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B.S.F.C is brake specific fuel consumption

7260 Views 6 Replies 3 Participants Last post by  Steve Jarvis
Does anyone know how B.S.F.C is brake specific fuel consumption compares to an a/f ratio? RC fuel injectors states

"In most cases a naturally aspirated engine will have a B.S.F.C of .50. This means that the engine will use .50 lbs. of fuel per hour for each horsepower it produces. Turbocharged engines will want to be at .60 lbs. per hour or higher."

Thanks in advance for the help, Steve
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Complicated question! BSFC is a measure of how effecient the engine is in making hp. The lower the bsfc, the less fuel it takes to make 1 hp. A/F numbers tell you the ratio of air to fuel. As an engine tuner you want to make the most hp at the lowest bsfc without detonation. The type of fuel used has a huge effect on this. A richer mixture will reduce detonation. A good rule of thumb is to run an A/F of 11.5 to 12.0 on pump gas below 20 psi, and 12.0 to 12.5 on race gas. We have run as high as 12.7 to 12.9 when everything was on the line and we were going for broke.
I believe that if "The lower the BSFC, the less fuel it takes to make 1 hp" is correct than the engine is closer to the ideal a/f. Given this it would still make sense that the lower the BSFC the leaner your running. I think most well tuned Supra's at around 20psi or less will have an a/f of about 11.5:1 or 12.0:1. I know the recommended BSFC for a turbo car is .60 to .65. Given that we tend to be able to run our Supra's a little leaner than the typically turbo car (which would be 11.0:1 to 11.5:1) because of the cylinder head design and a lot of other factors do you still think that .60 to .65 BSFC is still appropriate or would say a .55 to .60 would be more represetative of our cars.

Thanks for the response, Steve
Been looking at the fuel injector worksheet here ?

One thing I noticed, that if our cars are making 600bhp, we would need 800cc injectors to stay under 80% duty cycle. Could that really be right?
What fuel pressure did you assume? It should have been about 60psi at 20psi of boost. I would also use .60 for BSFC until I understand it correctly. This would mean that if you made 600bhp at 20psi than you would need 671cc injectors at 80% duty cycle. It kind of puts in perspective how much us single turbo guys with stock fuel systems are pushing our luck. Now imagine what my duty cycle was at 619bhp with stock 550cc injectors and 20.5psi of boost. :eek:

Later, Steve
Steve, I assumed 43 psi because thats the stock fuel pressure. The pressure difference between the rail and boost is what counts right? FP rises with boost. One guy on the list is running 100 psi total fuel pressure :eek: He says the Supra's side feed injectors can support it better than other kinds.

I saw your post in another thread, please tell us what you find out about enlargening the stock injectors.

So your sure your AVC-R was hooked up right when it read 97% duty at 520rwhp? Cause I've heard someone say 80% at 480, and all sorts of different readings.

Wait, just saw this on the list. Blitz PN#31249 is a side-feed 800cc injector, Dusty says he sells them for $155 each.
I spoke with RC fuel injection and he told me that on the Supra you should use .55 for the BSFC. This meant that on my 619bhp (526rwhp) dyno run I was at about 91% duty cycle. He didn't recommend running over 85% duty cycle. He said that he didn't recommend modifying the stock injectors. It would cost about $175 ea. and you could just buy a good set of new ones for that. My AVC-R isn't giving me the proper injector duty cycle, so I was just use the injector calculator to estimate my duty cycle.

Later, Steve
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