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turboomni

Driving Technique Question

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I live in a very hilly area of the country and the roads in question are mostly at 35 to 45 mph.

Long up hills and of course long downhills. with my 97 ls400 in drive and going down the long hills many times I must just ride the brakes which I hate as I am used to manual transmissions and would downshift to the next lower gear to save my brakes.

My question is if I put the car into 3rd gear on the auto transmission going down these hill [which works fine] am I wearing out the tranny faster?

One side of my brain says "use the brakes as they are cheaper to replace than the transmission"

What you guys think ? Sacrifice the brakes or the transmission by going into 3rd?? or is that not a factor to use the transmission that way.

Frankly any other car I have owned in my life I wouldn't have given a rat's butt what I did ,,but this car is different for some reason......

Any thoughts would be great.

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When driving down long highway grades, I always shift down one gear to save on the brakes.

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Same here, the last thing you want is brake fade caused by overheating,the transmission will be happy to do the braking for you with no ill effects.

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Nothing is for free. When you select a lower gear the transmission's sprag is locked up, and the driveshaft drives the trans and engine to a higher rpm. The torque convertor works harder, increasing trans oil temperature. The engine spins at a higher rpm.

Predicting wear rates is impossible. Whether the drivetrain wears more than the brakes cannot be known. But consider that all of the heat that would have been generated by the brakes will (and must be) generated in the drivetrain instead and dissipated through the cooling systems of the trans and engine. Simple physics.

Your choice.

I use the brakes in cars. That's what they are for. In semi's I use the Jacob's engine brake - and the engine temperature is maintained every time.

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Nothing is for free. When you select a lower gear the transmission's sprag is locked up, and the driveshaft drives the trans and engine to a higher rpm. The torque convertor works harder, increasing trans oil temperature. The engine spins at a higher rpm.

Predicting wear rates is impossible. Whether the drivetrain wears more than the brakes cannot be known. But consider that all of the heat that would have been generated by the brakes will (and must be) generated in the drivetrain instead and dissipated through the cooling systems of the trans and engine. Simple physics.

Your choice.

I use the brakes in cars. That's what they are for. In semi's I use the Jacob's engine brake - and the engine temperature is maintained every time.

Thanks for all the replies. SRK is a Jacobs engine brake a way to use a diesel's very high compression without fuel to brake a big truck?

I have heard a distictive sound before from big semi's and assumed this must be the case. I do know from past experience that when going down a hill and in a car in a lower gear with a manual tranny that the exhaust note did not change when I turned the ingnition off or on,, the sound of the engine did Not change [i removed the ingition off steering lock so I could steer !!} I am assuming the compression of the engine is the braking force ,and the less fuel the better the braking force. I am also assuming a vent to atmosphere when you do this in a big truck as it is much louder. My assumption on my car is the the cars computer shuts off all fuel in that situation and as a result the tone does not change with the ignition on or off..

If that is the case then at the time this event happens the motor and oil pump are getting a free ride from trasmission[sucking in a expelling cool air that has not been fired with fuel} and there is no load and very little or no fuel going to the motor and as a result it is cooling off but controlled by the thermostat which must shut down somewhat.

What is the point of this reply?? I have no fricken idea But I would like to know how a Jacob's engine brake operates and find out if my hunch is correct. Also in the Lexus,, when in 3rd on a down hill if the engine is making no or very little heat on it's own which would help the situation.

Sorry ,,just rambling thoughts and I appreciate your time.

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A "Jake Brake" opens the exhaust valves at the top of the diesel compression stroke, turning the engine into an air compressor, which is why the exhaust note or tone changes so dramatically. The heat of compression keeps the engine from over-cooling, and that heat is what would otherwise be in the wheel brakes.

In a modern gasoline engine the braking provided by the engine is far less, as the valves are opened at the bottom of the power stroke, so compression-expansion makes the deal about even, and the fuel is shut off entirely on deceleration, which adds a bit to the braking effect. It's mostly the high rpm friction that causes braking effect, and it's minimal compared to a Jake Brake for sure.

The point of my reply is that to slow the vehicle the kinetic energy is turned to heat energy - through the brakes or the driveline. Take your choice. But both must generate heat to slow the vehicle. And my further point is that the brakes are designed to do this, and Lexus brakes, at least after 1992, are quite capable of slowing the vehicle without damage.

