Injector size formula
03-21-2005, 12:11 PM
#1
Staging Lane
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Injector size formula inside I just thought I would post the formula for sizing an injector for a specific engine combination since I have seen multiple questions on the matter.
The formula goes as follows: (realistic flywheel HP X brake specific fuel consumption) divided by: (number of injectors X duty cycle).
Ex: (480 hp X .5)/(8 X .85) = 35.3 lb/hr injectors or the next step up which would be a 36.
BSFC= .5
Duty cycle= 85%
A BSFC of .5 is a good place to start for an efficient engine with good heads such as the LS1. BSFC ranges from around .4 for a really efficient engine to about .6 for turbo or supercharged engines which need a richer A/F ratio to live.
I hope this helps someone.
The formula goes as follows: (realistic flywheel HP X brake specific fuel consumption) divided by: (number of injectors X duty cycle).
Ex: (480 hp X .5)/(8 X .85) = 35.3 lb/hr injectors or the next step up which would be a 36.
BSFC= .5
Duty cycle= 85%
A BSFC of .5 is a good place to start for an efficient engine with good heads such as the LS1. BSFC ranges from around .4 for a really efficient engine to about .6 for turbo or supercharged engines which need a richer A/F ratio to live.
I hope this helps someone.
Last edited by DragLS1; 03-21-2005 at 12:32 PM. Reason: more info
03-21-2005, 05:01 PM
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if you do (480hp * .5)/(8*.7)= 42.857 lb/hr injector with a 70 percent duty cycle.
I didn't think that for safety reasons you wanted your injectors running above 80% if that.
For (450*.5)/(8*0.7)=40.17 lb/hr
anyways, i can't remember all this exactly but there is a webpage that has a calculator that you can just enter your information and it tells you what to get, I will post it up soon, i gotta get it from my dad.
I have 28.8 lb/hr injectors (stock) and I'm running close to or around 400 something at the motor and i have no clue what duty cycle that i'm running but its getting close to the limit
I didn't think that for safety reasons you wanted your injectors running above 80% if that.
For (450*.5)/(8*0.7)=40.17 lb/hr
anyways, i can't remember all this exactly but there is a webpage that has a calculator that you can just enter your information and it tells you what to get, I will post it up soon, i gotta get it from my dad.
I have 28.8 lb/hr injectors (stock) and I'm running close to or around 400 something at the motor and i have no clue what duty cycle that i'm running but its getting close to the limit
Last edited by ZL1Killa; 03-21-2005 at 05:19 PM.
03-21-2005, 06:49 PM
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Staging Lane
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No sweat on the post...It's nice to just punch in numbers but it is better to be able to do the math and come up with the right number without having to have a web calculator. And yes it is probably better to only push your injectors to 75% or so and that is the beauty of the formula...just change the numbers to get what you want.
03-21-2005, 07:15 PM
#5
hold on somethings off here how is it the many people are at 400 Rear wheel horsepower using 28-30 injector with less than 80% duty cycle. If you say just 400 at the fly using that formula, you come up with 31.25 using .5bsfc and an 80% duty cycle. assuming a 25% of power lost through the drivetrain your flywheel power would be around 535 horsepower if your making 400rwhp and would need a 41.7 pound injector according to that formula. so somethings off shouldn't fuel pressure be involved in this equation?
03-21-2005, 07:51 PM
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Please note:
Most injectors will max out at 80% duty cycle and this is the accepted industry standard.
I did not make this up! This is from SAE - Society of Automotive Engineers.
The website article is http://www.rceng.com/technical.htm
and of course the website is http://www.rceng.com
Most injectors will max out at 80% duty cycle and this is the accepted industry standard.
I did not make this up! This is from SAE - Society of Automotive Engineers.
The website article is http://www.rceng.com/technical.htm
and of course the website is http://www.rceng.com
03-21-2005, 08:04 PM
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Originally Posted by Cody Brooks
hold on somethings off here how is it the many people are at 400 Rear wheel horsepower using 28-30 injector with less than 80% duty cycle. If you say just 400 at the fly using that formula, you come up with 31.25 using .5bsfc and an 80% duty cycle. assuming a 25% of power lost through the drivetrain your flywheel power would be around 535 horsepower if your making 400rwhp and would need a 41.7 pound injector according to that formula. so somethings off shouldn't fuel pressure be involved in this equation?
naw, 400 at crank/flywheel
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03-21-2005, 08:08 PM
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ok, so everythings cool. the originaly equation posted is right also. I was just confused. but there is the site and here it is again just in case :
Please note:
Most injectors will max out at 80% duty cycle and this is the accepted industry standard.
