EFI INJECTOR CHANGE Question
 

Changing my (8) injectors from 60 LB Hrs to 80's. With my cam and head changes, the 60's have reached their max duty cycle, hence the change to 80's. I'm looking for a starting point on my fuel MAP for the 80's. Since I had a good MAP for the 60's my guess would be to decrease the fuel MAP 22% for 80'S as a starting point on the DYNO. 22% is the increase in size on the injector from 60 to 80's. Is my thinking correct? The engines are 540 SCI with MEFI 4B and MSD program.

How much power are you making? (8) 60's are good to over 1000 hp. On a Whipple where the injectors are in the rear of the blower (on the suction side) I have made 1100 with (8) 60's. Sure, they are maxed out and it takes it's toll on the injectors, but it can be done.
If the injectors are in the intake runners, then they will flow less since they are on the pressure side. You can still make over 900 hp with them in the runners.
The 80's get VERY touchy to control at idle. Sometimes you just have to settle for the best that you can get it at idle since the allowable amount of change in the Mefi isn't fine enough to get it perfect.
On the Mefi, you can go into injector characteristics and just enter a percentage that you want the injectors to run. In your case, change the multiplier to about .80 to start. This will give you an idea of what the engine will want. You can either leave the multiplier at less than 1, or you can go back and change the fuel tables by what ever amount you had in the multiplier and put themultiplier back to 1.
Good luck. Let me know if I can help.
Eddie

Eddie, Thanks for your help. The injectors are in a plate on top of the 871's. Then the air only valves bolt on top of the plate just like carbs would. They are bank fired from the MEFI. If you think the 60's will be fine, I'll leave them alone. I've been told that the 60's will max on their duty cycle for anything over 850 HP. Fuel pressure is 50 PSI with boost reference.

60's @50psi are just under 65's. At about 90% duty cycle and a .55 BSFC they'll support 850ish ok. You can run a higher duty cycle and more fuel psi to get more power out of them like Eddie said but it's harder on the injectors and you've lost control of them at that point as well. Being a roots deal, the 80's likely won't have too much of an effect on the idle especially with a boost referenced reg. What's your ms on time under boost for the last 500rpm + any corrections/modifiers? That will tell the story on how bad your injectors are maxed out.

I'm going to be pushing over 900 HP with 10lbs boost. So I tend to agree 60's are pushing it at best. I think if I cut my initial fuel MAP 20% on the dyno I'll have a starting point to set the overall MAP. I just don't want to fuel wash it down while I adjust it to idle and begin all RPM settings.

900 + the 120hp it takes to turn the blower.. If you've got a good curve then you'd probably be alright making a general % drop like that. Or try tweakin the modifier like Eddie said to start with.

Since its MSD ingenious software, I'll have to check on its ability of a modifier. In the past, I've adjusted the MAP fuel chart in 200 RPM incriments to achieve desired AF ratios. In closed loop program follows set AF ratios but it's important to have base program close to program ratios. Boost compensation tables can be adjusted to achieve correct ratios as well.

Since they are in the top of the blower and not in the intake ports, you can just change out 4 of the 60's for 4-80's. That way you can have a little more injector, not have to make a huge change in pulse width, and save some money. Just spread them out.
The injectors on top of the roots are not going to support as much power as they will in the rear of a Whipple, so you would probably be better off with some larger injectors if you are looking to make over 900 hp. with the roots. I shouldn't have assumed it was a Whipple when you said SCI. My bad for assuming.
Eddie

Thanks, that makes sense

who's top plate are you running?