Reducing Cruising RPM - How Important To MPG ?
Discussion
I'm building a V8 pick up so fuel efficiency isn't exactly my top priority but I'm looking at the gearbox choices and my preferred option (for 1-5) has a lower ratio 6th so 80mph would be 3k whilst I another option would mean it would sit at 2k
How much is that extra 1000rpm costing me ?
Essentially it's just the power to spin the engine at 1000rpm as the actual speed of the truck isn't changing
How much is that extra 1000rpm costing me ?
Essentially it's just the power to spin the engine at 1000rpm as the actual speed of the truck isn't changing
Going from 2000 rpm to 3000 for the same power output, the friction losses would increase by about 50% and the pumping losses would increase fractionally. Friction losses might account for as much as 30% of the total fuel consumption so a rough guesstimate would be maybe 10-15% difference in fuel consumption. It will also be quieter and more relaxed. However, you might find yourself changing gear more often since 2000 rpm is towards the bottom of the comfortable working range of a typical engine.
GreenV8S said:
friction losses would increase by about 50%
Friction losses might account for as much as 30% of the total fuel consumption so a rough guesstimate would be maybe 10-15% difference in fuel consumption.
Really ?Friction losses might account for as much as 30% of the total fuel consumption so a rough guesstimate would be maybe 10-15% difference in fuel consumption.
Did mpg increase that much when 5 / 6 speed gearboxes came out ?
I suppose a quick "guesstimate" could be done by resetting my Vito when cruising in both 5 and 4 it's a diesel V6 but it's the only car I own that has such "advanced technology"
Tony1963 said:
I’d say if it’s a good size V8 with plenty of torque from 1,000rpm upwards, I’d go for the higher ratio. If not, or if it’s only going to be used as a local runabout, go for the lower ratio. Will it be fully loaded/near maximum weight very often?
It's the Lexus 4.3 coming out of a GS430, as standard 5 speed auto geared for 43, 74, 108, 140, 187 mph but obviously has better aero than an old Ford Ranger my preferred choice is 36, 63, 93, 113, 138, 159 but that means a 3k cruise and it would easily sit at 2k with the right gearingMy issue is that it's mostly a toy for blatting about but I might want to use it to move my bikes around which would mean a lot more motorway cruising - ideally a close 4 with a high 5th would be ideal but that isn't available - there are adaptor kits to mate various gearboxes (IS200 and most BMW) to the 3UZ engine
Chances are it will make almost no difference. It will take the same power to drive it at 80mph regardless.
Although if you intend to use it a lot, the additional noise from higher cruising rpm might be more annoying.
And it would be running in closed loop at stioch either way as it would still be very light load.
Although if you intend to use it a lot, the additional noise from higher cruising rpm might be more annoying.
And it would be running in closed loop at stioch either way as it would still be very light load.
GreenV8S said:
Overdrive was popular for a reason. A 50% increase in revs is quite a lot. You'd probably need to compare 3rd vs 5th or 4th vs 6th to get an equivalent comparison.
It will make a big difference to driver comfort, noise etc etc. Less so economy.If it takes 50hp to cruise at 80mph...it takes 50hp whether it's at 2000rpm or 3000rpm. Any posses from friction or whatever will be very small.
I've had many setups in mine, with lots of different ratios etc....overall mpg has never really changed.
You can literally see the improvement in economy on modern cars with a MPG readout driven in 6th vs 5th on a motorway, it's not a massive difference but it definitely helps. Lowering the RPM for the same power output means you need more fuel/air in the cylinders for each engine cycle, which not only reduces pumping losses but also gives a higher dynamic compression ratio which helps thermal efficiency.
stevieturbo said:
It will make a big difference to driver comfort, noise etc etc. Less so economy.
If it takes 50hp to cruise at 80mph...it takes 50hp whether it's at 2000rpm or 3000rpm. Any posses from friction or whatever will be very small.
I've had many setups in mine, with lots of different ratios etc....overall mpg has never really changed.
At a first approximation yes. But then at first approximation it doesn't matter whether it's a 1.5 triple or a 5.0 V8. Losses from friction, pumping etc are considerable.If it takes 50hp to cruise at 80mph...it takes 50hp whether it's at 2000rpm or 3000rpm. Any posses from friction or whatever will be very small.
I've had many setups in mine, with lots of different ratios etc....overall mpg has never really changed.
Interesting question OP.
Here's my engineering take on it - I've got some exams for this somewhere so not totally clueless.
1) Petrol engine operating at stoichiometric will use more fuel at 3k rpm than 2k rpm, directly proportional to the engine speed.
