Injector Size & Horsepower

[Bo Young]

Working on some calculations, & not getting anywhere... please send help..

The stock injector size for my '05 S-197 GT (w/ 4.6L V8) is listed as 24 Lb/Hr- Assuming this refers to the amount of fuel the injectors can put out (max) at the 85% duty cycle.

Given that the engine burns a certain amount of fuel to produce x amount of horsepower, you can calculate the expected engine output:

(Amt fuel/Hr) x (BTU amt/unit fuel) x (1 HP/2545 BTU-H) x (0.3 or 30% efficiency) = Expected HP output

With that said, see the attached image of the calculations I've done with 24 lb/hr, excluding the # of cylinders, and using the incorrect value of (114,000 BTU/Lb of gas) but actually got close to the rated 300- 310 Horsepower rating of the engine.

Then, I worked through the same calculation, including the amount of cylinders and the duty cycle, and got 376 HP- way off the mark.

Take a look & give some feedback- what am I missing? A side note- when one simply changes the 24 Lbs into 24 gallons/hr, the math works out quite nicely.

Please let me know. Thanks.

 

[Dino Dino Bambino]

Stock injectors put out 24lb/hr of fuel at their maximum 100% duty cycle.
That's 192lb/hr total.
One US gallon contains 114,000 BTU and weighs 6.14lb, so that's 18,566 BTU per lb.
At 192lb/hr, that's 3,564,820 BTU/hr or 1400 HP at 100% duty cycle and 100% thermal efficiency.
If you assume 30% thermal efficiency for the 3V engine, the stock injectors will max out at 420 crank HP (370rwhp in a manual GT). At 300 crank HP, the stock injectors are only operating at a little over 70% duty cycle.