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A "Jake Brake" opens the exhaust valves at the top of the diesel compression stroke, turning the engine into an air compressor, which is why the exhaust note or tone changes so dramatically. The heat of compression keeps the engine from over-cooling, and that heat is what would otherwise be in the wheel brakes.

In a modern gasoline engine the braking provided by the engine is far less, as the valves are opened at the bottom of the power stroke, so compression-expansion makes the deal about even, and the fuel is shut off entirely on deceleration, which adds a bit to the braking effect. It's mostly the high rpm friction that causes braking effect, and it's minimal compared to a Jake Brake for sure.

The point of my reply is that to slow the vehicle the kinetic energy is turned to heat energy - through the brakes or the driveline. Take your choice. But both must generate heat to slow the vehicle. And my further point is that the brakes are designed to do this, and Lexus brakes, at least after 1992, are quite capable of slowing the vehicle without damage.

Thanks for the explanation . So there is a different exhaust valve cam profile when the Jake Brake is used? Very interesting stuff.

Back to the LS ,mine is a 97 and the brakes look rather huge especially the fronts for such a sized car.

Thanks again for the explanation of the Jake Brake,,,,

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A "Jake Brake" opens the exhaust valves at the top of the diesel compression stroke, turning the engine into an air compressor, which is why the exhaust note or tone changes so dramatically. The heat of compression keeps the engine from over-cooling, and that heat is what would otherwise be in the wheel brakes.

In a modern gasoline engine the braking provided by the engine is far less, as the valves are opened at the bottom of the power stroke, so compression-expansion makes the deal about even, and the fuel is shut off entirely on deceleration, which adds a bit to the braking effect. It's mostly the high rpm friction that causes braking effect, and it's minimal compared to a Jake Brake for sure.

The point of my reply is that to slow the vehicle the kinetic energy is turned to heat energy - through the brakes or the driveline. Take your choice. But both must generate heat to slow the vehicle. And my further point is that the brakes are designed to do this, and Lexus brakes, at least after 1992, are quite capable of slowing the vehicle without damage.

Thanks for the explanation . So there is a different exhaust valve cam profile when the Jake Brake is used? Very interesting stuff.

Back to the LS ,mine is a 97 and the brakes look rather huge especially the fronts for such a sized car.

Thanks again for the explanation of the Jake Brake,,,,

well most manufacturers say not to ride the brakes, not sure though but my 92 ls manual claims to use these settings for engine braking. In fact the manual calls them engine brakes, i use to shift with these in my previous car which was an automatic when racing and my tranny in my other car is outlasting my grandparents minivan, in fact they just replaced there tranny and mine is fine and they are both dodge.

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Downshifting to 3rd gear if necessary to control speed and avoid riding the brakes saves the brake pads and brake rotors without compromising transmission life.

Downshifting doesn't heat up the fluid. See for yourself - pull the dipstick after using 3rd gear on a long downgrade and you'll find the fluid is the same temperature as when driving on level ground for along period.

EVERY Toyota owners manual from 1965-2010 advises owners to downshift the automatic transmission to control speed on downgrades.

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Downshifting doesn't heat up the fluid. See for yourself - pull the dipstick after using 3rd gear on a long downgrade and you'll find the fluid is the same temperature as when driving on level ground for along period.

You can tell the difference between say, 190F and 230F with your fingers? I don't think so.

So if the trans fluid doesn't heat up, where does the heat energy appear? Apparently simple physics eludes you.

I'm not saying downshifting is bad - only that one has to choose which component gets hot in converting braking energy. And I use the brakes.

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To me the issue is not to extend the brake pads life, but to make sure that when I need the brakes they are not overheated and faded from having been drug all the way down a 5 mile 7% grade.

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I often drop down a gear when going down long grades if there are people in front of me. If not, I just let it roll. If doing this would damage the transmission in any way, I imagine they wouldn't put the gear selection on there.

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I also use the downshift method cheers.gif Never had a worry about the tranny. I also have a tranny cooler installed wink.gif

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Yeah if you drive in the mountains routinely, I think a tranny cooler would be a good investment.

For me its really the rare trip where I go to WV to visit family where I really have a lot of mountains to think about...

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The cooler is inside the radiator. As I said, check the fluid temp on the dipstick yourself before and after going down long downgrades and you'll see it doesn't change and is usually cool enough to handle (except in hot weather).

It is possible to get many hundreds of thousands of miles of life from BOTH your brake rotors and transmission if avoid using the brakes continuously or almost continuously on long downgrades on to control speeds.

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