Brake Specific Fuel Consumption (BSFC) For Turbo BMW Engines

[mcoupemindy]

Has anyone calculated or researched the BSFC for a turbo BMW engine? Most evidence shows ~.50, but there's no real testing, evidence, theory to show.

I'm quite aware at the high efficiency of BMW turbo platforms, but really how high is the efficiency?

Mind you, I'm a bit of a n00b when it comes to this, so I'd like to hear your information.

Thanks!

[Shuasha]

What HP level are you looking at? There is enough experience on here to tell you what you need for an injector.

[mcoupemindy]

What HP level are you looking at? There is enough experience on here to tell you what you need for an injector.

I'm a theory guy :) You should see some of the nerdiness that I create in my spare time.

[Shuasha]

I'm a theory guy :) You should see some of the nerdiness that I create in my spare time.
Sure, but according to the BSFC calculators I've seen tried, you need monster injectors for even 430 hp. I just go with experience instead, it's less depressing. :)

[pbonsalb]

Could you get a reasonable estimate by dynoing the car to determine rwhp while measuring injector duty cycle?

[mcoupemindy]

Could you get a reasonable estimate by dynoing the car to determine rwhp while measuring injector duty cycle?

Yes, I already have the groundwork laid for that.

I have a consistent dyno and have been running stock cars to get a true loss.

In time, I'll be able to back into the number.

[pbonsalb]

At 463 rwhp from a centrifugal that was sapping considerable crank horsepower, I was at 77% duty cycle with 60 lb injectors (rated at 3 bar; 65 lbs at BMW 3.5 bar fuel pressure) and AFR were very high 11s to about 12.0. I could only guess as to crank hp since the blower drag is unknown. AA suggests 25% for a centrifugal and 18% for a turbo instead of the usual 15% loss from rwhp to crank, as general numbers.

From the research I did, I think the BMW engine is very efficient, but did not feel I could come up with reliable numbers. If I had to guesstimate, I think naturally aspirated BSFC could be as good as about 0.42, turbo BSFC might be 0.48 and centrifugal BFSC maybe 0.50. I don't think these motors get anywhere near the 0.55 to 0.65 BSFC that you see commonly touted for forced induction, at least not for motors in the 500 rwhp range. The BSFC may well go up when they are pushed to the point where the stock head becomes less efficient, unless a fancy head is used.

[mcoupemindy]

^ Great information. Thanks for sharing.

[mcoupemindy]

^ Great information. Thanks for sharing.

[nucci]

BSFC is a "curve" which follows volumetric efficiency, or in simple terms, the engine's torque curve. BSFC will be lowest at peak torque and generally highest at redline since most engines have fallen out of their cam/intake manifold/cylinder head tuning RPM range by then.

It is also very much affected by AFR and ignition timing. Most of the setups on this board are not running MBT timing because they don't have enough octane to do so with the boost they're running. Retarded timing raises BSFC as it is reducing cylinder pressure from what it could be.

I'm using some simulation software and finding predicted BSFC with 11.6:1 AFR to be around 0.47 at peak torque, tapering to 0.50-0.57 at redline. If the setup has backpressure > boost, boost taper, and/or a restrictive exhaust system, think 0.55-0.57. If the setup has a big turbo with no boost taper and a 4" unmuffled exhaust, think 0.50-0.52.