Acceleration slow or fast? 2003 Chevy Impala

[justjohn]

Yes, I'm surprised more people haven't done this already. I'm planning on doing it when I eventually get an MPGuino.

Don't forget that slower accelerating methods will cover more ground so just going by how much fuel is consumed to get to 55 isn't a "fair" method.

I was debating on how to account for this and I was just going to compare mpg after accelerating to 55 and holding at 55 at least long enough to reach the distance required for the slowest method. I'm not sure that's a perfect solution but at least it accounts for the main differences.

[Thymeclock]

Originally Posted by abogart

Alright, I'm reviving this thread. I have been pondering this very same question for.... well, a long time.

It seems sometimes as though we drivers of automatics are shunned from the efficiency discussion. Unfortunately, my good ol' 3100 with the 4 spd is all I have for the time being and I have to make due with what I can get from it. Nevertheless, I see no reason not to at least TRY for better fuel economy, even if the transmission isn't the best tool for the job.

Sometimes we drivers of automatics are deliberately shunned from the discussion here.

I'll tell you the best tips I know for getting better FE with older vehicles that are automatics and less sophisticated (read: Chevys). (I have two of them.)

For highway driving the idea is to accelerate moderately briskly to get into the highest gear and enter torque converter lockup ASAP and stay there. Usually that is around 45 MPH on a Chevy, so maintaining a constant speed of 45-55 will do best. If you go much faster than that you are using more fuel than at that ideal speed for efficiency. If you are driving slower than that the automatic is also inefficient due to the shift points of it.

City driving is a real challenge, and your FE will automatically be lousy (no pun intended). Any time you can avoid coming to a total, dead stop, do so, for that really eats gas up, especially on a heavy vehicle. (But watch out for cops at stop signs and red lights, as getting a ticket is not worth the gas saved!) What does help some is coasting in neutral. But I recommend you only do it when you know you can coast to a near-stop. Most automatic transmissions are potentially susceptible to damage if you re-engage them back into drive at higher speeds.

Let's say there is no traffic and you are driving along a long block with a stop sign at the end. Accelerate briskly enough to cover the distance needed to coast, then shift it into neutral and coast to the stop sign. However don't do this when approaching a red signal, because if the light turns green your shifting back into drive at speeds of over 10 MPH might damage your tranny.

That's about all you can do with an automatic without putting it at risk.

[abogart]

Originally Posted by justjohn

Don't forget that slower accelerating methods will cover more ground so just going by how much fuel is consumed to get to 55 isn't a "fair" method.

On one hand I agree with you, and on the other I don't. Basically, the idea is simply to determine how much fuel each technique uses to accelerate to cruising speed. Once there we make decent MPG, so any amount of time or distance after that is irrelevant.

However, the overall goal is to use less fuel over the entire distance of the trip. Hypothetically, if harder acceleration was more efficient than gradual acceleration, there should be less fuel used over the same amount of distance. The problem with this method is that although it would determine which technique is more efficient, it adds a certain amount of distance at cruise to the techniques that take less time to achieve cruising speed. This could be considered "unfair" to the harder acceleration techniques.

I think the best way to go about this is to drive a set distance for each test. It shouldn't be too much longer than the technique that yields the farthest distance covered before reaching cruise speed, so as not to skew the results by variations in cruise MPG. I think 1/2 mile should be sufficient. If the slowest technique yields the best results, longer testing distance may be required.

[justjohn]

Getting to cruise faster is part of the advantage that harder acceleration has in real life vs slower acceleration. The question is whether it makes up for the fact that more gas is used in the short term.

But either way, we arrived at the same conclusion for how to test, so that's probably the best option.

[abogart]

Originally Posted by Thymeclock

Let's say there is no traffic and you are driving along a long block with a stop sign at the end. Accelerate briskly enough to cover the distance needed to coast, then shift it into neutral and coast to the stop sign. However don't do this when approaching a red signal, because if the light turns green your shifting back into drive at speeds of over 10 MPH might damage your tranny.

That's about all you can do with an automatic without putting it at risk.

