To overdrive, or not to overdrive?

1.8-liter VVTL-i (2ZZ-GE) and VVT-i (1ZZ-FE) engine, transmission, exhaust, intake, and performance tuning discussions

To overdrive, or not to overdrive?

Postby VibeInTx » Wed Jul 03, 2002 3:44 am

I just recieved my Vibe yesterday (base, auto, moon+toons, power, 6 disc), and I'm wondering if anyone has comments on the overdrive (i.e. when to use/not use it). Has anyone noted a gas mileage difference?

thanks

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Re: To overdrive, or not to overdrive? (VibeInTx)

Postby d_m_kolb » Wed Jul 03, 2002 3:55 am

Lockup converters contain another part: a torque converter clutch. When the clutch engages, it allows the converter to lock the engine to the transmission input shaft, providing a direct 1:1 communication from the motor to the transmission. Why is this necessary, or even desirable? In many ways, it comes down to fuel economy. Lockup torque converters have become popular since automakers have been stuck in a sort of Catch-22: Car buyers overwhelmingly prefer automatic transmissions, but automatic transmissions are not as fuel efficient as manual transmissions and automakers must meet government-mandated corporate average fuel economy targets. Overdrive transmissions have been one step along the path toward increased fuel economy from an automatic transmission. Overdrive transmissions allow the motor to spin at a lower rpm during cruising speeds. A higher final drive ratio (numerically lower) does the same thing. However, when the engine spins more slowly, it creates more slippage within the torque converter, and more slippage creates more heat. Heat within the torque converter reduces fuel economy and can harm both the torque converter and the transmission. The solution is to allow the converter to lock up at a 1:1 ratio. Lockup eliminates the slippage, which reduces heat and improves fuel economy. Do you want to know if your car has a lockup torque converter or if it's working properly? Try this: Drive along at 50 mph or a slightly higher, steady cruising speed. Depress the brake pedal ever so gently (not enough to actually apply the brakes, but enough to turn on the brake lights). See if you experience what feels like a slight downshift. Then release the pedal very slowly and see if you feel a slight upshift. If you do, the lockup mechanism is working properly. Usually, lockup converters are used in stock applications, but not in higher-performance vehicles. However, some drag racers choose to run lockup torque converters, too. It is possible to lock up the converter at wide-open throttle manually (by using a switch) or automatically (via a racing computer chip). It has been estimated that locking up the converter at WOT in a relatively stock doorslammer, such as a Buick Grand National, can pick up about a tenth in quarter-mile ETs; however, it also speeds up wear and tear on the torque converter clutch.
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Re: To overdrive, or not to overdrive? (d_m_kolb)

Postby NSimkins » Wed Jul 03, 2002 3:59 am

quote:
You don't have to use it in the city but for the best gas mileage you should. The over drive will shift in and out when it needs to with out any input from you while driving.

In regards to city driving... does this in any way "wear out" the overdrive gear by the unusual frequency of the stop-and-go driving?

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Re: To overdrive, or not to overdrive? (NSimkins)

Postby d_m_kolb » Wed Jul 03, 2002 4:12 am

I have heard of this happening in older cars but not so much with newer ones. Of course everything wears out and I know for a fact the quickest way to wear out or destroy a lock up converter is to have dirty trany fluid flowing through the trany. Change your trany fluid every 30,000 miles and filter if you use a non-synthetic trany fluid. I use synthetic (Amsoil) and change my fluid 60,000 miles.

I have never worn one out yet and have lived in a city most of my life. I also use a good synthetic trany fluid (Amsoil) that keeps the friction down in the trany and also keeps the trany cooler. The synthetic also helps increase gas mileage. Keeping a trany cool is the key to long life. Clean fluid is a good thing also. Heat is the worst enemy of a trany.

It wouldn't be a bad idea to buy a transmission cooler instead of having the main radiator do this. Reason being if your engine ever over heats your transmisson just over headed also and you might lose your trany because of it.
http://www.amsoil.com

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Re: To overdrive, or not to overdrive? (VibeInTx)

Postby HenryTS » Thu Jul 11, 2002 7:44 pm

The manual is a little vague about the overdrive. In the section where it talks about the overdrive on/off button it says to to use this feature for better gas mileage does it mean when it's on it get's better mileage and if so it gives no clue why anyone would want to turn it off?
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Re: To overdrive, or not to overdrive? (HenryTS)

Postby NSimkins » Thu Jul 11, 2002 7:55 pm

I also noticed the same.

Quote from manual:

quote:
Use this feature for better fuel economy. Fast starts use the most fuel while gradual starts give you the best fuel economy.

Other than that, the manual just states how to turn O/D on and off.

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Re: To overdrive, or not to overdrive? (NSimkins)

Postby NovaResource » Thu Jul 11, 2002 11:38 pm

Overdrive is just another gear ratio. Your automatic trans has 4 gear ratios, 2 are underdrive, one is direct (1 to 1) and one is overdriven. If you drive with overdrive off, you are basically driving a car with a 3 speed trans.

