BSFC - Confusion Cancellation

[Dusty]

BSFC

Is it the third most misunderstood number in the engine performance field?

By

Dusty Kennett

It’s been my experience – even this early in my career, that the two most commonly confused numbers are virtually joined at the hip. These being torque and horsepower. This does not so much hold true for serious enthusiasts who, all to often, are in deep enough into the subject to make the effort to find out and understand the relationship of these two numbers. What is a little surprising is that even big time race engine builders with big time race wins under their belt still fail to fully understand what I perceive as the third most important number - Brake Specific Fuel Consumption is, and more importantly what it is not!

Here’s an 350 small block Chevy engine I built for a PHR story (son of Sledgehammer) published about September 2007. Brake specifics on this were good in as much as they were low numbers. However if these low numbers had been interpreted as a lean mixture about 15 of the 470 hp would have been jetted out!

Let’s start with what it is not. It is not a measure of the mixture ratio. I have heard engine builders say they like to see the BSFC in the high 0.3’s to low 0.4’s as if this is better than say 0.28 or what ever. An interesting story here is that while lecturing at UNC Charlotte some years ago editor Vizard, during a Q& A session, was asked by a successful Cup Car engine builder what he liked to see in the way of BSFC figures. Realizing this was one of those situations where the person concerned was under the impression there was an optimum somewhere in the 0.3-0.45 range DV turned the question around and threw it out to the audience. Of some 115 attendees (standing room only in a 100 seat auditorium) about 100 were engine builders and the rest students and one member of the press. Of all the answers given only our press guy, Johnny Hunkins, now editor of Popular Hot Rodding, got the answer right. The optimum BSFC is – zero. What this means is you have an engine which is producing power and using no fuel to do it! Let’s be honest here you can’t get more efficient than that!

Hopefully the point I have just made is starting to revise the way you might be thinking of the BSFC. First it is called ‘Brake’ because it is measured on the ‘Brake’ now more commonly called a dyno. It is ‘Specific’ in as much as it applies specifically to one horsepower generated for one hour. In other words it is the amount of fuel, in pounds per hour, consumed by one hp of the engine being tested. That makes the number useful as a means of comparison regardless of the displacement of the engine.

At this point however I probably have not convinced a die-hard of anything of significance. The truth is that the BSFC figure can change while the mixture goes totally unchanged. If that is the case then the BSFC figure is hardly applicable, let alone reliable, as a gauge of the mixture ratio. Let’s look at an example here throwing in some numbers to show how things are breaking down.

Assume we have a test engine that makes 500 hp at 6000 rpm and at this point it looses, in pumping losses and internal friction, 100 hp. Attached to this engine we also have a means to simulate added internal friction. This it achieves by means of a disk brake on the front of the engine. What this allows us to do is to simulate the loss of another 100 hp in terms of internal friction.

At this point we dyno the engine and, with a 13/1 air/fuel ratio, the engine uses 200 lbs of fuel per hour. This means the BSFC is 200 lbs per hour of fuel divided by 500 hp – answer 0.400 lbs per hp per hour. So for every hp generated the engine uses, in one hour, 0.4 lbs of fuel.

If the brake at the front of the engine is now applied so as to simulate a 100 hp loss at 6000 rpm we will see, as measured at the flywheel, by the dyno, only 400 hp. The induction system has no idea that power is being absorbed by something other than the absorber on the end of the flywheel so it continues to feed in identical amount of fuel (200 lbs per hour) and mix it with an identical amount of air. Since neither the flow of fuel nor air has changed the air fuel ratio is still exactly 13/1. The same cannot be said for the BSFC though. If we now work out the number it is 200 lbs of fuel per hour divided by 400 hp. That comes out at 0.500 lbs per hp per hour. We have made no change in the mixture but the BSFC has changed by 20%. So we can with total certainty say that the BSFC is not a measure of the mixture ratio!

Now some of you old timers who have been looking at BSFC to determine if the engine is rich will be pleased to know that one of the symptoms of a poor (ie big number) BSFC is an overly rich mixture. However a poor BSFC could also be the result of the ignition timing being way out or any other of a number of faults for that matter.

A really good spark ignition piston engine can get down to the low 0.3’s. An example here is the compound turbo engines used in the last derivatives of the Lockheed Constellation airliners of the late 50’s. These had a BSFC of 0.32. Getting much below that seems hard to do but there are developments in the pipe line that could crank the barrier down to an estimated 0.28 – but a discussion on that is off limits at the moment.
Suffice to say at this point the best way to measure mixture with the minimum of unaccountable error is buy a wide band O2 system. One we use on the dyno regularly is the system by Innovate Motorsports (949-502-8400) in Irvine California.

Here’s a GFN hydraulic roller valve train 383 small block Chevy test engine we have been working with recently. The way we get the mixture right is to use a wide band O2 unit. The best option is to have one in each cylinder but failing that an excellent job can still be done with one in each collector as indicated here. This motor made over 530 hp and just marginally missed 500 lbs-ft on a hydraulic roller and out of the box cast Pro 1 Dart 215 cc port heads. Engine turned flawlessly to 6700 rpm!

Dusty Kennett

[Jesse Lackman]

-BSFC during engine dyno sweep testing can be thrown off by normal fuel flow meters lagging behind carb booster fuel flow.

If you don't believe this do both an upsweep and downsweep and compare the BSFC, fuel flow, and dyno calculated A/F ratio readings. Caculated A/F ratios can be especially off because airflow turbines can lag behind RPM changes during sweep testing just like fuel flow meters.

Steady state testing will produce the most accurate BSFC numbers.