Cosmo Kramer

My SC build is coming along OK but I have a small set back. My 30 lb Accel injectors that came with my used setup have 2 duds and my car started on 6 cylinders. I have done all the necessary troubleshooting with the connectors and spark and I have found the injectors are the culprit.

I have the 928SP RRFPR and no FMU. My instructions say to use 30 lb injectors with this setup, but my stock 24 lb injectors are still working well. Could I use these with an increase in fuel pressure with 5 psi of boost?

toofast928

Yes, using a rising rate fuel regulator, max Fuel Pressure will be around 75-80 psi

Cosmo Kramer

What would be a good baseline to start with for the fuel pressure? With the 30 lb the instructions said to set the pressure at 29 psi at idle.

PorKen

The 928 Specialists billet FPR (Kirban Performance Products #4500) is not Rising Rate, just Adjustable.

Try hitting the errant injectors with 12-14V directly a few times. Even new, they may be stuck. You should hear a good 'clack'.

Stock LH2.2 fuel pressure is 36±3 psi. (With a cold engine, engine off/fuel pump relay jumpered, FP will be 39 psi, post traffic hot, 33.)

55psi (S4 FPR, AFPR set 58 psi cold) turns the stock 24# into roughly 28#.

29 psi will make the 30# injectors much 'smaller' than they are rated for.

Cosmo Kramer

I jumped my fuel pump relay and put my vac tool on my FPR. There was roughly a 20 psi difference from 0 hg to 20 hg, an when I applied a small about on pressure (blowing into the tube) it rose a few more PSI. Would this not indicate a rising rate?

PorKen

The stock FPR will do that too. It rises at a linear rate, whereas a true big body RRFPR will increase the fuel pressure faster with boost pressure.

Cosmo Kramer

OK I was wondering. I know FMU's usually increase pressure at 8:1 under boost, I guess my regulator would only increase at 1:1.

How would you recommend setting the fuel pressure? Cold with relay jumpered or running at idle under vacuum? When you give the pressure numbers how do you measure it?

Imo000

Ted, FMUs are calibrated with a diaphram and spring to certain rations. Most popular for V8s is the 8:1 but I've seen others with different ratios.

PorKen

I recommend setting the FP with the engine cold, engine off, and the relay jumpered.

Warm idle FP is unpredictable, and is less than with the engine off. Engine vacuum will pull down the pressure 6+ psi at idle.

The 29 psi you were recommended was probably a warm, mid range, pressure. Engine off or on, though?

Cosmo Kramer

Cold, engine running at idle so full vacuum to the FPR.

jorj7

If your 30 lb-hr injectors flow 30 lb-hr at 43.5 psi (usual rating pressure) and the same
with the 24 lb-hr injectors, then the 24 lb-hr injectors will flow the same at 45.33 psi
as the 30 lb-hr injectors at 29 psi using the flow formula

effective flow rate = rated flow rate x sqrt(new fuel pressure/rated fuel pressure)

that I found in the Kinsler.com catalog.

Cosmo Kramer

Well did some troubleshooting tonight. I had an 18v battery from my cordless drill and a 9v battery and a test lead with a connector (thanks Imre). The two bad injectors would fire with 0 fuel pressure with the 9v battery but required the 18v drill battery once fuel pressure was present. I am thinking these two are bad as the 9v should have fired them when under fuel pressure. How many volts/amps go to the injectors from the LH?

One other thing, currently I do not have my alternator belt on but the battery is well charged. That wouldn't make a difference would it?

Cosmo Kramer

Would this setup not run rich at idle and part throttle? Would the LH be able to shorten the pulse enough to keep the mixture stoich with that big of an injector and that much pressure?

John Speake

If you have access to a Sharktuner then yes, you can tune for a good idle with 42# injectors.

Imo000

Do you know if there are there any Shartuners in Canada or near Ontario? I'm not asking for the names as that is confindential, just curious to know if there are any here.