When to go to 24#'s

89 droptop · Aug 3, 2004

I read in a magazine that 19# injectors are good for 400 horspower. But I see eveyone going to 24#'s that are not making that much power. I got all the stuff in my sig and plan on getting the BBK intake manifold later and of coures like a 75mm TB, and I know I wouln'd have near 400 ponies. Will I need to upgade to the 24#? Thanks for your help.

super302 · Aug 3, 2004

I've heard you don't need to get bigger unless you're over 300rwhp...if you do upgrade make sure you get the mass air sensor recalibrated for them.

65ShelbyClone · Aug 3, 2004

89 droptop said:
I read in a magazine that 19# injectors are good for 400 horspower.QUOTE]

That is a gross overestimate. They might shoot 400hp worth of fuel at 100psi and 100% duty cycle, but that is far beyond what they were designed to do. Factory Ford injectors are rated at either 38 or 40psi with a max duty cycle of 80%(open 80% of the time, closed 20%).

If you ont mind ramping up the pressure, then 19s might top out at 300fwhp. I prefer to run the factory pressure or as close to it as possible, so I got 24s. I still had to raise the pressure up around 45 to keep the engine from missing, which it still does slightly. Maybe 30s are in order....

89 droptop · Aug 4, 2004

Ok, sounds good thanks for the info, SO i can recalibrate the mass air, the car comes with, or do I have to buy an after market piece calibrated for the 24's at the same time?

5spd GT · Aug 4, 2004

Buy injectors and MAF at the same time...

Here is how I look at it...87-93 5.0 got 19lb injectors with about 225 horsepower...the 93 Cobra got 24lb injectors (from the factory) with about 260-270 horsepower at the motor...so judge accordingly...

I would feel sorry for the 19lb injectors (actually the car for that matter) that was running with 400hp with those injectors...probably wouldn't run for long

MadMatt5oh · Aug 4, 2004

So by that system would it be a good idea to go with 30# for a built 306 with afr165s, systemax intake and an FTI cam set for the most?

drgn5.0 · Aug 4, 2004

no, i would go with the 24lb's. 30lb will still run too rich.

stang2841 · Aug 4, 2004

isnt it, the injectors will only pump as much as needed?

or is what this guy above me said is true?

_jb_ · Aug 4, 2004

stang2841 said:
isnt it, the injectors will only pump as much as needed?

or is what this guy above me said is true?

The injectors will overload the chamber, resulting in backfires and such. The computer manages the injectors and it will pump in what it believes is 19# when in fact the injectors pump out 24#, 30#, ect. depending on what you are running. Or so I understand it lol.

Michael Yount · Aug 4, 2004

Go to Pro-M's website - www.pro-flow.com, and select the meter sizing tab. Scroll down and you'll see Pro-M's recommendations for how much HP various injectors will support at stock pressure, and how to calculate the increase in HP supportable by increasing fuel pressures.

5spd GT · Aug 4, 2004

drgn5.0 said:
no, i would go with the 24lb's. 30lb will still run too rich.

I'm going with 30lb injectors...302/AFR 165's, FTI custom cam, Performer intake...I would rather have to much (rich) than to little (lean)...but then again...I'm not against the 24lb injectors...if only FRPP made 27lb injectors

remat457 · Aug 4, 2004

5spd GT said:
I'm going with 30lb injectors...302/AFR 165's, FTI custom cam, Performer intake...I would rather have to much (rich) than to little (lean)...but then again...I'm not against the 24lb injectors...if only FRPP made 27lb injectors

if you get an afpr with a bigger fuel pump you can tune it to your requirements

even though you have decided on bigger injectors i think that 19lb'ers with an afpr+fuelpump would be fine....however, i would go with the 24's....

just my 2cents...

FoxChasis · Aug 5, 2004

Michael Yount said:
Go to Pro-M's website - www.pro-flow.com, and select the meter sizing tab. Scroll down and you'll see Pro-M's recommendations for how much HP various injectors will support at stock pressure, and how to calculate the increase in HP supportable by increasing fuel pressures.

Their Suggested Injector Sizing At Stock Fuel Pressure (8cyl.) table puts injector duty cycle at 100% and assumes BSFC is on the low (lean) side of average. (Notice the formula in the text beneath the table.)

#1 Injector duty cycle should NEVER be at 100%, save for a VERY short periods of time at WOT throttle.
#2 (A wrong) assumption is the mother of all *****ups. Unless BSFC has been measured, assume high (rich).

Maximum HP = Injector Size * # of Cylinders * Injector Duty Cycle / Brake Specific Fuel Consumption

T_5.0_N'works · Aug 5, 2004

I was doing a HP calculator and it woudl tell you your injector duty by determing your power through ET and MPH and weight.

Well I was rated at having 259rwhp. The Injectors were at 18.***lbs. That's almost maxed out. Take that for all it's worth. When the 351w goes in on boost, here come the 42lber's.

Michael Yount · Aug 5, 2004

A lot of the choices here that make the most sense depend of application. For racers only, err on the side of a low duty cycle - larger injector. For street cars - as a practical matter - most spend very little time at w.o.t. I prefer to err on the side of the least injector I can put in that will support the HP requirements at a relatively high duty cycle - 90-95%. As with cams and carbs, there are many over-injected vehicles out there.

remat457 · Aug 5, 2004

Michael Yount said:
A lot of the choices here that make the most sense depend of application. For racers only, err on the side of a low duty cycle - larger injector. For street cars - as a practical matter - most spend very little time at w.o.t. I prefer to err on the side of the least injector I can put in that will support the HP requirements at a relatively high duty cycle - 90-95%. As with cams and carbs, there are many over-injected vehicles out there.

Well said!

When to go to 24#'s

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