Hi all. We're working with a local shop on a 2002 Corvette which had the A&A Corvettes C5 supercharger kit installed. The car runs awesome, except for a minor "stumbling" issue between 1000-2000 RPM, with cruise throttle position.

The A&A kit has the MAF positioned after the SC, and uses a bypass valve that vents to the atmosphere.

To reproduce the problem in a controlled manner, we put the car on a chassis dyno with a wideband, and were easily able to reproduce it easily.

We spent a lot of time trying different tuning changes (increase, reduce timing advance, VE table adjustment, MAF calibration adjustment, injector flow rate constant), none of which affected the issue other then adjusted how much the PCM compensated to get the AFR correct.

We did notice, however, then when the stumble occurs, the AFR topped out momentarily at 18:1 meaning the stumble is likely caused by some intermittent condition that caused the PCM to reduce the fuel supplied, or there was a sudden, unexpected increase of airflow for which the PCM couldn't instantly compensate for.

We later learned the MAF had been descreened by the owner. We also found that "clocking" the MAF (e.g. rotating the MAF sensor to a different position) caused it to run very poorly.

We currently suspect two things: 1) A problem with the bypass valve, or 2) A problem with the MAF's ability to meter incoming air due to the removal of its screen.

Any thoughts from the experts on this one? Anybody else run into problems like this?

Thanks

 

How close is the MAF to the BPV? Some have had stumbling or surging problems with the MAF on the pressure side mounted close to the BPV and the BPV vented to atmosphere. It may sometimes cause issues with the airflow through the MAF. Being a descreened MAF also will change it's flow so the MAF tables may need to be adjusted somewhat. If you choose not the make MAF table changes and the MAF has only been descreeed, then maybe you can find a screen to replace it and see how it runs. This way, you can completly rule out the MAF scree factor. Also, may be dumb question, but all your piping completly sealed and not other vac leaks?

Ken R.

 

If you can rotate the MAF and change its measurements, that says that airflow is not uniform across its cross-section. I'd put a screen back in there to try to fix that, or reposition it if possible. It's ideal to have a long stretch of straight piping before the MAF to smooth out airflow, but that's not really practical in an engine bay, much less when you have to work around a supercharger- the screen is in place to bandaid that problem.

 

Thanks for the help. We'll look at putting a screened MAF back in and verify BPV function.

 

Finally. This is resolved.

I did try a screened MAF, all that did was removed about 40 HP, presumably due to the intake restriction.

The problem ended up being related to the 60# injectors that came with the kit. It turns out at very low pulsewidths the injectors can end up "misfiring" occassionally. Because they flow so much fuel, at idle and other "cruise throttle" situations, the injector pulsewidth ends up being very small.

There are two tables in the PCM called "minimum injector pulsewidth" and "default injector pulsewidth". As the name implies these tables set the smallest pulsewidth used, by RPM. For some reason, from the factory, GM made this value very small from 1200-2800 RPM, on the Corvettes.

There was a post on the Corvette forum discussing this problem, and they suggested making the 1200-2800 RPM value higher, matching the rest of the table. I did this and the surging went away. At idle and cruise this car runs as smooth as it did before the supercharger went on it. ;-)