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Curious: injector sizing & BSFC

DR1665

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Joined
Oct 19, 2005
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4,642
Location
Iowa City, IA
Brake specific fuel consumption (BSFC) is a function of pounds of fuel used per hour relative to power output (lbsFuel/HP) and is used to calculate target injector size, but I wasn't able to find anything regarding BSFC figures for the Galant here with the search. General consensus seems to be turbocharged engines should be 0.60-0.65 BSFC, but where does that figure come from? I'm not looking to go overboard with the math, since there's a point where injector needs take a backseat to market forces. (ie; Why pay more for 700cc injectors than for commonly available 720s?)

Assumptions:
BHP = 300, BSFC = 0.60, numINJ = 4, DC (duty cycle) = .80 (80%)

(300*0.60)/(4*0.80) = 56lbs/min * 10.50 = 590cc for 80% duty cycle at 300bhp

As BSFC increases, so too does injector size for a specified target.

I'm just curious. Stock injectors are 450cc, right? Do the math with 100% duty cycle and 300bhp is theoretically possible on 472.5cc injectors. I know. I know. This isn't an ideal situation, and I'm not suggesting some rookie asstard runs out and tries this in his barely maintained 4-door DSM, but it seems to me 280bhp would be possible on stock injectors, assuming the pump could keep up.

Don't really have a question with this one. More open to discussion, I guess. Seems like you could get into plenty of trouble on a set of 750s and a stock engine; pretty close to 400bhp, anyway.

Just spitballing.
 

AnotherNewb

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Apr 25, 2010
Messages
1,472
Location
Orlando, FL
I don't have much to add, however here is a link to RC's website with many other useful formulas in deterimining proper injector sizing for HP goals.

Click
 

the bsfc comes from how efficient the engine is. Many factors will affect it including cam profiles, compression ratio, ignition timing, etc. the .60 to .65 for a turbocharged engine is going to be fairly accurate, however I would lean toward the larger end of your choice of injectors. a slightly small injector will limit you a lot more than a slightly large injector. another thing you can use to manipulate injectors is fuel pressure. assuming your pump and regulator are capable, you can add pressure to make a smaller injector flow more. i generally run 50-60 psi depending on application.
 

AnotherNewb

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Joined
Apr 25, 2010
Messages
1,472
Location
Orlando, FL
You have to be careful how much pressure you run on your injectors, you can hold them shut with too much. Adjusting fuel pressure is a coarse adjustment, fine changes happen in the fuel maps. Increased fuel pressure isn't a replacement for undersized injectors.

You run 50-60 lbs 'base' pressure?
 

any aftermarket injector, and probably all oems since thats what aftermarkets are based on, will handle 75psi base pressure as long as there isnt anything wrong in the rest of the system like low voltage.
yes i generally will run 50-60 psi base depending on what my fuel pump can handle. Im not suggesting anyone else do it, im just saying its being done with no ill effects.
 
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