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You can have very low manifold vacuum even at low percentages of throttle position. This happens when you are using a higher gear and less throttle to accelerate. If you have the ability to read manifold vacuum this will be about 1 to 2 inches of vacuum reading. If you are in the correct range of gearing you will accelerate efficiently.
Conversely, if you are reading MAP then it will be above 80-90% when you are in higher gears and accelerating at the best RPM range. If you go to the next gear higher and do not achieve the desired rate of acceleration then the lower gear is the best choice. This is because you can take much longer to reach the desired speed if you choose too high a gear. Fortunately I found the best rate of acceleration to be right at what traffic normally accelerates from a stop. This way you are neither pushing the car in front or impeding the car behind you, which in either case can cause the drivers around you to do stupid things that increase your danger of an incident.
You can alter your rate of acceleration by using higher or lower gears, but as long as you maintain a consistent load at 80-90% you will get to speed with the least fuel. You can go too far in either direction to high a gear and you do not accelerate rapidly enough and spend to much time using to much fuel. To low a gear and your RPMs go beyond your best BSFC range.
Think of climbing a grade. As the grade increases you are forced to decide to downshift or loose speed. When the gear you are in can no longer maintain your desired speed then you go to a lower gear while maintaining the same load and BSFC. Some engines might do well at 1500 RPM while others will need to be at 2500 RPM. Looking at your specific BSFC chart will show you the best range of RPM to maintain highest efficiency. Just keep the load at the 80-90% point. It may be only 30-40% of the available range of your throttle position, but if you are at the correct MAP or vacuum, then you are getting the most for your energy use.
While it may seem to be you are using to much fuel at that instant when you are at best BSFC, you must understand that best BSFC is never lowest fuel consumption overall. It is the most power you can create for the least amount of fuel. Increasing the velocity of you vehicle is storing energy in the faster mass of the vehicle itself. Then you spend that energy coasting with no fuel consumed (engine off) or very little fuel consumed (engine idling). While engine off is the best, if your car is an automatic and you must keep the engine idling to operate the oil pump in the transmission you can still coast with very high MPG.
Lets say my Maxima is coming off the Interstate at 65 MPH. I shift into neutral with the engine idling. If my engine uses .3 GPH idling, and I could coast for an hour like that, then I would get about 200 MPG coasting for that time period. It would be infinite if I turned the engine off but it would also destroy my transmission, so I let it idle.
regards
Mech
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