Fuel Injector Upgrade - Ford Muscle Forums : Ford Muscle Cars Tech Forum
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post #1 of 5 (permalink) Old 04-06-2008, 11:07 PM Thread Starter
Join Date: Dec 2001
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Fuel Injector Upgrade

I thought I'd post some pictures of my little fuel injector replacement project this weekend on Project 67. I had been running stock Ford 30-lbs/hr "red top" injectors since I converted the car to EFI. I had sourced the injectors used (e.g. cheap) and had no idea about their previous usage. Injectors are sort of mystery that way anyway. Their history, in terms of mileage or the type of motor they were on, doesn't reveal much about their condition - they either work or they don't, or so I thought. In actuality there is a middle ground, which is that they work poorly due to contamination (dirt, small particles) inside the passage way and needle valve. Since you cannot really diagnose this, you go nuts trying to figure out why the motor has intermittent rough idle or poor air-fuel ratios.

A cut-away of a typical fuel injector reveals an assembly that is prone to damage from dirt and particles in the gasoline - just like a needle and seat assembly in a carburetor. (image source: carbibles.com)

After unsuccessfully trying to clean up a rough idle on my 331cid EFI motor, I broke down and decided it was time to get a set of new injectors. No more rolling the dice with second hand units. A new set of Ford Racing (made by Bosch) red-tops run about $300. However I came across a new injector type from ACCEL. They are new units, not remanufactured or modified OEM, and apparently have a different fuel metering design then found in conventional injectors. They claim this results in significantly better fuel atomization as well as less variance between injectors. Perhaps the biggest enticement was that they were priced about $20 less than the Ford units. I went with the part number 150830 (30-lbs/hr), which is said to support 400-450 crank horsepower. These are high-impedence (14 ohms resistance), with catalog ratings based on 43.5 psi fuel pressure. They come in incremental sizes from 14 up to 62 lbs/hr. Check the ACCEL catalog for part numbers.

Shown are ACCEL 30 lbs/hr (PN 150830) These injectors drop right in place of Ford (Bosch) injectors, but have an improved internal design and better atomization.

A look at the fuel spray tip of the ACCEL injector shows a marked difference compared to the OEM design. Gone is the plastic "pintle cap" that is prone to cracking and clogging.

Swapping out injectors is not a big deal. Aside from the fuel spillage it is an easy task that took only 20 minutes. I will caution however, on a stock 5.0L/5.8L EFI motor with the upper and lower manifolds, the task will take much longer since getting to the injectors on the passenger side require removal of the upper manifold.

Don't forget to put a little petroleum jelly on both o-rings of each injector. This will make for easy install and removal from the fuel rails, and prevent o-ring tear or slip-off.

On my '67 the injector swap took under 30 minutes. I de-pressurized the fuel system, then pulled the fuel rails up, removed the old injectors, sopped up the fuel spillage and install the new injectors. It will take a bit more time if you have a late-model style EFI upper and lower intake manifold.

The biggest reason I wanted to share this tech piece is that these injectors actually do make a noticeable difference in throttle response. The engine is much crisper and reactive to quick jabs of the throttle. Incidentally, my wide-band air fuel meter is now showing very steady idle A/F ratios, whereas before they'd bounce with the cam lope between 12.5 and 14.0:1. I am now seeing almost rock steady 13.8-14.0 on both banks (I'm using a FAST dual-bank wide band.) I haven't had the car to the track or dyno in a while, so I can't comment just yet on whether there are actual measurable performance increases.

A little info on how to select injector size.
The formula for injector size is derived from the amount of fuel the engine requires in pounds per hour (a gallon of fuel weighing about 6 pounds.)

Fuel requirement = Crank Horsepower x Brake Specific Fuel Consumption

Both the crank horsepower and BSFC need to be calculated on a dyno. Getting the horsepower value is not too difficult, you can even chassis dyno your car and then take the peak horsepower at the wheels and multiply by a drivetrain loss factor to get a pretty accurate peak crank horsespower. Brake Specific Fuel Consumption (BSFC) is much harder to calculate. Engine dyno software can usually calculate this value, which represents the amount of fuel consumed per unit of power produced. Fortunately there are fairly good rules of thumb that can be used based on the engine compression and induction type. For naturally aspirated gasoline engines between 8 and 10:1 compression the BSFC will run between .50 and .60. Most tuners simply use .55. Forced induction engines can use a multiplier of .55 to .65, while really low compression engines have a BSFC between .45 to .55.

So, if our engine is estimated to have 400 horsepower, we multiply by .55 which gives us 220 lbs/hour of fuel consumption. Now to figure out what size injector we need. We need to divide the total fuel consumption by the number of injectors our motor uses, in this case eight for a V8 engine. However, we also need to factor in the "duty cycle", which is the percent of time the injector will remain open. This is important because electromechanical devices, like a fuel injector, cannot operate at 100% for long period of time - they would overheat and fail quickly. We therefore want to build in a buffer, which is generally a duty-cyle of 85%. So we will factor that into the requirements for each injector as follows:

Fuel Injector Size = total fuel requirment (lbs/hr) / (# of cylinders)x(0.85)

So for our 425 horsepower motor which needs 220 lbs/hr total fuel, we divide that by 6.8 (8 cylinders x 85%). The result is 32 lbs/hr per injector. Note that if we simply divided 220 by 8 (not factoring in the duty cycle) we'd end up with a 27 lbs/hr injector. That injector would have to be constantly open to meet the fuel demand, and therefore would fail quickly. The duty cycle factor of 85% yields a slightly larger injector than actually needed in order to give us a margin for error.

A closer comparison of the ACCEL and stock Ford injectors. A close look reveals a cracked pintle cap on the Ford injector. Though it should not affect performance of the injector, the broken caps often fall into the manifold and hinder fuel spray. The ACCEL design eliminates the cap altogether.
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post #2 of 5 (permalink) Old 04-09-2008, 06:37 PM
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Re: Fuel Injector Upgrade

did you retune or computer? or how are you firing the injectors?


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post #3 of 5 (permalink) Old 04-09-2008, 07:59 PM Thread Starter
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Re: Fuel Injector Upgrade

No need to retune since the injectors are still 30lbs (same as before), the computer is already chipped for a bigger mass air meter and 30lb injectors.
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post #4 of 5 (permalink) Old 04-13-2008, 01:34 PM
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Re: Fuel Injector Upgrade

Originally Posted by 67stang View Post
No need to retune since the injectors are still 30lbs (same as before), the computer is already chipped for a bigger mass air meter and 30lb injectors.
opps, yea I thought you installed bigger injectors then what you had before


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post #5 of 5 (permalink) Old 04-21-2008, 03:10 AM
Join Date: Dec 2006
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Re: Fuel Injector Upgrade

Great input 67, I had seen the Accel injectors and was debating using them in my Bronco 351 EFI rebuild. I'm going to give them a try.
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