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Mazda5 and mountains

13 messages,  Last post on Aug 23, 2008 at 7:41 AM

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What is this discussion about? Mazda MAZDA5, Van

Messages Page 2 of 2

#9 of 13 Re: Mazda5 and mountains [bobw3] by maltb

Jul 14, 2008 (8:21 am)

Replying to: bobw3 (Jul 13, 2008 9:15 am)
"And when you're on a long downhill grade, being in 5th gear at 1000rpm or 4th gear at 2000rpm uses no more gas since the throttle is basically at the idle setting in either case. "
 
That really depends on the % of grade and the engine load. Being that the throttle and fuel management are 100% electronic, you really don't know what position the throttle is in and how much fuel is being supplied to the engine to keep it running smoothly.. Effectively using engine braking does reduce fuel consumption and manufacturers are pursuing this to reduce MPG.

#10 of 13 Re: Mazda5 and mountains [maltb] by bobw3

Jul 14, 2008 (9:05 am)

Replying to: maltb (Jul 14, 2008 8:21 am)
Even without any hill, if I'm coming up to a redlight and I let off the gas completly to allow engine braking to slow me down, there is no requirement of any combustion in the engine to keep the engine spinning, since it's the forward motion of the car that spins the engine, so that's why I'd think that the engine is at it's lowest throttle setting, unless it has a fuel cutoff setting.
 
But I'd say there's no reason that engine braking would use any more gas than putting a car in neutral and letting it coast, since in both cases both engines are basically ideling.

#11 of 13 Re: Mazda5 and mountains [bobw3] by maltb

Jul 14, 2008 (3:17 pm)

Replying to: bobw3 (Jul 14, 2008 9:05 am)
I dissagree but until we see a fuel system map or some real time injector pulse data, this one is up in the air.
 
Has anyone hacked their Mazda5 PCM?

#12 of 13 by dean1966

Aug 23, 2008 (7:41 am)

Took a trip out west this summer, ( milwaukee to mount vernon WA, down through idaho, and back up through yellowstone and home), and went through several mountainous areas.
 
I found my 06 manual shift 5, despite being heavily loaded with about 800 lbs people and gear actually got slightly better milage in the mountains, about 31.5 instead of the 31 I'd been averaging, and zero problems with power.
 In fact, at one point I was " racing " a group of harleys up the cascades doing about 75 mph up the 7% grades, ( 3rd and 4th gears) and the bikes simply did'nt have the power to get around me on the short straights.
  When they finally did get around me on a long straight, ( prob doing 95 mph in the process), they found I was thereafter running up their tailpipes on every corner after as the 5 could get through the corners faster than they could. I still averaged 29 mpg for that tank.
 
It must have ben a bit embaressing for them to have such trouble getting around a obviously heavily loaded minivan.
 
As far as the discussion of fuel cutoff on throttle lift, all modern cars do this already for the purpose of emmisions control. A engines emisions go through the roof in these conditions if the fuel is'nt cut off. I would imagine the fuel mapping shuts off whenever it would be transparent to the driver, like in periods of engine braking over some minimum rpm.

Messages Page 2 of 2

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