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I read through this whole thread this morning and would like to add my thoughts.
Best BSFC is a function of manifold vacuum, not throttle position. At 0 vacuum throttle position is irrelevant unless you go to WOT which is above best BSFC.
When I accelerate my Echo (53 MPG average) I just watch the vacuum gauge and try to stay within the range of 1 to 2 inches off 0 vacuum. I am in 5th gear at anything over 30MPH (car is geared poorly at 2550 RPM at 60 MPH-wish it was 2000)
This matches my power to the BSFC map, as long as I keep the RPM in the range where the map tells me the BSFC would be highest.
Load is measured by manifold vacuum. At the exact same throttle position as the load is increased the manifold vacuum will drop, until at max load the manifold vacuum will be 0.
0 manifold vacuum is the top of the map where the maximum power is produced but mixture is richer at WOT.
This is because the engine can produce no more power at that RPM. If you want to increase the acceleration you would have to use a lower gear.
So in conclusion there are two ways to determine if you are in the best BSFC range.
Stay in the RPM range of best BSFC which almost universally is between 1500 and 2500 RPM (good general rule-probably lower for diesels).
Choose the highest gear that will meet your acceleration requirements. Never use WOT unless it is a diesel.
Choose the vacuum level that comes as close as possible to the 1 to 2 inch range.
That will give you optimized BSFC.
An example;
You are climbing a hill in your highest gear with your THROTTLE POSITION FIXED. The hill becomes increasingly steep (more grade) as you continue climbing. Your RPM is in the best BSFC range.
At some point you can no longer maintain your speed (max load for throttle position). This would be where your manifold vacuum would be 0 and your load would be 100% for that throttle position and RPM.
Even though your throttle position might be only 25% of max, increasing the throttle position would not really give you a significant increase in power unless you went WOT and used fuel enrichment.
Lowest measured manifold vacuum without WOT enrichment is your best BSFC range.
To maintain your speed up the increasing steep grade you have two choices. Increase throttle position to WOT or use a lower gear. Choose the lower gear, because you do not want to use WOT and loose your ideal fuel mixture.
Your efficiency is poor when you have significant manifold vacuum, because the vacuum is robbing your engine of it's highest effective compression by reducing the available atmospheric pressure to the cylinders.
If you have the ability to read manifold vacuum. then you can easily get best BSFC by using the RPM limitations and maintaining the manifold vacuum at 1 to 2 inches any time you are accelerating.
That's it.
regards
Mech
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