EMP as related to the VNT 17/22

[jackbombay]

At 1300 RPM The EMP is more than 3 times the IMP. Maybe the TDI club folklore was right after all ;-)

Cool to see it all graphed out! The EMP was even lower than the IMP right after the turbo spooled, I never would have guessed that.

I'm looking forward to the logs from a 15 and stock 17, will be interesting ot se how they stack up.

Thanks for taking the time to put all this together!

 

[hatemi]

That system would be killer when finetuning a turbo. You could realy see what the changes in the setup make and not just guess. Looking 2 gauges at the same time isnt very acurate at all when doing highway speeds

 

[Fix_Until_Broke]

At 1300 RPM The EMP is more than 3 times the IMP. Maybe the TDI club folklore was right after all ;-)

Yeah, that's it, uh huh, because we all drive around at 75%+ throttle at 1300 RPM. . Remeber - there is ~2sec lag in the VagLog data so the 1300 RPM point is really ~2000 if you slide the black RPM trace to the right ~2 seconds. I should do a few more runs but starting at various RPM's (1200, 1400, 1600, 1800, etc). Part throttle accelerations (going from 10-50% throttle) gives some decent IMP overshoots compared to where it settles down at.

hatemi - you seem to have done quite a bit of playing around with this on your own - is there anything you'd suggest that I look at or certian way to evaluate the setup? I've got IMP:EMP as well but just not plotted (think it peaked at ~2).

 

[nicklockard]

FUB, I'm a bit confused. If IMP < EMP, by definition isn't that surge territory?

Or are you saying that that is simply indicative of lag?


Sorry for my ignorance. Can you plot differential [EMP - IMP] and EMP/IMP, ratio? Or give raw data and I'll do it.

Why is the vag log data 2 seconds lagged? I missed that. Can you do anything to time-correct it?

Noted: TIP is under 2 psi vacuum for almost all of your accelerative run. That is a suprise since you are running a 3" custom TIT, isn't it? I wonder where the restriction is coming from...airbox/airfilter?

Do you run your stock intake snorkel, or have you removed it?

 

[LurkerMike]

Anyways, sorry for derailing the thread.
EMP's are the effect of too much flow for a given volume, the volume is the constant and the flow is the variable. Now because these turbo's have variable vanes you can actually change the volume, and flow rate that makes it to the turbine wheel. Where the problem lies with these turbos is that even with the vanes wide open the volume is still to small to support the flow rate of the exhaust. So the volume maxes out but the flow rate keeps increasing and thus the pressure increases as a natural side affect.

That system would be killer when finetuning a turbo. You could realy see what the changes in the setup make and not just guess. Looking 2 gauges at the same time isnt very acurate at all when doing highway speeds

This is where the idea of adding an exhaust waste gate to our VNT's might prove very useful. It would not need to be more than say a 1" to say 1.25" bypass pipe to bleed off the excess EMP to a point downstream in the exhaust pipe.

It would be especially well suited for long periods of WOT operation.

Tuning it and the ECU would be a mother, but the end result would no doubt be a nice "free" increase in WOT power output. By reducing the exhaust backpressure, and even with a no overlap cam, cylinder purging at high rpms should improve and parasitic pumping losses lowered by the pistons not having to use as much power to push the exhaust slugs out.

Me? I'm PERSONALLY CHALLENGING ALL VENDORS HERE to a mud wresting contest at the 2008 TDI Fest! You are all a bunch of wusses and I will make you all eat mud suckers!

Who among ye are man enough to enter the mud pit with the Lurker?

Grrrrrrrrrr!

 

[LurkerMike]

FUB, I'm a bit confused. If IMP < EMP, by definition isn't that surge territory?

I have heard as much as 2:1 EMP to IMP is normal for some applications. The deal is that you have frictional losses, vane bypass losses, inertial losses and other losses that are going to require a greater EMP to overcome them in order to produce a given IMP. Turbo's are NOT efficient in terms of the exhaust pressure needed to produce a given charge air pressure. Turbos are just overall more efficient than belt or gear driven superchargers.

