Originally Posted by 360view
A question that has been endlessly debated in internet forums is:
should the gasoline engine throttle be mostly closed (high vacuum)
or mostly open ( low vacuum)
for best fuel economy?

The (complicated) answer is:
there should be no throttle and the intake valve movements should adjust cylinder filling to suit load.

It should be more discussed that:
a particular type of engine used to have no throttle (diesel)
and thanks to the EPA
most new diesels now have throttles
and diesel fuel economy has gone down
putting more CO2 out into the atmosphere
( and slightly less NOx)


I don't think it really matters to most if not all manufactures because the most important goal is to meet the emissions regulation using the FTP (EPA/CARB Federal Test Procedure) which the CAFE fuel economy results are derived from. They have to meet the FTP emissions reg and cannot put "ringers" into a fuel economy test that would be "tuned" for best MPG and then peg the NOX or HC or any other pollutant that's why the CAFE results are from the manufacture's FTP certification results. And all of the certification data is "Public Record" for EVERYONE to see at any time including all of the other manufactures, greenies and just plain you and me. Here's all of the information down to axle ratios, weight HC and everything else:

2022 EPA FTP and CAFE emissions and fuel economy results


And here from 2022 all the way back to 1984 (ironically the year of "Big Brother" laugh2 ) https://www.epa.gov/compliance-and-fuel-economy-data/data-cars-used-testing-fuel-economy