HP Limits for the Following!

Rails are good for about 550-600rwhp, try to keep the injector 42lb or less.

42lb injectors it depends if it is a boosted or NA car, boosted cars can make close to 600rwhp NA they can make more.

Rearend I don't think you can put a power rating on it. It depends on usage...will you be using slicks. I've stock cars break rear ends and I've seen stock ones last on high hp cars.

The stock fuel rails will work fine with an adjustable regulator at 450 hp, but the fuel lines are questionable. At this level, we’ll be right on the edge of requiring a larger ½-inch feed fuel line. You won’t need a larger return line because you can use your old 5/16-inch feed line as your new return.

rear end weakness is the factory 28 spline axles i have seen stock mustangs snap these axles on a sticky track with drag radials at 230 hrse power and 260 ft lb of torque then i have seen 350 horse power 420 ft. lbs of torque launch in same conditions and last forever its just a matter of launch RPM's the way i figure.....i would say if you launch over 3500 rpm's or have 350 and up hrse power change to the 31 spline.



(number of injectors x injector size) / BSFC = Max safe hp
BSFC (Brake Specific Fuel Consumption) varies with application.
Normally aspirated cars use between .45 and .55,
Supercharged cars use between .55 and .65

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...... naturally asperated/////////////// Supercharged
19 lb...................... 276 hp //////////////////////// 338 hp
24 lb...................... 349 hp ///////////////////////// 427 hp
30 lb...................... 436 hp ///////////////////////// 533 hp
36 lb...................... 553 hp ///////////////////////// 640 hp
42 lb...................... 611 hp //////////////////////// 747 hp
55 lb...................... 800 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 to 40 which are the typical test pressures.

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.

the above info is from my reasearch, opinion and experience.

On the inj. thing you have to remember you have to run stock PSI at the reg. with a tweecer. There is no FMU (as the tweecer can be setup better without one) and the amount the inj. hp wise can handle kinda gets put in the same "area" as the maf.

It depends....

You can make a "best guess" and go with the 42's but you very well may run out, it seems each combo is diff. and some just get away with less inj.

I think 42's will be a great starting point though. If you do need more inj. there is PLENTY of a market out there to sell the 42's and not loose your arse.

A few local guys run a f150 tloc with extra clutches or 03 cobra 31 spline tloc with great results along with 31 spline axles. You could build a fairly stout rear (like low 10's high 9's prob. with welded axle tubes and a guirdle) for under 700$ easy.

Like said the rails are not as a big issue as the line.

bill302 said:

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...... naturally asperated/////////////// Supercharged
19 lb...................... 276 hp //////////////////////// 338 hp
24 lb...................... 349 hp ///////////////////////// 427 hp
30 lb...................... 436 hp ///////////////////////// 533 hp
36 lb...................... 553 hp ///////////////////////// 640 hp
42 lb...................... 611 hp //////////////////////// 747 hp
55 lb...................... 800 hp//////////////////////// 978 hp

Click to expand...

you got that backwards....

Injector Size...... Supercharged /////////////////// Natually Asperated
19 lb...................... 276 hp //////////////////////// 338 hp
24 lb...................... 349 hp ///////////////////////// 427 hp
30 lb...................... 436 hp ///////////////////////// 533 hp
36 lb...................... 553 hp ///////////////////////// 640 hp
42 lb...................... 611 hp //////////////////////// 747 hp
55 lb...................... 800 hp//////////////////////// 978 hp

and thats not necessarily true either, just best guess. i was running 24s and putting down 390rwhp/360rwtq. FMU, 255lph, t-rex, adjustable fpr all on stock lines and rails.