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Why is boost pressure so erratic (sometimes high, sometimes low)?
I'm trying to figure out why my boost pressure is so erratic. Sometimes I'll see respectable boost levels at up to 12.6 psi which seems reasonable with the breathing mods. Other times it will max out at 11psi and spend most of the RPM band in the 9s-10s. I can definitely feel that the car is a lot peppier when it's actually making full boost. I was on the dyno a few days ago and made 3 pulls at around 490-500rwhp and 2 pulls at around 550-560 rwhp on a Dynojet. IATs stopped at around 110-115 degrees at the end of each pull and are in check. Fuel pressure is good at 69-73 psi at WOT. I am logging from OBD2 with only RPM/PSI checked and understand that it's slow but figure it has to be in the ballpark.
The mods are 77mm clutched pulley, headers with high flow cats, 3" exhaust, 82mm throttle body, 550cc injectors, cooling mods, ecu/tcu tune, and belt wrap kit. I took a video of the tensioner and also the supercharger pulley during a dyno pull (see below). The tensioner doesn't bounce around the magnet/pulley seem to be locking up during the pull. The car otherwise drives fine with no codes, hiccups, or issues to speak of.
Here's a list of some troubleshooting ideas that I've already checked.
- Smoked tested for vacuum leaks at 5 psi
- Air tested for vacuum leaks at 15 psi
- No blowby coming from oil filler cap
- Tried 3 different supercharger belts
- Had anti-slip coating applied to supercharger pulley
- Installed new belt tensioner
- Did an engine compression test
- Adjusted supercharger shims a bunch of times
- Tested supercharger bypass valve in DAS
- New MAP sensor
- New accelerator pedal/sensor
Honestly running out of ideas of what could be going on here. My only last guess is that the supercharger bypass valve isn't closing completely under throttle or is leaking. Has anyone ever tried to install a stronger spring in this unit or a new butterfly plate that seals 100%? I don't think that it's belt slip as there's not much dust build up around the pulleys and the belt is very tight.
The boost falling that way is fine,your on a stock blower and the drop off as fast as they rise up..I feel your belt is not tight enough you ill need a tighter belt .5 less or so and rpm belts are not that great.Stick with a green hd belt .
The boost falling that way is fine,your on a stock blower and the drop off as fast as they rise up..I feel your belt is not tight enough you ill need a tighter belt .5 less or so and rpm belts are not that great.Stick with a green hd belt .
He is not attempting to figure out why his boost is dropping off through the rpm range. He is trying to figure out why pressure is inconsistent one pull to the next at the same RPM. It varies by almost 2 psi at the same RPM on different pulls with as far as I can see, no other changes.
[QUOTE=drothgeb;7394706]He is not attempting to figure out why his boost is dropping off through the rpm range. He is trying to figure out why pressure is inconsistent one pull to the next at the same RPM. It varies by almost 2 psi at the same RPM on different pulls with as far as I can see, no other changes.[/QUOTE
Yes he did .Logs posted read again as he stated 11 psi and 9 psi to 10 psi so I answered the question about that and the reason for his changes too>
To the other person get a tight belt it will solve it..
He is not attempting to figure out why his boost is dropping off through the rpm range. He is trying to figure out why pressure is inconsistent one pull to the next at the same RPM. It varies by almost 2 psi at the same RPM on different pulls with as far as I can see, no other changes.[/QUOTE
Yes he did .Logs posted read again as he stated 11 psi and 9 psi to 10 psi so I answered the question about that and the reason for his changes too>
To the other person get a tight belt it will solve it..
I sopposed you can lead a horse to water but can't make him drink
I tried the Gates HD green belt 525, Gates RPM 525 belt, Continental Elite (Gatorback) 525 belt, and Gates plain MicroV 530 belt. The Gates green and Gates RPM were about equal when looking at the logs, the 530 produced a tiny amount less psi, and the Elite was horrible which produced a ton a dust everywhere after two pulls.
