air is being forced into the combustion chamber, so backpressure will hinder performance, not enhance it. Someone correct me if I'm wrong.

 

[KLockliear]

Scott, that's called drive pressure, not backpressure. Backpressure is the pressure between the turbo outlet and the atmosphere... drive pressure is the pressure between the cylinder and the turbo.

 

[SRadke]

Thanks Kieth. I still don't see how what we've now defined as backpressure will affect combustion efficiency? What is the relationship between decreasing backpressure and the drive pressure?



-Scott

 

[Hawkeye 04]

What about reports of 5 inch exhaust systems robbing power from a stock deisel? Wouldn't that come from to much free flowing air? Just wondering...

 

[nickleinonen]

Originally posted by Hawkeye 04

What about reports of 5 inch exhaust systems robbing power from a stock deisel? Wouldn't that come from to much free flowing air? Just wondering...

there was some company here [i can't remember the name] that dyno'd a truck and noticed that the truck was down on power with exhaust on it until they put a boost fooler in place [i think it was a VA box on a 03] so with a free flowing exhaust the map sensor needs to be cheated...

 

[OT-OF-HERE!!]

Several weeks back Ted Jannetty of Jannetty Racing did a post and had all kinds of dyno supporting info. The bottom line was that with the defueling on these new trucks you actually loose power going to the 5" exhaust. That's per his post. Do a search on it if you want the detailed data. Also on a sidebar, ... . recently I spoke with TST and I believe it was Mark who said he had tested all kinds of aftermarket exhaust and the stock ran as good as any. Put a chrome tip on it and chill is what he suggested.

 

[KLockliear]

You have to think about WHEN you are looking at this backpressure/drivepressure thing. At idle, you have none of either. A very low backpressure system will allow the exhaust to escape quickly from the turbo, which will also slow down the buildup to drive pressure. This means it will take slightly longer to build boost, which is what the computer needs to see to deliver fuel.



Once up on boost, backpressure isn't much of an issue. The less the better, to let those EGT's escape.



It's a balancing act... take from the bottom to give to the top - just like a turbo upgrade.