Efficient driving question. (Torque converter lock, hills, etc.)

[CCCMikey]

G'day

I do a lot of rural driving.

Up until recently I drove an old 3 speed Auto bluebird, and it was pretty straight forward. 70 was a good casual speed in that car.

I notice in the commodore if I do 78kph, I'm in 4th, doing nearly 2,000rpm. Once I hit 80 the RPM drops back to about 1500, still in 4th due to the locking torque converter. (I think!)

So, a couple of questions.

1 - When driving on the flat, I'm guessing it's most efficient to drive at just over 80kph? (I'll do 100 or pull off if there's traffic around.) If I'm doing 75 I'm slowly cooking the tranny?

2 - When driving up a hill and 80kph is pushing it (I'm on LPG) am I better to stay in 4th until the tranny decides it wants third, or should I drop it down to 3rd manually and let it rev away at it. (One hill in particular is about 4km long and traditionally I end up doing about 60 by the time I get to the top from a 90+ start, without adding much more throttle.)

3 - When descending but not requiring brakeing, is 'foot off the gas' the best action? I know some hypermilers would drop into neutral in older vehicles but if I understand correctly the motor will turn off the injectors if they're not needed?

4 - Probably a stupid question: If I turn on the aircon while descending will it give me an increase in engine brakeing, or does the computer just blindly up the revs anyway - I guess that depends in part on question 3

Cheers, Mike.

[hako]

1....Correct
2....Leave it in auto - the car is designed to change back when needed in the most efficient manner.
3....Foot off the gas....the injectors are virtually off on a trailing throttle.
4....it would cause additional braking (minimal) - just change back to 2nd on steepish decline or 1st on really steep decline....if you leave it in D the revs drop to about 1000rpm and the car seems to freewheel. (presuming you are using cruise).

The way I figure it is that when the car was designed, GMH used many highly paid engineers to design the vehicle to perform best under all conditions.....some dudes figure a twitch here and a twitch there may improve things and maybe so, but it will always be at a cost. Best method of improving fuel economy/efficieny is your right foot IMHO.

[The1]

Yes stock Tune TCC lockup is at that speed in 4th, goto manual 3rd if you want to lockup around 60km/h.

Drive shift times do lag a bit from 4th to 3rd at light throttle on inclines, can be changed in the tune to change back at higher speed, Holden Designed it this way, if you want it to change back use your right foot, otherwise it's for light coasting.

There's quite a few conditions that have to be met to enable DFCO(deceleration fuel cut) one being the revs need to be over 1200rpm in the stock tune when the pedal is released, other than this the Torque Converter will unlock in the stock tune when throttle is released also, the overrun clutches in the auto then just let the car coast.

If your not coasting and either in engine braking via transmission or DFCO, then aircon on will help slightly, i have done this lots of times going down a hill in manual 2nd and aircon on helps with engine braking, ive only noticed it with no load and by myself though.

If you do alot of highway driving, lean cruise can be remapped as well, 20:1 in the stock tune is pretty lean you'l use more fuel keeping pace then saving running a richer mixture.

There's always two sides to the coin, the Engineers are paid to build the most cost effective car while beating the competition and passing emissions and manufacturing laws. There not interested in economy and longevity, as long as it better than other cars and lasts the warranty then it's in the bag, Aftermarket tuners are not constrained by manufacturing standards and how your car runs can be improved and changed to how you like it. If this was not the case then there would not be dyno tuners in every city.