View Full Version : Manual Transmission power loss vs. Auto..


Sir Aristrotle
12-06-04, 02:44 PM
Hey,

Basically I need to know why it is that manual transmission eats less power than an auto. I came here rather than asking (again) on the automotive forum I am a member on because I need a detailed answer.


Personally thought that it was just the Automatic Transmission's torque converter that caused the loss of power but someone just told me there is more than that to it, what else is there?


Thanks in advance!

spidergoat
12-06-04, 03:47 PM
I would think it uses power from the engine to shift the gears, that robs power going to the wheels. Also, there are more moving parts, causing more friction.

Sir Aristrotle
12-06-04, 04:55 PM
Thanks for the reply, any more theories?

cosmictraveler
12-06-04, 05:59 PM
I'd think that a properly adjusted automatic tranny would not use that much more HP to operate it. I'd think that it would be a plus to race with autos for then you wouldn't lose the time to shiift and lose HP when doing the shifting.

In general, automatic transmissions are known as being more useful for lazy commuters than for hardcore racers. But it may be surprising for some to know that many pro drag-racing cars have heavy-duty auto gearboxes. That's because the brake-torque launch is an automatic specialty. This launch involves keeping the car stationary by flooring the brakes with the left foot, while using the right foot to rev up the engine against the torque converter. In technical terms, this preloads the entire drivetrain with the stress of a launch, allowing the engine to rev closer to its power and torque peaks at the starting line. Brake-torquing is also beneficial for turbocharged engines as it allows boost to build up before the launch, reducing turbo lag. The only problem is that there is a lot of stress on the transmission, and the consequent heat build-up can destroy your automatic gearbox. Unless your car has too much power for the tires to handle, a brake-torque launch usually will not spin the wheels. This is because the automatic transmission absorbs the shock by design, and brake-torquing actually reduces stress on the rest of the drivetrain. Instead of a sudden massive load, the drivetrain has the torque applied slower instead of one huge jolt.

If your car has power brakes, you could apply the brake-torquing technique even better. At the starting line, shift your auto gearbox into neutral and floor the brake pedal with your left foot. Rev the engine once and quickly get off the throttle. You will feel the brake pedal sink further to the floor. This greatly increases the braking force. Now shift back into gear. You will now be able to rev up the engine even higher against the torque converter.

The downfall of standard automatics is their easy-going nature while shifting, so you lose power during the actual race, where a manual would have allowed for more aggressive upshifts. However, the shift points of an automatic can be professionally modified for drag racing, and some new automatics even allow for manual shifting or have a sport setting for quicker gear changes. In fact, you could shift up through the gears even on most conventional automatics for a little more oomph.

http://www.modernracer.com/tips/dragracinglaunchtechniques.html

Sir Aristrotle
12-06-04, 07:36 PM
Thanks again. More replies are welcomed!

Sirius Is Serious
12-06-04, 08:45 PM
It is a myth :rolleyes: that auto trans results in higher fuel consumption. I hv worked with some of the best Japanese Auto Mfgs and drove cars with both types. Infact for congested city traffic driving, the auto transm performs better than the manual with less strain on the gear box and fuel. :)

slotty
12-07-04, 04:52 PM
Most auto's work along with a fluid flywheel. This is basically a chamber with two fans immersed in fluid. Say you filled a bowl with water, placed a propeller on a pivot at the bottom. Now if you lowered another fan in the water that was attached to an electric whisk and turned it on, the current it generates in the water will make the fan at the bottom turn. Thats the basic idea of a fluid flywheel. Most of the power is lost through the sheer effects of the liquid.Autos use a thing called a torque converter to up the effiency of the flywheel, and after a certain speed of rotation is reached, they are pretty much as effective as a manual.

Stokes Pennwalt
12-08-04, 11:11 PM
Autos use a hydraulic fluid coupling called a toque converter rather than a clutch, in order to serve as a buffer between the engine crankshaft and the gearbox to prevent damage to both from speed/torque disparities. Anytime you have a fluid coupling in an open looped system there is a certain amount of energy lost in the form of heat within the coupling, as it slips to accomodate speed differences.

Think of how inefficient an airplane propeller is as opposed to a driven wheel at propelling a vehicle along the ground, and you'll get the idea. Driveline loss in autos is significant enough to cause a decrease in fuel economy across the board by a significant amount.

edit: slotty's explanation might be a bit better

Repo Man
12-09-04, 12:45 AM
Some autos have locking converters for greater fuel efficiency at highway speeds. But there is also the parasitic drag of the oil pump inside the transmission, as well as the friction loss from the torque converter already mentioned.

cardiovascular_tech
12-10-04, 09:30 PM
ok if you have ever looked inside both a auto and a manual tranny you would have a pretty good understanding of the power difference in each. On the manual side you have gears and no bands and such like you do in a automatic tranny, sure you have clutches on both but they act differently where is in a auto tranny its done by the tranny its self where a manual you have to engage the clutch, there is alot more going on inside a automatic then a manual, but with the solid gears of a manual you will get more power with few moving parts. but if you was to go with say a powerglide transmission which is a auto you can beef it up enough to match or even beat a manual tranny.

slotty
12-12-04, 08:56 AM
ok if you have ever looked inside both a auto and a manual tranny you would have a pretty good understanding of the power difference in each. On the manual side you have gears and no bands and such like you do in a automatic tranny, sure you have clutches on both but they act differently where is in a auto tranny its done by the tranny its self where a manual you have to engage the clutch, there is alot more going on inside a automatic then a manual, but with the solid gears of a manual you will get more power with few moving parts. but if you was to go with say a powerglide transmission which is a auto you can beef it up enough to match or even beat a manual tranny.

