Alright, i’ve sat down and done some mathmatizin here about a set of six fuel injectors that i have been holding to that came from a Buick 3.8l supercharged. The injectors are bosch 28 lbs/hr. (295 cc/min.) injectors. Now these will be run on a 2.5L BMW m20 engine running forced induction to make 300Hp.
theoretical fuel flow (cc/min.)= (HP * K) / C
HP= max. HP
C= # of cylinders
K= 5.6 for forced induction
Injector static flow (cc/min.)= (TF100)/(NM)
TF= Theoretical fuel flow
N= # of injectors per cylinder
M= injector duty cycle
My results were:
280 cc/min. (300 max hp, 6 cyl.)
350 cc/min. (1 inj. per cyl., 80% duty cycle)
Now this formula was used for a sequential fuel injected engine, however this setup will be on a batch fire engine running two groups of three injectors, my question is; is this still a valid calculation for my setup or is there a different formula?
Any feedback would be helpful. I will post this on other boards also to look for some more answers, just looking for your guys’ answer too.
Should have thrown this in, running 43 lbs. of fuel pressure (factory spec) and if injector fuel flow is too high or fuel pressure is too low HP loss is probable along with loss in efficiency.
The formulas still all work out the same as far as require fuel flow, however if you’re sticking with batch fire injection and adding larger injectors, you should recalculate fuel flow for a bit higher fuel pressure, somewhere around 3.5 bar(~50psi). Larger injectors added to a batch fire system can(doesn’t always pending fuel system setup) cause pulse problems in the fuel rail that can disrupt fuel flow at higher levels due to the immediate dump of two or more injectors simultaneously on the rail, rather then the individually timed sequential setups. Increasing stock FP just a little bit helps to smooth this pulse out at upper fule demand levels, as long as your fuel pump can provide the flow at that pressure.
The injectors you have are red tops I believe from the Fords. More people believe they flow like 30lbs vs 28, give you a bit more play room. Ran those same injectors in my dodge back in the day on the first turbo setup, batch fire + stock fuel timing, 55psi, and had no problems making 300hp.
From your post it sounds to me like you’re sticking with the stock systems pressure and fuel timing and just adding larger injectors to compensate for the required fuel increase???
i know its not on a BMW, but this was done a stock 10:1 c/r 2.8 VR6, using ford 30lb/hr injectors, with a 3bar FPR, stock pump, all stock engine, with a 57trim T3/T04E @ 11.5psi with 2.5inch downpipe, stock VR6 exhaust, 92 octane pump. car made 271WHP on a mustang dyno, while reading 12.7:1 AFR average, never went over 13:1 while in boost.
i doubt this helps, but it shows the potention of the 30lb injectors @ 3Bar ~55psi