24 lb/hr injectors?

Topic: 24 lb/hr injectors? (Read 5226 times) previous topic - next topic

24 lb/hr injectors?

July 11, 2008, 04:32:13 PM

Ok I've been doing research and can't really find an answer to this so maybe someone here will chime in. I know that the stock HO (19 lb/hr) injectors will support up to 280 hp at the factory fuel pressure setting (40 psi) My engine makes ~280 hp and it runs fine with the stock 19 lb/hr injectors (runs great, no lean codes ect.). Now my question is if 19 lb/hr injectors can support my engine why did Ford factory fit 93-95 GT40 Cobra engines with 24 lb/hr fuel injectors? Was Ford just being overly cautious? This is a curriosity question and I'm hoping someone has an answer

24 lb/hr injectors?

Reply #1 – July 11, 2008, 04:48:57 PM

double post

24 lb/hr injectors?

Reply #2 – July 11, 2008, 04:51:37 PM

19lbs aren't designed to run much above 300 hp and while you can run them the increased fuel pressure, as well as their increased rate of firing will over time cause an eventual failure of the injector. So as you're aware of yes you can run 19lbs, but you're operating them outside of the range they were designed for. But you're hot rodding so you should expect to always be buying parts.

Although with you're combo you should be fine for quite a while. And yes Ford put a lot of time and effort into their fuel injection system.

BTW I'm going to be running 24lbs with a 331 and abouts 450hp so yeah I'm pushing the limits as well.

24 lb/hr injectors?

Reply #3 – July 11, 2008, 05:44:53 PM

This is straight from the official ford mustang 5.0 technical reference and performance handbook:
At 39psi, OE 19lb injectors can feed a normally aspirated 5 liter up to 300hp. At 60psi, OE 19lb injectors can feed up to 330hp. Of course as Grannysbird mentioned they are being overworked at this level and will experience a shorter service life. The use of 24lb injectors in the 93-95 GT-40 Cobra engine was intended to provide reserve capacity in extreme operating conditions. Because they are being utilized for only a small portion of their capacity most of the time they are operating, the high rate injectors in this OE application may be considered overkill. (The same is true for using 24lb injectors in any OE or near OE 5.0 liter engine). Operated in this application only within a small portion of the overall range they are capable of handling, the 24lb injectors become less accurate at low speeds and idle. At 39 psi, production 24lb injectors can feed a normally aspirated motor up to approx. 400hp. Installing high rate injectors (24lb) on an otherwise stock (less than 320hp) engine will result in loss of low speed driveability and overall economy without any increase in performance.
Replacement injectors must always be in the same impedance range as the OE injectors (13.5-19ohms) to avoid potential damage to the engine computer.
Hopefully this answers your question. I basically picked out the pertinent info from the book but it goes into great detail about the stock system as well as what to do when modifying any part of that system.

24 lb/hr injectors?

Reply #4 – July 11, 2008, 11:49:06 PM

Throwing the figures around the way that is done in that book is surprising to say the least. It's a wee bit more involved than that. BSFC, duty cycle, and volumetric efficiency all have impact on this;amongst other variables. True, too low a pressure/duty cycle will cause ssue. The same can be said of too high as well. Trust me. I've experienced injectors in full static.

Quote

At 60psi, OE 19lb injectors can feed up to 330hp......At 39 psi, production 24lb injectors can feed a normally aspirated motor up to approx. 400hp

An interesting figure due to the fact that:

New flow rating = [square root of (new pressure /old pressure)] x old flow rating

Which would be 23.56 pounds/hour. Now, if the fuel pump was big enough to support the pressure and needs of the engine I would say there is a bit of a discrepancy there.

Keep your 19's. They're fine.

24 lb/hr injectors?

Reply #5 – July 13, 2008, 02:33:25 PM

Quote from: V8Demon;226987

Throwing the figures around the way that is done in that book is surprising to say the least. It's a wee bit more involved than that. BSFC, duty cycle, and volumetric efficiency all have impact on this;amongst other variables. True, too low a pressure/duty cycle will cause ssue. The same can be said of too high as well. Trust me. I've experienced injectors in full static.

An interesting figure due to the fact that:

New flow rating = [square root of (new pressure /old pressure)] x old flow rating

Which would be 23.56 pounds/hour. Now, if the fuel pump was big enough to support the pressure and needs of the engine I would say there is a bit of a discrepancy there.

Keep your 19's. They're fine.

