VNT Rabbit tuning: intercooler + more boost

[fspGTD]

OK, today I just tweaked the fueling up a bit more via a LDA cone rotation.  I figured, OK it's time to see what this thing's like set to a reasonable and stock-like "whisp" of smoke fueling level at wide open throttle.  That was my target smoking level.

I popped the LDA cover, and found that the stock '84 Jetta TD setting of the LDA cone rotation was perhaps 20% rotated towards rich.  For the first change, I tried rotating the LDA cone to all the way to rich.  On the first wide open burst of testing, I found smoking was evident and trailed off behind the car, a little thicker than I wanted to leave this daily driver set at.  So I promptly made a second LDA rotation adjustment - dialing the cone rotation back halfway between full rich and stock (we could call where I set it 60% rich.)  A further road test found that now smoking was just barely noticeable - exactly where I wanted it.  And driving the car around a little bit more, I found the performance of the car was quite noticeably increased, particularly at lower RPMs, where the VNT reacted well to the extra fuel by spooling up more boost.  Based on just a short drive, the car seemed overall quite noticeably ballsier.  More usable low-end torque!  Can't wait to hear back later from dad what he thinks about it.  

[TDIMeister]

I'd love to see pictures around the overall car!

[VWRacer]

Hey Jake, how about throwing your dyno on that baby for some updated numbers?  

Looking at the "old" graph, I see that the fueling appears completely stock...i.e., the governor starts pulling fuel out so that power starts falling off rapidly above about 4300 RPM.



The other factiod that catches my eye is that the HP slope goes from 60 HP at 3000 RPM to 80 HP at 4260. That implies that if the governor wasn't taking fuel out the engine would hit 100 HP at about 5500 RPM...right where I'd like to be!  

[fspGTD]

TDIMeister - will put on my list to take some car pics for ya next time I see the car.

Stan - I have not increased the revv limiter on this motor, and I probably won't either since it's dad's daily driver and reliability is important.  Removing the intermediate spring seemed to have helped right before the main governor kicks in, so that probably would be in the high 3000/low 4000 RPM area for the (stock) max RPM screw setting it has.  You can also really feel it when the main governor kicks in, it's kind of like "OK, fun's over.  Time to shift!"  :lol:

This volumetric efficiency plot, taken from the 1.5 D SAE paper, might be useful for our "bench engine builds".  :wink:


You can see the 1.5D hits a VE peak of 92% around 3000-4000RPM.  This drops down to 84% at 5000RPM (An 8.6% dropoff), with presumably the dropoff in VE continuing with higher RPMs.  Keep in mind the 1.5D, like the 1.6D naturally aspirated, has those long, tuned intake runners, which would probably agument VE around the peak, so without them or perhaps with a tuned runners slightly shorter, perhaps the falloff in VE might not appear to be as drastic.

Despite the VE dropoff, I have come to some interesting findings based on the 1.5D SAE VE plot.  In excel, I have done an analysis of the expected shape of the HP curve, given this VE plot and assuming torque is directly related to VE.  Despite the dropoff in VE above 4000RPM, the HP should keep climbing as RPMs increase all the way to the end of the given VE numbers at 5000RPM.  And extrapolating the VE curve above 5000RPM, if we were to assume it trended in a straight line with the same slope as it has between 4000-5000, I found that the HP peak would occur at 6250 RPM (with the VE being approx 70%.)  The HP peak there would be 18.5% higher than at 4000RPM.

I don't know if it's reasonable to assume in a diesel that torque is directly related to VE though as it is in a gas engine... this assumption entails that fuel injection efficiency doesn't fall off at high RPMs, which might not be realistic.  But it would be an exciting thing to try, to see if we can modify the fuel injection system to get a HP peak around 6000'ish RPM.

[VWRacer]

How well I remember the dreaded 4000 RPM fuel cut in my first VW diesel, a 1980 Rabbit 1.5D. Even bone stock I can still remember hitting that wall...!  :lol:

Great illustration! Blessed be those oh-so-thorough VW engineers, eh?  :wink:

I would guess that your analysis of the impact of the long runners is correct, and that shorties would work better, but keep in mind that once one adds boost to an engine, VE ceases to have the impact that it does on NA engines. Now, before anyone jumps on their keyboard to point out to me that boost has no real relationship to VE, rest assured that I'm aware of that. OTOH, boost has everything to do with getting air into a cylinder. And if you're boosting your diesel at one bar and your VE is a disgustingly low 70% you're still getting an effective 140% VE compared to the same engine NA, which goes a long ways toward explaining why our seemingly low RPM engines will happily rev to stratospheric levels if given fuel.

I agree that even an untouched head should be able to reach 6250 RPM if given fuel. Just think of what a proper porting job would do for VE at that figure...

I don't think that torque is as directly related to VE in diesels as it is in gassers. Gas engines have such a narrow a/f window that their torque/VE curve is practically linear. Diesels have a much broader a/f window, and I suspect therefore are less sensitive to VE. As VE drops the engine may make more smoke, but I suspect it would continue to develop torque well past where our gasser experience would lead us to believe that the torque curve should fall.

[Turbo Turtle]

man your car is so sweet

[fspGTD]

Little Mikey: thanks and welcome to the board!  As a side note, I wanted to bring to your attention that you are using an avatar that is 500 x 374 pixels.  You really should scale it down so to fit within 100x100 pixels so that it meets our guidelines and doesn't skew the display of posts.  Thanks in advance!

[fspGTD]

Update on the VNT Rabbit:

Its oil consumption problem was progressive getting worse and there was noticeably blue smoke out the back, so suspecting the oil scraper rings weren't working right, we tore into the motor to give it some new rings.

