What injectors do I need for my build?

[JasinC19]

Hey guys!

I'm trying to determine what size injectors I need for my build.

This is my first build and it's been really fun so far. I want to have all my parts on hand by the time the block is done at the machine shop.

I plan on installing the Trick Flow top end kit. Others power mods so far will simply be I/H/E.

My goals are modest. Basically whatever extra power the top end kit provides. This is my DD and probably won't ever see the track.

What size injectors do I need?

I have a new set of stock 19# injectors, but I'm thinking they won't quite be enough. Would a set of 24# be sufficient?

How much will these upgrades affect fuel economy?

Thanks for the help,
Jason

 

[VibrantRedGT]

The 24's should be fine. Also have an adjustable fuel regulator so you can adjust pressure, 255 or 340 fuel pump and calibrated MAF for the 24's. I tried to get away with 19's after Intake, Heads and Cam, no dice. Some have had no problems with all that and 19's.

Fuel economy will be based on how hard you use your right foot. I know fuel is expensive but you don't get a Mustang for MPG's, you get one for MPH.

 

[JasinC19]

Makes sense. Thanks a lot for the advice.

 

[NIKwoaC]

There is this really neat thing called Google...

Let me google that for you

First hit:
RC Fuel Injection

 

[JasinC19]

Maybe you should google how to not be a ****.

If you don't like my post, go somewhere else.

 

[NIKwoaC]

Maybe you should google how to not be a ****.

If you don't like my post, go somewhere else.

Do you want my help or not? I just provided you with a resource to help you make an educated decision on sizing your injectors.

We get the "what size injectors do I need" question (along with some others) very, very, very frequently. I would be amiss if I didn't razz you for it a little.

Welcome to StangNet.

 

[NIKwoaC]

FYI.

http://www.stangnet.com/mustang-forums/825480-fox-talks-really-common-questions-thread.html

It's a work-in-progress, but you should check it out if you haven't already.

 

[jrichker]

Fuel injector sizing & injector photos

Revised 20-May-2011 to add MAF and custom tune requirements for use with larger injectors

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.

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

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

 

[tannerc91gt]

Do you want my help or not? I just provided you with a resource to help you make an educated decision on sizing your injectors.

We get the "what size injectors do I need" question (along with some others) very, very, very frequently. I would be amiss if I didn't razz you for it a little.

Welcome to StangNet.

let me google that for you rubs people the wrong way 99% of the time . surprising how little people use it, i google EVERYTHING. Generally because when i look at some of my first threads on stangnet, it was all the most common questions. Tech stickies need to be 72 font or something

 

[Noobz347]

let me google that for you rubs people the wrong way 99% of the time .

Go figure.

Oh.. and what's better? An H-pipe or and X-pipe?

 

[NIKwoaC]

let me google that for you rubs people the wrong way 99% of the time . surprising how little people use it, i google EVERYTHING. Generally because when i look at some of my first threads on stangnet, it was all the most common questions.

There is a ton of crap that goes on with my car, but members of this forum would never know. Google and the forum search tools know, though.

Tech stickies need to be 72 font or something

Try as we might, nobody reads the stickies.

 

[JasinC19]

Yeah I appreciate the help. I just don't like being made to look like a moron.

I had originally said in the post that this was my first build and I want to make sure I know that I'm doing the right things, so forgive me if I asked a question that was answered in a roundabout way somewhere else buried in the site.

Now let me ask for more help!

I'm actually still not 100% sure that the 24#ers are the right choice. The TF kit claims 350hp (crank?) and JRichker's post states that 24# injectors at 85% duty cycle are good for 326hp at the crank.

This sounds to me like I might want 30# injectors, but I've seen other people use similar builds with 24# injectors.

So to me, there are a lot of unknowns. I don't want to spend a ton of cash and time on this project to let injector size be a huge problem.

Is there a definitive answer?

Thanks,
Jason

 

[NIKwoaC]

These calculators and formulas are often on the conservative side. You can probably get away with 24s on your build, a lot of people with similar stuff do. Bumping the fuel pressure with an adjustable fuel pressure regulator would help if you find that the 24s are stretched too thin.

30s are harder to find and more expensive than 24s, but would just add a little more wiggle room and potential for a little growth. FWIW, my 302 has 30s on it, but I have a completely different top end than you.

 

[JasinC19]

I see.

So as an example, if I were to put 42# injectors on my car, would I be able to use an adjustable FPR to supply just the right amount of fuel to my injectors? And not run rich?

Or does it not work like that?

(I know 42# is overkill)

 

[VibrantRedGT]

I see.

So as an example, if I were to put 42# injectors on my car, would I be able to use an adjustable FPR to supply just the right amount of fuel to my injectors? And not run rich?

Or does it not work like that?

(I know 42# is overkill)

It doesn't work like that. The 42's would dump so much fuel on your combo you would be shooting streams of fuel out the tailpipes.

For the top end kit 24's would be perfect. Should put you close to 300HP at the wheels with other supporting mods (70MM Throttle body, 77MM Mass air, full exhaust, etc.). I had 30lb injectors on my 347 with AFR 185's, SystemaxII Intake, Custom Cam and they were perfect for my combo. I ran 42's on my Supercharged 302 that made 455HP. Before the 347 I did what you are about to do and tried 19's first, ran really lean, switched to 24's and was fine. Car made 292HP to the wheels (Edelbrock Peformer Intake, GT40X Aluminum heads with a little work, Crower 15511 Cam with 1.7 rockers, full exhaust, pulley's, timing).

 

[JasinC19]

I gotcha. I'll go with the 24's then and keep my fingers crossed.

Thanks for the advice guys.

 

[NIKwoaC]

I see.

So as an example, if I were to put 42# injectors on my car, would I be able to use an adjustable FPR to supply just the right amount of fuel to my injectors? And not run rich?

Or does it not work like that?

(I know 42# is overkill)

Eh, it's kind of one of those things where you can only go so far with it until you get out of the injector's ideal operating range. Consider that changing fuel pressure not only changes fuel flow, but it also alters the spray pattern of the injector.

Also, without tuning, extreme changes to fuel pressure can really throw off operation at non-WOT conditions. Example: if you have to raise fuel pressure a lot to get the engine at a happy A/F while at WOT, you're likely to have over-rich conditions at part-throttle.

Ideally, you just shoot for an injector that will be close, with maybe a liiiittle on the "big" side. Trying to get an overly huge or tiny injector to work will only lead to headaches.

 

[tannerc91gt]

Go figure.

Oh.. and what's better? An H-pipe or and X-pipe?

This might help