 

[jackbombay]

Yeah, that's it, uh huh, because we all drive around at 75%+ throttle at 1300 RPM. .

I wasn't serious as its fairly irrelevant, who goes WOT at that low of RPM on a regular basis?

Clearly the old TDIclub folklore of 3:1 EMP IMP ratio has gone down in flames except for very high throttle applicationi at low RPM, and nobody does that with any regularity anyway, or they shouldn't.

It would be interesting to see a "roll on" from 2000 RPM, something more indicative of normal driving.

 

[Rub87]

This is where the idea of adding an exhaust waste gate to our VNT's might prove very useful. It would not need to be more than say a 1" to say 1.25" bypass pipe to bleed off the excess EMP to a point downstream in the exhaust pipe.

It would be especially well suited for long periods of WOT operation.

Tuning it and the ECU would be a mother, but the end result would no doubt be a nice "free" increase in WOT power output. By reducing the exhaust backpressure, and even with a no overlap cam, cylinder purging at high rpms should improve and parasitic pumping losses lowered by the pistons not having to use as much power to push the exhaust slugs out.

Me? I'm PERSONALLY CHALLENGING ALL VENDORS HERE to a mud wresting contest at the 2008 TDI Fest! You are all a bunch of wusses and I will make you all eat mud suckers!

Who among ye are man enough to enter the mud pit with the Lurker?

Grrrrrrrrrr!

Bypass would only be good to help the peaks at turbo spoolup..

Think about it..

High EMP at high rpm WOT is because your vnt17 is to restrictive.. When you would add the bypass it would just close it's vanes a bit more to keep up with the desired boost.. and the emp would be +- the same.. it needs that much EMP to keep supplying the desired boost at 4200rpm..

Solution is more efficient charger or less desired boost..

 

[DanG144]

Actually on my BEW (with RC1+) when at high RPM and high fuel, the boost is way over desired, even with the vanes in the absolute minimum position.
There would seem to be some benefit in an exhaust bypass.

 

[Rub87]

Or a bigger turbo

This could indeed be a solution for boost creep situations ..

 

[LurkerMike]

The big turbos like the VNT-20's suffer at low rpms where there is insufficient exhaust flow to keep the compressor spooled... a daily driver's nightmare, probably as bad as trying to drive a 1967-1969 Z28 Camaro in traffic... not enough low-end torque to pull a string out of a (insert metaphor)... but cut down on it and it hits harder than a 300 shot of nitrous.

The smaller turbo's are ideal for city driving and fine cruising at 100+ mph on the highway -except for the high EGT's...

 

[hatemi]

VNT20 is a small turbo. Smalest singe turbo option for 1,9 even considerable for a "mild" performance upgrade. It can produce more torque down low to destroy anything else.

 

[Rub87]

I found out.. After 3600rpm a vnt20 chokes a stock engine with 1.7 bar boost.. with chokes I mean EMP>IMP..

I can get 1.5bar at just over 2000rpm easily.. If I let of the pedal than I get the nice surging sound.. makes the bov wannabees very jalous :d

 

[nicklockard]

Dumb question time...

So, I guess there's something dumb I need to clear up: if EMP is lower than IMP, but the turbo does not stall out (surge) and spin backwards, this means that the surface area (effective surface area upon which the gasses push) of the exhaust exducer must be larger than the surface are of the compressor inducer by quite a lot, since :

1. F driven = F drive
2. P = F/A

If P ex < P in, then
Aex > A in

by at least a factor of: (P exhaust minimum)/(P intake maximum.)| pre-stall pressures.

Or am I missing something entirely?

 

[jackbombay]

You are also dealing with a larger volume of gas through the turbine that through the compressor, for say 100 L of compressed air produced by the compressor there are 200 L (?) of exhaust going through the turbine. Not sure if that was addressed in your post, or how it would affect what you posted, but it is something to think about.

 

[LurkerMike]

VNT20 is a small turbo. Smalest singe turbo option for 1,9 even considerable for a "mild" performance upgrade. It can produce more torque down low to destroy anything else.

Then why do people bother with hybrid 17/22's or 18/XX's?