Rats. I thought I was the first one to discover those tricks...

I have been able to achieve about 32 in town using these and other fuel conservation tricks, compared to my all-time record highway trip MPG of 38 (thank you, no applause necessary). It never fails to amaze me when I see other drivers accelerate to get right on my bumper on a descending highway on-ramp, just to hit the brakes for the corner ahead, as I merrily coast along at 25 MPH or so . It just goes to show how inefficient the standard driving style in this country really is.

I shift in and out neutral at 70 MPH all the time, it gets me some good time at over 120 MPG on a good stretch of highway exit lane. There is definitely some skill involved though. From steady-state cruise, lift your foot off the throttle. The trick is to shift to neutral at the exact moment that the engine crosses the RPM that would match the transmission if the TC were locked (about 1500 for my car), Scangauge comes in handy for this purpose. This is usually about a second or two delay as RPM momentarily increases as the TC unlocks, then slowly drifts down toward idle. If you catch it at just the right time, you can't even feel the change. Shift before and you'll feel the car lurch backward because the engine was still providing forward power, shift after and you'll feel the car lurch forward because the engine was providing engine braking.

Shifting back into overdrive is pretty basic for the most part, just shift to drive and the PCM will automatically find the right gear and (relatively) smoothly engage it. However, I like to try to match the speed at it engages, so as to further prevent wear and tear. Shift to D, and immediately apply VERY light throttle, this will spin the engine up just enough to match the transmission. It takes about a second at cruise to engage. This is tricky to master, too much throttle causes the transmission to "slam" into gear (not good).

I have found that below about 35 MPH, there is no engine braking effect whatsoever with the transmission in drive - in fact it actually pushes the car a bit - and fuel consumption and MPG are comparable to coasting in neutral. So the only time I shift to neutral below this speed is when I know that I will be coming to a complete stop, otherwise it's just excess wear and tear. Also, after coasting down from cruise speed to a stop sign or light where I will only be stopped for a short time, I pop the gear back into D at about 10-15 MPH. It will smoothly reengage at that speed, rather than coming to a complete stop and getting the typical automatic "clunk" when shifting into gear.

[roosterk0031]

With my Malibu, same engine, probably same tranny, it coast just as good in gear as Neutral so I don't bother with N on it. My money's against slow accelerating, I don't like seeing single digit mpg, and you'll see them longer.

I'll do a few 3/4 mile runs tomorrow and record my results with the Stratus 2.4 I4 recording TFC vs max RPM.

[abogart]

I just got done programming the trip distance as an x-gauge. One thing that concerns me is that the distance only displays down to 0.1, and I'm not sure if I can increase that resolution. This means that there could be up to a .09 mile variation in results.

Also, I have decided to use the TFC gauge because (at $4.20/gal) it will have much higher resolution than the fuel used. However, after a quick run around the block, I am guessing that all results will still be within about a $.04 range.

[justjohn]

If you use the odometer on your car you should be able to see when it is about to roll over to the next tenth so you should be able to start and stop exactly as you hit the mark, giving you the exact 1 mile or whatever you were going to do.

But yeah, .1 possible error is waaaaaay too much for a .5 mile or even 1 mile test.

[Fat Charlie]

I favor hard acceleration, but then I would- I just love stomping on it and making it go. Instant MPG suffers, but it's not too much lower than moderate acceleration and the distance spent at that lower MPG is a lot shorter than the distance spent at the almost as bad MPG of moderate acceleration- at least for my car.

It gives me a lot more distance in glide. It has the added benefit of catching back up to the truck I was drafting that pulled ahead of me on my long coast to the tollbooth. Even if it didn't work for me on its own then catching back up to the truck would make it work.

[roosterk0031]

Up you gas cost to $10 a gallon, then use TFC for each run should give enough accuracy.

I'm going to go gravel road to gravel for turn around, picking something a drive or sign little short of the mile to record TFC, once each direction at rpm's yet to be determined, probably 500 rpm spreads from 2000-3500. Initial data should narrow the area to focus on for max. efficiency.