Why is AT less efficient?

[caprice]

I know its well known that automatics are less efficient, but why? Is it because of the torque converter (slipping), and pump (extra work), shift points? At a constant cruising speed with the TC lockup engaged, would a AT get the same mileage as a MT with the same gearing?

[MetroMPG]

You're right on all points, and "yes" to your question.

Automatics restrict how much freedom the driver has in choosing how much to load up the engine. (EG you can't short-shift an automatic.)

Also most automatics are ill-suited to being used for one of the most effective ecodriving techniques: engine-off coasting. (Most autos rely on an input shaft-driven pump for lubrication. With the engine is off, these can be damaged by excessive coasting.)

[skewbe]

As for "why", there are several reasons:

1. Slippage
2. Heavier
3. Usually less gear selections
4. more internal losses
4a. has a larger internal oil pump
4b. takes power to shift gears
4c. takes power to decide what gear to use
4d. more moving parts and gears
4e. electrical drain to lockup the converter

and
5. like Darin says, they have a mind of their own. Not necessarily FE biased either and unable to anticipate the environment.

[skewbe]

Also I would guess that they burn more fuel at a stoplight at idle since they are still applying torque to the rear wheels.

[MetroMPG]

Good point - rarely mentioned.

[diamondlarry]

Originally Posted by MetroMPG

Automatics restrict how much freedom the driver has in choosing how much to load up the engine. (EG you can't short-shift an automatic.)

I am by no means promoting automatics but, I think there are things you can do that will force the transmission to shift where/when you want it to.

[landspeed]

If you put them in Neutral at a stoplight you will use less fuel at idle

[MetroMPG]

That's true, Larry, within limits (IE lift to shift; but there's little you can do to achieve tc lockup beneath the set speed).

[diamondlarry]

I'm REALLY glad I have a manual.

[rh77]

Yup, I miss the manual dearly. Although, on many Honda automatics, you can short-shift from 1st to 2nd, and overcome the hill-logic control by shifting down into 3rd to shift into 3rd on a grade. Then D4 is where it has a mind of its own. TC lockup wants 25-35% TPS input for full-lockup in sub-60F temps -- I don't drive more than 20% if I can help it...and 3-4 and 4-3 shifts are unpredictable.

I don't know what kind of fun stuff is going on in there with engine-off coast. I guess ignorance is bliss until it quits working. From a financial standpoint, I may have to decide on trans. longevity and/or a manual swap if/when it fails all vs. FE. It's one of the only ways to to get the Teg to achieve reasonable in-town FE. Ugh, what to do...I'm not selling selling it, so that should save some posts.

RH77

[Lincoln]

With the 4T65E-HD (4-sp automatic) I can control shift point, TC lock-up, and other parameters with my tuner.
I am sure there are ways to adjust settings on most electronically controller automatics.
For the older automatics (example: 92-95 Civics) has anyone tried changing gears to lower the highway cruising RPMs?

[cfg83]

diamondlarry -

Originally Posted by diamondlarry

I'm REALLY glad I have a manual.

My brain just ain't working right. I thought you meant manual as in Haynes or Chilton or something.

CarloSW2

[rh77]

I located the shop manual and noted that I can override the TC engagement through manually engaging 1 or both of the 2 TC lockup sensors via 5V power. Judging by the schematics, it may bypass the VSS and TPS to make the final decision to lock up through the switch override -- I'd just have to be mindful of speeds and coolant temps to prevent further damage to the trans. Of the 3 settings of full, partial, or zero lockup, one sensor allows partial, and the other engages full and uses the Hill/Fuzzy-Logic adaptive computer for shifting (argh). So, during the colder months, perhaps 2 toggles could manually engage these at lower TPS settings once the speed allows. Purely theory.

I'll tell ya, that 50-page section on the automatic transmission looks more complicated than open-heart surgery. Sheesh.

If you're stuck with an auto, I reccommend finding a shop manual that describes exactly what parameters the transmission requires. Further inspection could perhaps find a way to take control of that vicious dictator: the Transmission Control Module.

RH77

RH77

[caprice]

Wow thanks for all the responces! I could swap in a Camero 6 speed transmission. I've had two 5 speed manual cars before. On both, I notice if I short shift, I can get better than the EPA highway milage, in the city. I have a 700r4 transmission in my caprice, I would have trade in the newer electrically controlled one and program differnt shift points.

[skewbe]

Originally Posted by caprice

Wow thanks for all the responces! I could swap in a Camero 6 speed transmission. I've had two 5 speed manual cars before. On both, I notice if I short shift, I can get better than the EPA highway milage, in the city. I have a 700r4 transmission in my caprice, I would have trade in the newer electrically controlled one and program differnt shift points.

