I’ve noticed that many of your tech questions are about carburetors but I have an EFI question for you. When choosing fuel injector size for a supercharged engine, the BSFC number is always higher than for normally aspirated engines. Bolting a blower on an engine with a BSFC of 0.5 should not change how the engine burns the fuel, which is why I don’t understand why the blower engine BSFC number is always higher. I’m sure there is a good explanation but I don’t see it.
D.T.
This is an excellent question that will first demand, of course, that we lay the groundwork and define BSFC for those readers who may not be familiar with this concept.
What Does BSFC Mean?
Brake Specific Fuel Consumption (BSFC) is a measure of the engine’s ability to convert pounds of fuel (in this case gasoline) into heat which the engine turns into observed horsepower. Notice that this definition starts with observed horsepower, which is the power the engine makes before we correct the numbers for atmospheric pressure, temperature, and humidity standards.
The easiest way to explain this process is by an example. So let’s say we have a typical small block Chevy V8 on the engine dyno. Most dynos are equipped with a fuel flow meter that measures the amount of fuel the engine uses at each specific rpm data point. So let’s say that after testing, the engine makes 440 horsepower and when we look at that rpm line, we see that the engine is using 226 pounds of fuel per hour to make this power.
If we divide the amount of fuel by the observed horsepower, we come up with a fractional number. The math looks like this: 226 lbs. of fuel per hour / 440 horsepower = 0.513 BSFC. This number is expressed as 0.513 pounds of fuel per horsepower per hour (lbs./hp/hr).
Let’s say that now we make some changes where we optimize compression and employ cylinder heads with a better combustion chamber design. Now the engine makes 450 observed horsepower at the same rpm but uses less fuel at only 205 pounds of fuel per hour to make that additional power.
Now the math looks like this: 205 lbs./hr. / 450 lb.-ft. = 0.466 lbs./hp/hr. BSFC. This reveals that our engine is now more efficient because it can now make more power with less fuel. Most hot rodders just look at torque and horsepower.
There’s nothing wrong with this but engineers and professional engine builders—especially road race, circle track, and endurance engine builders—watch the BSFC very closely because if they can make more power on less fuel, they can go farther on the same load of fuel in the car. And that will win races.
All this goes to explain how BSFC is used to measure the engine’s efficiency. Keep in mind that this is not necessarily an evaluation of how lean or rich the air-fuel mixture is. Every engine will tell you—if you are paying attention—what air-fuel mixture is best for overall power. Some engines want to run lean while others make best power running a richer air-fuel ratio. It’s your job as the tuner to give the engine what it wants.
The Effect of Supercharging on an Engine’s BSFC Number
Now let’s address your question. You are correct that if a given normally aspirated engine generates a BSFC of let’s say 0.51 at peak horsepower, that bolting on a supercharger should not change what happens in the combustion chamber other than pushing a denser charge with boosted air into the cylinders. If we want to split hairs, the charge will likely produce BSFC changes because of a higher inlet air temperature and perhaps changes in mixture density due to how well the blower distributes the air and fuel, and perhaps another half dozen variables but we’ll put those aside for now.
What does drastically affect the BSFC number to determine fuel injector size is the fact that a supercharger demands crankshaft power to drive it. This additional power requires work from the engine that is absorbed by the supercharger and therefore is not measured at the flywheel as power.
But the blower’s power demand is most definitely there. Let’s use some numbers to illustrate this. The fuel injector and fuel pump industry uses a typical 0.50 as the BSFC number for a normally aspirated engine running on gasoline. That number changes to 0.60 for an engine running a supercharger on gasoline. Here’s why that supercharged BSFC number is higher—indicating reduced efficiency.
Let’s say we bolt a blower on the above small block and now that normally aspirated engine that made 440 horsepower now makes 600 for a gain of 160 hp or 37 percent. We might expect the BSFC numbers to change a little but we discover that now our fuel usage has increased from 226 lbs./hr. to 355 lbs./hr. which is more than a 50 percent increase. Yet the engine should be able to make power at its normally aspirated BSFC of roughly 0.50.
The math would look like this: 600 hp x 0.50 BSFC = 300 lbs. of fuel. Yet our engine consumed much more at 355 lbs./hr.
What the numbers are telling us is that the supercharger was demanding power to drive the blower and squeeze the air. This is not a free ride. Let’s make some assumptions and say that the blower required 90 hp to make boost at peak horsepower. That requires fuel—but that additional power is eaten up by the blower and is not measured at the flywheel. But the engine still had to make the power and use fuel to accomplish this task.
This additional 90 horsepower load on the engine means we will need an additional 45 pounds of fuel per hour to feed the power demanded by the blower using that 0.50 BSFC number. So when we see the engine making 600 hp at the flywheel—the engine is really making roughly 690 hp.
When we read the lbs./hr. of fuel consumed at 355 lbs./hr.—the result is: 355 lbs./hr. / 600 hp observed = 0.592 which is very close to the standard 0.60 BSFC recommended by the fuel injector and fuel delivery companies.
Sure, it is entirely likely that your next supercharged engine will be able to generate a much more efficient BSFC number of—let’s say 0.55. But you have to build the engine and select the fuel injectors before you test the engine. It’s always better to use a slightly larger fuel injector (or slightly larger fuel pump) than your engine requires to be on the safe side. What you never want to happen is where the injector is too small and the engine runs lean at peak power. That’s when bad things can happen.
So it’s always best to be on the rich side of the selection process for either fuel injectors or fuel pumps. You can always shorten the duty cycle on the injector to deliver less fuel when it runs rich. But if an injector is too small—there are very few things you can do to compensate and those solutions usually are accompanied by unintended consequences.
Turbocharged engines should be tuned with similar attention but keep in mind that while turbos make boost by using “free heat” from the exhaust, there is no such thing as a free lunch. All turbos require engine power to push the exhaust out. This is most often measured as backpressure in the exhaust. The engineering phrase is “pumping losses.” Again, this backpressure at the turbocharger requires power to push the exhaust out—which requires fuel to make the power demanded to overcome the backpressure. It’s easier just to lump turbos in with superchargers to be on the safe side.
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I hope this answers your question on why supercharged engines demand higher BSFC numbers. Once you understand how the system works—you can make more intelligent decisions about your next engine project.
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