81 Rabbit LS AAZ (2023: now tdi-m)

[RabbitJockey]

i did measure, with the nylon spacer the boost pin can't move down far enough to pinch the feeler pin

the pump is a giles pump i bought second hand, i had to open it when i first installed it because the governor weights were stuck from sitting... you can see it has some type of governor mod with an added spring.

[RabbitJockey]

also that'd be an odd though, 3500 is where it really begins to make power after losing it.

[libbydiesel]

What is the boost pressure at 3500? 

One suspicion of mine would be that the dip in power would be the governor cutting back fuel before the turbo starts to push the boost pin down.  Then the boost pressure builds enough to push the pin down and the power takes off again.  I'm not convinced that makes sense, though. 

[RabbitJockey]

i gave up working on it tonight because i am having trouble with my tach... but it builds 10psi and kind of holds that until 3500 then nearly instantly goes to 20psi.  which is what has me thinking it could be something with the governor as well.  im not sure how much boost is a factor, even at 8psi(stock wg no boost controller) i still had a similar dip in power i can post a picture of that

[RabbitJockey]

torque had already peaked at 2800, then drops off, then peaks again at 4100, both at around 105 ft lbs, peak hp is at 4100 83whp... this is 8psi stock wg

[libbydiesel]

If you are already seeing 10 psi prior to the dip then it is not LDA related.  The AAZ boost spring is weak and will be completely compressed by then. 

Any chance the wastegate valve is opening partially?  Then the exhaust flow is enough to overcome the leak and the power rises again. 

[RabbitJockey]

well, it is possible the spring is not original its very tall and pretty stiff .  as far as the wastegate i have no idea, but that is a possibility, but i wouldn't expect that to create such a sudden and abrupt gain in power.

[RabbitJockey]

i really need to experiment tonight before i speculate too much more, but it could still be the lda, it has a tremendous amount of preload, i think i have to press it down an inch or more before i can start screwing in the lda lid, so it could be that power flattens out as non enriched fueling reaches it's peak then when the lda starts to receive enough boost to move it suddenly comes to life.  but i just need to try some of the stuff discussed here, 1 thing will be to just remove the spring that way only my foot controls max fueling.

[RabbitJockey]

so tonight i eliminated the possibility of it being the lda, i removed the spring and drove it that way and also dynoed it... also keep in mind the scaling of the gtech is not very accurate so torque numbers and actual rpm points are not correct, but the curve shapes and hp numbers are correct, just not necessarily at the exact rpm it shows.  so removing the spring did not change anything other than creating lots of black smoke at low rpms.  but this is now the third time i dynoed over 130whp, and it was well over 80 degrees at the time, with no intercooler, i think thats impressive, but this dead low rpm *** is not haha.

this run was 131.7hp @ 4600 and 150 ft lbs @4559, how ever it does peak at 150 @ 3300 prior to that, then drops to 105, then back up to 150 at 4559



picture of the boost pin and spring...

[libbydiesel]

How does the engine sound at that time?  Maybe the dynamic advance is too much, it gets overly advanced at 3500 rpms, the power dips, but with even more rpms, the advance runs out and the engine catches up so the timing is good again.  Power then increases.  Just a theory...

What about the turbo going into surge for that drop?  How's it sound?

[RabbitJockey]

it really doesn't make any odd noises turbo or otherwise, you can definitely feel that power flattens out and then the big jump in power though.  i think its possible that it simply isn't efficient down that low its a pretty big compressor 42.5mm inducer and 60mm exducer and its a billet wheel which would push everything further to the right and move the efficiency to high flow rates... i just would still expect a more linear power curve and not an actual loss... ill map it again and see where everything sits

[theman53]

Check into a shorter and softer spring if the other stuff doesn't pan out. OR test the spring movement with air pressure from a regulator so you know at x psi it moves this much.

[RabbitJockey]

Check into a shorter and softer spring if the other stuff doesn't pan out. OR test the spring movement with air pressure from a regulator so you know at x psi it moves this much.

i don't think its anything lda related, i drove it with the lda spring removed, and it made no difference like the last dyno i posted shows. with out the spring there is not fuel limiting so i was getting full fueling no matter what the boost psi was.


i also plotted my engine on the compressor map.  the blue and red lines are at 70 degree ambient temp, blue line is 18 psi 2000-5000 rpms, red line is 8 psi 2000-5000 rpms, each dot is 1000 rpms, starting at 2000.

the green line would represent a more realistic example of where i am on the map, its at 85 degrees outside temp, and has the rpm and boost levels listed... 3500 rpms is where i enter 70% efficiency.  but regardless, i have the same issue at 8 psi as i do at 18, so i think this problem is definitely pump related.  i am thinking sticking advance piston or something wrong with case pressure relief.  or just a ***ty setting of the throttle lever and max fueling screw.

[libbydiesel]

If it is a pump issue I would think it was due to an issue with case pressure and dynamic advance.  It does not sound like a max fuel or accelerator lever issue.

Maybe try retarding the static timing and see what the graph looks like. 

[RabbitJockey]

i agree, i will try a can of diesel purge, but if that doesn't work i will put the stock pump on and clean this one out, the pump head seal is weaping anyway.