We took the pistons to our local VW Diesel machinist to have them cleaned them up on the sandblaster, and get some rings for them.  (We went with grant top and bottom rings and total seal gapless middle rings.)  He gave us a deal we couldn't refuse on three new pistons which were compatible in bore size and weight (to within about 10 grams.)  We used them to replace the three pistons that had some pitting on the combustion chamber surfaces, near the pre-chamber port.

But trouble lurked in the bottom end: we found a trashed thrust bearing surface that the clutch release rod would load when the clutch pedal was depressed.  The thrust bearing was the one-piece style integrated main/thrust that Dr. Diesel reported similar bad luck with.  At .033" axial play (about twice the wear limit,) all the soft metal had already been worn through and it was already starting to gaul the smooth thrust surface on the crank, but dealing with that was much more of an undertaking than we wanted at the moment, so we just put it back together with a fresh thrust bearing (unfortunately, the only style we could find quickly locally was the one-piece), and will keep our fingers crossed it will last for at least a little while.

I learned that starting the engine with the clutch pedal depressed is hard on thrust bearings, and I will be changing my ways.  Doing this loads the thrust bearing at a time when there is no oil pressure.

Yesterday, after getting the the engine put back together with fresh honed/deglazed bores and the oil system pre-lubed with non-synthetic nondetergent 30 weight break in oil, it fired right up.  After allowing it to warm up, I gave it a few shots of decent loading and boost to try and get the new rings nice and seated in, and it felt pretty good.  Fingers are crossed that those rings seat and stop the oil consumption.

Thanks to LarryB for lending me the bottle hone and a few other necessary tools.

[Maarten]

I popped the LDA cover, and found that the stock '84 Jetta TD setting of the LDA cone rotation was perhaps 20% rotated towards rich.  For the first change, I tried rotating the LDA cone to all the way to rich.  

Can you explain that a bit more, I noticed that the cone is not symetric and there is a dot on the rubber, standing infront of the car my GTD pumps' dot was at 12 o'clock so I dropped it in that way into my 1.9 AAZ pump. But which way is rich, and what is all the way to rich?

Keep us posted on the great project

[fspGTD]

Moving the cone toward the end of the pump with the injection lines will allow more fueling when your foot is pressed down far enough.

[veeman]

>> so suspecting the oil scraper rings weren't working right, we tore into the motor to give it some new rings.

What a bummer to have to tear back into the engine just after finishing the "tuning" portion...  Did you end up pulling the motor?  

>>At .033" axial play (about twice the wear limit,) all the soft metal had already been worn through and it was already starting to gaul the smooth thrust surface on the crank,

Does that mean that the crank would need to be turned as well?

 >>(unfortunately, the only style we could find quickly locally was the one-piece), and will keep our fingers crossed it will last for at least a little while.

What's the alternative bearing option?  Is there a different design or material in a two piece?  

>>I learned that starting the engine with the clutch pedal depressed is hard on thrust bearings, and I will be changing my ways.  Doing this loads the thrust bearing at a time when there is no oil pressure.

I had no idea... I wonder if the gassers are the same.  That'll change my thinking on starting the FSP car.  I had never heard that, but it seems possible.

>> Fingers are crossed that those rings seat and stop the oil consumption.

Hopefully that'll be peace of mind for another 150k miles....

[VWRacer]

I learned that starting the engine with the clutch pedal depressed is hard on thrust bearings, and I will be changing my ways. Doing this loads the thrust bearing at a time when there is no oil pressure.

Hmm, that is interesting... :?

Neither of my VW diesels will start without the clutch pedal being fully depressed.  :roll:

[jackbombay]

I learned that starting the engine with the clutch pedal depressed is hard on thrust bearings, and I will be changing my ways. Doing this loads the thrust bearing at a time when there is no oil pressure.

Hmm, that is interesting... :?

Neither of my VW diesels will start without the clutch pedal being fully depressed.  :roll:

  You can "fix" the switch so the car will start wit your foot off the clutch, I did it witha smal piece of wire bent into a "U" shape and held it into the plug for the clutch switch with a zip tie.

[fspGTD]

What a bummer to have to tear back into the engine just after finishing the "tuning" portion...  Did you end up pulling the motor?  
...
Does that mean that the crank would need to be turned as well?
...
What's the alternative bearing option?  Is there a different design or material in a two piece?  

This was just a quick and dirty re-ring.  We didn't remove the block from the chassis, or the crank from the block.

Ideally, (IE: if this motor were completely rebuilt "correctly"), the crankshaft thrust surfaces would be restored so as to fully tighten up the axial play to like-new clearance spec (which is in the neighborhood of .002".)  I can't say if it would be as simple as just machining the crank, or it it might take welding... but there becomes a point where the crank is not worth saving, and the one in this motor may be in that category.

There is a main bearing set available that has the 3rd main bearing separated from the thrust surfaces.  It looks like this:

Here is a close-up of the thrust washers:

It is from what I hear a better design, although it is more costly and harder to find.

Compare that with the integrated 3rd main/thrust style, which is the kind that was in the car and it looks like this:

[lord_verminaard]

I'm pretty sure that all OEM main bearing sets have the separate thrust washers.  At least every set that I ever sold at VW did.    

Did the old set that you took out have the separate washers?  If not then that engine has been dug into before.

Interesting.  

Also, a good reminder seeing that this could be caused by clutch pushrod loading over time, is to stop the habit of holding your foot on the clutch at a stoplight.  I've never done it as I was always told not to while learning but I know a LOT of people who do.  

Brendan
84 Scirocco 8v
00 Camaro L36 M49