Ok, we've got a lot of great formulas, worksheets, and calculators to determine fuel needs for our cars. One problem: they all rely on a value for BSFC. This value is a BIG deal. You change it just a little, and everything else changes. And most everyone is just guessing at the value. Not only that, but it changes while driving, it's NOT a constant. It's only really valid at max torque. So basically, I'm asking why any of these great resources we have do us any good, if we don't know the exact BSFC for our exact engine?

Example, play with the BSFC calculator at the bottom of this page: http://www.rceng.com/technical.htm#WORKSHEET
Put the known constants in, but use different "System Fuel Pressures". Assume you know what HP your engine makes.

Remember...we're *using* BSFC to determine needed fuel pressure, injector size and whatnot, yet there's no way to know what BSFC is or what Fuel Pressures to run without ENGINE (not chassis) dynoing. And even if we could dyno the engine itself, we couldn't extrapolate the other values for partial boost, idle, vacuum, etc. because the BSFC is no longer valid for those loads.

Also, assuming I was comfortable guessing a "general" BSFC value for my engine... what should it be for a Turbo 420a with 8.5:1 comp dished pistons? Would it be the .525 Corbin uses in his example problem, or the .60-.65 recommended by RC Engineering for turbo cars?

Obviously, I'm confused. :D

Corbin actually got that .525 value from someone who had done some testing on our engine I believe (maybe RC) and they said that it should stay the same as long as the compression is the same.

If you lowered your compression, I would assume your BSFC would go up b/c you're losing efficiency. I'd use the higher numbers (~.60 in your example and according to RC) to be on the safe side. I'm sure there will be some more opinions on this one. Good question though.

I got that 0.525 BSFC from RC Eng. Its just their best guess based on the design of our engine. When it comes to fuel calcs...there are alot of complex equations, constants, and variables. Don't get too caught up in exact numbers; they will get you close enough. And I agree that BSFC should rise as comp ratio decreases (all else equal). The BSFC #'s RC lists on its site are VERY general and are SAFE for any engine (in other words, they don't apply to our engine).

Corbin

'95 ESI-T
HRC+FMIC+ETC...



Gimme fuel...Gimme fire...Gimme that which I desire