One word: Drivability

 

[hatemi]

Just few words. Lack of ability to tune it to the maximum... The nice surge Phew sounds described when letting go of the throttle mean that you are getting close to whre it should be! Nothing to worry about. Normal and will do no harm. If you get the CHA CHA CHA while still on the pedal then you have surge. CHA CHA CHA is bad. PEHW is good. A turbo that makes PEHW is tuned nearly right. If the boost response is bad in your tuners mind then you should question his/hers methods...

VNT20 is the smalest turbo that can make the rest of the engine live and survive. There is no point/use of more torque or responsiveness in a FWD car... No point and no practical benefit.

 

[Mark@MaloneTuning]

Then why do people bother with hybrid 17/22's or 18/XX's?

One word: Drivability

Overall cost and ease of install are two big reasons. VNT-20 requires new exhaust manifold, different downpipe, new ic pipes, etc. A VNT-20 is a pretty small turbo IMO, it's quite daily driveable with enough low-RPM response if configured correctly. Heck, joevat says his VNT-20 with a larger compressor produces boost quicker than a VNT-15 (although that's normally not the case). The 18/xx hybrid (aka SS185x or Hammerhead) from Kerma is at least as large as a VNT-20 from what I'm told.

Happy new year!
Mark

 

[LurkerMike]

VNT20 is the smalest turbo that can make the rest of the engine live and survive. There is no point/use of more torque or responsiveness in a FWD car... No point and no practical benefit.

Overall cost and ease of install are two big reasons. VNT-20 requires new exhaust manifold, different downpipe, new ic pipes, etc. A VNT-20 is a pretty small turbo IMO, it's quite daily driveable with enough low-RPM response if configured correctly. Heck, joevat says his VNT-20 with a larger compressor produces boost quicker than a VNT-15 (although that's normally not the case). The 18/xx hybrid (aka SS185x or Hammerhead) from Kerma is at least as large as a VNT-20 from what I'm told.

Happy new year!
Mark

Thanks! I remember seeing some dyno graphs that made the 20 look really bad on the low end as compared to the straight-17. Perhaps the tuning was not right on the 20 that was tested (on an ALH if I remember correctly)?

And you aren't kidding about a VNT-20 currently NOT being 100% bolt-on plug and play. That may eventually change because I know KERMA has a cast iron VNT-20 exhaust manifold and the KERMA website shows a DIY exhaust splice pipe kit...

If the VNT-20's come down in price to where the hybrids are, would you prefer a 20 over a 17XX or 18XX hybrid? Or even a 20XX hybrid?

 

[Rub87]

Thanks! I remember seeing some dyno graphs that made the 20 look really bad on the low end as compared to the straight-17. Perhaps the tuning was not right on the 20 that was tested (on an ALH if I remember correctly)?

And you aren't kidding about a VNT-20 currently NOT being 100% bolt-on plug and play. That may eventually change because I know KERMA has a cast iron VNT-20 exhaust manifold and the KERMA website shows a DIY exhaust splice pipe kit...

If the VNT-20's come down in price to where the hybrids are, would you prefer a 20 over a 17XX or 18XX hybrid? Or even a 20XX hybrid?

Cast iron vnt20 manifold already is there for a long time.. and its really nice looking too.. I only didn't remember who sell it..

 

[DanG144]

Jack,
Good point to emphasize. Combustion gasses increase exhaust volume considerably.
Dan

 

[nicklockard]

Jack,
Good point to emphasize. Combustion gasses increase exhaust volume considerably.
Dan

Yes, Jack. Thanks. I think it makes more sense now.

[P in*v in]*compressor efficiency factor = [p ex*V ex ]

But, I still wonder if the blade areas are different.

When IMP > EMP, it should smoke a lot because at those rpm's (see FUB's graphs above), there isn't much fuel injected, especially once you just tip into the accelerator.

 

[Fix_Until_Broke]

FUB, I'm a bit confused. If IMP < EMP, by definition isn't that surge territory?

Or are you saying that that is simply indicative of lag?


