My Injector Duty Tops out at 81% Duty Cycle on BW S366
 

First off let me say I have no desire for more power at this point, thus I will not be raising the boost. I am running 1000ccprecision/1600ccbosch injectors on my BW S66 at about 25psi on pump fuel with water injection. My Haltech E8 normally states d/cylce at about 75-78% at full boost(25psi) at high rpm. Now that the weather is cooler and the ecu is doing more compensation i will sometimes see as high as 81%. I always hear not to run injectors higher than 80%. Is this a cause for concern, or can I let i be. This is an occasionally driven street car and has a good tune and runs well. I really dont want to touch anything.

you are fine around 80. the injector tends to overheat if it is run wide open. sustained above 85% is generally considered pushing it.

a S366 at 25 psi is quite a ride.:)

Thanks for the feedback. Good to hear that my current injector setup is good enough.

Howard, typically how much rotary hp wIll my 1000/1600 injector setup support?

" how much rotary hp wIll my 1000/1600 injector setup support?"

5200 Gross minus 15% duty and 13% lag = 3845 net max into the motor

3845 = 1.01 Gallons per minute X 6.35 = 6.41 pounds per minute

6.41 X 11.3 afr = 72.4 pounds per minute air

72.4 X 14.471 = 1047 CFM / 1.92 = 545 rotary rwhp SAE

the S366 (note this is not the flow for the SXE 66 which is higher) can make 76 pounds at 25 psi and 79 at 37 so be a bit careful here.

HC

Thanks for that info.

i wouldn't worry about 81% duty cycle, we ran the miata on E85 in 2015 with the stock injectors, we were running like 99.9% duty, and the only trouble spot was the ecu would limit current at 100%.

we have actually fried injectors before so it is possible, but we ran 100% duty for more than 12 hours straight to do it.

that being said, the 80% rule of thumb isn't a bad idea at all, and i wouldn't go pushing it harder if it was mine.

Cool, thanks.

Howard,

is it cool if you can school me a bit on why you:

1) multiplied 6.35 to 1.01 gallons
2) divided 1.92 to the 1047 CFM

im just confused on where you got 6.35 and 1.92 from and or what they represent.
thank you very much for your time and consideration.:icon_tup:

-KiD

my bad Howard, i just read this thank you again for your knowledge :icon_tup:
https://www.rx7club.com/single-turbo...e-e85-1056358/

"school me a bit on why you:

1) multiplied 6.35 to 1.01 gallons
2) divided 1.92 to the 1047 CFM

im just confused on where you got 6.35 and 1.92 from and or what they represent.
thank you very much for your time and consideration.

-KiD"

sure, happy to explain.

it is possible to determine rotary rwhp for a given amount of gasoline burned.

start w total (Gross) injector capacity...

in this case 5200 CC per minute.

since we normally don't want to run injectors 100% let's use 85%.

5200 times .85 = 4420 after our arbitrary duty cycle limit of 85%.

the 5200 assumes the injectors are wide open. below 100% open there is delivery slippage due to the opening and closing process (often called lag).
this slippage varies due to a number of reasons but let's just use 13% which will get us in the ballpark.

4420 times .87 = 3845 NET into the motor after duty and lag.

now that we know the net fuel we can calculate the net air. once we know the net
air we apply our 1.92 divisor (empirically derived) and we have our power.

first, change to cc per minute to gallons per minute

3845 CC per minute is 1.01 gallons per minute.

multiply 1.01 GPM times the weight of a gallon of gas (one of your questions)

1.01 X 6.35 = 6.41 pounds of fuel per minute NET into the motor.

the AFR or air fuel ratio is the ratio between the weight of fuel and air into the motor.

so let's use a common AFR of 11.3...

6.41 pounds of fuel X 11.3 = 72.45 pounds of air.

now that we have the air we can calculate rwhp.

72.45 X 14.471 = 1048 Cubic Feet per Minute.

divide 1048 by 1.92 for rotary RWHP

1048/ 1.92 = 545

the 1.92 has worked very well and as i mentioned has been empirically derived.

another method is to take the 72.48 X 10 / 1.3 = 557

close enough:)

Going off topic but curious since I'm going water meth on pump fuel. What fuel setup are you using?