Fuel economy shift points ?

[Stanley]

All this time, when I want MPG, I let the AT shift itself (about 2200 to 2nd and 2500 into 3rd, when easy on the gas pedal), but maybe I had it all wrong. After all, stick usually beats AT for mpg. Because of shifting at a higher rpm? For performance, I shift about 4700 - per dyno run and the operator's advice - for my setup, sorta stock - this worked well at the strip, beating reported best times for AT stocker by a couple secs. So when to shift for economy? To stay in the best torque zone? Guess it would would be similar for a stick, a little different with 1 or 2 more gears.

Concerned with around town driving, on the highway wind resistance (increase per square of velocity) would govern and I would be in good torque zone anyway.

It's counterintuitive, but maybe I should shift manually and let it rev more for better economy?

[Arne]

I don't think RPM is a significant factor - you need to maximize torque converter lockup. On an early, non-electronic automatic, that is difficult to control manually. I suspect the greatest benefit to fuel economy is to just drive it gently, which it sounds like you already do.

[Stanley]

What's torque converter lockup? I did adjust the bands to factory spec.

[doradox]

Best fuel economy occurs at large throttle openings accompanied by low rpm. With an AT this is hard to do unless you have a manual valve body so you can force shifts. Otherwise what you are doing sounds good.

Steve

[TomoHawk]

"Auto trans beats the manual" is no longer applicable to modern electronically-controlled transmissions- for fuel ecomony at least. Because of advances in the design, manufacturing and electronic control, it no longer makes any difference if you are considering purchasing a new car. So now it's only a matter of nostalgia or hormones, as in "MEN don't use automatic transmissions" or "real sportscars have manual transmissions."

The "lockup" refers to the torque converter. At certain times, like on the highway, when there is a minimum load on the engine, the transmission can operate without any slip in the torque converter, so it will literally lock up so there is no difference between engine RPM and transmission input RPM, like when you have a clutch engaged. That way you get the best gas mileage on the highway. It also depends on the intake vacuum, transmission hydraulic pressure and some other stuff, depending on the transmission, so if you press the gas pedal, it will unlock itself, and function normally again. You can have torque converter "lockup" in city driving too, but not usually.

A video: Lock-Up Torque Converter -

If you want to mess with shift points, you will need to do some research or contact certain companies that deal with performance transmissions, specifically for the one you have. Usually, they do things like modifying hydraulic fluid holes, or changing hydraulic valve springs, gasket mods, etc. There are lots of "shift kits" for Chevy, ford, Mopar, etc. transmissions that can do the same stuff. But for the average driver tire pressure, air temperature, terrain, and what you do with your right foot has the most affect on gas mileage or performance, and your library has some old books on the subject.

Edited January 20, 2013 by TomoHawk

[Stanley]

Thanks for the primers. No problem shifting manually and any mods to the trans would be over my head and way down the list anyway. The Z gets good mileage even with the lead foot but at around $4 per gal for 92 octane in CA (owner's manual recommends 96) every bit helps.

[TomoHawk]

Shifting an automatic shift trans could damage the governor, so you might want to consult your transmission expert on how to shift it or if the should be any (mods) to keep it working well.

Other than that, having the transmission in top operating condition is the best thing you can do to get good gas mileage, besides your driving style.

the Video on the lockup torque vonverter is: Lock-Up Torque Converter - YouTube

Edited January 21, 2013 by TomoHawk

[Stanley]

Got the owner's manual for the 73, it says you can shift the AT manually, but it warns not to downshift to 2nd over 70 mph (maybe cause it could redline?).

[TomoHawk]

You could also damage the pump inside the transmission by causing back-pressure, which is not what it was designed to do. The only times you'd really shift is to go to 2nd or first for going down a steep grade. In a situation like that, you'd probably be going relatively slow anyway. I wonder why they don't teach that in Driver's Ed any more? Maybe because it might now be better to rely on the brakes to slow the vehicle (a car) when going downhill, especially since many come with 4-wheel disc brakes or good drum brakes.

Edited January 21, 2013 by TomoHawk

[Stanley]

Gotta correct my error: 73 owner's manual say's don't downshift to 2nd (or 1st) over 75 mph, not 70. Also it says if you downshift to 1st it will shift to 2nd and stay there until you get to 30 mph. Will have to try that. Also it recommends not shifting to 1st over 45 (I think) mph because it could cause a slide. Which might be just the thing for some driving situations - like a rally.

BTW before a trip to the drags I asked my mechanic (who has many years of race and resto experience) if it would hurt the AT to rev it up a launch with the brake.

He said no problem for the transmission at all (might be different for other AT's). If anthing let go it would be in the drive train behind the transmission and that wouldn't happen unless the parts were worn out or defective or you had a heavily modded engine. I like the AT OK but can see a disadvantage in racing - you can't (or shouldn't) downshift for that 90 mph corner. When I was a dumb kid, J.D. and I would roadrace almost every day down through the foothills, in our parent's station wagons. Mine was automatic and I slamed it through the gears to slide every corner. Never hurt the trans - but went offroad a few times.

[TomoHawk]

For Drag racing there is a thing called a trans brake, which allows you to rev the engine and engage the trans with no problems, but it involves some minor mods that a trans guy should do. I tried double-footing the Jatco trans, and it doesn't seem to like that, and NO transmissions like the old neutral-drop.

WikiPedia page: Transbrake - Wikipedia, the free encyclopedia

[Stanley]

Yeah, the neutral drop sounds like something for rental cars only. Not sure how this thread went from gas mileage to AT's and drag racing unless going fast is more fun than saving gas, but I'll take partial responsibility. Hadn't heard of the trans brake, there's also the line lock. No drag strip nearby so it's once a year for me - no plans for mods like that.

The street racers want a strip - so do the cops, to minimize street racing - but it never seems to happen. It's been many years since there was legal drags in the South Bay, so there's a lot of street racing. I've gone against everything from Mazarati's to moms in SUV's. They see a Datsun and just want to race.