Fuel Injector Formula - Is This Crazy?
Thread Starter
Team Owner
Joined: Sep 2002
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From: OBAMA IS HITLER
Fuel Injector Formula - Is This Crazy?
Looking for a reasonably priced set of Bosch replacements, I come across this crazy formula: http://www.corvette.racenet.net/induction.php
I know this has been beaten to death. If I assumed 400chp (slightly conservative), then the formula dictates I'll need 30# injectors! I simply can't believe that.
My gut feeling is 24# would be the right selection. Certainly the stock 22# ones would be overtaxed. Someone please help the blind man.
I'm still out to lunch on those $99 LS1 injectors from SLP.....due to both good and bad posts in terms of fitment (L98). Any more comments about these?
http://www.slponline.com/view_produc...RTNUMBER=23051
I know this has been beaten to death. If I assumed 400chp (slightly conservative), then the formula dictates I'll need 30# injectors! I simply can't believe that.
My gut feeling is 24# would be the right selection. Certainly the stock 22# ones would be overtaxed. Someone please help the blind man.
I'm still out to lunch on those $99 LS1 injectors from SLP.....due to both good and bad posts in terms of fitment (L98). Any more comments about these?
http://www.slponline.com/view_produc...RTNUMBER=23051
Former Vendor
Joined: Jun 2001
Posts: 7,204
Likes: 6
From: St Marys PA
Re: Fuel Injector Formula - Is This Crazy? (bradvette)
Bradvette.
I have been trying to pound this into everyone's thinking caps.
Base your injector sizing off similiar setups that are making about the same power and running very well.
24# injectors will be more than enough for you at around 400chp. Again look at what we ran with 24# injectors. Low 11s to high 10s. :thumbs:
I brought the information here: And the assumptions are what makes these formulas so wrong. Since the MAIN assumption is BSFC, and it has more bearing on the formula than anything else. Take a look:
Engine HP must be a realistic estimate of engine output.
BSFC is determined from engine dynamometer measurements. It typically ranges from 0.4-0.6 for gasoline powered engines. A BSFC of 0.5 is a reasonable initial estimate for most engines.
The 0.8 multiplier of the "Number of Injectors" helps us derive a practical, maximum "Injector Flow Rate" for each injector based on an effective real world injector operating pulse time and fuel flow. It's unrealistic to establish the fuel flow to the engine based on an injector operating pulse time of 100% (wide open all the time). This formula uses an injector operating cycle based on 80%. Some full race engine management systems may operate at 85-95% duty cycle, but doing so for some time increases the likelihood of overheating the injectors which may cause irregular fuel rates or a decrease in low rpm operation.
Injector Flow Rate (lb/hr) = Engine HP x BSFC divided by
Number of Injectors x 0.8
For example, to calculate the individual injector size for a 650 HP V8 using 8 injectors and assuming a BSFC of 0.5:
Injector Flow Rate (lb/hr) = 650 x 0.5
8 x 0.8 = 50.78 lb/hr
Using the formula above injectors with a flow rating of 50 lb/hr static flow at 43.5 psi (3bar) fuel system pressure would be the correct injectors.
If you have a known injector fuel flow rate you can solve the above equation for a rough estimate of fuel system capacity like this:
For example, using the same estimated values from above:
Engine HP = 50 x 8 x 0.8
0.5 = 640HP
Using there formula and guessing the BSFC at their above range of .4-.6 we can solve the problem with both to show how far off one can be:
We will use his 400hp estimate:
400 x .4 divided by 8 x .8 = 25lb/hr injectors
400 x .6 divided by 8 x .8 = 37.5lb/hr injectors
You see how loose these formulas are with the assumptions we have to make. 12.5 lb/hr swing is ALOT. You would be better off statistically probably to put 22#, 24#, 30#, and 36# in a hat and pull one out to get the right injector.
The formulas are outdated and do not properly represent todays efficiencies. The sure way to get what you need is to follow proven setups with similiar setups. If it worked once, it will work again. :cheers:
[Modified by ski_dwn_it, 8:26 AM 3/2/2004]
I have been trying to pound this into everyone's thinking caps.
Base your injector sizing off similiar setups that are making about the same power and running very well.
24# injectors will be more than enough for you at around 400chp. Again look at what we ran with 24# injectors. Low 11s to high 10s. :thumbs:
I brought the information here: And the assumptions are what makes these formulas so wrong. Since the MAIN assumption is BSFC, and it has more bearing on the formula than anything else. Take a look:
Engine HP must be a realistic estimate of engine output.
BSFC is determined from engine dynamometer measurements. It typically ranges from 0.4-0.6 for gasoline powered engines. A BSFC of 0.5 is a reasonable initial estimate for most engines.
The 0.8 multiplier of the "Number of Injectors" helps us derive a practical, maximum "Injector Flow Rate" for each injector based on an effective real world injector operating pulse time and fuel flow. It's unrealistic to establish the fuel flow to the engine based on an injector operating pulse time of 100% (wide open all the time). This formula uses an injector operating cycle based on 80%. Some full race engine management systems may operate at 85-95% duty cycle, but doing so for some time increases the likelihood of overheating the injectors which may cause irregular fuel rates or a decrease in low rpm operation.