On just the transmission side an auto is much simpler in theory. There is just an annulus, planet gears and a sun gear. the ouput shaft of each is joined to the next and a wide variety of ratios is availible. But like you said,the down side is the gubbins that lock the various gears up. I used to work on a 7 train epicyclic geabox that was electronically controled and hydraulicly acctuated. It was a sodding nightmare when it went wrong :(

a_ht
12-12-04, 11:01 AM
Maybe automatic transmission comsumes more fuel than a very well *caught* perfectly *caught* driven manual car, but if the user has an heavy foot or is latent to switch to higher gear, then I doubt that the manual car comsumes less.

slotty
12-12-04, 11:18 AM
Absolutley. But nowdays auto's seem to be faster, why else would F1 use them? A standard road car now comes with a six speed auto box, controlled by computer. Much quicker than humans. Paddle change like F1, much faster than the old stick method.

el-half
12-12-04, 12:06 PM
I'd think that it would be a plus to race with autos for then you wouldn't lose the time to shiift and lose HP when doing the shifting.

Automatic gearboxes are for pussies.
They are practical for people who are too lazy to drive a manual but come on, racing with automatic gearboxes? Shifting with those damn paddles on the steeringwheel is already lame enough.
Or maybe I am a nostalgic fool.

Sir Aristrotle
01-02-05, 12:27 PM
Personally, I feel the lost is in hydralic system which causes drag and in the torque converter not being a direct 1:1 coupling initally. Thats why autos don't have the best acceleration, then again it varies from design to design. I hear GM auto transmissions are better than Hondas.

slotty
01-02-05, 02:14 PM
I have a Honda Prelude, the auto is faster than the manual.

freakinout220
01-10-05, 04:58 PM
In an automatic transmission, each gear possesses a device that, under specific rpm and engine vaccuum pressure, will change the gear. In a manual transmission, you are that device. Also, manual transmissions are geared differently, to make up for operator error when shifting. The differences are gears that are easier for the engine to turn, due to circumference of the gear. A higher gear robs horsepower. :D

slotty
01-10-05, 05:33 PM
In an automatic transmission, each gear possesses a device that, under specific rpm and engine vaccuum pressure, will change the gear. In a manual transmission, you are that device. Also, manual transmissions are geared differently, to make up for operator error when shifting. The differences are gears that are easier for the engine to turn, due to circumference of the gear. A higher gear robs horsepower. :D

Yeh, about 30 years ago. I am a qualified technician before i changed direction in life, and now the irritating thing is that i can't touch my motor because of the myriad of computers controlling every aspect of it. My motor has so many sensors on it to control the gearbox, it even impresses me. I have a friend with the manual version and mine pisses all over it every time :D

Nomadd22
02-25-05, 07:13 AM
If you look under your car, you'll see cooling lines going from your auto transmission to the radiator. They're not there for looks. It needs them because it produces more heat than a manual. That heat doesn't come from heat fairies. It comes from engine power. The power from your engine going into the transmission goes two places. To the drive shaft, and to heat. Since the auto tranny makes more heat, it makes less power to the wheels.

boppa
03-05-05, 10:06 AM
im not sure about the brand new ones, but for the older non lockup in top gear ones-they may/maynot be faster in the city cycles-but on long distance non stop driving the autos are definately thirstier on the fuel

2 identical hyundi excels from sydney to brisbane-about 20 bucks more for the autos

and that works out to be almost a third higher as the manual used only just over 40$ but the auto used $60

slotty
03-06-05, 09:10 AM
Yeh , older autos are much thirstier than the manuals, but nowdays i don't think theres much in it, but if your using a fluid coupling its bound to be less efficient until you reach lock up speeds.

CANGAS
10-12-05, 07:25 AM
In addition to all the excellently thought out and presented ideas already extant, here is one more, although one of minor but measureable importance. When we are stopped, as when chatting with a nosey policeman, or, at a red light, our automatic transmission, if left in gear, will drag down the motor idle speed a hundred RPM or something like that. LOWER MOTOR SPEED MEANS LESS GAS USED! Alternatively, our manual transmission, unless we are doing something really weird with our clutch, will allow motor to run at its usual higher RPM. HIGHER MOTOR SPEED MEANS MORE GAS USED!