sammy340

I was just wondering if there was a point where a diesel would have too much exhaust flow. Is any backpressure needed on a diesel engine? Is straight piping hard on the engine?

thanks for the info

jeff

jiMMyCTD

might be hard on your brakes, lol,

Forrest Nearing

turbo = backpressure

wannadiesel

Straight piping won't hurt anything. The turbo works best with no backpressure, the more pressure/temperature drop across the turbo the better.

Monty

It has been said that a 5" system on a stock truck will actually lose you hp, however, I have nothing to back that statement up.

loch

its my understanding that a 5 inch on a stock truck allows the exhaust gas to cool off very fast, the result cool air creates turbulence thus making back pressure, but thats just a story i heard.

edwinsmith

In my hot-rod days it was the length of the exhaust tube which made the difference. When the pulse of exhaust exits the end of the tube a negative pressure wave goes back up the tube. You cut the tube length so that the negative pressure wave hit when the exhaust valve opens on the next rev. at your chosen RPM. This is why headers are sold as "equal length" although I haven't seen much about them being tuned. (probably over the heads of most wrench turners.)

I assume that the same could be done for a diesel but the turbo plumbing would be a problem.

I seriously doubt that excessive cooling of the exhaust would cause any more turbulence in the pipe as long as there are no sharp edges or bends to disturb the flow, bigger is always better from a resistance standpoint.

In fact, the megaphone exhaust on Motorcycles is designed to allow the exhause to expand even faster to reduce backpressure and increase the scavenging effect. We even used megaphones on 2 cycle engines which required some backpressure. There was a little tube at the end of the megaphone.

From my understanding the turbo uses the heat difference from input to output to generate the spin. Bigger pipe=more expansion=greater cooling=greater turbo spin.

Edwin

sammy340

I ended up chopping off my muffler-back on my exhaust today. It was new 3" too there so i just chopped it were it went to the old piping and bought a 60 degree bend so it comes out in front of the passenger rear tire. It's just 3" straight pipe but i noticed a drop in egts and a 3 psi increase in boost. I can hit 23 PSI now. The rim gets black after awhile, but who cares. I bought some PODS since they were on sale at piers and now just need then installed, i was hoping that my 3" shorter straight pipe will help the EGT's for the PODS until i get a 4" exhaust. Anyone know what the PODS should increase my boost too?


Jeff

jleonard

The changes to your exhaust gave you similar results that I got when I put in a 4 inch system (with a muffler) to replace my stock exhaust (I still have the 3 inch downpipe).
PODs will give you more boost...you'll probably be in the mid 30s (?)
Jay

jiMMyCTD

I have yet to hit 30 with my set up. im thinking either an air leak somewhere or my auto is sucking up alot of it. jiMMy

DDD

Edwin speaks the truth. ANY 4 stroke engine performs better with less backpressure. It’s a myth that the ol’ Chevy 350 needs a little backpressure to run right. I’m sure people have bolted on bigger exhausts and lost power, but it’s because of others factors like not re-optimizing their carburetor jetting, or using pipes that are tuned to the wrong power band, as Edwin explained above. When other factors are equal, less backpressure = more output, and higher efficiency, no exceptions. On a turbo engine, the less resistance downstream of the turbo, the better. I don’t know much about 2 strokes theory, so I can’t speak for them.

Forrest Nearing

well, on a non-turbo setup, too large of a pipe causes reversion, which acts as backpressure... so you're right that engines don't need backpressure, but that doesn't mean that the biggest pipes are the best.

Forrest

Lil Dog

From what I read on here a while back, the fluid flow is also important to remember. The bigger pipes with low volume do become less lamilar flow than the 4" setup. With the flow velocity of the 4" system in the 2-400 Hp range, the inertia of the flow in the pipe helps out more than just having more pipe diameter.

Its hard to explain but the 4" system seems to scavange better than the 5". And as for tuning the exhaust length, that is important too, not only for flow, but for that dreaded drone everyone has to deal with. I remember fixing the drone on my 89 with a little longer pipe and a turndown, the effective length of pipe was longer and made the drone out of my driving speed range.. Blah blah blah..

J-eh