Injector Flow Rate (lb/hr) = Engine HP x BSFC divided by
Number of Injectors x 0.8
For example, to calculate the individual injector size for a 650 HP V8 using 8 injectors and assuming a BSFC of 0.5:
Injector Flow Rate (lb/hr) = 650 x 0.5
8 x 0.8 = 50.78 lb/hr
Using the formula above injectors with a flow rating of 50 lb/hr static flow at 43.5 psi (3bar) fuel system pressure would be the correct injectors.
If you have a known injector fuel flow rate you can solve the above equation for a rough estimate of fuel system capacity like this:
For example, using the same estimated values from above:
Engine HP = 50 x 8 x 0.8
0.5 = 640HP
Using there formula and guessing the BSFC at their above range of .4-.6 we can solve the problem with both to show how far off one can be:
We will use his 400hp estimate:
400 x .4 divided by 8 x .8 = 25lb/hr injectors
400 x .6 divided by 8 x .8 = 37.5lb/hr injectors
You see how loose these formulas are with the assumptions we have to make. 12.5 lb/hr swing is ALOT. You would be better off statistically probably to put 22#, 24#, 30#, and 36# in a hat and pull one out to get the right injector.
The formulas are outdated and do not properly represent todays efficiencies. The sure way to get what you need is to follow proven setups with similiar setups. If it worked once, it will work again. :cheers:
[Modified by ski_dwn_it, 8:26 AM 3/2/2004]
Thread Starter
Team Owner
Joined: Sep 2002
Posts: 22,209
Likes: 12
From: OBAMA IS HITLER
Re: Fuel Injector Formula - Is This Crazy? (ski_dwn_it)
Jesse, thank you, and that makes perfect sense, look at the real world. You can consider my thinking cap pounded as well, much appreciated! :cheers:
Drifting
Joined: Jan 2002
Posts: 1,261
Likes: 4
From: Middle River MD
Re: Fuel Injector Formula - Is This Crazy? (bradvette)
Speaking of 24lb injectors, this place sells the Ford 24 lb'ers for $199. And they fit too.
As stated they are more than enough but you can grow into them if you ever decide to upgrade in the future. I would suggest using an AFPR with these.
http://www.buyfordmotorsport.com/
[Modified by HeeHaw89, 8:29 AM 3/2/2004]
As stated they are more than enough but you can grow into them if you ever decide to upgrade in the future. I would suggest using an AFPR with these.
http://www.buyfordmotorsport.com/
[Modified by HeeHaw89, 8:29 AM 3/2/2004]
Team Owner
Joined: Dec 2000
Posts: 21,094
Likes: 282
From: Phila., PA burbs
Re: Fuel Injector Formula - Is This Crazy? (HeeHaw89)
Speaking of 24lb injectors, this place sells the Ford 24 lb'ers for $199. And they fit too.
As stated they are more than enough but you can grow into them if you ever decide to upgrade in the future. I would suggest using an AFPR with these.
http://www.buyfordmotorsport.com/
[Modified by HeeHaw89, 8:29 AM 3/2/2004]
As stated they are more than enough but you can grow into them if you ever decide to upgrade in the future. I would suggest using an AFPR with these.
http://www.buyfordmotorsport.com/
[Modified by HeeHaw89, 8:29 AM 3/2/2004]
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Re: Fuel Injector Formula - Is This Crazy? (bradvette)
What Jesse said. PLUS, if you find yourself a little "short" you can "crutch" it by raising the fuel pressure. Also bear in mind, if you go for Ford injectors, they are rated at a lower pressure. A 24# Motorsports injector flows over 25# (close to 26#?) at the 43.? pressure we're used to. Good luck, and...
RACE ON!!!
RACE ON!!!
Thread Starter
Team Owner
Joined: Sep 2002
Posts: 22,209
Likes: 12
From: OBAMA IS HITLER
Re: Fuel Injector Formula - Is This Crazy? (CFI-EFI)
Yup, just checked with Ford racing tech line (586) 468-1356....their 24# injectors are rated at 40.0 psi. This will be a technical fact along with a bunch of other things for the chip tuner (guess who).....
Advanced
Joined: Jan 2001
Posts: 71
Likes: 0
From: suprise,arizona,usa
Re: Fuel Injector Formula - Is This Crazy? (bradvette)
I have been considering the slp injectors too. A friend of mine got them and gave me the part #'s GM/Delphi # 12561462 and bosch#0 280 155 931. I searched yahoo for these #'s and found 2 sites selling the same injectors, only rebuilt, and they claim 28 lbs per hr at 43.5 psi instead of 24 that slp claims. I have an LT-4 which requires 28 lbs. I believe they are Bosch Gen III.
Anybody know which lbs rating they actually are and if they are any good?
Anybody know which lbs rating they actually are and if they are any good?
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