Engine/Transmission (1998.5 - 2002) Can Somebody Explain Drive Pressures & Measurement?

[HeavyHauler]

Hey guys - drive pressures have become a pretty hot topic, but I've got to admit that I still haven't caught on to the whole concept.



Can someone explain what drive pressures are, what negative effects higher ratios can have, and how they are measured? I see that some people run a gauge just to measure drive pressure - how is this done and is it necessary?



I basically know that you want your drive pressure to be as close as possible to actual boost pressure, and higher ratios can blow head gaskets.



Enlighten me!



Thanks!

 

[SRadke]

Drive pressure is the pressure in your exhaust manifold. Measure it just like boost.

 

[nps]

Drive pressure is the pressure inside of the exhaust manifold created behind the turbine wheel's position in the exhaust passage.



As drive pressure goes up but boost pressure does not, then it means that you are building up a more exhaust manifold pressure thus choking off the engine without producing any more fresh air coming in - not good for making additional power.



A larger housing will reduce drive pressure because there is more volume to flow the exhaust gas through. But it will also cause slower spooling for the same reason.



My understanding of the issue - others may vary. :-laf

 

[RustyJC]

Think about what happens during valve overlap. If the pressure in the exhaust manifold is higher than the pressure in the intake manifold, what happens to airflow through (and pressures/temperatures in) the cylinder during the valve overlap event?



Rusty

 

[SRadke]

Think about what happens during valve overlap. If the pressure in the exhaust manifold is higher than the pressure in the intake manifold, what happens to airflow through (and pressures/temperatures in) the cylinder during the valve overlap event?



Rusty

You get hot, oxygenless air coming in from the exhaust manifold instead of cool oxygen rich air from the intake manifold. Is this the air you want in your combustion cycle?

 

[nps]

You get hot, oxygenless air coming in from the exhaust manifold instead of cool oxygen rich air from the intake manifold. Is this the air you want in your combustion cycle?

And if you have a 12V without 60lb valve springs you could also float the exhaust valves because the springs are not strong enough to close them against the manifold pressure.

 

[HeavyHauler]

So the inertia of the turbine wheel is what actually causes excess drive pressure? Ok, this is starting to make sense if that's the case.



How and where would you plumb a gauge to measure the pressure? It wouldn't be in the exhaust manifold would it?

 

[nps]

How and where would you plumb a gauge to measure the pressure? It wouldn't be in the exhaust manifold would it?

Yep - that's where it goes.

 

[SRadke]

Not really the "inertia" of the turbine but rather the resistance of flow the turbine causes as it gathers the needed energy for the compressor on the other end of the shaft.



The pressure can be measued from anywhere pre turbine. A pyro hole is the easiest.

 

[HeavyHauler]

Not really the "inertia" of the turbine but rather the resistance of flow the turbine causes as it gathers the needed energy for the compressor on the other end of the shaft.

Oh ok, I gotcha. That makes more sense.

The pressure can be measued from anywhere pre turbine. A pyro hole is the easiest.

Ok, treat me like a dummy :-laf - so when most people measure drive pressure, do they disconnect their pyro probe and insert some sort of gauge for the drive pressure and then go for a spin? What about the folks that have a permanent gauge for measurement? Tap another hole? I would assume some special tubing and gauge is required because of the temperature?

 

[SRadke]

Ok, treat me like a dummy :-laf

Well gee dang I can do that!



Just tap a second hole for the pressure port. Run a length of 1/8 copper tube from a compression fitting, leave it a little curly to dissipate the heat then either go into the cab to your gauge or adapt it to a plastic boost tubing after you've gotten far enough away to dissipate the heat. Just use a generic presure gauge or a boost gauge with similar scale to your boost gauge.

 

[59Turbo]

So can high drive pressures cause headgaskets to blow? Speaking from experience... :{

 

[SRadke]

High drive pressures, high boost pressures, and high cylinder pressures will all contribute to head gasket failures.

 

[HeavyHauler]

So is the only way to lower drive pressures to increase the size of the turbine housing? Would a performance cam help with the pressure or could this actually raise the ratio?



Is it primarily big fuel trucks with undersized turbos that have the biggest problems?

 

[SRadke]

A cam helps air move throughthe engine but it will still come up agnst the same blockage in the exhaust that is creating the pressure. It would be of no help but a larger housing or wastegate adjustment can.



Big fuel/small turbo is a classic recipe for high drive pressures but it can also be caused by moderate fueling increases if the wastegate function has been comprimised.

 

[HeavyHauler]

The reason I mentioned the cam is I wondered if the increased flow out the exhaust valves would actually create more pressure in the manifold.



So when you say an adjustment of the wastegate, do you mean a reduction in the max boost pressure or an increase? If you adjust the gate to flow more air, would that not create more air flowing through the engine which would increase drive pressure if the exhaust housing isn't large enough to let it out?



I think I may be confusing myself now... I probably should have left well enough alone!

 

[SRadke]

Open up the wastegate a bit, bypass more exhaust. You may find to a point that the boost will stay up there as the drive pressure begins to drop. It's a common misconception that boost = power... . not so

 

[HeavyHauler]

Ok, a few more questions:



Other than head gasket failure, are they any other problems that can be caused by high drive pressure ratios?



Also, is it typical for drive pressure ratios to increase exponentially as boost pressure increases? If so, would this mean that drive pressures really only become an issue if high boost levels are frequently seen? And if that's the case, I suppose that would mean that drive pressures really aren't an issue on trucks that are mostly daily drivers relative to trucks that see harder use.



This sounds really over-simplified, but as it turns out, this really isn't that hard of a concept to grasp.

 

[SRadke]

High drive pressures cause high cylinder pressures, high egt's, and all of the other things that come along with the previous two.



Increase exponentially with boost? No. Actually more proportional to boost. Will you have high drive pressures at low boost, not likely. You're realy going to find high drive when your pushing a turbo beyond what it was designed to do. This is paticularly true with tight exhaust housings.



I read a post by sombody... . thinking it was Evan Beck... that had measured the drive on his HY35 and found anything over the high 20's for boost would yield drive pressures so high you were in the land of diminishing return. For a turbo with a 9sq. cm. exhaust housing I believe it.



Give up your pyro for a couple days and stick a pressure gauge in there temporarily. It may surprise you.

 

[Fishin Guide]

A cam can help EGTs even if you have a tight housing, but it will do little to help the drive pressure issue. The cams claim to fame is reducing pumping loss. At least in mild engines.



A cam coupled with a slightly larger housing will usually yield the same spool chacteristics as the smaller housing, but add even more EGT relief. Also, you usually will make more power as you are not running nore boost than required. The side benefit of all this is that the exhaust exits even quicker. This stops your engine from suffering from natural EGR effect in the form of drive pressure. And those cylinders can be filled with a cooler, denser charge.



It's a viscous cycle, but I love it!!



Dave