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MG MGA - Fuel usage

I have a question that pertains to any auto not just MG's. I am interested in differences in fuel usage under the following three conditions. Cruising at 50 mhp and 2000 rpms, coasting (throttle closed) at 50 mph and 2000 rpms, coasting (throttle closed) 50 mph and idle rpms
Stan

Can't help, I don't drive in any of these styles, and you don't have anything for HARD driving
dominic clancy

I had to use these tame numbers to make coasting at 50-2000 reasonable
Stan

Many (all?) modern cars will shut of the fuel supply completely in your 50/2000 throttle closed scenario so no fuel will be used. (Called Trailing Throttle Cut-off I think). A small fuel supply will be turned on again when the revs drop to idle to maintain that engine speed, whatever it is set at.
So most fuel will be used for scenario 1, least for 2 and a little bit for 3.

In an MGA of course this doesn't apply but the SU's will be pretty much shut down in 2 as you have a high manifold vacuum. In 3 they don't know how fast the car is going so will treat it as a normal idling situation.
malcolm asquith

I have to admit I am struggling with the question. The 2 coasting scenarios will be momentary happenings unless descending an incline at a steady state. In which case, if i was so worried about my fuel usage in an MGA I would switch off the engine (no power steering nor servo brakes)and bump it into life again at the bottom of the hill!

And yes, my Father-in-Law did unwittingly try it in a power steering / servo braked car many years ago (they don't pay our pensioners much). Scared the hell out of him!

Steve
Steve Gyles

Interesting topic. I had a 1965 SAAB Monte Carlo 850 that actually had a free-wheeling clutch, so when you let up on the gas, it coasted and the engine dropped back to idle RPM. This feature was supposed to save gas and also protected the engine (two cycle) from lack of lubrication on long downhills. It also made downshifting less effective for slowing down.
HJ
Harley

<<<snip>>>
So most fuel will be used for scenario 1, least for 2 and a little bit for 3
<<<snip>>>

I think 2 and 3 are reversed for carb equipped cars.

As explained all modern cars, i.e. Fuel injected, will completly shut the fuel off when the throttle is closed until the engine reaches idling speed. It's part of the emissions system.

With a carb equipped car and throttle is closed, the vacuum is higher and thus MORE fuel is sucked through the jet. That's because the intake vacuum can't get the air it wants due to high engine speed so, it pulls in more fuel.

On a carb equipped car, the fuel usage would be most for scenaro 1. The other two might be a tie.

For FI cars, the least fuel used would be throttle off at 50mph.

Regards,
Blake
Bullwinkle

This thread was discussed between 27/06/2005 and 29/06/2005

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