Vacuum VS Rpm

[MDCmotorsports]

Ive been contemplating this for a long time now.

Which would save more fuel @ 14.7:1 AFR

1.) Running 3,000rpm @ 10 vacuum

OR

2.) Running 4,000rpm @ 20 vacuum


So would less load (more vacuum) be more fuel efficient or would less RPMs and more load (less vacuum) be more fuel efficient?

Discuss.

:icon_razz

 

[figgie]

less rpm will always be more fuel efficent than higher rpms. Basically a matter of what saves more fuel

10 x 10 ms injections or 100 x 10 ms injections

 

[inline6]

Is that true if you step it up even more tho? 3200 RPM and 10psi vs. 4000 RPM and 10mmHg. I'm just playing devil's advocate but I've wondered this before also?

 

[TurboStreetCar]

well u want a ballance of both, just gota find that ballance, at a lower RPM the injectors open less times but at a lower load they dont stay open as long, so u would need to find out how long and how many times at diffrent rpm's in diffrent gears.

lower RPM isnt always better for fuel economy.

 

[figgie]

well u want a ballance of both, just gota find that ballance, at a lower RPM the injectors open less times but at a lower load they dont stay open as long, so u would need to find out how long and how many times at diffrent rpm's in diffrent gears.

lower RPM isnt always better for fuel economy.

actaully it always is
otherwise we would still be running around with 2 speed powerglides as transmissions.

 

[aljordan]

I say vacuum. Load is bad for mpg. Low load and low rmp is best for economy.

Also, injectors don't open longer the higher the revs. The time (milliseconds) between closing and opening just shrinks.

 

[3p141592654]

There is a lot of papers on this in the SAE journals. Low speed high load for best efficiency. Does anyone remember the shift arrow light that VWs used to have. It would tell you when to shift to get best fuel efficiency, and basically you were lugging the engine all the time if you followed it.

 

[TurboStreetCar]

actaully it always is
otherwise we would still be running around with 2 speed powerglides as transmissions.

so if ur lugging the motor at 2000rpm and say the injector duration is 20ms and 5 inhg of vaccum, then u downshift and get the motor to 3000rpm much higher in the engines torque curve and are cruising with 15inhg of vaccum but the pulsewith is 5ms ud be running more fuel efficient at the lower rpm?

if u free reved the motor to 3000rpm and recorded the gallons per minuite fuel consumption rate then took the same engine to 2500rpm and put it under 30-40% load and recorded its gpm fuel consumption rate do u really think the 3000rpm free rev would be less fuel efficient?

i agree that lower rpm means theinjectors are open less times per minuite, but if u have to be at half throttle to cruise at 1500rpm and 1/8 to cruise at 3000rpm, ur gona get better gas mileage at 3000rpm

and i think the reason we steped up from powerglides is for the gearing, not for fuel economy. say ur cruising with a powerglide and ur at 2500 in second at 60, now u go to a hill and the motor starts lugging so it downshifts, now ur crusing at 60 at 5000rpm to maintain speed up a hill, with a conventional 4 speed the downshift would only take u to 3000 or so

(all above numbers are hypothetical)

 

[TurboStreetCar]

less rpm will always be more fuel efficent than higher rpms. Basically a matter of what saves more fuel

10 x 10 ms injections or 100 x 10 ms injections

thats not entirely right either, the question was 3k to 4krpm, it woudl be more like

10 x 10 or 13.3 x 10

but it would be more diffrent becasue of the lad diffrence, more load more fuel so itwould really be more like

10 x 15 or 13.3 x 10

witch shows that the less load, higher rpm would have open time of the injectors or less fule comsumption.

im not trying to bash u figgie, im just stating my opinion in this discussion. no beef man lol

 

[3p141592654]

Internal and pumping losses go up with rpm. For the same shaft horspower, the lower speed will give better efficiency.

 

[TurboStreetCar]

is it really that great of a differnce of 1000rpm?