So fuel injectors?

89LXakaSally · Apr 12, 2012

Im going to run a GT40 intake upper and lower. 80mm Cobra MAF, K&N intake. and a cam not sure what size... so 24lb injectors and 255 pump or?
I Bought the car on my R&R from afghanstain. so i cant do the physical work untill october when i get back to the states

jrichker · Apr 12, 2012

The stock 19 LB injectors will do fine with your combo.

Fuel injector sizing & injector photos

Revised 11-Dec-2011 to add larger injector sizes to 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
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/ 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

89LXakaSally · Apr 12, 2012

well its just startin alsom i plan on doin alot more after i get out on afghanstain

arco777 · Apr 12, 2012

255 pump is too much for just bolt ons. You'll burn it up if you run low on fuel too. Go for a 110 or 155lph until you get forced induction.

nine3cobra · Apr 12, 2012

255 worked fine on my car when it was n/a, its been in the car since '04. hasn't burnt up yet, and i haven't had to buy new fuel pumps everytime i changed my setup.

f8tlfiveo · Apr 12, 2012

arco777 said:
255 pump is too much for just bolt ons. You'll burn it up if you run low on fuel too. Go for a 110 or 155lph until you get forced induction.

I don't agree with this.. The fuel that does not get used gets returned to the tank.. Go with the 255 and don't look back..

jrichker · Apr 12, 2012

There is problem with using a 255 LPH pump if you live in the southern states or southwest. It get hot here during the summer - very hot. Certainly the excess fuel recirculates back to the tank, but it also picks up heat and starts to boil in the return lines and tank. Far too many people have yanked out the carbon canister plumbing in ignorance because they think it makes the car go faster. That's another ricer fable that just makes trouble. Without a properly working vent system, the boiling fuel evpaorates out of the tank, making a gasoline stink, and shooting down your gas mileage. At $4+ a gallon, it makes sense to save every drop of gas, be it liquid or vapor.

How the carbon canister affects the fuel vent system:

Charcoal canister plumbing - one 3/8" tube from the bottom of the upper manifold to the rubber hose. Rubber hose connects to one side of the canister solenoid valve. Other side of the solenoid valve connects to one side of the canister. The other side of the canister connects to a rubber hose that connects to a line that goes all the way back to the gas tank. There is an electrical connector coming from the passenger side injector harness near #1 injector that plugs into the canister solenoid valve. It's purpose is to vent the gas tank. The solenoid valve opens at cruse to provide some extra fuel. The canister is normally mounted on the passenger side frame rail near the smog pump pulley.

It does not weigh but a pound or so and helps richen up the cruse mixture. It draws no HP & keeps the car from smelling like gasoline in a closed garage. So with all these good things and no bad ones, why not hook it up & use it?

The purge valve solenoid connector is a dangling wire that is near the ECT sensor and oil filler on the passenger side rocker cover. The actual solenoid valve is down next to the carbon canister. There is about 12"-16" of wire that runs parallel to the canister vent hose that comes off the bottom side of the upper intake manifold. That hose connects one port of the solenoid valve; the other port connects to the carbon canister.

The purge valve solenoid should be available at your local auto parts store.

Purge valve solenoid:

The carbon canister is normally mounted on the passenger side frame rail near the smog pump pulley.
Carbon Canister:

89LXakaSally · Apr 12, 2012

i already took out the smog $hit so should i run a 155- with 24lb with the 80mm Maf from a cobra and the gt40 intake

FastDriver · Apr 13, 2012

Ok, a lot of bad information in this thread, bro. Sorry, tech here is usually pretty sound.

While what Jrichker says is accurate, I have never personally, nor with any of my friends, experienced boiling fuel or any problems from running a 255 LPH fuel pump. I've run the Walbro 255LPH pump (GSS307, and GSS340) in 3 different cars without issue - all in the south in FL and GA 100*+ temps. I have a friend who I've helped with his cars run them in 2 different mustangs. There are tens of thousands of modified fox-bodies running these pumps. Go with it, and you will not be sorry. Going with a 110 or 155 will only limit the power that you can make in the future. There will be no other noticeable change.

On injectors, again Jrichker's advice is spot on, and you would be ok with the 19s, but IMO they would be at the absolute limit of what you should run. Yes, you can turn up the fuel pressure to compensate for small injectors, but this puts added stress on the fuel pump and lowers the volume of fuel that it can move (http://mustangpartstech.com/WalbroFuelPumpFlow.html), and if too high, the pressure can cause the injectors to go static, unable to close. With a 255LPH pump, I wouldn't be too worried about bumping the fuel pressure up to something reasonable (I'd say about 60psi max). My recommendation here is to go one of two ways. Either stay with the 19lb-hr injectors initially until you really need to bump up to bigger injectors, or go with 42 lb-hr fuel injectors. They are absolutely overkill on your combo, but if you are going pay the money for a new set of injectors, and you don't know where you're modifications will stop, you might find them useful in the future. A custom tune or the ability to tune yourself is always a good idea regardless, but with a matching MAF, it wouldn't be absolutely required. Look at spending about $500 to pay for a tune on a dyno, or 300-500 for the tuner hardware/software of your choice. Do not buy a mail-order tune. Unless it's built for your specific car, it's a waste of money.

