Manual shifting auto
Discussion
As a result of previous discussion about whether downshifting manually ready for an overtake can damage the auto box I asked some questions of a transmission engineer and received the following reply,
"When you move the lever from "D" to "3" to "2" etc the lever via a linkage operates a spool valve in the valve body. As fluid is routed from one set of clutches and bands to another the rate of flow is controlled by an orifice that is calibrated, so in short the answer is there is no harm and there is no damage".
I find manual downshifting is the best way to avoid the rear end breakaway that can happen at kickdown. Quicker too as the transmission is prepared and ready for the manoever.
"When you move the lever from "D" to "3" to "2" etc the lever via a linkage operates a spool valve in the valve body. As fluid is routed from one set of clutches and bands to another the rate of flow is controlled by an orifice that is calibrated, so in short the answer is there is no harm and there is no damage".
I find manual downshifting is the best way to avoid the rear end breakaway that can happen at kickdown. Quicker too as the transmission is prepared and ready for the manoever.
Some more info on this subject from the Q&A pages of Road & Track,
Q. "I don't believe it is advised to make a habit of shifting a traditional auto into lower gears to compression brake. I have not detected any noticeable throttle blip in my G35, a la Ferrari, which would match the higher engine RPM with the lower gear during a downshift. Am I flirting with disaster?"
A. "Not to worry. It's a traditional auto with a torque-converter; thus it's a natural shift-cushioning device. Our only caution is to make the brakes the primary device to slow the vehicle, with the transmission a secondary slowing tool. Banging the transmission down a gear at high engine RPM without also applying the brakes is asking too much of the transmission. At the least such antics over-heat the transmission fluid and can apply high loads to the clutch packs, accelerating their wear. Brakes are cheaper to replace than overhauling transmissions."
To my mind this confirms that a smooth manual downshift in the auto Vette with some throttle to match the engine revs can be useful before an overtaking manoever and should do no harm. But don't use the shift lever as a high speed hand-brake!
Q. "I don't believe it is advised to make a habit of shifting a traditional auto into lower gears to compression brake. I have not detected any noticeable throttle blip in my G35, a la Ferrari, which would match the higher engine RPM with the lower gear during a downshift. Am I flirting with disaster?"
A. "Not to worry. It's a traditional auto with a torque-converter; thus it's a natural shift-cushioning device. Our only caution is to make the brakes the primary device to slow the vehicle, with the transmission a secondary slowing tool. Banging the transmission down a gear at high engine RPM without also applying the brakes is asking too much of the transmission. At the least such antics over-heat the transmission fluid and can apply high loads to the clutch packs, accelerating their wear. Brakes are cheaper to replace than overhauling transmissions."
To my mind this confirms that a smooth manual downshift in the auto Vette with some throttle to match the engine revs can be useful before an overtaking manoever and should do no harm. But don't use the shift lever as a high speed hand-brake!
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