Sorry for my ignorance. Can you plot differential [EMP - IMP] and EMP/IMP, ratio? Or give raw data and I'll do it.

Why is the vag log data 2 seconds lagged? I missed that. Can you do anything to time-correct it?

Noted: TIP is under 2 psi vacuum for almost all of your accelerative run. That is a suprise since you are running a 3" custom TIT, isn't it? I wonder where the restriction is coming from...airbox/airfilter?

Do you run your stock intake snorkel, or have you removed it?

IMP's can be > EMP's without surge: Surge is a function of shaft speed and compressor differential pressure and has nothing to do with EMP's directly if I understand correctly. Remember these are not positive displacement air pumps/motors - they rely on accelerating/decelerating the gasses so if you can't spin the wheel fast enough to accelerate the air to a high enough velocity that it will continue moving out of the compressor housing, then it will surge. The exhaust side is a bit different with the variable geometry as the vanes change the angle that the incoming air contacts the exhaust wheel. The same "EMP" can have significantly different ammounts of energy transfer to the wheel depending on what angle the gasses hit the wheel. When the vanes are just opening there is more restriction but the gasses hit the wheel at the outer perimeter and ~perpendicular. The EMP's are measured before the vanes so that pressure is not necessairly what is acting on the exhaust wheel. I don't know how to measure the pressure after the vanes but before the wheel.

I did not notice the lag in the VagLog data until I posted it. I can subtract the difference between where the RPM's start to fall and where the IMP and EMP's fall (assuming these points correlate) and re-plot/post it. For some reason my trigger's must have a delay in them or something or I just screwed up the merging somehow.

IMP-EMP's are in the plot (purple) unless you specifically want EMP-IMP which I can do if you want. I've got EMP/IMP data (or the other way around - don't remember off hand) but did not plot it as the graph was pretty busy as it is. It did not exceed ~2.0 IIRC

My guess on the TIP is the airfilter is the main restriction, but I don't know. I have the snowscreen removed, but otherwise stock. This is measured before the MAF and my 3" inlet pipe so the actual at the turbo will be even less (absolute) pressure. The MAF readings were pegged at ~1275 mg/stroke early on in the run and tapered off as RPM's increased.

This is all about mass airflow as volumes don't make much difference due to the Pv=NRT thing and we are dealing with energy transfer here. I don't think IMP's or EMP's or their relationship to each other have much to do with smoke. That's just mass airflow and fuel injected, primarly, along with injection timing and RPM's, secondairly (sp?).

I hope I answered all your questions (and a few others). I'll try and do some more runs on my car since everything is hooked up (maybe put in my new air filter) and try and work out some of the bugs (filter the EMP and IMP signals to reduce the "fuzzy" traces, trigger synchronization, different driving conditions, downpipe pressure, automated VagLog data merging etc).

I think what we really need is a GTG (when or where it's warm) with a variety of cars with various equipment where we can take a bunch of measurements and do some comparisons (R32 Airbox/MAF, OMI, FMIC/SMIC's, Downpipes, Turbos, etc). We'd also need a nice piece of road where we can do runs on or a controlled load dyno - Hey DGD, what do you think about another GTG in April ? (I say that because it's only ~1hour away, I'm selfish and I probably can't drive too far to another GTG since the last one kept me away for 20+ hours which made for some interesting home life for a few days after).

 

[nicklockard]

FUB,

Well, this clarifies one thing for sure: at a level of tune similar to yours, one needs to reduce restrictions in the upstream air intake pathway (upstream of turbo.)

So, bigger airboxes and bigger filters and better snorkels are going to give a small payoff for you.

But, that may true because now you've gone and reduced all of the exhaust-side restrictions, and now the intake side restrictions are visible, where before they were secondary and hidden from view.

You answered a lot of my questions. Thanks! I'm trying to think of conservation of energy in terms of P-V work done on each side of the air pump(s) (turbo and engine as a whole)

 

[fishmando]

FUB, Thanks for doing all of this work! It is great to see that this thread came back around. Great info!!

 

[LurkerMike]

What do you meen by remove wazoo out of tha car?

......