SN95 What Size Injectors Should I Run

Karri94gt

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Mar 25, 2017
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What size injectors should I run. 306 stock ho cam, cantfield 192cc heads,track heat intake,1 3/4 " headers, 75mm TB,76mm maf, msd ignition, 150 shot plate kit, 255lph boost pump
 
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What size injectors should I run. 306 stock ho cam, cantfield 192cc heads,track heat intake,1 3/4 " headers, 75mm TB,76mm maf, msd ignition, 150 shot plate kit, 255lph boost pump
 

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Well the nitrous system should supply its own fuel so you just need to supply for the motor. I would think 24's should work. You're combo is probably around 260 rwhp n/a supply needs to match that area. If you want to make sure go up to 30's(overkill but can be tuned to work).
 
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as always solid advice from Todd.

To the OP, hope you dont mind me asking but can you tell me a little more why/who chose the combination? Not to tear it apart, but it appears to be a lot of mismatched parts- stock cam, 75mm TB, 1 3/4 headers... track heat intake...

You're giving up a lot of low end imo.
 
Fuel injector sizing & injector photos

Revised 26-Dec-2014 to add statement about figures are for flywheel HP and not rear wheel HP

Injector HP ratings: this flywheel HP, not rear wheel HP.
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
36/.5 = 72, 72 x 8 = 576 HP
42/.5 = 84, 84 x 8 = 672 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
36/.5 = 72, 72 x 8 = 576 HP x .85 = 490 HP
42/.5 = 84, 84 x 8 = 672 HP x .85 = 571 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 http://users.erols.com/srweiss/calcpchg.htm 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.

If you did turn up the fuel pressure, you might be able to change the injector duty cycle to get the air/fuel mixture ratio you want since the injectors will have extra fuel delivery capability.

With larger than stock injectors or higher that stock fuel pressure, you will need an aftermarket MAF that matches the injector size. The MAF “lies” to the computer to get a fuel delivery schedule that meets the engine’s needs and isn’t too rich or too lean. The best strategy is an aftermarket MAF and a custom tune to insure the best air/fuel ratio over all the RPM range.

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



Diagram courtesy of Tmoss & Stang&2birds
Ford_Injector_Guide.jpg


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ 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
 
as always solid advice from Todd.

To the OP, hope you dont mind me asking but can you tell me a little more why/who chose the combination? Not to tear it apart, but it appears to be a lot of mismatched parts- stock cam, 75mm TB, 1 3/4 headers... track heat intake...

You're giving up a lot of low end imo.
 
94 GT auto 4:11 gears, 306 flat tops,192cc heads ,60cc chamber,2.02-1.60 valves stock .444 lift 260° cam,TFS track heat intake with plate kit, full Mac exhaust 1 3/4 headers to 2.5" pro chamber to tail pipe, bbk 75 mm TB bbk 76mm maf ,295/55r15 drag radials car is weekends and track car. Would like 300-350 rwhp n/a
 
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94 GT auto 4:11 gears, 306 flat tops,192cc heads ,60cc chamber,2.02-1.60 valves stock .444 lift 260° cam,TFS track heat intake with plate kit, full Mac exhaust 1 3/4 headers to 2.5" pro chamber to tail pipe, bbk 75 mm TB bbk 76mm maf car is weekends and track car. Would like 300-350 rwhp n/a
You need more cam to get to and over 300rwhp(you need to be able move more air). If you want to wait on a camshaft change,just take it to the track and get a baseline. Suspension,chassis,and tires are where you need to concentrate. With you running nitrous I don't know why you shouldn't be able to run that number as is. Depending on how much you are spraying you could see around 450whp.
 
You need more cam to get to and over 300rwhp(you need to be able move more air). If you want to wait on a camshaft change,just take it to the track and get a baseline. Suspension,chassis,and tires are where you need to concentrate. With you running nitrous I don't know why you shouldn't be able to run that number as is. Depending on how much you are spraying you could see around 450whp.
You need more cam to get to and over 300rwhp(you need to be able move more air). If you want to wait on a camshaft change,just take it to the track and get a baseline. Suspension,chassis,and tires are where you need to concentrate. With you running nitrous I don't know why you shouldn't be able to run that number as is. Depending on how much you are spraying you could see around 450whp.
Thanks for the input was what I was looking for
 
My rule of thumb is based on what I've seen from OEMs. The stock mustangs made about 190-200 rwhp, and came with 19lbs/hr. The Cobra came with 235 rwhp and came with 24 lbs/hr injectors. The LS1s came with 29 lbs/hr injectors. I see a trend of ~1lbs/hr injectors for every 10 rwhp. It's just a rule of thumb, but it will work for you. 24s to 30s in your case.