engines, speed and economy....

[E38Ross]

McSam said:

es - I touched on this with mentioning the complications for working out an ideal cruise speed. Increase speed by x%, gaining efficiency of y%, but at cost of drag of z% etc. and that's the important bit.

I was using the BSFC stuff mostly to say where the ideal sort of point for most engines is and why it's there, but the really interesting one is what it implies about the most efficient way to accelerate or deal with hills and things. If you're coming up a steep incline in 5th and at 2000rpm and are having to give it quite a lot of boot, you sort of instinctively think that's inefficient and you should change down to get a lesser throttle position. In fact, that makes it much worse, and being almost wide open in the higher gear at 2000rpm would be better!

that's very interesting and useful to know! with my car, when it's on cruise control i sometimes leave it in auto, if it has to climb a steep hill it will often shift into 4th gear to maintain the speed, sometimes i leave it in manual mode and bung it in 5th and leave the cruise on to stop it shifting down, for the sake of a couple of mph it makes for a slightly more relaxed drive. it rarely happens (only on pretty reasonably steep hills on the motorway to be honest) but now it would appear leaving it in 5th on manual would be the more efficient method. as said, 70mph comes in at around 2050rpm so it's geared very tall.

thanks!!