A camaro 6 speed will cost you more than a metro

[cfg83]

caprice -

Originally Posted by caprice

Wow thanks for all the responces! I could swap in a Camero 6 speed transmission. I've had two 5 speed manual cars before. On both, I notice if I short shift, I can get better than the EPA highway milage, in the city. I have a 700r4 transmission in my caprice, I would have trade in the newer electrically controlled one and program differnt shift points.

I googled "camaro 6th speed manual gear ratios" and found this :

1996 CHEVROLET CAMARO
http://www.media.gm.com/ca/gm/en/pro...996Camaro.html

(Paraphrased from URL)
TRANSMISSION : 6-speed manual w/overdrive
Gear Ratios:
1st 2.66
2nd 1.78
3rd 1.30
4th 1.00
5th 0.74
6th 0.50
Reverse 2.90

CarloSW2

[caprice]

and with my gear ratio and tire size, at 75 MPH the engine would be turning 1500 RPM 50 MPH is 1000 RPM. With the low end torque of a 5.7 liter engine, I'd have no problem If I want a little passing power, I could just down shift.
http://www.f-body.org/gears/

[caprice]

It would be cool driving a huge car with a manual short throw shifter. Like the one in Underworld.

[brucepick]

How about synthetic transmission fluid?
I've seen some posts elsewhere saying that's definitely the way to go.

Any benefits to FE, or just that it lasts longer and probably protects the tranny better?

[MetroMPG]

From a FE perspective, the main benefit of synthetics is their cold performance (viscosity). They don't thicken as much as mineral fluids as temps drop.

People who live in a year-round warm climate would probably see no FE benefit from synthetic use.

Note that no major synthetic brand claims fuel economy improvements with their use.

[Snax]

I've noticed that our F150 simply will not engage the lockup in topgear until it heats up which is typically a good five minutes or more down the road. During this time, economy drops from about 18 mpg to 16. I can only guess that the ATF viscosity is just too high to engage the lockup until warm and that extra 2 mpg is essentially diverted to heat it up. So the question is, will synthetic ATF improve the time lag until lockup is possible?

[caprice]

When I did my engine swap i didn't reconnect the tranny oil line to the radiator. I can't because I lost the fitting

The tranny doesn't ever warm up and so for some reason it was holding it in low gears. OD doesnt enguage till 60 MPH...

[brucepick]

What's short shifting?
A couple posts in this thread mentioned it.

[MetroMPG]

Short shifting just means upshifting at low RPM to maintain a higher load under acceleration. Theoretically, it permits the driver to keep the engine near the BSFC happy zone longer.

It's the opposite of what an automatic will do when you stick your foot in it - since the AT shifts based on load, it will hold a lower gear longer (or even downshift) for better accel.

[brucepick]

OK. I've been doing what I can to keep my AT at lower rpms. Basically, a light foot.

I also slacked off the adjustment on the kickdown cable a couple turns. So it doesn't jump down a gear at the drop of a hat.

[MetroMPG]

The cable adjustment should help, yes. I think RH77 played with that on his Acura (unexpected results?) - he may have more to offer.

[rh77]

Originally Posted by MetroMPG

The cable adjustment should help, yes. I think RH77 played with that on his Acura (unexpected results?) - he may have more to offer.

Bruce- Holy cow, don't forget to tighten it up once you've found a good adjustment! Mine slipped into a tragic "Non-TC-Lockup" situation about a year ago. Man did that suck.

Experiments for me showed that a loose cable gives the tranny less pressure to shift, and could slip and wear prematurely -- also it took further up the RPM band to decide to shift. I have mine tighter-than-stock, and I'm getting great FE results, although with firmer shifts. Does your car have a torque converter?

RH77

[SVOboy]

Slackening off, is, I think he means, adjusting it so that it tugs less on the transmission, so that it registers less throttle input and shifts sooner/doesn't kick down as quickly.

[rh77]

Originally Posted by SVOboy

Slackening off, is, I think he means, adjusting it so that it tugs less on the transmission, so that it registers less throttle input and shifts sooner/doesn't kick down as quickly.

It's actually the opposite for me. Forget that it had popped off of its connection point, and bear in mind that a "Grade-Logic" hill control system is at work here (basically calculation of Load, TPS, and VSS).

I tightened the cable adjuster so that it pulls more on the transmission cable with more throttle input. With the OBD-II feedback loop, it takes the TPS reading and communicates with the TCU. The result is full-TC lockup at exactly 35 mph, whereas stock is closer to 40. A pressure sensor in the transmission will help decide when to make the shift, and with the cable tighter, it upshifts sooner. Downshifts are based primarily on Load and TPS input (generally 45%+ TPS, with 70%+ loads) or, of course, vehicle speed as it slows. As it stands, TC lockup will hold with the added pressure from the cable actuation over the input from the TPS/VSS -> TCU circuit.

Long story short, a shop manual will translate how the transmission "thinks", and what modifications are required to excercise some control over the slushbox.

RH77