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what size and brand of injectors? BSFC

PostPosted: September 24th, 2011, 4:47 pm
by grumpyvette
"hey grumpy?
I'm building a TPI 383 corvette engine and I'm sure the old factory fuel injectors I have are both well worn and more than likely to small for the new application, how do I find the correct injector?"

, ITS not a random guess its an easily calculated figure that you need to know to get the cars engine to run to its full potential, knowing the cylinder head port flow rates and compression ratio and cam timing will help a great deal, here, an engine will burn between 12.6 and about 15 pounds of air for every pound of fuel, the flow rate you need will be fairly easy to calculate if you know the approximate hp level your dealing with.
I don,t know why but its common for guys to assume they can install bigger heads and a radical cam and a free flow exhaust and still keep the stock injectors, the truth is that you MUST maintain a max .80 duty cyscle and a 12.5:1-13:1 f/a ratio and that can easily require a significantly larger than stock injector size upgrade
the sub links in the thread below have 90% or more of the info, youll need the stock computer will require you stay with similar TYPE of injector,
measure your current injector resistance its most likely near 12-14 ohms, thats what the computer wants to see, you can get 24lb, 30lb, 32lb, 36lb,38lb, 40lb, 42 lb,44 lb etc. what you need is an injector that supplies fuel at about 80% pulse duration and about 42 psi of fuel pressure that will supply your horsepower level at about a .50 BSF level
lets assume your building a 450hp engine

THERES DOZENS of SOFTWARE programs to help you estimate your combos power potential,


if that was the power range it will require about a 36lb injector ,but a 39 lb or 44 lb would give you some growth room for the future,I,d suggest you use the calculator to verify, but keep in mind the computer varies the pulse duration (the amount of time the injectors spraying fuel based on sensor input like engine temp and oxygen sensor feed back, it can shorten the pulse far easier than it can add fuel above the 80% duty cycle so its best to select the injector size as close to what you need as possible but if your in doubt its better to go up a small step that get an injector that maxs out flow on the upper rpm ranges leaning out the engine to much, a slight too rich mix hurts mileage a slightly too lean mix might damage rings and pistons
GASOLINE 6.073 pounds per US Gallon.

Divide the pounds of gas used per hour by the horsepower that is produced.

Use the following example equation as a reference for your calculations:
An engine produces 200 horsepower while consuming 80 lbs. of gasoline (13.17 gallons)in an hour. Therefore, 80 divided by 200 would equal a BSFC of 0.4.
To calculate the injector size for a particular application:
Injector Flow Rate (lb/hr) = Engine HP(1) x BSFC(2)
Number of Injectors x Injector duty cycle(3)

Injector Flow Rate (cc/min) = Engine HP(1) x BSFC(2) x 10.5
Number of Injectors x Injector duty cycle(3)

Inj Flow Rate (@ 40psid) Naturally Aspirated hp
19 lb/hr 258 hp @ 85% Duty Cycle
24 lb/hr 326 hp @ 85% Duty Cycle
30 lb/hr 408 hp @ 85% Duty Cycle
32 lb/hr 435 hp @ 85% Duty Cycle
39 lb/hr 530 hp @ 85% Duty Cycle
42 lb/hr 571 hp @ 85% Duty Cycle
47 lb/hr 639 hp @ 85% Duty Cycle
60 lb/hr 816 hp @ 85% Duty Cycle

BOSCH, ACCEL or LUCAS all sell a decent injector , I personally prefer the bosch injectors

don,t forget the high pressure fuel pump may help here
Bosch High Pressure Pump 0-580-254-984

Rated at 44 GPH @ 90 PSI 750+ Hp This pump is identical to the Accel part number 74702 which they boast will handle 870 Hp! ... _pumps.htm
read this thread, and its sub linked info

heres related info










Re: what size and brand of injectors?

PostPosted: September 25th, 2011, 12:39 pm
by Indycars
It wasn't obvious that two numbers were being divided until I tried to do a calculation.

Hopefully this will make it easier for others to see exactly what the formula says.


Re: what size and brand of injectors?

PostPosted: December 8th, 2011, 9:50 am
by grumpyvette
guys when selecting injectors its best to keep a few factors firmly in mind
I recently had a guy in the shop with a random miss that drove him insane, it was the result of defective injectors which I firmly believe was the result or at least strongly contributed to by him running too small of a flow rate that resulted in a 90% duration and frequently higher duty cycle, where some injectors effectively locked open , or just stopped flowing fuel at times, and in some cases when the rpms dropped lower it took a good deal of time for them to start functioning correctly again

(1) its generally best to max the injector flow rate on any direct port injector you select at no more than a 75%-80% duty cycle at peak rpms
simply because as the rpms increase the TIME required and available for the injector spring to close the injector between duty cycles drops significantly
while at 2000rpm a 85% duty cycles no big deal in most cases the injector will never effectively close at 7500rpm with a 85% OR HIGHER duty cycle because there not enough time for the spring to close the injectors pindle valve before its re-energized


(2) many injector designs have irregular flow rates when pushed to over 80% -85% duty cycle, in fact its common at anything over 85%-90% in many types of injectors, and can easily result in engine damage

(3) duty cycles lower than 50% at peak hp generally indicate the injectors too large for the application

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