Easy and accurate test (based on relativity) ?

[some_other_dave]

Also, don't forget what BSFC stands for: Brake-Specific Fuel Consumption. That means (pretty much) fuel consumption per horsepower, or per foot-pound of torque, or per unit of load. (Those measurements are somewhat interchangeable, at least in this context.) So you have to calculate or guess at the amount of HP, the amount of TQ, or the actual load on your engine to get an idea of BSFC.

My gut feeling is that you would make so many assumptions to make the numbers calculable by a reasonable person that the numbers would be slightly better than guesses. You'd have approximations of approximations of approximations...

In general, you can use injector opening time as a proxy for fuel usage. But it is very much a relative measure, it is an approximation. As noted, other factors (pressure differential across the injector valve, flow while the valve is opening and closing, etc.) can have a pretty significant effect. The more conditions differ between two measurements, the less reliable the approximation is.

Note that the MPGuino does just this, uses injector-open time to work out fuel consumption. But it has a lot of fudge factors built in, and most of its users (hopefully) realize it is an approximation.

But once you start multiplying approximations by estimates and dividing by guesses, the reliability of your results get more and more questionable.

-soD

[user removed]

I can't remember any fuel injected car that did not have different fuel pressures depending on load (manifold vacuum). Older cars (maybe still some newer ones) use a vacuum pressure regulator with a return loop for excess pump pressure.

Fords in the last few years use a sensor, without a return loop for fuel back to the tank.
The sensor sends a signal to the ECU which sends a specific amount of power to the fuel pump control module, which controls the pressure by controlling the pressure created by the fuel pump itself.

I am pretty sure my VX had the same system as Nissan from the same era. Wet pumps inside the tank itself with 65 PSI of pump developed pressure regulated to 30-40 PSI at the injectors, using engine vacuum. In turbo setups, the positive pressure would increase the fuel pressure beyond the point of atmospheric pressure.

It may be that recent developments eliminated the necessity to change fuel pressure, in much the same way some modern systems have eliminated the EGR valve.

regards
Mech

[user removed]

I may have replaced 1000 regulators over the last 30 years. Nissan started using them in the Z cars in 1975, the first year they were fuel injected.

regards
Mech

[user removed]

One thing that is fairly constant at RPMs and loads that are in the best BSFC range is mixture ratio. It does change when you go WOT, but in other areas of engine operation it is fairly consistent.

MAP or the reverse, vacuum is a good indication of the amount of air, relative to atmospheric pressure that actually enters the combustion chamber. I like to call this effective compression, because it can be calculated by knowing atmospheric pressure and intake manifold vacuum readings (or MAP).

This volume of air can be fairly accurately measured, and with that quantity know, the corresponding volume of fuel can also be calculated.

Best BFSC will be found at lowest manifold vacuum (highest MAP) that the fuel injection system will allow, without enrichment as you approach WOT.

The only other factor is RPM, and if you know the range where BSFC is highest you can stay in the range of RPM and load to minimise consumption per unit of power.

regards
Mech

[autoteach]

Originally Posted by Old Mechanic

One thing that is fairly constant at RPMs and loads that are in the best BSFC range is mixture ratio. It does change when you go WOT, but in other areas of engine operation it is fairly consistent.

MAP or the reverse, vacuum is a good indication of the amount of air, relative to atmospheric pressure that actually enters the combustion chamber. I like to call this effective compression, because it can be calculated by knowing atmospheric pressure and intake manifold vacuum readings (or MAP).

This volume of air can be fairly accurately measured, and with that quantity know, the corresponding volume of fuel can also be calculated.

Best BFSC will be found at lowest manifold vacuum (highest MAP) that the fuel injection system will allow, without enrichment as you approach WOT.

The only other factor is RPM, and if you know the range where BSFC is highest you can stay in the range of RPM and load to minimise consumption per unit of power.

regards
Mech

And that is exactly what I was referring to when I said that load % coupled with a good BSFC map (or graph, if you will) will give you a better idea of the mpg than the pulse width or duty cycle of the injector.

Does the difference in pressure from the fuel side to the air side of the injector, if maintained with the same differential, maintain the same flow for the same duty cycle?

Does a duty cycle of 30% for 1500 rpm and 3000 rpm consist of the same amount of fuel flow?

[gone-ot]

Originally Posted by Old Mechanic

Best BFSC will be found at lowest manifold vacuum (highest MAP) that the fuel injection system will allow, without enrichment as you approach WOT.

...called, "...following the bathtub curve..." between idle-rich (closed throttle) and power-rich (WOT), back in the days of carburetors.

[UFO]

Originally Posted by Old Mechanic

I may have replaced 1000 regulators over the last 30 years. Nissan started using them in the Z cars in 1975, the first year they were fuel injected.

regards
Mech

And I own and modify that same Z car. And I can say with 100% certainty this very common fuel system maintains a constant fuel pressure across the injector. So a duty cycle of x% represents the same fuel flow at a particular rpm regardless of engine load.

Reference your fuel pressure gauge to the manifold and you will see what I mean.

[abently]

I wonder how much opening and closing times change with duty cyle?

Because say the duty cycle is within 5%, you would assume the error margin would be negligible?

Or has any oscilloscope'd injectors at different duty cycles and posted the difference?

[user removed]

Originally Posted by UFO

And I own and modify that same Z car. And I can say with 100% certainty this very common fuel system maintains a constant fuel pressure across the injector. So a duty cycle of x% represents the same fuel flow at a particular rpm regardless of engine load.

Reference your fuel pressure gauge to the manifold and you will see what I mean.

I have heard the same statement for other sources. It's been a while since I worked on them.

My memory was the pressure range was 32-40 PSI, with pump pressures of 65PSI.

Maybe I am not following your logic correctly but with atmospheric pressures ranging from say 26.99 (cat 5 hurricane) to 31 (seriously high pressure cell at sea level), I can't follow the logic that the range differential in pressure at the injectors follows the range of available air pressure into the cylinders.

With vacuum readings of 20-22 inches, your range of available air pressure to the cylinders would be 4-9 inches (depending on atmospheric pressure) and 26 to 31 inches at WOT.

Maybe my math is rusty but that would be hard to match perfectly with a range of 8 PSI at the injector, but I am always open to additional learning .

Here is my 76 280 at the National Convention in Knoxville 1992.

regards
Mech

[UFO]

Factory fuel pressure across the injectors is typically around 40psi. The pumps will dead head over 80psi. I went to a Porsche pump that will put out over 100 psi so I have the overhead to maintain fuel pressure over 20 psi of manifold pressure. I am still running about 45 psi at the injector over all manifold conditions.

I'm sorry for the tangent, but I think it points to the simplicity of the OPs goal of comparing the efficiency of one gear over the other. Fuel injectors take a relatively long time to open and typically cannot reliably deliver fuel under 5% duty, so your efficiency comparison may break down at lower duty cycles.