Oh I'm not getting rid of the 19s to swap to 24s as the 19s are supporting the engine fine at stock fuel pressure and I don't get any lean codes. My question was why Ford factory equpied the 5.0 Cobras with 24lb/hr injectors when it was overkill. I can't see why

Quote

the use of 24lb injectors in the 93-95 GT-40 Cobra engine was intended to provide reserve capacity in extreme operating conditions.

when 19s would have been fine and would not be maxed out by the engine, even at full throttle. My engine makes more power than the 93-95 Cobra motors and runs fine with stock HO injectors. It just seems stupid when someone sticks an E303 cam in a stock HO and puts on 24lb/hr injectors because the stock injectors "won't support it"

24 lb/hr injectors?

Reply #6 – July 14, 2008, 11:06:22 AM

Ok so I think I figured out why 93-95 Cobras came with 24 lb/hr injectors. Lets say the hp an injector will support= [(injector size)x(# of injectors)x(duty cycle)]/brake specific fuel consumption. Lets say BSFC is .50 so at a 90% duty cycle 19 lb/hr injectors will support 275 hp. I'm betting that Ford didn't want to run the injectors at a 90% duty cycle so they went with something like an 80% duty cycle. If they did pluging in the numbers a 19 lb/hr injector will support 243 hp at an 80% duty cycle. 243 hp is only 3 hp above what a 93-95 Cobra engine was rated at hence the reason why Ford went with 24 lb/hr injectors on the Cobra. Man I put too much thought into this

24 lb/hr injectors?

Reply #7 – July 14, 2008, 06:43:49 PM

It's been said the Cobra motors were heavily underrated power-wise. As I've never sriven a car with a bone stock Cobra motor, I couldn't tell you.

You're on the right track though

Most auto manufacturers shoot for an 80-85% duty cycle.
Funny thing though. The 3V 4.6 motors with a 300 HP rating come with ............ you guessed it, 24's.

BTW, another formula:

Fuel requirement in lbs./hr = (Max HP x BSFC) / (number of injectors x duty cycle)

24 lb/hr injectors?

Reply #8 – July 14, 2008, 07:20:25 PM

Quote from: V8Demon;227367

It's been said the Cobra motors were heavily underrated power-wise. As I've never sriven a car with a bone stock Cobra motor, I couldn't tell you.

Here's something else I found in that book relating to what you said:
"By 1993 new appraisal standards, the cobra v-8's rating worked out to 235hp, or 30 more than the rating assigned to the standard 5 liter H.O. These conservative numbers were delegated despite the fact that the same GT-40 parts assembled on an otherwise OE 5 liter engine had already produced 270hp in a well publicized non factory evaluation. In another test, based on the quarter mile trap speed recorded by a 3380 pound car, the GT-40's output calculates to 260hp. According to July 1992 issue of Hot Rod, adding GT-40 cylinder heads and intake system to a 87-92 5 liter reduced quarter mile clockings by .6 second. This is consistent with a computer model of a 270hp 5 speed mustang."

24 lb/hr injectors?

Reply #9 – July 15, 2008, 12:47:30 PM

Quote from: slicksport88;227371

Quote from: V8Demon;227367
It's been said the Cobra motors were heavily underrated power-wise. As I've never sriven a car with a bone stock Cobra motor, I couldn't tell you.

Here's something else I found in that book relating to what you said:
"By 1993 new appraisal standards, the cobra v-8's rating worked out to 235hp, or 30 more than the rating assigned to the standard 5 liter H.O. These conservative numbers were delegated despite the fact that the same GT-40 parts assembled on an otherwise OE 5 liter engine had already produced 270hp in a well publicized non factory evaluation. In another test, based on the quarter mile trap speed recorded by a 3380 pound car, the GT-40's output calculates to 260hp. According to July 1992 issue of Hot Rod, adding GT-40 cylinder heads and intake system to a 87-92 5 liter reduced quarter mile clockings by .6 second. This is consistent with a computer model of a 270hp 5 speed mustang."

Yep like I said I'm guessing that my combo makes about 270-280 fwhp. The shiznitty stock AOD and 215 section tires are really killing my 1/4 mile et.

I'm wondering if there is a way to tell if I'm maxing out my injectors with my combo (ie 100% duty cycle)? I'm actually currious to see how well the stock HO injectors are working with my combo.

24 lb/hr injectors?

Reply #10 – July 15, 2008, 07:52:01 PM

Quote from: thunderjet302;227491

Quote from: slicksport88;227371

Yep like I said I'm guessing that my combo makes about 270-280 fwhp. The shiznitty stock AOD and 215 section tires are really killing my 1/4 mile et.

I'm wondering if there is a way to tell if I'm maxing out my injectors with my combo (ie 100% duty cycle)? I'm actually currious to see how well the stock HO injectors are working with my combo.

My best guess on that one would be to have the car dyno'd and watch the air/fuel curve for lean spots. If additional tuning doesn't help, then I believe bigger injectors and/or fuel pump would be needed.