Robin, I do not fully follow your question. Lets first repeat the subject we
are trying to answer: For a given loading, what is better, higher rpm and
lower load (i.e., more vacuum) or lower rpm and higher load (less vacuum). The
answer to this is as follows. Assuming the load is not high enough that you
are in the fuel enrichment area for knock suppression, the higher load (less
vacuum) is more efficient (also, the lower rpm means less engine friction).
The reason why the lower vacuum increases efficiency was explained earlier.
Now, if you include the fuel enrichment part, its hard to tell for sure which
operating condition is more efficient. The converter slippage adds another
variable.
Quoting Robin Giesbrecht <robings@xxxxxxxxxxx>:
> it is quite simple. More air flow equals more fuel flow and less
> vacuum.Simple. High vacuum equals less air flow equals less fuel flow.
That is true assuming we are at the same rpm. Remember, the high vacuum
scenario is actually at a higher rpm. Remember, the two operating conditions
we are comparing have to have the same power delivery, so the lower load/higher
vacuum case is at higher rpm.
On further analysis of your statement, remember, we are talking about efficiency
here. Efficiency is defined engine power per flow rate of fuel. Closing the
throttle for a given rpm reduces fuel flow, but also reduces power. As a
result, at very low loads, the efficiency decreases. At the extreme, at idle,
the efficiency of the engine is zero, because there is no useful work
produced.
D^2
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