greentrees

I grabbed a Road & Track today for reading while traveling, and page 103 has an interesting article called "Saving Fuel - And Still Having Fun", which seemed to be the whole theme of the mag. It was a good read on various real and bogus ways to help MPG. This paragraph was interesting:

In routine acceleration, a fairly heavy foot and short shifting reduce pumping and throttling losses. Given that traffic allows it, this can save as much as 20 percent in city/suburban driving. Plus, it's fun to practice.

This validates mine and many of our discoveries that aggressive driving really doesn't hinder MPG as compared to grandma driving.

The article also had a good link with scientifically-proven data on products/methods that supposedly improve MPG:

Good, Better, Best: How to Improve Gas Mileage

BAC5.2

Actually, grandma driving rarely helps.

High RPM + closed throttle = REALLY HIGH pumping losses. More energy is sapped maintaining a vacuum in the manifold than making power.

The only way grandma driving helps is high throttle at low RPM. It reduces pumping losses and maximizes the translation of fuel to thrust. The flip is that the engine is less efficient during low RPM than high RPM. Plus, it's bad for rod bearings.

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The Aggressive™ -- You don't know about it.

1998 Forester S - Big brakes, Big Tires, 5-gears.

Forester 2.5 XTi

You mean I HAVE been saving gas all these years? Nice!

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USMC - One Shot, One Kill. You can run, but you'll die tired.

Dan

Supposedly BMW did a study in the 1980's that showed exactly that - quicker acceleration got you into taller gears sooner. The key is to use a heavier foot but don't wind out the motor. The result was something like 2/3 throttle plus shifting at 3000 RPM or below netted significantly better mileage than babying the throttle. I'm trying to find a link but have had no luck so far.

Edit: IIRC, the R&T article also mentioned that windows versus AC isn't as simple as switching over at 35 MPH or so, since the compressor creates drag on the drivetrain. It mentions up to 20% worse economy using AC. I'm guessing that's an extreme case.

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SHIFT_to_Subaru
'04 XT, 5spd

The Rookie

Of course at high rpms the fricitional losses are significantly higher than at lower rpms.

The big variable is enrichment, some manufacturesr richen the mixture at high load/low speed, others stay at Lambda1, richening up by about 10% (from Lambda1) gains about 5% more power, so a loss of efficiency of 5%, but if it stays at Lambda1, then yes its more efficient at wide open throttle than a light load.

Simon

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98 S-turbo, approx 220 BHP on a TF035-6

SuperRu

I remember reading that in the 80's. Wasn't it in R&T too.

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Richard
04 XT.... 98S

Dan

I think both R&T and C&D have referenced that test in the past. I'm surprised he didn't bring it up in this latest article.

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SHIFT_to_Subaru
'04 XT, 5spd

abacall

OK, but how does boost and load effect this theory?
Open throttle + high load = high boost. Sooo, wouldn't there be more fuel dumped?

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Forester: the car that put the BOX in Boxer

Want to see my Fozz?

Dan

The key is to shift early before you get into big boost. And also around 2/3 throttle, not full-on foot-to-the-floor glory. Other than that, I don't know for certain. I'd say you'd have to be in a lot of stop-and-go traffic for it to make much difference.

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SHIFT_to_Subaru
'04 XT, 5spd

gandapanda

W/ stage2 TD04, I'll be shifting at 1500rpms. Plus I'd imagine you'd be more prone to knock w/ such a high load at low RPMs, kinda like trying to make it up a steep hill in top gear at low speeds.

My driving style is moderate revving w/ no boost and I'm content w/ 21mpg.

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- sonny
97 200sx - SR20DET
05 FXT - AP2

Dan

Yeah, not sure exactly how boost comes into play. In general, the sooner you're up to cruising speed, the better. I wish I could find a link to that study.

Edit: the study may have lost some relevance with modern motor controls. Dunno.

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SHIFT_to_Subaru
'04 XT, 5spd

adc

Very misleading. Heavy throtle wastes fuel bc the computer gives it a higher enrichment and from an acceleration standpoint..acceleration takes extra energy. Try it and then use 15% or less on the TPS (Throttle Position Indicator) I can guarantee you that less throttle will give better mileage.

I get 28 mpg with my forester in town driving.

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Be careful what you wish for.

The Rookie

ON a turbo car you want to get to atmospheric pressure no more, ideally have a dual setting EBC and set it to wastegate boost only for economy drives....whan doing long distances I pull off the BCS to intake hose and plug it to limit me to WG boost, resist the temptation...I too have full boost (.75 bar) at about 1750rpm.

Simon

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98 S-turbo, approx 220 BHP on a TF035-6

Jphank

I graduated last year from a program in vehicle design (Western Washington University VRI). It was design from a mechanical stand-point, not the artsy, industrial design per se. We learned, at least for an NA car, that the burn and coast method is a great way to get better mileage. This is normally feasible on long, straight, and lonely freeways. You drive at full throttle as long as you want (normally in top gear to stay in burn-mode for longer) then shut off the car and coast down to whatever speed you want, start the car, and do it all over again.

As mentioned before, pumping and throttle losses are at a minimum at WOT. Your car's engine is least efficient at idle (this is why you turn off the engine while coasting).

Don't do this when it's busy. While you're coasting you don't have any vacuum for brakes....

abacall

While that may be efficient, it is also quite .
If you want better as mileage that bad, find another car. Don't compromise your safety.

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Forester: the car that put the BOX in Boxer

Want to see my Fozz?