2) But it's probably not running stoichio at 2k (prob a bit rich) - so I would expect it to use more fuel at 3k than 2, but not 50% more - I dunno, say 25%? But more than 0% and probably less than 50%. Given that friction losses will increase at higher RPM, 0% looks unlikely, so say between 10% and 50% more fuel.
3) Diesel has different characteristic, and never runs stoichiometric anyway - essentially it's always 'lean'. In this case I would expect the difference between 3k rpm and 2k rpm to be much less significant, and the 'only' difference in fuel consumption to be related to a) increased friction etc, and b) reduced efficiency (if applicable) of the turbo. Nevertheless, I guess friction losses would still account for maybe a 10% increase in fuel consumption.
I'd take the longer ratio all other things being equal.
Here's my engineering take on it - I've got some exams for this somewhere so not totally clueless.
1) Petrol engine operating at stoichiometric will use more fuel at 3k rpm than 2k rpm, directly proportional to the engine speed.
2) But it's probably not running stoichio at 2k (prob a bit rich) - so I would expect it to use more fuel at 3k than 2, but not 50% more - I dunno, say 25%? But more than 0% and probably less than 50%. Given that friction losses will increase at higher RPM, 0% looks unlikely, so say between 10% and 50% more fuel.
3) Diesel has different characteristic, and never runs stoichiometric anyway - essentially it's always 'lean'. In this case I would expect the difference between 3k rpm and 2k rpm to be much less significant, and the 'only' difference in fuel consumption to be related to a) increased friction etc, and b) reduced efficiency (if applicable) of the turbo. Nevertheless, I guess friction losses would still account for maybe a 10% increase in fuel consumption.
I'd take the longer ratio all other things being equal.
Dr Jekyll said:
At a first approximation yes. But then at first approximation it doesn't matter whether it's a 1.5 triple or a 5.0 V8. Losses from friction, pumping etc are considerable.
Having had at least a dozen different configurations on my car....mpg differences from what are being discussed are not considerable.In fact they will barely change unless you're doing something extremely dumb with tuning.
bearman68 said:
Interesting question OP.
Here's my engineering take on it - I've got some exams for this somewhere so not totally clueless.
1) Petrol engine operating at stoichiometric will use more fuel at 3k rpm than 2k rpm, directly proportional to the engine speed.
2) But it's probably not running stoichio at 2k (prob a bit rich) - so I would expect it to use more fuel at 3k than 2, but not 50% more - I dunno, say 25%? But more than 0% and probably less than 50%. Given that friction losses will increase at higher RPM, 0% looks unlikely, so say between 10% and 50% more fuel.
3) Diesel has different characteristic, and never runs stoichiometric anyway - essentially it's always 'lean'. In this case I would expect the difference between 3k rpm and 2k rpm to be much less significant, and the 'only' difference in fuel consumption to be related to a) increased friction etc, and b) reduced efficiency (if applicable) of the turbo. Nevertheless, I guess friction losses would still account for maybe a 10% increase in fuel consumption.
I'd take the longer ratio all other things being equal.
1. No it wont. It will be ingesting similar amounts of air, dictated by the power needed to maintain that cruise speed...ie throttle.. And there is no reason whatsoever for it to be running rich at such low load. If anything you could easily lean it out a little.Here's my engineering take on it - I've got some exams for this somewhere so not totally clueless.
1) Petrol engine operating at stoichiometric will use more fuel at 3k rpm than 2k rpm, directly proportional to the engine speed.
2) But it's probably not running stoichio at 2k (prob a bit rich) - so I would expect it to use more fuel at 3k than 2, but not 50% more - I dunno, say 25%? But more than 0% and probably less than 50%. Given that friction losses will increase at higher RPM, 0% looks unlikely, so say between 10% and 50% more fuel.
3) Diesel has different characteristic, and never runs stoichiometric anyway - essentially it's always 'lean'. In this case I would expect the difference between 3k rpm and 2k rpm to be much less significant, and the 'only' difference in fuel consumption to be related to a) increased friction etc, and b) reduced efficiency (if applicable) of the turbo. Nevertheless, I guess friction losses would still account for maybe a 10% increase in fuel consumption.
I'd take the longer ratio all other things being equal.
I tried a few different final drives in my old E30. From shortish to very long, like barely able to maintain speed up a slight incline long. Overall I didn’t find a massive difference in real world full tank mpg. Somewhere around the manufacturer’s original ratio was best compromise overall for the car (shocker eh!)
I’d go for more gears if poss to get a longer top gear but sticking a long final drive on the original box is a compromise everywhere but the motorway i rekon.
I’d go for more gears if poss to get a longer top gear but sticking a long final drive on the original box is a compromise everywhere but the motorway i rekon.
Edited by buggalugs on Friday 25th December 10:53
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