Determining Brake Specific Fuel Consumption for a 951?
03-24-2005, 02:57 PM
#1
Three Wheelin'
Thread Starter
Determining Brake Specific Fuel Consumption for a 951?
When calculating injector requirements for certain HP numbers, the BSFC must be entered. It's suggested for turbocharged engines to use 0.60-0.65, but this can make a pretty big difference in your resulting injector flow rate requirement. A turbocharged engine should have a higher BSFC due to running richer under boost, but has anyone come up with a BSFC number for a properly tuned (read: proper A/F ratio) 951 engine?
Last edited by Mike Murcia; 03-25-2005 at 03:23 PM.
03-25-2005, 03:23 PM
#2
Three Wheelin'
Thread Starter
I did some calculating based on the stock confuguration HP numbers, and the results are not making sense.
Engine Hp x BSFC = Injector flow rate in lbs/hr
# of cylinders x Duty cycle
Rearrange this equation to calculate BSFC. Based upon this equation, a stock 220hp engine (at the flywheel), 80% max injector duty cycle and the stock injectors (rated at 37.1lb/hr @ 2.7bar Fuel pressure, which equates to 35.7lb/hr at our 2.5 bar stock fuel pressure), our cars fom the factory come with a BSFC of 0.519. Can this be right? That's extremely efficient for a turbocharged engine since most people suggest using values of 0.6 to 0.65 on calculations for a turbocharged engine.
If you use a 100% duty cycle for the stock injectors in the above scenario, the BSFC does go up to 0.65, but I seriously doubt that the factory would design a system that maxed out the injectors. Otherwise we would not be able to increase boost safely beyond stock.
If you rearrange this equation to calculate HP and put in the stock injector flow rate, 80% duty cycle, and the suggested 0.65 BSFC, our engines should be around 190HP.
So what is wrong with the equation above? Does anyone have any thoughts?
Engine Hp x BSFC = Injector flow rate in lbs/hr
# of cylinders x Duty cycle
Rearrange this equation to calculate BSFC. Based upon this equation, a stock 220hp engine (at the flywheel), 80% max injector duty cycle and the stock injectors (rated at 37.1lb/hr @ 2.7bar Fuel pressure, which equates to 35.7lb/hr at our 2.5 bar stock fuel pressure), our cars fom the factory come with a BSFC of 0.519. Can this be right? That's extremely efficient for a turbocharged engine since most people suggest using values of 0.6 to 0.65 on calculations for a turbocharged engine.
If you use a 100% duty cycle for the stock injectors in the above scenario, the BSFC does go up to 0.65, but I seriously doubt that the factory would design a system that maxed out the injectors. Otherwise we would not be able to increase boost safely beyond stock.
If you rearrange this equation to calculate HP and put in the stock injector flow rate, 80% duty cycle, and the suggested 0.65 BSFC, our engines should be around 190HP.
So what is wrong with the equation above? Does anyone have any thoughts?
03-25-2005, 07:43 PM
#3
Burning Brakes
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Mike, I've wondered the same thing and have gone through the same process you have to come to the conclusion that our engines are around the .5 BFSC mark, at least in the stock configurtion. Think about the S which has a rated 247 bhp and uses the same injectors. There are other turbo engines, particularly in airplanes that have published BFSC numbers of .47. I take the fuel injector calculators suggestion of using .65 as a very conservative number to keep the injectors sizes safely too large. The only way to tell for sure would be to have a very accurate flow meter inline while running the engine on a dyno. Porsche probably knows the exact number.
03-25-2005, 09:47 PM
#4
Three Wheelin'
Thread Starter
Thanks for the reply Dan. I know that some of the upgrade packages for the 951S from Autothority included a 3 bar FPR to allow higher boost levels, so maybe the S is getting close to maxing out the flow under stock configuration.
In the archives, I've noticed a lot of guys that have upgraded to 55# injectors and larger have noticed extra rich A/F ratios initially. Obviously this depends on what boost levels you are running and how much your turbo flows, but it seems like 55# injectors might safely support more HP than the online sites suggest.
Based upon the suggested online BSFC value (0.65), a 951 engine running 55# injectors (with a 3bar FPR) at 80% duty cycle would put out 270HP.
If you change only the BSFC value to the one calculated for a stock 951 (0.5), the resulting power is 352hp. That's a big difference. I know firsthand how good these cars are, but are 951 engines that efficient? Someone here must have maxed out the 55# injectors and know the corresponding duty cycle and HP. From there we can get another data point other than stock. Anyone?
In the archives, I've noticed a lot of guys that have upgraded to 55# injectors and larger have noticed extra rich A/F ratios initially. Obviously this depends on what boost levels you are running and how much your turbo flows, but it seems like 55# injectors might safely support more HP than the online sites suggest.
Based upon the suggested online BSFC value (0.65), a 951 engine running 55# injectors (with a 3bar FPR) at 80% duty cycle would put out 270HP.
If you change only the BSFC value to the one calculated for a stock 951 (0.5), the resulting power is 352hp. That's a big difference. I know firsthand how good these cars are, but are 951 engines that efficient? Someone here must have maxed out the 55# injectors and know the corresponding duty cycle and HP. From there we can get another data point other than stock. Anyone?
03-26-2005, 02:36 AM
#5
Addict
Rennlist Member
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If the purpose of the exercise is to find appropriately sized injectors for a given power level, the simplest way to go is to assume that the Porsche engineers selected the 37 lb/hr injectors for the 951S appropriately and just scale to the desired flywheel HP. So if you eventually want 2*247 = 494 fwHP you’ll need 74 lb/hr injectors.
Going from 2.5 bar fuel pressure to 3.0 bar gives you SQRT(3.0/2.5) – 1 = 10% more fuel.
Initially I had concerns about the idle characteristics of my 72 lb/hr injectors, but they are just fine at idle and believe others have a similar positive experience.
Don’t forget the fuel pump. I calculated the OEM pump to run out of flow just a little later than the injectors.
Laust
Going from 2.5 bar fuel pressure to 3.0 bar gives you SQRT(3.0/2.5) – 1 = 10% more fuel.
Initially I had concerns about the idle characteristics of my 72 lb/hr injectors, but they are just fine at idle and believe others have a similar positive experience.
Don’t forget the fuel pump. I calculated the OEM pump to run out of flow just a little later than the injectors.
Laust
03-26-2005, 11:21 AM
#6
Three Wheelin'
"Don’t forget the fuel pump. I calculated the OEM pump to run out of flow just a little later than the injectors."
Nobody has ever posted info about this before. Would you mind posting detials or a rough description of the procedure you used so that I could repeat it?
Nobody has ever posted info about this before. Would you mind posting detials or a rough description of the procedure you used so that I could repeat it?