If you look at the picture below, you can see that the belt tensioner is pretty much wide open and you can't put a shorter belt on. I tried a 521 belt it was too short and there's nothing in between the 525 and 521 sizes.
It does bother me that the boost is in the 9s at low/high RPM but I suppose that's just a function of the blower/clutched pulley design unless someone else tells me different or has logs to show me. I'm trying to figure out why I can hit 12.6 psi sometimes and other times it's 11psi given same ambient and IAT temps. Aside from max boost numbers, you can compare psi numbers throughout the band and see how different they are between logs. As noted, the car dynoed at 550 and 500 on the same day so I feel that there is something hardware related going on here.
After all the trouble shooting you've done it's tough to think of anything else.
Maybe try to verify the bypass is not getting any type of open signal from the ECU? I don't know if you can monitor bypass valve voltage with star during a couple pulls but you could do that or just get primative and figure out a way to splice a voltage lead onto the correct wire for the bypass valve and monitor a meter through a couple pulls and verify the values are consistent? I don't know what the voltage range for the valve is though but you can probably find it.
I am almost positive the ECU uses the bypass for torque management. From my own observations on street pulls I never get as high of a peak pressure on a single pull for any gear any time the traction control has activated on that pull. My guess has always gone towards ECU cracking the bypass valve. Just speculation obviously but might be worth checking if it may be the issue you are having.
Right, I'm kind of at a dead end here. If I never come to a resolution, I'd be tempted to reinstall the stock upper pulley and install a 180 crank to see if I get the results that I was initially expecting to see.
I know how to actuate the bypass valve in DAS and step it in 10% increments to test for functionality but never seen an option to monitor it in rael time with "actual values". I know that the tune has the bypass programmed to be shut 100% during WOT. However, I was told that it never closes 100% as it's not a water tight seal. Also, the spring or whatever keeps it closed may not be strong enough when seeing increased boost levels from stock causing it to leak air during build up. On older supercharged Mini Coopers, there was a company that modified the stock bypass valve with a stiffer spring and adjusted butterfly valve to hold higher boost pressure without leaking. Also on the CTS-Vs, you can adjust the butterfly valve and gain a couple of PSI to tighten it to prevent leaking. However, there doesn't appear to be any way to adjust our factory BPV.
This belt tensioner on the vehicle currently is an OEM product and only has about 800 miles on it. It's not dancing around excessively during a pull/WOT, so I doubt there's anything wrong with it. I actually have a brand new 77mm clutched pulley here that I can try and log. I'm also tempted to try and install the stock upper with correct belt (while leaving everything else the same) and see what kind of boost it produces in comparison to the 77mm. In theory, this should be safe to do since we're only talking about a 1-3 psi increase/decrease and it should just richen the tune up about if it sees a little less boost, correct?
I would log with the stock upper pulley. Have you removed the 77 and looked at the clutch contact points, and surface area? Obviously they typically cook the paint on the drum when they're slipping real bad, but it would be interesting to compare.
My tensioner is held in place by another spring loaded tensioner that I made. Hulk was waiting for a picture so I will try and upload what I did...not pretty but works and that's all that matters.
My tensioner is held in place by another spring loaded tensioner that I made. Hulk was waiting for a picture so I will try and upload what I did...not pretty but works and that's all that matters.
Si ,post post that pic I was waiting to see it too lol..thanks man.
Not pretty but don't really care. I just set it in place ,adjust the spring tension and it keeps the pulley loaded all the time. And as bad as it looks...boost will stay consistant right to the top of the rpm band. Another big difference is when I spike the throttle the boost is higher right out of the gate...no slip when engine rev is spiked quickly.
E55AMG0220....It looks like Trabots just moved the bolt hole up more to get more "wrap" around the pulley. He also has a much bigger lower pulley on his kit and that will help a lot as well. I only have the smaller pulley.
Lightning a guy named Finny made a really nice set up on his car. You can find it under the "from stock to awe" post's he did a while back.
03-01-2018, 09:34 PM
Again, read his tittle and beginning ..The belt has to be just in between op.
