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Here is a good formula to find out what size injectors you need
(H*B) / (C*D)= estimated injector size

H=horsepower (Rear wheel)
B=*B.S.F.C. (see below)***
C=number of cylinders
D=duty cycle (constant .80-.85)

B.S.F.C.=
Low to med. performance = .50
Performance engine with head work= .45
High performance with lots of head work= .40-.45
S/C or T/C engines= .55-.60

Here is an example using my car,
(300*.45) / (8*.80) = 21.09

Guessing that my 8cyl. car with heads is 300rwhp, and at 80% duty cycle would need somewhere around or above 21lb injectors. That is at 80% duty cycle, which is the most that you would want to run on injectors (to be safe), so 24lb. should be good for my application.

You can use this equation to find what duty cycle your current injectors are running. For example: using x = duty cycle
(300*.45) / (8 * X) = 24lb injectors
x= 70.3% duty cycle

***B.S.F.C. = brake specific fuel consumption, it is the number of pounds of fuel it takes to make 1 horsepower for 1 hour. B.S.F.C values vary from engine to engine depending on its efficiency. The more efficient the lower the B.S.F.C. value, which translates into needing less fuel to make 1 horsepower. This changes for forced induction applications due to the engine needing fuel to cool the combustion chamber, which helps prevent detonation.