IIRC, i read somewhere (possibly HJ's column?) that to get the best fuel economy from a vtec engine, it is better to keep the revs higher than normal. Is this correct, and why? My honda 2 litre vtec accord tourer does about 28 mpg around town and about 34 on longer journeys. Anyone got any tips on economical driving? I am not BTW, a heavy footed driver.
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let me be the last to let you down....
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Because it makes it's best power higher up in the rev range, so at those revs you're not loading it too much or revving it too high.
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thank you for a succinct reply! But does not higher revs mean more fuel burnt?
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let me be the last to let you down....
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Apparently not! I have no idea why, probably just the engine working at it's optimum 'frequency'.
No doubt a guru will have a better answer...
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With VTECs, its not so much about 'higher revs than normal' but driving in the 'optimal' rev band, with some Honda models having lights to indicate this rev-range!
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I have always assumed that for economy you should keep the revs down, on a vtec or vvt engine - essentially at higher revs the engine has more breathing capacity and resembles a high performance engine; at lower revs there is less lift and overlap so better and more economical running than would be the case with a traditional high power engine. It's trying to be 2 engines in one, in other words.
My own experience is that keeping the revs down is much more economical on the vtec.
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Any engine has better efficiency when it is operating "on cam". If you keep the revs too low by being in too high a gear, you lug the engine and need to keep the throttle wide open just to maintain speed. This is not economical. Conversely, if you are in a low gear, the engine runs through more cycles in the same length of road, burning up more fuel. The optimum is somewhere in between, I read somewhere that most petrol engines has maximum thermal efficiency at around 3000rpm. Of course, 3000 rpm in top gear may not be the most ECONOMICAL speed, because drag increases with the square? of speed. I expect ~3000RPM is also true for a VTEC because it would be operating in its "economy" setting rather than "performance" setting during our hypothetical test.
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Should have read: 3000 rpm in top geat may not be the most ECONOMICAL engine speed, because drag increases with the square of road speed.
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Here's some info I posted in a similat thread some time ago, I appreciate that it doesn't fully answer the question because it only talks about getting power per unit fuel burnt rather than covering distance per unit fuel burnt.
In order to rigourously understand how to get the best efficiency from an engine, you need to arm yourself with some dynamometer test results, and plot out a number of graphs. (It is highly unlikely you will be able to get enough data to do this outside a manufacturer's engine lab!)
If on the x axis of the graph, you mark engine speed in rpm
and on the y axis, mark up in BMEP (Brake Mean Effective Pressure - consider it as a measure of load)
On the graph plot contours of constant BSFC (Brake Specific Fuel Consumption - how much power you get per unit mass of fuel burnt)
The graph looks a bit like the skins of a sectioned onion. You get the best power per unit mass of fuel burn in the centre of the "onion".
For petrol engines, the centre of the onion is typically close to the maximum torque speed, and about mid load. This area of optimum economy is quite small for a petrol engine.
For a diesel, the "onion" is more oval, stetched out in the direction of the speed axis - they tend to be less sensitive to manifold tuning effects as they typically run so weak. The centre of the onion is also closer to full load than in the case of the petrol engine.
So labouring along in too high a gear may not be making the best use of your fuel - equally, screaming along the back roads near the red line may also not be conducive to good economy.
In the absence of all of the required data, and the time to plot it all out, biassing your driving around the maximum torque speed of the engine is not a bad place to begin IMO.
I suppose that with a v-tec having two humps in its torque curve, one below the switching point and one above, that for economy one should run nearer to the peak of the lower engine speed torque peak. The rev range near the switching point gives relatively poor torque.
Number_Cruncher
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