H/C/I injector size??

stangjunkie · Apr 13, 2007

OK well combo is in sig im just curious what injectors to run?? im on 19's and i think they are hurting me (well the stock maf is hurting me but if im gonna upgrade i might as well do injectors) so 24's or 30's?? I'm just gonna ballpark and say i have 350wheel hp. im gonna do a little summer work so i should be there anyway. i found this

19 lb injectors/.5 = 38 - 38 x 8 = 304 HP
24 lb injectors/.5 = 48 - 48 x 8 = 384 HP
30 lb injectors/.5 = 60 - 60 x 8 = 480 HP
36 lb injectors/.5 = 72 - 72 x 8 = 576 HP
42 lb injectors/.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.

304 HP x .85 = 258 HP for 19lb injectors
385 HP x .85 = 326 HP for 24lb injectors
480 HP x .85 = 408 HP for 30lb injectors
576 HP x .85 = 490 HP for 36lb injectors
672 HP x .85 = 571 HP for 42lb injectors

But i don't understand preferred duty cycle?? also is this engine HP or Wheel HP??

so again 24's or 30's??

thanks everyone in advance

Jason 302 · Apr 13, 2007

I vote 24's, they should do just dandy. It is a 302/306 right? If you plan on stroking, spraying, or boosting, go bigger.

I have 290rwhp (about 340-350 crank horsepower) and my 24's keep up just fine, I had to back off my fuel pressure to lean it out a bit, and I get 23mpg on the highway.

stangjunkie · Apr 13, 2007

i do plan to spray it in the future but it will be a wet kit

jrichker · Apr 13, 2007

I'm guy that posted the figures, so 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.

TN89LX5.0 · Apr 13, 2007

I suggest getting this book and reading through it. I have learned a few things from it.

Mustang Performance Handbook by William R. Mathis.

There is a formula to figure proper injector and fuel pump size.
To save space and keep from having to give a reference I will tell you what they figure up to be.

A 350 hp motor will need bare minimum 22 lbs-hr. The book suggests only 80% flow max of the injector so that means a 28 lbs-hr injector would be safest. 24's would work but the book says basically if you work the injector close to its max that you will have to replace them more often.

As for the fuel pump, it recomends going to a 155 lph pump for 350 hp motors.

This guy has been racing and building mustangs for a long time and designs and tests many components for the fox mustang. He has his own company High-G performance that deals with Ford performance engines and handling...Just FYI.

Anyone else read or have this. I thought it was pretty good reading!

Good luck with your upgrade there Stangjunkie.

5spd GT · Apr 13, 2007

Yeah Jrichker got you covered

Those are engine horsepower numbers (bhp).

I would go with 24lb injectors, I went with 30's and I wish I didn't. Rich idle, rich fuel curve - causes some bucking at low rpms. I don't think it is affected gas mileage at all though. It gets pretty good mpg untuned...

Sicarius428 · Apr 13, 2007

I vote 24 lbs for NA but if you are going N02 I suggest 30s. I would also suggest a 190 fuel pump but a lot of people like the 255 for future mods and just do it once.
Kevin

98BLKGT · Apr 15, 2007

5spd GT said:
Yeah Jrichker got you covered

Those are engine horsepower numbers (bhp).

I would go with 24lb injectors, I went with 30's and I wish I didn't. Rich idle, rich fuel curve - causes some bucking at low rpms. I don't think it is affected gas mileage at all though. It gets pretty good mpg untuned...

+1
I am in the process of down grading from 30's to 24's. Rich all around at 32psi fuel pressure. Bad idle.

stangjunkie · Apr 18, 2007

thanks alot guys i've been planning on 24's but just wanted to be sure. Im sure i may be starving the car quite a biit now so hopefully the track times will drop a bit now too.

mustanggt5pt0 · Apr 18, 2007

I too just did H/C/I and most certainly do not want to run lean, so I'm figuring I'll have to get 24's. Fuel pump is a must if upgraded to 24's?? If so what size fuel pump. I saw three different numbers, 155, 190, and 255.

5spd GT · Apr 18, 2007

With h/c/i either one of those will work. I chose the 190lph. I drove with the stock fuel pump for a couple weeks of 95lph and didn't notice anything. I wouldn't do that though, be safe and upgrade.

The larger the fuel pump the more 'draw' of amperage it uses.

mustanggt5pt0 · Apr 18, 2007

And the larger draw means a larger alternator. Great. This rebuild keeps getting more and more expensive.

Jason 302 · Apr 18, 2007

stangjunkie said:
thanks alot guys i've been planning on 24's but just wanted to be sure. Im sure i may be starving the car quite a biit now so hopefully the track times will drop a bit now too.

When I went from from 19's and a stock MAF to 24's and a 75mm MAF I saw a gain of 2 tenths and 1 or 2 mph. This was on a modified engine and mostly credited to the extra flow of the MAF.

cobradvm · Apr 18, 2007

98BLKGT said:
+1
I am in the process of down grading from 30's to 24's. Rich all around at 32psi fuel pressure. Bad idle.

How well is the car tuned? Do you have any datalogging on it with a Tweecer or a PMS? Or are you going off a dyno AF. Your AF ratio is determined by what your computer decides is the right amount of fuel to inject based on various inputs like ACT, mass air voltages, ECT, and TPS. Once you are in closed loop, the computer wil alter the injector pulses to give it an AF that it thinks is right. If it is too rich and you drop to a smaller injector, the computer will still want the same AF, and will compensate by holding the injector open longer. I'm not sure that simply changing to a smaller injector will solve your rich problem. In open loop at WOT, the computer will still use tables that are adapted based on the info it was getting in closed loop.
At lower rpms, the larger injectors will have a harder time giving accurate fuel pulses since their pulses may have to be really short and hence they become less accurate. That might explain an improvement in general driveability if you switch to a smaller injector.

I am no expert, but I have been doing some reading on this lately, and those were just a few thoughts I had.

98BLKGT · Apr 19, 2007

cobradvm said:
...At lower rpms, the larger injectors will have a harder time giving accurate fuel pulses since their pulses may have to be really short and hence they become less accurate. That might explain an improvement in general driveability if you switch to a smaller injector.

I am no expert, but I have been doing some reading on this lately, and those were just a few thoughts I had.

Thats the only thing I was going by... still working on fixing my idle but the injector swap was free. Plus I had a bad injector seal so I had to pull them anyway.

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