Should you make EFI super-lean at cruise?

[OldMachinist]

Correct me if I'm wrong but an engine is most efficient making about 75-80% of its power as the throttle is more open filling the cylinders more and raising the effective compression ratio. This would mean 4 cylinders would be more efficient at cruise than 8 because the cylinders under power are operating at a higher effective CR. It would follow that your engine would be even more efficient on 2 cyls. at cruise as you would be close to that 75-80% but the engine would probably be objectionable from a smoothness standpoint.

[Beagle]

It would seem to me your not going to find much trying to kill cylinders unless you can kill friction losses with it. You have to make X power to Go Y MPH with the same friction and losses involved.

that's kind of what I was wondering - how to bleed pressure in your unused cylinders. Since you aren't firing BTDC on those holes, wouldn't you lose some of your friction / pumping losses?

I wouldn't want the intake open, but the Exhaust might be a candidate. That would require something besides a cam / spring setup for valve actuation though. As long as I'm daydreaming, a servo based valve actuator would be a total hoot. Programming up a new "cam lobe" with the laptop.. I guess it wouldn't really be necessary.

BMW did a study a while back about running WFO and short shifting to get improved economy, I'm not sure what they ever did with the data. I tried to emulate it with the Mustang, didn't see any significant change from normal driving.

[milner351]

Don't forget pumping losses (and as Oldmachinist stated - throttling losses) and associated heat, unless you can control the valve actuation to the "dead" cylinders to bleed off pressure - the "DOD" idea doesn't work too well.

[TheSilverBuick]

Don't forget pumping losses (and as Oldmachinist stated - throttling losses) and associated heat, unless you can control the valve actuation to the "dead" cylinders to bleed off pressure - the "DOD" idea doesn't work too well.

Going back to post #24.

Pumping losses are still big with the valves going, especially with big displacement.

This is the angle I'm shooting for. I've gone as lean as I can to support combustion in the cylinders, so if I cut half the cylinders out of fuel, it forces the remaining four to work harder and thus require more fuel, and dropping the AFR's, which opens the possibility of using less fuel over all. Basically the limiting factor in reducing fuel into my engine is to support combustion because running the cylinders even leaner will result in mis-fires, now if I take all the fuel currently being used but distribute it to only 4 cylinders, effectively doubling the fuel in the cylinders that are getting fuel (for cruising), my AFR's at that point can then be leaned out some amount again because those 4 cylinders will likely be running at 13-something to 1 AFR's. If I can cruise highway speeds, on something leaner than 13-something to 1 AFR then it could net some gains. Shutting the valves off is definitely the most ideal and would likely net big gains, 25%?, where as this will likely be 5%(?) or so. So definitely splitting hairs at this point, and I think is really only practical on large engines that have so much volume per cylinder there is a higher minimum fuel requirement per cylinder just to support combustion, not to contribute HP. Remember, four of my cylinders is 3.8L, or 231cid, more than adequate to support highway cruising, and yeah the pumping losses are going to eat at least 75%+ of the gains according to my gut feeling.