Author Topic: High gear and low revs or low gear and high revs?  (Read 2417 times)

Jocko

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High gear and low revs or low gear and high revs?
« on: August 03, 2019, 06:07:52 PM »
I always monitor the Instantaneous mpg while driving. My Mk 1 doesn't have this option on the dashboard display (Fibometer) but I believe it was available from the Mk 2 on. I use my ScanGauge E.
What I have found, is that selecting a higher gear returns better mpg than a lower gear. This works for hills, flat roads, city driving, whatever. I find I get better mpg with the engine "slogging" than free revving. I put slogging in inverted commas as I don't have it labouring, just working hard.
Obviously you don't get the acceleration a lower gear would give, so it is not for all traffic conditions. But a high gear, and low revs, give me my best numbers.
Try it and see if you find the same. I know many of you never even consider selecting top around town, but give it a go. My car pulls happily in 5th gear from 22 mph, it is just a case of choosing the right throttle position.

Ralph

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Re: High gear and low revs or low gear and high revs?
« Reply #1 on: August 03, 2019, 06:35:12 PM »
What I have noticed since swapping from manual to cvt is that the auto will not let you “slog” the engine it will rev at the slightest provocation where as the manual with it’s gear changing arrows would want  you to change into top gear as soon as possible even if you were going uphill is this the reason that I get less mpg from my cvt than the manual I had before. It seams the auto doesn’t practice what the manual preaches

sparky Paul

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Re: High gear and low revs or low gear and high revs?
« Reply #2 on: August 03, 2019, 10:38:26 PM »
The i-DSI really pulls from low down and encourages you to stay in higher gears, peak torque must come in really low. I don't like the low revs though, think of your crank bearings!

Jocko

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Re: High gear and low revs or low gear and high revs?
« Reply #3 on: August 04, 2019, 08:58:17 AM »
Maximum torque is delivered at 2800 rpm but the torque output at 6000 rpm is the same torque that is developed at 1800 rpm. So 1800 rpm has the same load as 6000 rpm but without the added factor of rotational speed.



The last thing you need to worry about is the load on the crank bearings at low rpm. There is more stress put on your engine at other times in its operation. As long as you are well above idling speed and the engine is not labouring, then the load is fine. As you can see from the above graph, the engine is only developing half the torque at 1000 rpm as it does at 2000 rpm. And I am not talking about driving with revs anywhere near as low as 1000 rpm.
I have regularly run engines to over 200,000 miles with no issues, and I am sure the Honda engine is good for the same.

John Ratsey

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Re: High gear and low revs or low gear and high revs?
« Reply #4 on: August 04, 2019, 12:23:59 PM »
What I have noticed since swapping from manual to cvt is that the auto will not let you “slog” the engine it will rev at the slightest provocation where as the manual with it’s gear changing arrows would want  you to change into top gear as soon as possible even if you were going uphill is this the reason that I get less mpg from my cvt than the manual I had before. It seams the auto doesn’t practice what the manual preaches
That's caused by the Atkinson cycle mode on the Mk. 3 and the vehicle deciding that you want more power and thus revving the engine to get out of the Atkinson cycle mode (the switch-over is at around 3,000 rpm). To stay in the more economical Atkinson cycle mode requires very gentle pressure on the right foot. The alternative is to accept a short burst of power and then ease back so the engine goes back into the efficient mode. I read somewhere that this behaviour was adjusted in the Mk 3 refresh so it might be less frustrating. It was most likely originally set up to score well in the standard fuel efficiency tests.

I had the Mk. 3 CVT for a year and found this behaviour somewhat tiring and traded it in for the CVT HR-V which doesn't have the Atkinson cycle mode. It's been a much more pleasant vehicle to drive with a very progressive delivery of power. Most of the time the engine is between 1,000 and 2,000 rpm and the lifetime average mpg is about 50.

I'm sure the Honda engines are set up to be most efficient at around 1,500 - 2,000 rpm. The maximum power is at much higher revs but is delivered less efficiently. Remember the Mk. 3 Jazz which got from Land's End to John O'Groats on one tank of fuel (but at a steady 40 mph). I sometimes wonder if the HR-V could repeat that stunt given that it's got a 25% bigger tank but don't fancy 20 hours of steady 40 mph driving needed to test this theory.
2022 HR-V Elegance, previously 2020 Jazz Crosstar

sparky Paul

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Re: High gear and low revs or low gear and high revs?
« Reply #5 on: August 04, 2019, 01:54:23 PM »
The last thing you need to worry about is the load on the crank bearings at low rpm.

It's not the load on the engine I worry about, it's the oil pressure. The crank bearings rely on that thin film of oil to prevent metal to metal contact.

Jocko

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Re: High gear and low revs or low gear and high revs?
« Reply #6 on: August 04, 2019, 03:11:20 PM »
An engine should not have low oil pressure at idle speed unless the oil pump is faulty, the engine is worn, the oil is too hot, or the oil is too thin. The lubrication system on a modern engine is designed, with a huge safety margin, to provide adequate bearing lubrication at all engine speeds and loads. But if it worries you, don't do it. it doesn't worry me, so I do.

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