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Old 02-25-2009, 11:55 AM   #21 (permalink)
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Originally Posted by Mogal View Post
I judge my accel on location and how heavy traffic is...

If the distance between lights is long, I will accel faster to get to a good cruzin' speed. Shorter distance between lights, slower accel.

And as always, I judge all that with how heavy traffic is...
I've found that on a diesel engine accelerating (specifically my 1982 6.2ltr diesel suburban) the slower I accelerate the better my MPG, I can get 20mpg in town with that rig. (which is within 1mpg of the highway)

On gasoline engines it REALLY varies, my buick gets better accelerating slowly, at least when its not bloody cold, my Dodge crew cab is abysmal but generally I need to get out of 1 asap 2nd rapid but controlled, 3rd slow and steady.

What I've found on the Dodge is that the 1st and 2nd gear you start accelerating slow and then ramp up to get out of the gear, basically the lower the RPM the less throttle you use but you have to try to get out of 1st and 2nd relatively quickly or your FE really really sucks. So I am saying your throttle level is variable in any given gear under acceleration.

Good Luck, Its more complicated than many make it seem, and I've found if I just accelerate slowly through all the gears my FE sucks.

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Old 03-06-2009, 01:25 PM   #22 (permalink)
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I think I have found that my Smart responds to acceleraton at or near minimum BSFC. I found a Mitsubishi publication that was published in conjunction with the release of that engine; it includes a curve of BSFC vs. BMEP at 2000 rpm that seems to show that minimum BSFC at that speed occurs at about 63% of max BMEP.

Now, LOD on the Scangauge is more closely related to IMEP. It doesn't go to zero at zero shaft torque like brake MEP would. I found that at zero torque LOD is about 17 between 2000 and 3000 RPM. So to get a rough target value for LOD under acceleration, I interpolated to find a value that is 63% of the way from 17 to 100 (69).

I try to accelerate at an LOD of around 70-75 whenever possible, shifting at no more than 3000 rpm. I don't have good hard data, but I can tell on the Scangauge that it's better than feather-footing it all the time.
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Old 03-25-2009, 04:15 PM   #23 (permalink)
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Judging by the BSFC charts I've seen, I think it is best to keep intake manifold vacuum at 5 to 10 inches of mercury and the engine speed in the 2000 to 3000 RPM range on most engines. If slower acceleration is required, then the shifting should be done earlier. Most cars will run smoothly if the transmission is shifted when the engine reaches 2000 RPM. After the shift the engine will probably be running about 1300 RPM. It depends on the transmission gear ratios.

I think it would be a good idea to avoid a heavy load when engine RPM is below 1500 because the oil pressure may be low and a heavy load could increase wear on the connecting rod bearings.
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Old 03-25-2009, 09:57 PM   #24 (permalink)
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Im not so sure about higher load accelerating in turbo cars such as mine, because Im pretty sure if you give it over a certain amount of throttle it goes to open loop. I know the boost maxes out at any more than half throttle or so above 2000 rpms. 55mph is around 26-2700 rpms in 5th so thats about the only time you have to really worry about that. Its geared pretty low (.8 5th, 3.85 final) so accelerating below 2000 rpms isnt much of a problem.
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Old 03-26-2009, 09:55 AM   #25 (permalink)
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I often read terms like 75% of throttle, 80% load while talking about the brisk acceleration that is supposed to be most efficient.

Is there a way to figure these in the absence of a scangauge? Any trick with MPGuino to hit the sweet spot?

Any approximations for situations where the stock (and only) instrumentation is a tacho and a speedometer?
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Old 03-26-2009, 08:48 PM   #26 (permalink)
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well, 75% throttle is exactly that: the throttle is 3/4 of pedal travel to wide open. engine load is a calculation based on several factors, but your ear can probably tell you what the engine is doing. if it sounds like the engine is working hard, your under heavy load. Instrumentation, even a vacuum gauge, makes it the easiest and takes away the guessing.
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Old 03-28-2009, 08:54 AM   #27 (permalink)
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For my 36 and 37 mpg tanks I was accelerating as slow as possible, for this tank I am getting up to speed more rapidly and thinking it might actually be better to do this. The longer it takes me to get up to speed, the more effort the engine is putting in at low rpms trying to get there. The faster I get to 5'th with out going wide open throttle, the less gas I'm hoping to spend because I'll be at cruising speed at a relatively low rpm sooner.

It makes sense, so I'm hoping it pans out. Maybe I'll go to a 1 gallon fill up to see how it goes? BRB!

Back!... Wow.. uhh I don't know what to say!

I went 55 miles this morning, and I just topped it off.. 1.280 gallons @ 55.2 miles...

43.125 mpg accelerating using 65-70% throttle. Coasting down hills, and 1 turn-off for about 1 mile this morning.

for me as it turns out, using 70% to accel, has resulted in a significant mpg increase apparently.

Last edited by Drive Stick; 03-28-2009 at 09:06 AM..
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Old 03-28-2009, 07:54 PM   #28 (permalink)
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Best acceleration is 0

Best is to find a hill, that for the speed you want to travel:
  • has an up gradient that matches the best BSFC of the engine and gearing; and
  • has a down gradient that allows you to roll at the speed you want.
Go up the hill. Turn off engine. Roll down the other side at the same speed.
Acceleration = 0
(you may have a little trouble finding the hill, but it must be there somewhere)
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Old 04-07-2009, 01:09 AM   #29 (permalink)
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I just got my MPGuino in the mail. I won't be hooking it up for a while cause it needs to warm up, and I need to figure out what's wrong with my wheels before I do anything else to the car. Should I get a vaccuum gauge also? Or will a MPGuino tell me what I need to know to optimize acceleration?

Which brings up the question, what numbers would I be looking for on either instrument to know if I'm at optimum acceleration rates?
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Old 04-07-2009, 01:16 AM   #30 (permalink)
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Originally Posted by MechEngVT View Post
I have read that BMW instructs drivers to accelerate at 75% engine load and shift when the engine's mean piston speed reaches 1200-1500 feet per minute. Mean piston speed is a function of engine stroke and RPM.


Vp= mean piston speed
RPM= engine speed
S= engine stroke length, inches

This keeps you in closed-loop operation (for EFI) at high volumetric efficiency with low frictional horsepower loss for as short a time as possible before you can achieve steady-state cruise and/or EOC. It is what I like to say is accelerating "smartly."
Using this formula, and my engine stroke length of 3.1", I came up with an ideal range of 2322-2903 RPM... is is really possible that it's that high?

It would explain my dramatically low mileage compared to my fellow Taurus drivers. I try to always keep the engine under 2000. Never over 2500.

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