I am having an issue with boost dropping off at high rpms with a TVS supercharger. In second gear I will start about 12 psi and the boost drops off from there as i approach 7k rpms to below 10psi instead of climbing like it should. I can then shift into third gear and the boost will be about 15 psi and climb as I rev out 3rd or it will suddenly drop 5 psi and stay low. It seemed to be like belt slip because it made sense the boost would drop and stay there once the belt started to slip. However the belt seems to be as tight as I can get it. I have the ZZP adjustable tensioner and I don't think i have enough travel left in the cylinder to drop a setting and still get the belt back on even with the cylinder fully bottomed out. I removed the tensioner cylinder and it seems to still have full spring force as far as I can tell when comparing it to a new spare unit I have. Based on what I have observed it definitely doesn't seem like I can get the belt any tighter, if anything may have it too tight because the tensioner has barely enough excess travel to get the belt on.

I have ruled out the bypass solenoid as I disconnected it from the actuator and it still pulled boost.

I can only think of 3 other possibilities that could cause the boost to act this way. The most obvious is belt slip though I don't see how it could with how the tensioner is set. The other is my intake filter is restricting flow to the point it is pulling a slight vacuum on the actuator and causing it to go into partial bypass mode. The last is that my tune is actually telling my throttle body to close some or my throttle body isn't reporting it's position properly though I don't see why this could allow for a sudden drop and the same goes for the restrictive filter idea.


Is there anything I could be missing that would cause boost to drop like this? I make more power along with more boost in low rpm high load situations. Everything seems to work normally with lower rpm high load. I haven't revved 3rd gear completely out but I have had it maintain proper boost sometimes and others it drops off.

Does anyone make a belt size that is actually bigger than the stock LSJ belt? If I had a slightly bigger belt I would have more room to play with the adjustment on the adjustable ZZP tensioner. Right now I am in the loosest setting but it doesn't seem I have enough room to go up a notch. I wouldn't be able to get a smaller belt on.

Things affecting my belt setup are TVS with 2.9" pulley, phenolic spacer, 1/2" supercharger spacer between SC and IM, Overdrive ATI harmonic balancer, stock LSJ belt, and ZZP adjustable tensioner on loosest setting.

Any ideas or feedback would be appreciated.

 

Have you tried using any sort belt dressing?

 

Dont have any experience with the stuff. I assume it would improve grip at least temporarily and be good for diagnostics. Thanks for the idea.

 

I've put on a new belt and I don't think it is belt slip. I've put on a new filter and insured the filter is not being blocked by anything. Today I did 4 pulls and for 3 of them the boost was pulled down about 5-8psi. On one of the pulls I had full boost. All in 3rd gear. With the boost bypass solenoid and belt slip eliminated as a possibility I don't see what could be intermittently pulling boost like that. The only possible causes I can think of are that the throttle body is partially closing (either the wonky tune or the throttle body is faulty itself), or my crank pulley key stock is sheared off. Though I would think if the key stock was sheared of it would slip in a different fashion if the pulley would even have enough friction to drive the accessories plus the supercharger at all. If my tuner gets back to me and lets me know what the ECU is doing I have a spare throttle body I can throw on and after that I guess I will drive the crank pulley bolt out and take a look to see if the keyways are aligned.

My tuner has been making adjustments to my tune in an attempt to get my car to idle correctly. Ever since I have installed my lightweight flywheel my idle went from shitty, where it would modulate up and down, to non-existant. When I pull up to a stop I have to womp the throttle a bit to keep it running. I wonder if it is possible he messed with something that could be throwing off the way the ECU communicates with the throttle.

Considering the supercharger is basically a fixed displacement pump I am running out of ideas for potential causes of intermittent low boost. If anyone can think of anything else that could potentially be a cause please let me know.

 

I dont think its a broken crank key, you would have low oil pressure and possibly some unpleasant noises.

 

Good point slowbalt, I forgot about the oil pump being driven by the hub of the crank pulley.

 

My tuner got back to me and my ECU is commanding the throttle body to close for some reason. He says there is an ETC (electronic throttle control) table that can limit throttle opening based on delivered torque and rpm and that I am hitting a table that is demanding 80% throttle. He is going to adjust it and we will go from there.

Man I wish I could use HPtuners. Alternatively I wish going standalone wasn't such a B on these cars.

 

ETC table has been adjusted and everything is working properly for now it seems.

 

I swore I posted this but maybe it didn't go through or it was in a different thread but in case anyone is looking there is a good reference right here on this site for different belts that will work on the LSJ. https://www.cobaltss.net/forums/2-0l...rs-lsj-318187/

Copypasta:
All are 6 rib double side belts with a 20mm width.

Ac Delco #12585151-71.25in(1809.75mm) stock
Ac Delco #12597993-69.56in(1767mm) stage 2. Those are best answers to length we found

Dayco #6060792- 70.19in(1782.82mm). Stock replacement

Gates DK060472- 48.5in(1233mm)
Gates DK060519- 53in(1348mm)
Gates DK060650- 66.125in(1680mm)
Gates DK060674- 68.5in(1741mm) closet to stg 2
Gates DK060702- 71.25in(1811mm) stock replacement
Gates DK060718- 73in(1853mm)
Gates DK060725- 73.75(1872mm)
Gates DK060729- 74.25(1885mm)
Gates DK060791- 80.375(2043mm)
Gates DK060867- 87.875(2232mm)
Gates DK060956- 95.75(2432mm)
Gates DK060973- 98.625(2504mm) __________________

I installed the Gates DK060718- 73in(1853mm) to work in combination with my 2.9" pulley, phenolic spacer between IM and Head, supercharger 1/2" spacer between SC and IM, ZZP adjustable tensioner on tightest setting and ATI OD harmonic balancer. It is a much better fit than running the stock LSJ belt with the loosest setting on the LSJ adjustable tensioner which was over-compressing the tensioner cylinder where I could hardly get the belt on.

 

That is good info to have, thats how I found the belt for my 3.8 pulley.

 

lmk when you wanna make that happen lol

 

What has me tripping is the crank sensor. From what I have seen no one really has a solid reliable way of measuring crank position. I think Trevor and Chris have tried a crank pulley trigger wheel but they weren't happy with it and last I new they were working on a way to measure position with a sensor on the end of the intake cam. What are your ideas for solving this issue? Otherwise running a harness I am not afraid of. It's just if I go to the trouble of doing it I don't want to trade off for other finicky BS.

 

You would have to use an external crank trigger or destroke to use lsj crank.

 

That's the part I have concerns with because I know nothing about what it would take to do that.

 

thats my external trigger on my l61 in my 944.

 

Is it bolted to your crank pulley? Does the imbalance cause much excess vibration? Do you have a drawing or layout for your pattern there?

 

the imbalance isnt detectable. if i was building another engine id have my balancing shop zero balance the trigger wheel .

its a universal trigger wheel i bought off ebay and its mounted on a stock crank pulley with spacers. i machined a slug to fit tightly into the damper and fit the center hole of the trigger wheel, and then i marked and drilled the mounting holes. i ended up with only around 0.010" ronout, so its well within acceptable tolerances for the sensor.