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Turbo Pressure Delta Factor

Practical Diesel Tuning

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Discussion and questions related to the course Practical Diesel Tuning

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Is there any benefit in increasing the pressure delta factor, will this give faster turbo response?


Let me come back on this next week. We are testing several turbo including wastegate and VNT on a small engine for a customer so ideal for this forum. Lambda EMP and MAP in relation and spool-up as well.

Peasy, I've not seen a table labeled as such. Would you mind posting a picture of it so we can get some idea of how it interacts?



Here it is, what do you think?


We tested different turbo on a small CR diesel engine. Both wastegate and VNT ones.

Testing was done under an NDA so just a general impression about the testing.

Application was hi power so relative big turbo for top end performance.

First conclusion. Wastegate and VNT turbo have same spool-up. Remember VNT is only used for EMP/EGR control. If no EGR is used VNT becomes more complicated in controlling it. Wastegate far easy and simple.

Now for EMP (exhaust manifold pressure) in relation to lambda. Hi EMP makes EGR working so less o2 for combustion.

Can I run lambda 1 on a modern CR engine? Yes @ no and low boost and low RPM. With VNT turbo EMP jumps up very easy eliminating power gain so essential to open the vanes to a point that the engine is able to vent off exhaust gas. We have seen 350Kpa EMP over 180Kpa boost with VNT turbo not making the same power as wastegate turbo @ 150Kpa boost producing better lambda numbers and no smoke so it’s all about VNT vane control.

Top end power. EMP over MAP. EMP about 30Kpa lower than MAP with wastegate turbo. VNT problematic on EMP control maybe due to shape of turbine housing and relative big turbine set-up for small engine.

Top end power lambda. 1.16 possible with no smoke and EGT within limits but better run 1.2 Lambda as for hard to tell what an alloy piton can hold on thermal stress.

Fast turbo response on VNT. We program most of them as a turbo kicker. Off power vanes closed to the max with very small delay on opening when engine power is needed. We have seen 150Kpa boost off power while breaking on our rally applications for small engines and this 0,5Bar boost as a start shoots up engine power very fast to a point that low end power is getting a problem for the engine and drive line. We calculate 250Nm per 1000cc but better not below 2200Rpm on hi revving engines with a redline of about 5500Rpm. The only thing I can think of is piston speed on small stroke engines is a lot lower than bigger engine and there for more critical handling/controlling combustion pressure.

Thanks Ruud

I haven’t played with my vane position yet and I think for me without a dyno and ability to control vane position in real time would be hard.

But with added fuel and boost would I want to open the vanes more also?

The table appears to limit turbine pressure based on boost pressure. I could be wrong here though, the description is pretty vague. If it has an exhaust backpressure (drive) sensor you could log boost and drive to see if the values correlate with this table. They seem kind of low for that honestly. You may try to raise the values by 15% and see if the truck spools up an quicker.

The truck should be targeting it's own boost vane position based on desired boost. If you want quicker turbo action down low I would first suggest raising your desired boost (assuming this table exists on your platform). Work in increments of 10%.


Thanks Nick.

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