Boatnik

I have been complaining about too much heat in the exhaust ever since I swapped injectors. I finally emailed HTT and asked if they had any idea as to why I can hit 1600 at 265 horsepower with a turbo that is supposed to support 400 hp. They responded and ask if I knew what my "drive pressure" was. That is a new one to me. What is "drive pressure" and where do I look? I am assuming they are referring to the VE and not boost. Am i right or am I in left field?

Rpainter

The easiest way to measure drive pressure is remove the egt probe out of the exhaust manifold and install a metal line going to a psi pressure gauge upto 100 psi and mount it temperary somewhere you can see it while driving. ideal pressure is one to one. example 50 psi of boost and 50 psi of drive pressure.

Boatnik

And I was thinking injector pump . Thanks, I have the stuff to do that plus I need to inspect the probe for the pyrometer as well. It is always the simple explainations that escape me.

Boatnik

I just ran a comparison of boost versus drive pressure by tapping the egt port in the exhaust manifold. After running a gauge comparison I have discovered the boost gauge is off five pounds light across the full range. I am actually running 37 psi boost at WOT and 52 pounds drive pressure. Cruising at sixty I see 2 pounds more drive pressure than boost. But that is not allowing for the five pound deviation between the two gauges. I feel that all my trouble is in the pyrometer and not the engine. This week the gauge started skipping like half of the step motor has quit. It is time for a whole new set of real gauges.

G1625S

is the 37psi with the 5psi added? Even then that's a 10psi spread which seems a bit excessive. Before my intercooler I saw 1:1 up to 40psi. I haven't run the DP gauge since the install but I would be surprised to see a 10psi difference...

Boatnik

Yep, that is the adjusted reading. Ok, why would there be big difference. How much drag should be felt on the turbo when spun by hand? I feel a little drag in the bearing, no play. The boost reading is from the port underneath the grid heater in the head. The reading on the turbo where the tap for a wastegate would go is only two pounds higher WOT.

kjewer1

The biggest factors should be turbine wheel and turbine housing size. Moving the housing up a size should lower the turbine housing pressure. How much though for these trucks, I have no idea, and if anyone had any real numbers on this I'd love to hear them.

Cowboy_Customs

Have you gotten back to HTT on your readings yet? And so what your saying is when u see 37PSI of boost, u are seeing 52PSI of drive pressure?

Boatnik

I sent the information to HTT but they haven't replied yet.

Cowboy_Customs

Anything as of yet? I was readin the "what to do with old injectors" thread n thought of this one. Im curious about the whole drive pressure thing myself. Maybe time to add another guage to the already monster list? lol

Boatnik

Nothing yet. I probably got him thinking that I haven't a clue when I asked how to get drive pressure. At the time I didn't realize he was talking about the turbo because my mind set for the conversation was IP. Now that you guys have me back on track I have been waiting for the new Pyrometer so I can see if the old one was out of calibration. I have a mind to send it back to Autometer with a notsonice letter so they can tell me why it skips and sticks. For whatever good it would do.

Ace

Have you closed the ground loop by running a ground strap from the VR to the engine block? That seemed to solve my flaky Autometer pyro problem. I used one of the studs joining the exhaust manfold to the turbine housing. It was easy to reach, already broke loose from the housing upgrade last year and only about a 20" run from the VR.

Cowboy_Customs

Oh i ment to ask, do u know what shape the thromcouple is?? There was an issue with one of the designs, the round tip "bullet" design. All ya had to do IIRC is send it back and they send u the new design. I THINK. Gotta call autometer though. I think Greg had one.

So how close is your drive pressure to your boost under normal conditions? just casual driving, like right on, or a consistant higher drive pressure to boost pressure ratio?
The 16cm ur running should allow the exhaust to flow nicely.

Dont feel bad, if I hadent seen someone running a drive pressure guage before I would have assumed IP too.

kjewer1

Some of the confusion might come from the name itself. I've never heard of it refered to as drive pressure until I came here. With the cars, we always just called it exhaust manifold pressure, which is a nice intuitive comparison to intake manifold pressure.

I may set up an exhaust manifold pressure guage before I do the swap to the 16cm housing from the 21, just to satisfy my own curiousity. Then again, my laziness is probably far superior to my curiousity.