Noobz347 · Apr 13, 2012

FastDriver said:
Ok, a lot of bad information in this thread, bro. Sorry, tech here is usually pretty sound.

While what Jrichker says is accurate, I have never personally, nor with any of my friends, experienced boiling fuel or any problems from running a 255 LPH fuel pump. I've run the Walbro 255LPH pump (GSS307, and GSS340) in 3 different cars without issue - all in the south in FL and GA 100*+ temps. I have a friend who I've helped with his cars run them in 2 different mustangs. There are tens of thousands of modified fox-bodies running these pumps. Go with it, and you will not be sorry. Going with a 110 or 155 will only limit the power that you can make in the future. There will be no other noticeable change.

On injectors, again Jrichker's advice is spot on, and you would be ok with the 19s, but IMO they would be at the absolute limit of what you should run. Yes, you can turn up the fuel pressure to compensate for small injectors, but this puts added stress on the fuel pump and lowers the volume of fuel that it can move (http://mustangpartstech.com/WalbroFuelPumpFlow.html), and if too high, the pressure can cause the injectors to go static, unable to close. With a 255LPH pump, I wouldn't be too worried about bumping the fuel pressure up to something reasonable (I'd say about 60psi max). My recommendation here is to go one of two ways. Either stay with the 19lb-hr injectors initially until you really need to bump up to bigger injectors, or go with 42 lb-hr fuel injectors. They are absolutely overkill on your combo, but if you are going pay the money for a new set of injectors, and you don't know where you're modifications will stop, you might find them useful in the future. A custom tune or the ability to tune yourself is always a good idea regardless, but with a matching MAF, it wouldn't be absolutely required. Look at spending about $500 to pay for a tune on a dyno, or 300-500 for the tuner hardware/software of your choice. Do not buy a mail-order tune. Unless it's built for your specific car, it's a waste of money.

I'll put some of my own spin on this reply and follow the, "absolutely overkill" line mentioned above. Stick with the 19s for now. There's now way on god's green earth that the intake and maf will be too much for those 19s. Map out your combo and THEN but the injectors that will suit your total plan. Upgrading to teh 42s now and coupling that with a dyno-tune would be a pretty significant waste of $$. Fastdriver treaded this already but I'll come right out and say... Put that money into an AFR meter ($300ish) and tuner ($300 - $1000)! These are two items that will accomodate nearly ANY mod you can throw at your motor, down the road.

What Driver says about the 255 is spot on. Buy it now and forget it. If you even THINK you might be adding boost, ensure that you buy the high pressure variation.

Last note on 19lb injectors: I've personally run a set of OEM 19s to 90 psi through an FMU with no issues. The point of saying that is because those injectors are more than capable to carry you through current and probably next to stages of modifications. Get your plan on paper then buy injectors once!

One last thing (while I'm doing the talkin parrot gig):

Do not buy a mail-order tune.

89LXakaSally · Apr 13, 2012

ok thanks guy much help

FastDriver · Apr 13, 2012

Noobz... just as a point of clarification, you are not running a differential of 90 psi across the injector. If you were running 20 psig boost, as an example, that would be a pressure drop of 70psi across the injector. The injector would be under the same conditions as an n/a car at 70psi of fuel pressure.... Still pretty insane, but not as insane as 90 psi.

To me, it's insane to run that kind of pressure anyway, because if you've got the money for the Weldon/aeromotive pump, or the T-rex+walbro+KB-boost-a-pump, why didn't you have the money to buy bigger injectors? If you were running a 255 walbro to that kind of pressure, it's not going to move enough volume.

Noobz347 · Apr 13, 2012

FastDriver said:
Noobz... just as a point of clarification, you are not running a differential of 90 psi across the injector. If you were running 20 psig boost, as an example, that would be a pressure drop of 70psi across the injector. The injector would be under the same conditions as an n/a car at 70psi of fuel pressure.... Still pretty insane, but not as insane as 90 psi.

To me, it's insane to run that kind of pressure anyway, because if you've got the money for the Weldon/aeromotive pump, or the T-rex+walbro+KB-boost-a-pump, why didn't you have the money to buy bigger injectors? If you were running a 255 walbro to that kind of pressure, it's not going to move enough volume.

At that point and time, it was something that was in the works. I didn't run it that way for long. I was in the midst of building a new motor and had put the blower ontop of the stocker long block. It was a hold-over measure until engine completion and subsequent tune along with a SD to maf conversion.

At any rate, the point was only to indicate that his 19s have MORE than enough headroom to deal with the intended intake swap and even a head swap and cam (if the need arose).

I put 90 psi on that fuel pressure pressure gauge everytime that that FMU closed KNOWING that the situation was slightly less than ideal. :rlaugh:

FastDriver · Apr 13, 2012

haha... ok cool

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