Overdrive means the output shaft of the transmission is spinning faster than the input shaft. Here are the Vibes gears:
1st gear = 2.804 (under driven)
2nd gear = 1.531 (under driven)
3rd gear = 1.000 (direct)
4th gear = 0.753 (overdrive)

First gear is underdriven because for every 2.804 turns of the input shaft, the output shaft only turns 1 time.

Third gear is a direct relationship. The input shaft turn once and so does teh output shaft.

Forth gear is overdriven because of the input shaft onlt turns .753 (or 3/4 of a turn) for every single turn of the output shaft.

A lockup converter will seem like another gear. This is because all torque converters have slip in them as designed. At low speeds, the slip is more but at higher speeds the slip is less. Usually about 10%. When you lock the converter you are removing the slip and that extra 10% seems like another gear because it slows the engine RPM down for the same wheel speed. Lockups were needed because overdrive gear ratios slow the engine RPMs down to a point wher the converter will slip more creating heat and reducing the effectivness of the overdrive.

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Re: To overdrive, or not to overdrive? (NovaResource)

Postby HenryTS » Fri Jul 12, 2002 12:52 am

Verr good description of the overdrive system and from the sounds of it you should never need to turn it off> However they provide a switch to turn it off_under what conditions would you want to turn it off?

Henry

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Re: To overdrive, or not to overdrive? (HenryTS)

Postby NovaResource » Fri Jul 12, 2002 4:25 am

About the only time turning off the overdrive is a good idea is when you do alot of city driving. Lugging the engine rpms down that much will make the car feel sluggish you you have to accelerate from a lower speed (about 30-mph or lower).
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Re: To overdrive, or not to overdrive? (NovaResource)

Postby d_m_kolb » Fri Jul 12, 2002 5:34 am

quote:
Overdrive is just another gear ratio. Your automatic trans has 4 gear ratios, 2 are underdrive, one is direct (1 to 1) and one is overdriven. If you drive with overdrive off, you are basically driving a car with a 3 speed trans.

Overdrive means the output shaft of the transmission is spinning faster than the input shaft. Here are the Vibes gears:
1st gear = 2.804 (under driven)
2nd gear = 1.531 (under driven)
3rd gear = 1.000 (direct)
4th gear = 0.753 (overdrive)

First gear is underdriven because for every 2.804 turns of the input shaft, the output shaft only turns 1 time.

Third gear is a direct relationship. The input shaft turn once and so does teh output shaft.

Forth gear is overdriven because of the input shaft onlt turns .753 (or 3/4 of a turn) for every single turn of the output shaft.

A lockup converter will seem like another gear. This is because all torque converters have slip in them as designed. At low speeds, the slip is more but at higher speeds the slip is less. Usually about 10%. When you lock the converter you are removing the slip and that extra 10% seems like another gear because it slows the engine RPM down for the same wheel speed. Lockups were needed because overdrive gear ratios slow the engine RPMs down to a point wher the converter will slip more creating heat and reducing the effectivness of the overdrive.


Your wrong about 3rd and 4th gear. The only time the a gear is direct to the engine in a auto trany is when the converter locks up. Then and only then is it not slipping and this only happens in the last gear.

The better gas mileage comes from the torque converter not slipping in 4th when locked. 1st, 2nd and 3rd gear will all ways have slippage of the torque converter, 4th will to until the torque converter locks up and then it's just like a stick in the sense that the gear is connected directly to the engine. Let off the gas on the highway and the converter will unlock so you can slow down and stop and not keep the trany in gear. If it didn't unlock the engine would stall like a stick does if left in gear and then come to a stop.

Lock ups aren't needed in a torque converter either. Many older vehicles do not have lock up torque converters. Any stock 350, or 400 chevy trany doesn't have a lock up torque converter. The chevy 700-R4 has a lock up torque converter though.

Also the lock up mechanize in the converter don't make heat. How could they? They don't spin. All they do is lock and unlock. The slippage of the T.C. itself is what causes the heat. That's why higher stall T.C. make lots of heat. They spin or slip to a higher RPM before the vehicle starts to move helping the engine get into it's power band before launching.

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Re: To overdrive, or not to overdrive? (HenryTS)

Postby d_m_kolb » Fri Jul 12, 2002 5:41 am

quote:
Verr good description of the overdrive system and from the sounds of it you should never need to turn it off> However they provide a switch to turn it off_under what conditions would you want to turn it off?

Henry


It's good to be able to turn the lock up feature off and on. In most cases it will be on but if in stop and go driving and you feel the trany lock up and then disengage, lock up and disengage, lock up and disengage every few seconds because your speeding up and slowing down it would be a good idea to turn the feature off. The more it locks and then disenages the more wear and tear on it.

