Describe in detail what it is and why out-of-whack drive pressure numbers are bad.

Drive pressure = backpressure on the engine.

With too much backpressure on the engine you are making it harder to breathe. Most people strive to have a 1:1 boost/drive pressure ratio.

Turbo is the inlet and outlet of the big air pump we call an engine.


If the inlet is cramming enough air in the engine but it can't get out the restriction(exhaust housing) drive pressures start to go up and usually turbo shaft rpm is way high.

Here's the best way to think about it, IMO.

The power you make is determined by the average pressure over the course of the powerstroke from TDC to BDC when the pressure times surface area of the piston is placing downward force on the connecting rod, turning the crankshaft, imparting torque into that shaft, the rate of which is your power output.

Well..... on the next stroke from there (BDC) back up to TDC again the exhaust drive pressure is atop the piston. So you have that average pressure times the piston surface area taking AWAY force/torque/power. The lower the drive, the less power your rob, or the more power you make, all else constant.

Furthermore, there's the issue of negative effects to cylinder VE. When the boost:drive ratio is bad out of whack, during any valve overlap period, the exhaust gasses will actually revert back into the cylinder from the manifold, right across the top of the piston and back up into your intake runner if things are bad enough. Some people have actually noted SOOT up in the intake runners. #1, this means that for any given displacement you HAD from the stroke x bore of the cylinder, you just pissed that much away because it never gets filled with fresh air, it's still getting sloppy seconds from the last combustion event still just hanging out taking up space that fresh air could have used. #2, it raises the in-cylinder temperature (burning gases present) potentially raising the temp enough to have a notable effect on the effective start of combustion, or in essence, advancing your timing.

This is where people like to point the finger at drive pressure for blown head gaskets. It's not the pressure, but the potential effect to cylinder temp, and it's subsequent effect to ignition delay..... or lack thereof as the case may be.

Hows this for detail:

TN Super T72...... over 2 to 1 drive ratio in this video:


2.8 legal S400 with about 1.2 to 1 drive ratio:


Exact same tuning. Watch and listen at the end of each run. Notice any difference? The high drive ratio literally choked the truck out.

Jason

Click here to see Video

1.7 PSI of boost : 1 PSI of Drive

That is a noticeable difference! What mount are you using for the S400?

Big difference in sound and performance there Jason...choked it out in the first video and it kept pulling hard all the way till you shut it down in the second.

And Nate, those compounds sure do sing!!!

That is quite a difference Jason, also the smoke seemed alot heavier in the first video.

High drive pressure causes exhaust gas reversion into the cylinder. If the exhaust gases can't get out, then how can you expect the fresh air to get in.

Dave

Charles I agree with your statement. It wasn't that long ago that that wasn't the normal thought for some people.