Mustang Math

[joker]

I came upon this when searching for a table to show needed injector size at different hp levels. Seems pretty helpfull.

http://www.pro-flow.com/tech%20info/mustmath.htm

[joker]

Some more good info on injector sizing:


















Fuel Systems

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The Fuel system is perhaps the most critical part of any high performance system. Without the proper Air/Fuel Ratio you can't make full power, or even worse - you can destroy a perfectly good motor really quick! The first thing to consider is that it's the fuel that makes the power. For every amount of air you can squeeze into the cylinder, you must also have the proper amount of fuel to go with it. Too much fuel and you drown, too little and you start burning things up. It's that fine line that you're after, if only it was that easy.

All too often I see cars with huge fuel systems, for no apparent reason other than to jump on the band wagon. Don't get me wrong. If you're planning ahead that's one thing, and it is better to be too big than to be too small. The sad thing is when people are pushed into theoretical ideas of what they need. While with the proper Fuel Management Unit (FMU) or Regulator you can run a huge Fuel Pump, it is the injector size that is most critical. This is the one piece that should definitely be matched to the motor. While I have fallen prey to what size injector I should be using, I quickly learned that doesn't always work in the real world. For every Mustang running 11's and 12's with 30 lb, 36 lb, and even 42 lb. injectors, there is another running 10's with 24 lb. injectors. Sure a 30 lb. injector will work with the right amount of fuel pressure, but at what cost? Typically, idle quality and mileage are what suffer. In that same motor a 24 lb. injector will work great by running a higher fuel pressure under load. This will allow your car to run properly, especially when not being pushed hard, which is very critical for a street driven motor. Huge injectors can be great on race only cars, but will be hard to tune around on a daily driver. Don't get me wrong, there is a point of no return, too.

In my case, I wanted to run more boost. My 5 liter Mustang has a motor compression of 9.5:1 and 14 lbs. boost. Going to 14 lbs from 8 lbs caused me to go from a 24 lb. injector to a 30 lb. injector. Unfortunately, my idle got worse and so did my mileage. I switched back to my 24 lb. injectors and obviously everything went back to the way it was - which was great. To keep up with the horsepower, I did what every magazine says not to do - run a lot of fuel pressure. Under full boost, I now run about 80-90 lbs of fuel pressure. Unfortunately, at this pressure, it's hard for the computer to properly control the injector. Actually, this is only one of the problems with running an injector at its limits. Another is that you are at the limit and are running a huge risk of motor damage due to lack of fuel. As if this wasn't enough, remember that you are completely limited by your injector size. If there isn't enough fuel, you'll never make more power. A little time on a DynoJet was more than proof of this.

With the boost back to 8 lbs. (actually 9 lbs.) and while running with 24 lb. injectors, my car ran into a limit of 437 rwhp (Rear Wheel Horsepower). It was obvious that the car was making big hp (close to 500 hp at the flywheel), but it was limited by my choice in injector size. When comparing my results to another car with a similar set-up (big Darts, Vortech S-Trim @ 12 lbs boost, etc...), my car was about 50 hp higher at 4800 rpm, but then hit the wall of my limited fuel system. As one would expect, both of our cars (his also had 24 lb. injectors) had very similar peak HP numbers - about 437 hp (mine) and 423 hp (his). I'd like to think that this is a perfect example of what kind of limit the wrong fuel system can have on an otherwise good set-up.

While it is important to understand the limit of certain injector sizes, it's still important not to go overboard. I know that I have had better street results using 24 lb. injectors, but I also know that I can't make full power using them. For this reason, I am somewhat forced into using 30 lb. injectors, which proved to be what I needed. Because a properly calibrated Mass-Air Meter is crucial to getting a set of injectors to work properly, I will be looking into a Pro-M to hopefully solve my slight drivability problems. While I have shown what works for me, to each his own. My biggest recommendation is to make your changes gradually. Remember to do the math - a 24 lb. is 25% bigger than a 19 lb, a 30 lb is 25% bigger than a 24 lb. , etc... I'm not saying to run small injectors, but explaining what worked for me. To help find the right injector size for your application, use the following formula:


(number of cylinders x injector size) / BSFC = Max safe hp

* BSFC (Brake Specific Fuel Consumption) varies with application.
For normally aspirated cars use between .45 and .55,
and for Supercharged cars use between .55 and .65
* If using more than one injector per cylinder, substitute
the number of injectors for number of cylinders.

As an example, a supercharged car with 24 lb. injectors can safely make 349 hp while a normally aspirated car with the same injector can safely make 426 hp.


Injector Size

19 lb.
24 lb.
30 lb.
36 lb.
42 lb.
55 lb.

Supercharged

276 hp
349 hp
436 hp
553 hp
611 hp
800 hp

Naturally
Asperated
338 hp
427 hp
533 hp
640 hp
747 hp
978 hp


While on the subject of injector sizing, it is also very important to know the relationship of fuel pressure to injector size. Increasing the supply pressure to the injector will simply allow more fuel to flow through the injector while it is open. This is how an FMU (Fuel Management Unit) on a supercharged car allows a smaller than normal injector to work properly. It uses the supercharger's boost to increase the fuel pressure at a set rate, such as six psi per pound of boost. This is also how an adjustable regulator helps to provide the few extra pounds of fuel pressure that can often times make a world of difference. Still, there are limits to the amount of fuel pressure you can safely and effectively use. Regardless, this really is an excellent way to increase the size of an injector for a particular application. For the effects of fuel pressure on fuel injector size, use this formula:

Divide the new pressure by the original pressure. Take the square root of that number and multiply it by the size of the injector. This is the equivalent injector size.


* For the original pressure use 38 which is the typical test pressure.

As an example, on the same supercharged car as before, but with 60 psi fuel pressure and the same 24 lb injectors, you can safely make 436 hp - which our red Vortech supercharged Mustang easily proved while using 24 lb injectors. Actually, we tested using slightly higher fuel pressures, and safely made well over 500 hp using these same 24 lb injectors and pump gas. This just goes to show how well this math works out.

Either way, I hope all this helps to explain the basics of choosing the proper injector size. Next time, I will explain a little more about the other fuel system components. Because the fuel system plays such a critical role in making the maximum amount of Horsepower, this is definitely one place that you don't want to start guessing. As usual, if you have any questions please E-mail me.



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Copyright© 2001, MotorSports Digest
msd@stic.net

[coupe]

The injector capabilities are a little bit exaggerated; or they are talking about extreme 100% duty cycle with FMU, etc. I just don't see 8 55-Lb. injectors doing 800 hp. Maybe 700. Anyone else with me? Maybe I'm off but I have a formula that works pretty much dead-on with turbo apps.

[TheJeanyus]

The injector capabilities are a little bit exaggerated; or they are talking about extreme 100% duty cycle with FMU, etc. I just don't see 8 55-Lb. injectors doing 800 hp. Maybe 700. Anyone else with me? Maybe I'm off but I have a formula that works pretty much dead-on with turbo apps.
Those are probably fly wheel numbers with no accessories attached.