injector size

[lnirenberg]

I have a Mass-flo efi on my 61 bel air 409 (overbored to 421) with about 510 hp at the crank using 42# injectors. The motor has the large port iron heads with hardened valve seats with about 10-10.5:1 compression that I run on 93 pump gas. The ecu is a fox mustang unit that is sold as "self learning". I was having trouble getting the right amount of fuel across the rev range--enough fuel at WOT the car runs rich at idle turn down the pressure to get the right af ratio at idle and it runs lean at WOT and knocks. To try to fix this I had a tuner install a Moates Qthorse so he could fine tune the af but its still running lean at WOT. I ran the injector size through an on-line calculator and I should be fine but the tuner is recommending larger injectors in the 55# range. Any help would be appreciated.

[Six_Shooter]

Using this calculator: http://www.rceng.com/technical.aspx

I come up with about 40 lbs/hr, at 43.2 PSIG rail pressure, 80% DC, and .5 BSFC.

What fuel pressure at the rail will allow for a rich enough mixture at WOT?

What fuel pressure do you need to run for stoich idle?

Are you using a manifold vacuum referenced fuel regulator?

I just have to mention I use Moates products as well (for GM ECMs mostly), and the parts work great.

[MonzaRacer]

Is this unit set up as N-alpha or does it have an O2 sensor? If its tunable it sounds like you need pull fuel on the lower end and increase it on top end.
Vacuum referenced fuel pressure regulator needs a clean manifold signal, like at base of carb.
Also if it uses a map sensor you need to check the position of the hose because if its on an intake runner you can get a "bouncy" signal and the computer will read it wrong. Most computers have a sort of "gain" adjustment that makes computer look less closely at it or basically it "filters" the signal.
I used to help TPI guys by strapping in resistors and capacitors on MAP sensors if they used too big of cams.
Most injectors are calibrated for 43.5 psi.
Now another trick to make sure your not having mechanical issues is unhook the vacuum reference and wing the throttle and watch a large/accurate fuel pressure gauge. if you can see more than the needle just moving you may have fuel delivery issues. on stock engines as long as its less the 1.5 lb drop your good. I use this to find bad pumps or stopped up filters.
Depending on the cam/intake setup (and you should be using a good wide band O2 sensor) tuning for idle at or near 14 to 1 AFR, but depending on cam CAN run as low as 13 to one or worse.
For WOT tune for 11.0 to 1 to 11.5 as a target and then fine tune from there. You can run as rich as you want but much lower than 10.5 and its gonna was motor out or foul sensor,,,,and just wont make good power.
Also which brand of injector are you using? Some of the brands on the market are pretty sloppy in fuel control. also if you can pull the rail complete with injectors make sure non are dripping if you prime the system , this will make it hard to dial it in.
I have started using Injector Dynamic parts and get much better fuel control.
Also get someone to make sure the injector drivers are turning off clean.
By Ford unit do you mean a Ford ECM or one actually made to go in a different car like a Ford?

[lnirenberg]

Thanks for all the input. We ran some diagnostics and figured out we had a bad ECU, the 2nd one I've had from Mass-Flo. Borrowed a ECU from a fox Mustang and we got the car to run fine. I give up on Mass-Flo's brains and bought a rebuilt one for $120 and should have it early next week. Once we get the Moates chip in the new ECU I think I will be good to go.