Cummins designed the ISB to operate at 3 pounds backpressure, no more. This should give an idea if larger eshaust is needed. A while ago I had installed a fitting in the turbo elbow to check this but was not able to find the max pressure. The Cummins service manual has it.

 

You are of course, referring to after the turbo, correct? Prior to the turbo there is considerably more pressure.... As for after the turbo, you want it to be way way lower for resonable performance. 3 psi after the turbo would be a deplorable exhaust system....


KP

 

Agreed, but what I am stating is from Cummins specs on the ISB. I would want it considerably lower. And I bet the stock system with the muffler on it is right there.

 

True. But keep in mind that the atmosphere is providing 14.7psi of free backpressure at sea level.

Makes me wonder if there was a way to find a really low pressure area on the truck (say at hwy speeds), and vent the exhaust to THAT location.

Wouldn't that help to reduce backpressure?

 

Early on TST tested the back pressure on a stock 24v. It was so low they couldn't measure it.

 

I have a problem with that one. How much pressure does it take to blow exhaust through a joint not clamped real good? Has to be some pressure to do that, expecially with a muffler and resinator.

If there was not a pressure differential, there would be no leakage. And three pounds pressure is not much. A boost gage would have a hard time with accuracy.

 

I tested my back pressure. I chould get 3 psi without pulling a load, Hate to see what is would do with a load at WOT. Off went the stock exhaust.

 

I see your point. I'm only going on what Mark Chapple said once at a May Madness seminar, when the question was asked.

I konw you don't see any power increase on the dyno, going to a 4" system on a 24v, until 375 to 400 hp. And then it's not much. I'd think the a 12v with a cat would be a different story. Time and time again it's been shown the biggest benefit of a larger exhaust system is reduced egts.

 

It's easy Justin... AeroTurbine lol

 

Not sure if this is even relavent to "backpressure".

I have a brake pressure guage in my truck that measures so called brake pressure when my exhaust brake is engaged. It is just a boost guage plumbed into the elbow off the rear of the turbo.
Yes during normal operation I can get readings up to 40 psi of backpressure, but the guage gets pressure all the time when the brake is turned off and it shows zero during normal cruise or even hammering it at WOT.
I even went so far as to put a smaller increment guage on it to see if I could get any kind of reading and didn't get anything.
I have a feeling that if there is any kind of backpressure present during cruise or WOT, it is so small or the guage can't measure it accurately enough.

Just thought I'd throw this out to the thread.

Jeff

 

I drove a customer's truck several years ago, with a brake pressure gauge on it. Under load at highway speeds that gauge would indicate backpressure of 3-5 psi. He wondered if that was considered high, but I really couldn't find a spec for him.

 

Torquefan the pressure is in the Cummins isb/qsb service manual. It list three pounds max.

 

Along the same lines, who has measured their drive pressures (pre-turbo), and at what boost pressures did they occur? Just curious.....working on a pressure differential BOV using drive pressure instead of vacuum, such as gas engine BOV's use. Curious to know the various findings out there for different turbos/fueling levels.

 

Motorcycle exhaust companies and the OEM are catching onto that.. designing the exhaust to exit into the slipstream created by bike & rider.. every little bit helps!

But on a truck like ours? doubtful.