MSefi.com read only forum (Jan.15,2018)

Engine is a 468”/7.4L big block Chevy making about 580 HP at 6,500 rpm. I am almost ready to order my MS-II but need some help with the fuel system so I can scope the full project.

Injectors: on line calculators suggest I need about a 46# injector at 44 psi. I want to use GM/OEM parts is possible because of the quality and price. I have read on this board that too large an injector can cause poor idle control, poor cold start and overall tuning difficulty. What is the point when an injector is too big and causes this? I can get Z06 Corvette injectors that are rated at 42# at 58 psi or ZR1 injectors rated at 52# at 58 psi. The 42 look too small but the 52 look too big. I plan on running about 45 psi so this may make the 52# act more like a 46# and I would be fine. Can someone give me a little guidance before I spend $400?

Fuel system: I plan to use the stock 3/8 feed line and 5/16 vapor line as the return line. At 7 psi this has fed this engine w/o issues. Can I use the Ford inline pump from the F150 from 89-90 era or this too small? If too small are there any inline alternatives I can get from a donor vehicle? I want to avoid cutting into the tank at all cost. The Walbro 255 lph looks about right and is reasonably priced - opinions? Are there any regulators I can get from a donor vehicle or is the Aeromotive the only good choice? I have an old Cartech unit that was never used

Thanks for your help sorting out the newbie questions. I have had enough of this Holley pouring fuel in at one rpm and leaning out at another!

pcf_mark,

MS-II is much better at idle pulse width control than MS-I (it has 100x the pulse width resolution).

For MS-I idle pulse widths, see this: http://www.megamanual.com/v22manual/mtune.htm#idlepw

For MS-II, your 52 pph injectors should be okay. I have run 76 pph injectors on my 406 chevy (~485HP) without problems. It is better to go a little bigger on the injectors than risk being too small (and being lean at full throttle, possibly damaging the engine).

If this what mine, I would use the 255 lph pump (and in fact that is what I have on my car).

Lance.

Thanks Lance that nails it. Walbro pump, Aeromtive regulator, ZR1 52# injectors.

One last question - I have a set of 24# 90's era V8 injectors. I could get a second set for almost nothing. Would it be worth it to run 16 of these or too much hassle for wiring and fuel rails?