I am starting over again with my L98 P600B ProCharger project. I am now building a forged 383 and have a question about the pro's and con's of an FMU or AFPR. I currently run an FMU and was told by the tuner that as long as I had sufficient vacuum it would work well.

Since I have everything apart and I am trying NOT to cut corners, I thought I would see what the rest of the world is running and why. Your thoughts are appreciated.

90Indy

 

The only reson I see to use an FMU is if you can't alter the ECM. Once you get to the point where you have to go into the ECM an do a custom calibration the FMU is only trouble. Another bad thing with an FMU is that the fuel pump has to deliver enough fuel at a much higher pressure.

An AFPR is no better than a FPR, how often do you adjust fuel pressure? The important thing is that the fuel pressure is stable, both at WOT and idle. The stock FPR is actually quite good and it can handle a lot higher fuel flow than stock.

You should get proper sized injectors and a custom calibration of your ECM.

 

If I remember right, the tuner said the ECM would only handle up 30 lb injectors which is what I was running at the time. I have not contacted him about the rebuild but by my calculations 30 lb injectors will not suffice. I want to get it right this time around which is why I am asking.

90Indy

 

I'm a relative newb to the forced induction scene, but here are my thoughts:

I would think you would want injectors at least in the 42-48 lb./hr range based on a guess-timate of 550 hp and the available injector sizing calculators.

I'm currently running Lucas 42 lb. injectors and no FMU along with the corresponding ecm tuning to provide appropriate enrichment. These injectors are running at around an 80% duty cycle to support approx. 6 lbs. of boost in my application, so I think they are a good fit.

As the injector size goes up, idling issues may arise due to the injector's non-linear response to very small pulse widths. This may entail some tuning of the injector offsets for battery voltage and low pulse widths, in addition to the standard injector constants.

With the Lucas 42s, I'm having no problem at idle with pulse widths in the vicinity of 0.75 to 1 ms with an old 88 MAF batch fire ecm. I did have to do some tuning of the offsets, and injector constants to get a stable idle with these injectors.

If you need to go much larger for big power, an FMU may be a good option to permit undersized injectors to idle better and still flow more due to the FMUs pressure increase under boost.

 

You are only limited by the size of the high impedence injectors offered with the factory ECM. They have come out with some newer, larger flow, high impedence injectors, but the largest I had purchased were 42#/hr units.

With regard to tuneability, I run 83#/hr injectors on my street car, and have set up other combos with large injectors with no issue. Granted they are being tuned with aftermarket ECMs, so a bit easier to tune.

Aaron

 

I've been running an FMU for five years now and it's failed three times.......the problem is a stainless steel plunger up against a aluminum cylinder.........the steel wins and small particles of aluminum jam the plunger..........just removing and cleaning fixes the problem.......but depending on how often you drive it, it must become part of your maintenance.........it's hard to watch the fuel pressure all the time....I had mine long enough I can tell right away when I get a lean pop.............tunning with big injectors is more fail safe ......if you can get it to idle......not kill after a/c is on and other issues.....you really have to be a tunner just to run high horsepower.

 

The problem with tuning a '85 thru '89 Vette is the flow range of the MAF. At 255 or so gms/sec you quickly peg the MAF with a blower and then what do you do? The '85 thru '89 MAF outputs a 1-5vdc signal, so using a newer style MAF with a frequency output won't work w/o a signal conditioner. I've looked into doing but it seems to me the market is very limited. Additionally even with the signal condtioner the ECM would have to be re-mapped to match the new range of say 450 gms/sec.
In the meantime the FMU works quite well given a proper pump set-up with sufficient head.
When you get into the high power applications it seems more effective to use say the FAST XFI, particularly since you can buy the Plug-N-Play adaptor for these years Vettes and be up and running in just a few minutes.
I for one would be interested in alternative solutions for the '85 - '89, particularly since the stock MAF sensor is prone to failure.
Best regards,
Greg

 

The 85-89 MAF cars can be tuned to run above the 255 gm/sec limitation. The typical method is to add the extra fuel in the power enrichment vs rpm table for the rpms where the MAF would be saturated.

For this to work, the excess airflow needs to be predicted and additional fuel added to compensate in power enrichment (PE).

In my case at approx. 6 lbs of boost, I'm assuming roughly a 40%-50% increase in flow over the 255 limit at higher rpm. I was typically running in the mid 240s N/A. Now, I'm adding between 40 and 50% PE enrichment vs rpm between 5200 and 6400 rpm to compensate for the predicted excess airflow. The PE % must continue to rise with rpm as long as the airflow is increasing. This can be done up to 6400 rpm. Over 6400 rpm, it would begin to lean out, since the PE can't continue to increase with the present design. For my application, this is not a concern.

For reference 0% PE would represent a 12.0 AFR target with the current coolant temperature settings, so 50% PE enrichment would approximate a 12.0 AFR target for 50% additional flow (383 gm/sec). The narrow band O2 sensor output suggests reasonable fueling with this approach (900 + mv) so its not too far off.

It is not a perfect solution, but it seems to be the standard method for dealing with excess airflow in these MAF ecms.

As another twist, I'm also not actually using any MAF at all currently. Instead I've tuned the default airflow calculation with good results. The airflow is calculated from a few basic inputs, IAC position, tps and rpm, a base offset, IAC scale factor and limited by the max flow vs rpm limit table. The actual MAF output can be reasonably approximated with this approach. This was done primarily to simplify the plumbing in the early stage of my installation. I've also played with this approach running N/A to serve as a reliable MAF back-up, in the event of MAF failure, but I think the blower helps to make the flow vs rpm vs tps even more predictable.

I've also considered going with a newer 3" (on the discharge side) or 3.5" (on the inlet side) LS1 digital maf and a high to low frequency turbo buick style MAF translator.

The ecm needs to be re-pinned (one pin, B6 I think? ) and the bin file must be configured for the digital MAF input (there is an existing switch in the software for $32 and $32B masks).

The airflow and fueling can then be rescaled internally for more flow/less resolution. By simply doubling the fuel and scaling the MAF output by 50%, you could run up to 512 gm/sec. The newer MAFs would also be easier to package in the duct work, since they are much thinner, which makes this very attractive.

This would cost roughly $300 for the new MAF and a translator. I may end up going this route for an interesting science project, but for now the "MAFless" solution is satisfactory.

To the OP, I apologize for going off-topic in this response.