[LostCause]

Originally Posted by RH77

Lower MAP figures = lower than ATM, which I assume is considered a vacuum of sorts. My question: when lifting from the accelerator, vacuum is apparently produced, and (assuming the injectors are not cut), FE increases per the gauge.

The only reason I can think of is that your engine is producing less HP when you lift the gas pedal. Lets say that during acceleration @ 45mph you are pulling 65HP, but cruising @ 45mph requires 15HP. Logically, less fuel is going to be consumed cruising than accelerating at a given speed. I don't think the arrival of vacuum is any benefit.

So acceleration closer to ATM is more efficient? This generally happens during WOT -- where the forced induction comes into play in non-WOT applications.

Driving with load is generally the most efficient, not necessarily accelerating. The idea is to keep rpm's low, but throttle demand high. I don't think the intake manifold ever sees 1 atm, though, so forced induction could still be useful while driving with load.

Forced induction would serve as a band-aid rather than a solution, in my opinion. Essentially, a constriction is being overcome with brute force (which requires additional energy). The most elegant solution would be to remove the constriction.

Still, we're only at the theory stage with this sort of inquiry. I recall a couple years back hearing of a guy with a Civic that ran 0-psi of boost from a turbo to reduce pumping losses or something similar.

Basically, I need more efficient, operability in the low RPM range (1500-2500).

RH77

I operate at the theory stage...

To gain low-rpm functionality, you either have to increase complexity or compromise. If neither option is desired, low-rpm efficiency can just be avoided via pulse and gliding.

In order of difficulty (easy --> hard).

• Create a new intake manifold with runners sized for low-rpms. By timing the intake pulses to coincide with valve overlap you increase cylinder scavenging...and consequently lower the effect of high vacuum. Without variable length intake runners, you'll hurt mid/top-end performance.
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• Add an oversized turbocharger with the wastegate set for 1 atm. This might be tough as I know piston-engine aircraft use a totally unique method of maintaining 1 atm. If worse comes to worse, a microcontroller could work.
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• You can gear higher to decrease engine rpm's and switch out the cam to shift peak torque to lower rpms. Switching the cam allows lower rpms to be used and increases efficiency in that speed range (usually through early valve closing). The result is degraded top-end power, which can be overcome with complexity (VVT).
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• You can adjust the ECU to lean out the mixture. Leaning the mixture decreases the HP for a given throttle setting, which requires the butterfly valve to open more. Honda lean burn engine use this strategy to decrease pumping losses (as do diesels, I'm assuming).
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• You can increase EGR levels to achieve the same effect as lean burn, but you'll start to run into ignition issues. This can usually be overcome by "swirling" the mixture by leaving an intake valve closed and using a hotter spark plug.
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• You can use advanced variable valve timing/ignition control like BMW to totally remove the butterfly valve. Good luck, though...
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• You can downsize to a smaller engine that will be operate at its most efficient rpm when providing power for cruise. Acceleration can be handled by using a turbo to increase compression (w/ ethanol injection) and/or increasing volumetric efficiency >100% .
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• Depending on where you desire low-rpm efficiency, you can kill the engine and use an electric motor to tool about (hybrid technology). Easier conversions would include a lowerable "trolling" motor, upgraded starter for short hops, or a beefed up starter (i.e. belted starter/generator) for low speed movement.

If anyone has information beyond theory or different ideas feel free to post them. I don't know how else to go about the problem.

- LostCause