I did not make this up! This is from SAE - Society of Automotive Engineers.
The website article is http://www.rceng.com/technical.htm
and of course the website is http://www.rceng.com
SCROLL TO THE BOTTOM OF THE PAGE AND YOU CAN SEE THE TABLES TO INPUT WHAT YOU WOULD WANT ....ALSO there is another table at the very bottom to calculate your Actual Brake specific fuel consumption in use.
Please note:
Most injectors will max out at 80% duty cycle and this is the accepted industry standard.
I did not make this up! This is from SAE - Society of Automotive Engineers.
The website article is http://www.rceng.com/technical.htm
and of course the website is http://www.rceng.com
SCROLL TO THE BOTTOM OF THE PAGE AND YOU CAN SEE THE TABLES TO INPUT WHAT YOU WOULD WANT ....ALSO there is another table at the very bottom to calculate your Actual Brake specific fuel consumption in use.
03-21-2005, 08:14 PM
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Originally Posted by Cody Brooks
hold on somethings off here how is it the many people are at 400 Rear wheel horsepower using 28-30 injector with less than 80% duty cycle. If you say just 400 at the fly using that formula, you come up with 31.25 using .5bsfc and an 80% duty cycle. assuming a 25% of power lost through the drivetrain your flywheel power would be around 535 horsepower if your making 400rwhp and would need a 41.7 pound injector according to that formula. so somethings off shouldn't fuel pressure be involved in this equation?
fuel pressure is involved......its on the page posted......THATS what was wrong with the above equation...oops missed that one
03-21-2005, 09:10 PM
#10
Originally Posted by ZL1Killa
naw, 400 at crank/flywheel
most people using 28-30 lbs injectors are making 400 at the wheels with less than 80% duty cycle.
03-21-2005, 09:19 PM
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Originally Posted by Cody Brooks
reread my post I said 400 at the fly would need according to your formula 31.25 lbs. to maintain 80% or less duty cycle.
most people using 28-30 lbs injectors are making 400 at the wheels with less than 80% duty cycle.
most people using 28-30 lbs injectors are making 400 at the wheels with less than 80% duty cycle.
that is why we are more than likely simply going to go from the stock 50psi to 60psi fuel pump and it will make enough of a difference to do what we want, but since we are going to upgrade things later we might as well (since we are getting the FAST intake and Nick Williams TB) go with larger injectors too.
03-21-2005, 09:28 PM
#13
used formula on site 530 crank horsepower, which is approximately 409 rwhp with 23% drivetrain loss, with .45 BSFC, 80% duty cycle, 60 psi rail pressure says should be using 31.73 lbs/hr injectors still a little off from what most people are seeing but closer than before.
03-22-2005, 07:21 AM
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Staging Lane
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Originally Posted by Cody Brooks
ok glad to see that there was something missing that I having seen yet. I knew the formula as orignally posted simply couldn't be right.
Last edited by DragLS1; 03-22-2005 at 07:27 AM. Reason: more info
03-22-2005, 12:11 PM
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Originally Posted by Cody Brooks
used formula on site 530 crank horsepower, which is approximately 409 rwhp with 23% drivetrain loss, with .45 BSFC, 80% duty cycle, 60 psi rail pressure says should be using 31.73 lbs/hr injectors still a little off from what most people are seeing but closer than before.
do you have any automatic??
i thought that there was anywhere from a 10-20% loss ; and yes i know its not a fixed amount, that its different but still i though that everyone came to a conclusion that it was somewhere close to 10-20%
03-24-2005, 07:20 PM
#16
17-23% in manuals and 20-25% in automatics is what I've always been told as far as drivetrain loss goes. yes my car is an automatic so i figured 23% drivetrain loss. of course draintrain loss is effected by lots of things such as rear axle ratios. The only way to know what your losing through your drivetrain is to dyno on the chassis dyno and on the engine dyno doing 3 pulls each and assuming both dynos are accurate but thats alot of trouble.