Low/No Boost 2003 BMW Z4 2.5i SC14 Supercharger Kit

Issue: installed supercharger kit on my car and boost gauge is reading low pressure (0-1psi) even at 4-6k RPMs. No boost leaks that I can tell, belt isn't slipping to my knowledge, supercharger rotors rotate when pulley is rotated by hand, and car is tuned for the new injectors, have all the supporting mods for the setup, but can't rack my head around why this isn't producing higher boost?

Car: 2003 BMW Z4 2.5i M54B25 (5spd Manual Transmission, RWD) inline 6/91-93 octane

Specs with some context:

- 32lb injectors from VF Engineering (flow tested, working great)

- Hyde Motor Works Brackets and install kit

- Ogura Clutch TX15 supercharger (aka SC14 with smaller 80mm pulley) -- rotors spin when the pulley is rotated by hand

- Smaller belt to account for the smaller pulley -- fits tight, doesn't appear to be slipping, but obviously can't tell when in operation, but don't hear any squeaking and have no debris from belt or glazing on belt.

- Replaced Hydraulic tensioner with mechanical tensioner to support belt tension

- A2A Intercooler (Believe it is fin/tube) -- installed on vehicle, but piping is disconnected as I diagnose my low/no boost situation. My supercharger outlet is currently going straight to intake manifold for troubleshooting.

- No boost bypass valve installed currently, but have a TurboSmart plumb back one ready to go on once low boost issue is corrected.

- AEM Wideband hooked to lower O2 that has been coded out. ProSport Digital boost gauge sensor installed on vacuum line on intake manifold (shows positive boost during leak test, so gauge is working properly during testing)

- Racing Dynamics Headers (no cats/muffler)

- Secondary air pump (SAP) has been removed and plate installed to block it off, this has been coded out

- DISA has been removed and a plate has been installed and coded out

- Working with a tuner, injectors are tuned for the setup properly

- Car has been boost leak tested and holds 10psi with a pressure test cap and leaks down 1 psi every 1-2 seconds from 10-7psi, then every 3-6 seconds for 5-4psi and then 30-40 seconds from 3 to zero psi. I've read a good leak down rate during a pressure test is 1psi per second, anything greater than a second per psi is considered great.

My thoughts - I'm no expert but that's plumbed the way I'd expect to do it myself

Have you been able to check the inlet and outlet pressure/vacuum to check there isn't a restriction upstream/down stream that's a restriction?

What is the actual drive ratio, the nominal (as far as I can find out) is 1.460 l/rev'n, and the engine nominal intake volume is 1.25 l/rev'n, add in pumping losses (see above) and it might matter - it's possible, if unlikely, that the s/charger is passing almost exactly the mass/volume of air through to match the engine's demand.

On that, have you tried different pulley sizes to see if it swings the manifold pressure either way?

Running one of the smallest pulleys you can run on this charger (the 80mm). So is the idea if I moved my boost sensor to the charge pipe, I might see higher pressure there? Or am I understanding that incorrectly? Basically, with this setup I should at the very least see 5psi, so I'd like to confirm that if possible.

Thank you!

Apologies for suggesting something basic, but have you confirmed the throttle isn't simply closing?

German cars are generally fitted with ECUs which when faced with excess airflow and/or torque, will close throttle to avoid generating more torque than desired. This can all be datalogged to confirm actual vs. target values.

That's a good thought MikeMcGinnis, as a throttle plate not opening fully would cause this too. I actually have a throttle position 1 and throttle position 2 error cropping up in my tuner's data logger. I will try to run that by them to see if maybe the throttle plate isn't opening fully and see what they think.

I did try resetting the throttle body adaptations and clearing code to see if that would help, but it did not.

This is a really odd issue, as I can rotate the pulley on the supercharger, and the rotors rotate by hand and rotate freely, I can hear just a little whine, but not much. They don't appear damaged, and no clunking noises or anything. At some points, according to my boost gauge when under load, I have seen 1-2 times of 3psi.

Since no bypass valve is fitted, the throttles may have been damaged by extreme pre throttle pressure common when you are driving a supercharger into mostly closed throttles, without a bypass.

Agree, Mike, also high charge temperatures.

However, it's a draw-through.

Ah Gord thanks, now I see the word "rotors" mentioned and agree it sounds like a roots style post throttle supercharger which would negate my concern about high pre throttle pressure.

Correct, the charger is post throttle body.

Since my last post, I've tried a one size smaller belt, didn't help. Thought it might be related to belt slip.

I've also taken charger off to get a look at gears by taking oil pan off back and they look good. Oil in charger was pitch black, so changed that out with fresh gear oil (should have done this initially). This didn't help after the fact. Rotors don't look great, as it appears the coating has come off on all lobes. I'm not certain they were really coated well to begin with as they were very dirty initially.

I have a refurbished charger on hand I'm going to try to see if it's just an issue with the current charger and will run fresh gear oil in that before mounting it.

Sounds like a plan. Were you able to confirm actual throttle position via datalogging before you go swapping parts?

The data logging software my tuner supplied has pedal % but not throttle and they are still working at developing that value. So I plugged in my OBDII scanner and the TPS % at WOT is ~89%. I've read that's normal. The % doesn't drop until foot comes off throttle, so I'm guessing it's not a throttle issue. Throttle codes have also gone away after clearing them and stayed away. Is the TPS the commanded value, or actual?

If you're using a generic scanner it's hard to guess. They may not even know.

Did you work this out? MAP sensor has appropriate scale range?

SOLUTION: Issue was supercharger. Prior owner had something inside charger or was placed there during shipping. It then made its way into the outlet and was stuck there blocking flow.

Replaced charger with refurbished unit and I'm seeing boost and have whine now. It also pulls a lot harder. I'd say this case solved minus my need to align pulleys better so belt doesn't fly off!

Thanks all for trying to troubleshoot this!

Awesome! Glad to hear it.