save the bunnies by shifting at 2500rpm

[dylan0451]

it was ages since i read about this through a load of corporate bks at work, but didn't some scientists work out the most efficient/economical way of driving was to drive WOT and shift at 2500rpm?

what i don't get with that is:

if oil pressure is directly linked to rpm, then surely it's putting more wear on the oil to keep film strength up over bearing faces at low rpm/pressure, WOT... in petrol cars atleast, diesels have to put up with quite a torque surge after 1800-2000rpm

and, in the case of fixed cam engines, surely peak efficiency is going to be closer to halfway between idle + redline - a balance between power output at x rpm and valve lift

i'm not a scientist, so presumably have missed the boat somewhat

discuss?!

[dylan0451]

davepoth said:

Peak thermodynamic efficiency, i.e. the petrol explosion efficiency, is most effective at lower speeds (more time for complete combustion). While power is down at those revs, power per gallon of petrol is at its highest.

If you cam the engine in such a way as to maximise the HP at lower revs (like all cars used to have when they didn't need to cheat to get high HP and also good fuel economy on the crappy EU scale) you can quite happily pootle around between 1750 and 2500 rpm at full throttle. Older cars may be a different thing because oil technology wasn't so great.

ahhhh, i see, i did indeed step off the pier

i'm guessing that would explain all those curious engines bmw made for the 3? series back in the late 80's for south african? markets - the ones with weak valve springs, less bearings, lower rev limit etc. x or E type or something

i still don't see though, that manufacturers have designed the actual production engines themselves to operate that efficiently at such low revs - over fueling, valve open/overlap i get that they could be designed that way, but wouldn't that need, long term, a higher compression - diesel?