VGT+WG compound drive pressure issue

JFettig

JFettig

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Dec 28, 2011
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I've been running compounds on my Passat TDI for the past few months, spending a ton of time tuning and most everything is worked out but the one thing that I'm baffled on is drive pressure in lower gears. In 5th gear drive pressure is roughly 1:1, 4th gear its roughly 1.1:1 until higher RPM and 3rd gear its 1.5+:1, 2nd gear is at least that or probably more. I'm running 60psi Autometer gauges and the drive pressure gets pegged approximately 70psi(straight down) in 3rd gear or below.

Fuel mapping is the same 3rd through 5th, timing, duration,etc. 2nd an 1st its reduced. I have a feeling people will say too much duration, not enough timing - maybe so, I have to start bumping timing up now that I have the other issues fixed but timing map was created based on engine sound as well as AFR. I run 18-19AFR at WOT, 45-50psi depending on tune. 20psi at atmospheric turbo(need to drop this a bit). I am running a 38mm wastegate to bypass the VGT, 18psi spring and pressure is modified by the vacuum signal to the turbo(ask for more boost, wastegate gets fed vacuum). At WOT and 50psi and 20psi at the inlet the vanes are 100% open, at 45psi and 15psi at the inlet vanes are about 80% open.

Any input/experience would be appreciated! I know it doesn't quite translate to trucks, maybe what I'm experiencing is common?

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My initial thought is the secondary is too large. It is not responsive enough in the lower less loaded gears to create boost quickly. What happens if you tighten the vanes up even more? Can you increase the response from the turbo by closing the vanes? What about removing some diesel from the tune? Would having less diesel lower the back pressure?
 
The secondary is sized well, 30psi by 2000rpm, 40psi by 2200rpm, spools super fast. The turbos are sized very well together(did all the math). I'm not talking about EMP spikes, I'm talking sustained EMP. Boost remains the same, no creeping or spiking. Less fuel will give a little less back pressure but my AFR is high and I'm not smoking.

Keep in mind, I'm cranking out 5900rpm - 30psi at 2000 RPM is next to impossible to achieve with any single.
 
JFettig said:
The secondary is sized well, 30psi by 2000rpm, 40psi by 2200rpm, spools super fast. The turbos are sized very well together(did all the math). I'm not talking about EMP spikes, I'm talking sustained EMP. Boost remains the same, no creeping or spiking. Less fuel will give a little less back pressure but my AFR is high and I'm not smoking.

Keep in mind, I'm cranking out 5900rpm - 30psi at 2000 RPM is next to impossible to achieve with any single.
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It does the same when rpms are kept steady at those lower gears ? Or only when accelerating ?
 
only accelerating of course, below 40psi its much closer to 1:1 even in lower gears. If I had gear dependent boost maps I could make an easy fix since I obviously don't need 50psi in 2nd gear :D
 
went from 16deg at 6000rpm to 19deg(38deg at the crank) timing and little to no change in exhaust pressure if any. Basically did +20% overall so mid range was also increased, going +10 didn't effect driveability but +20% mid range is a problem.
 
It has nothing to do with delay in vgt control - vanes are in their steady state position while the high EMP is happening.

Mike - the turbos are TD06SL2 20G(11 blade billet) over a BW BV43A
 
Good choice on the 20g, i loved it on my mitsu.

Is 1.5:1 drive pressure excessive in 1, 2 gear? I dont know what is normal in a vw, but load is way less in lower gears and stress on the engine isn as much. I would be tickled pink to have 1:1 in 4th and 5th gear where everything really has to work hard.
Ask any 12v guy here and you will see some miserably high drive pressure.
 
I'd say its at least that since my gauge maxes out around 70psi, I'll pick up a 100psi gauge soon. Above 4000rpm it drive pressure gets up over 60psi again in 4th gear.

Typical single turbo setups see a 1:1.5 spike which quickly drops to 1:1 to about 4000rpm where it rises depending on the turbo size.
 
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Ok, I have to ask -

What's going on with the perpendicularly mounted drivetrain?

edit, just googled you - interesting find!
 
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JFettig said:
It has nothing to do with delay in vgt control - vanes are in their steady state position while the high EMP is happening.
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Are you sure ? Not only from diagnostics.

There might also be delay in ECU maps, some engines have different maps for quick acceleration.
 
I got quotes for those quite a while ago, I'm planning on ordering a few :)
 
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