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Old 01-16-2011, 12:21 AM   #1 (permalink)
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Controversial idea to better FE

Greetings all! I have a theory, that I am certain will cause much ruckus among you. Now this may or may not apply only to cars with an automatic transmission, but I wouldn't think it would, since it seems pretty general of an idea. I may be wrong entirely about this too!

So, my theory is that if you are to 'punch it' and get to the speed limit, or 'cruising speed' as I call it, you would get better fuel economy as opposed to driving slowly up to that point. I watch my gauge closely, and if I slowly accelerate to my cruising speed, my average goes down because I am below average for a longer period of time. If I 'punch it' I get yo my speed faster, and see my cruising MPG more quickly, which means less of a chance for average MPG to go down. Basically, it is very bad MPGs for a short period of time against sorta bad MPGs for a longer period of time. Once it gets warmer, and I find a good road, I think I'll do a proper A-B-A test on this.

The reason I see this as controversial, is because it flies in the face of what we currently think, yet it makes sense (to me). Am I doing it wrong, or do I have a valid point?

Comment/riot as you please!

*Note: there are no big risks in when I 'punch it'. I am a careful, observant, and kinda timid driver. I try to separate myself from the normal 'punch it for no good reason' teen driver, although after doing that once or twice this thought occurred to me. But please, no criticizing me for being a dangerous driver. I am safer than the average teen, and lets leave it at that.

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Old 01-16-2011, 12:27 AM   #2 (permalink)
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Not controversial at all.

Many have found brisk acceleration to be most efficient; not "punching" it, but about 80% throttle. Some of the reasons for that are engines are most efficiently converting fuel to work at near wide open throttle BUT go too near WOT and fuel enrichment systems get activated thus reducing fuel efficiency.

This works best on m/t cars; with a/t cars one has to let up on the gas a bit so that upshifts can occur at lower rpms.

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Old 01-16-2011, 02:00 AM   #3 (permalink)
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The idea works differently on the two cars I drive... the Maus doesn't seem to accelerate any faster at 80% throttle vs ~30% ... that is, there is no "punching it" with the Maus. Helga responds much better to the 80% throttle compared to 20 - 30%, and so I use that method a lot more often with Helga.

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Old 01-16-2011, 02:33 AM   #4 (permalink)
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KITT222 -

What Frank and NachtRitter said. If you know your car's BSFC "sweet spot", then you can shoot for that in your acceleration :


Like Frank said, a manual has more control over that. I wonder if a paddle shifter would "obey" your wishes or shift when it wants to.


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Old 01-16-2011, 08:47 AM   #5 (permalink)
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Accelerating briskly is the way to go with a automatic.You want to achieve
transmission lockup as quickly and safely as possible, then use a light load to achieve your desired speed.

Case in point.

One of my vehicles is a 2000 f-250 work truck with a 5.4 and a/t. Up to 37mph the best mpg I can get is 9-11 range. At 38 mph lockup occurs and fuel economy jumps to 19-21 mpg range. (verified by scanguage and ultraguage)

A 100% increase in fuel economy...

Automatic transmissions have certain amount of slip built into the torque converter called stall speed. That certain amount is needed to keep the engine from stalling when the transmission is put in gear and also allows the vehicle to perform as required.

Drag racers want a higher stall speed (rpm) or what is also known as a loose converter. It allows the engine to enter its sweet spot for power before transfering maximum power to the wheels.

Passenger cars have a lower stall speed or what is known as a tight converter, it allows the engine to take advantage of its lower powerband for propeling the vehicle.

(Side note)

As vehicles with automatic transmissiom age, their stall speeds can increase due to wear on the internal parts of the converter. This in turn can lead to a increased loss in fuel economy before lock up occurs. (as in my case)

So, in the end........

The sooner the transmission is in lockup and the engine is at its BSFC "sweet spot", the better...

Having a scanguage, ultraguage or MPGuino is a indispensable tool when trying to figure out the best combination for your vehicle.


Last edited by redneck; 01-16-2011 at 08:56 AM..
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Old 01-16-2011, 09:17 AM   #6 (permalink)
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The risk in describing the "punch it" approach as "the best way to accelerate" is people may hear: "it's always better to accelerate briskly", and apply that across all driving situations.

However I guarantee you that if you have to brake shortly after "punching it" (ie. in city driving), your fuel consumption will be much worse than someone who feathers up to speed.

The "punch it" approach only pays dividends when the cruising period that follows lasts long enough that your average MPG has time to recover from the high fuel consumption hit experienced during the "punch".
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Old 01-16-2011, 10:10 AM   #7 (permalink)
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I have to be careful using this method, so that I don't inadvertently force my engine computer to go into open loop. My wideband O2 meter gives me an indication of this when it goes down from 14:1 or so, down to 12:1 or even 11.5:1 (ugh).

That aside, properly executing "punching it" will cause the engine to work more efficiently to accelerate your car to cruising speed.
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Old 01-16-2011, 10:45 AM   #8 (permalink)
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Accelerate at the highest rate that will not cause the transmission to stay in lower gears generally will work best. It also, in most cases is about the speed of average acceleration in traffic.

I would still do the ABA testing to see which method works best for your particular vehicle since you have the means to perfect the technique.

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Old 01-16-2011, 11:35 AM   #9 (permalink)
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Figured this out in the 1970's. My Dad's '74 V8-472 Cadillac definitely did better by acceleration from a standing start to achieve third gear (top gear) over the shortest time/distance versus a slow or medium pace. It was a matter of not delaying any of the shifts, overall. (And once a "shift kit" was installed, it was even easier). One had to know the exact point and have a good feel for it. (This was with a 2.94 rear gear; on a later 3.21 & V8-500 it was a bit different though not much).

He got better mileage than I did in driving that car, and he -- through his life -- averaged 90k on brake jobs.

Chryslers were easier to "feel" than the competition due to throttle return/linkages, IMO, but all could be "learned" for best mpg tactics. Big blocks were a world of their own.
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Old 01-16-2011, 12:20 PM   #10 (permalink)
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Well... I never thought it'd be generally accepted as it is. Surprise to me.

In the area I drive, which is a very spread out suburb, has a few lights, but speeds that at minimum are 45MPH, and the main stretch to my high school has a speed of 55MPH and only two stop lights.

I'll look for that 'sweet spot' in the Vibe. The thing can rev to over 5k RPMs before shifting, but when I saw the needle that high, I let off a bit. It's almost weird to watch me accelerate. I hit it, and it's loud, the car rear backs, but once I hit the speed limit, I'm a steady, calm driver, no matter how close that truck gets to my rear bumper. For the ABA, I'll check around on Google earth, but I may have to go to the nearby Native American reservation to find a good road.

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