injector sizing

[DukeGnarley]

I'm just finishing up a new engine for my '65. My original plan was to put on a supercharger, but for budgetary reasons I'm going to have to hold off until the car is back together and running.

For now, i was hoping to find some junk yard injectors to get me by while I'm naturally aspirated.

The engine combo is:
stock 5.0 bottom end
trick flow 170 heads
trick flow stage 1 cam
'93 cobra intake (tmoss ported lower).
70mm TB

Will I be ok with junk yard 24lb/hr injectors? Or should I just empty my wallet and go buy some new 42lb/hr injectors for when i eventually get a supercharger?

changing the MAF is a non-issue since I'll be running megasquirt.

 

[jrichker]

Fuel injector sizing & injector photos

Revised 07-May-2011 to add flywheel HP statement & improve injector table

Injector HP ratings: divide flow rating by.5 and multiply the result by the number of injectors. This uses a 100% duty cycle. These ratings are for naturally aspirated engines at the flywheel.

Example:
19/.5 = 38, 38 x 8 = 304 HP
24/.5 = 48, 48 x 8 = 384 HP
30/.5 = 60, 60 x 8 = 480 HP

The preferred duty cycle is about 85% maximum, so for a safety factor multiply the final figure times .85.

19/.5 = 38, 38 x 8 = 304 HP x .85 = 258 HP
24/.5 = 48, 48 x 8 = 384 HP x .85 = 326 HP
30/.5 = 60, 60 x 8 = 480 HP x .85 = 408 HP

Remember that the above ratings are at 39 PSI. Increasing the pressure will effectively increase the flow rating. Example: a 19 lb injector will flow 24 lbs at 63 PSI, and a 24 lb injector will flow 30 lbs at 63 PSI.

See Automotive Performance Software / Interactive Calculators to get the calculators used in these examples.

Here's the duty cycle explanation. Duty cycle is how much of the time the intake is open the injectors are turned on. The 85% figure means that for 85% of the time the intake valve is open, the injectors are spraying. The idea is that you want some percentage of the duty cycle left over so that you have some room to grow the process.

If you are at 100% and you need more fuel, all you can do is turn up the fuel pressure. That means the whole fuel curve from idle to WOT is affected. Maybe you are already too rich at idle, and turning up the fuel pressure makes it worse. If you had some injector duty cycle left to play with, a custom tune could use that where it is needed. That would not over richen the whole range from idle to WOT.

Don't forget to increase the fuel pump size with you increase injector size or significantly increase the fuel pressure



Diagram courtesy of Tmoss & Stang&2birds

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

 

[2000xp8]

24's are the right injector.
There guys out there that think you can "tune" the car to the wrong injector, but it's a half assed method, it's nearly impossible to get the fuel correct when you use injectors that are nearly double the size of what's needed.

 

[DukeGnarley]

Trick flow puts their top end kit at 360 hp, and I'm basically running the same thing with a different intake.

Are the numbers jrichker posted flywheel hp or rwhp? If it's flywheel hp then would the 24s be on the edge of being too small?

 

[2000xp8]

The trick flow kit done right should produce just about 300rwhp, some maybe a few hp higher and some a few hp lower.

Not sure if that info is rwhp or flywheel, but it doesn't matter, the stuff you see on paper doesn't always translate the same in practice.
I assure you, 24's are the right injector for you.

 

[Rick 91GT]

I agree id stick a 24# injector in that combo as well