Most of the time I wouldn't worry about it. But if you are pulling something you might want to turn it off also. Reason being the lock up part of the T.C. isn't that strong. It can't handle alot of pressure for extended periods of time. That's also another reason when pushing on the accelerator just a little bit harder will make it unlock. If it stayed locked under hard acceleration first off you wouldn't accelerate much faster but you could damage the lock up T.C.

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Re: To overdrive, or not to overdrive? (d_m_kolb)

Postby NovaResource » Fri Jul 12, 2002 8:09 am

quote:
Your wrong about 3rd and 4th gear. The only time the a gear is direct to the engine in a auto trany is when the converter locks up. Then and only then is it not slipping and this only happens in the last gear.

Read my reply again. I never said it was direct to the engine. I said a direct link from the INPUT SHAFT to the output shaft. You are correct that there is no direct link to the engine crankshaft unless the converter is locked. However, 3rd gear (1.000:1) is always a direct link between the input and output shafts. If the engine is turning 5000-rpm and there is 5% slippage the torque converter, the input shaft (that which what the torque converter rides on) is spinning 4750-rpm. In 3rd gear, the output shaft will also be spinning 4750-rpm. So therefor there is a direct link 1:1 of the input and output shafts.

In reality, there is no output shaft because the auto trans is actually a transaxle which houses the trans and final axle ratio (2.96 in the auto). So when the engine is turning 5000-rpm and the converter is not locked, the input shaft is spinning 4750-rpm. In 3rd gear the output of the trans (to the final axle ratio) is spinning 4750-rpm also. Then through the 2.96 final drive ratio, the axle half-shafts (and the wheels) are spinning approx 1600rpm (approx 120-mph).

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Re: To overdrive, or not to overdrive? (NovaResource)

Postby d_m_kolb » Fri Jul 12, 2002 9:27 am

I miss read and see what you were saying about 3rd gear. You are right. I thought you ment the T.C. was locking up then making it a direct connect. Sorry I called you on that.
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Re: To overdrive, or not to overdrive? (VibeInTx)

Postby ironchef » Tue Jul 16, 2002 5:33 am

Leave the durn thing on and let the highly evolved transmission figure out when to use it. I drive about 60 miles a day on the freeway, ranging from stop and go to 80mph. If I step hard on the gas the tranny _automatically_ downshifts! (sarcasm intententional).

That said, I have turned it off to let the engine do some of the braking on steep hills and have downshifted even even more as necessary. Works like a charm.

Larry

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Re: To overdrive, or not to overdrive?

Postby cmpltvertigo » Wed Jan 29, 2014 8:49 pm

Ok I just need some clarification. I drive with the OD Off when I do a lot of Stop and Go driving and my question is will the automatic transmission turn the OD Off automatically when I'm at the proper speed or do I need to press the switch on the shifter to turn it Off? Thanks in advanced.
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Re: To overdrive, or not to overdrive?

Postby KITT222 » Wed Jan 29, 2014 9:47 pm

cmpltvertigo wrote:Ok I just need some clarification. I drive with the OD Off when I do a lot of Stop and Go driving and my question is will the automatic transmission turn the OD Off automatically when I'm at the proper speed or do I need to press the switch on the shifter to turn it Off? Thanks in advanced.


If you click the OD button, it will stay off until you turn it back on, or until you turn the car off. Even at 70mph uphill passing a semi truck. Which means that you can indeed downshift your automatic to pass things.
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Re: To overdrive, or not to overdrive?

Postby JohnO » Thu Jan 30, 2014 3:43 pm

O/D off = you have a 3-speed automatic
O/D on = you have a 4-speed automatic
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Re: To overdrive, or not to overdrive? (NovaResource)

Postby Chiadog » Sat Feb 01, 2014 8:55 am

HenryTS wrote:Verr good description of the overdrive system and from the sounds of it you should never need to turn it off> However they provide a switch to turn it off_under what conditions would you want to turn it off? Henry

Probably the most important reason for the OD button is to keep the automatic transmission from overheating when pulling a heavy load, like a heavy trailer or another Vibe:) The transmission fluid pump is driven directly off the transmission input shaft, it always turns at engine rpm speed. The faster the engine speed, the more cooling capacity to the transmission. Slower engine speed (like while in OD) less cooling of the transmission. Pulling a trailer at low engine speed (in OD) and high load for too long will destroy the automatic transmission by overheating it.
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Re: To overdrive, or not to overdrive?

Postby Salsa Guy » Sat Feb 01, 2014 9:11 am

If your towing you should add a tranny cooler. Fairly cheap and easy to install. Going up and down small hills or taking on large mountains I found turning off the OD kept the thing from constantly shift up and down.
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Re: To overdrive, or not to overdrive?

Postby Water boy » Sun Feb 02, 2014 2:18 pm

:o
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That is 11 years, 6 months and 14 days totaling 4216 days.

Must be some worthwhile stuff here, no matter how old it is.
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