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#1
05-05-2010, 10:36 AM
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Shifting Down - What does it tell you?
Have you ever driven down a canyon or something and instead of roasting your rotors, you shift down (yes, I'm talking autos too) to slow your car down? I heard once that this can also be an indication of compression health? I have absolutely no clue at all if that's accurate but I heard that if you're cruising along, you shift down, and the car slows down considerably that means that your compression is in good shape. Conversely, I heard that if you shift down, you see your RPM's go up with the shift, but the car really doesn't decellerate much, then you might have compression problems (bad piston rings maybe?). Is this complete bunk, or is there any wisdom or logic to this? I'm sure gearing has something to do with it. When I shift down in the Jeep to go down a big long hill it really slows down the vehicle a lot but with the GTP, it seems to only make the engine louder.
 What are your thoughts?
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  2000 Pontiac Grand Prix GTP 3.8L SII V6 S/C 2000 Jeep Grand Cherokee Limited 4.7L V8 ---------------------------------------------------- 1996 Chevrolet Camaro Z28 5.7L V8 1993 Chevrolet Suburban K1500 5.7L V8 1991 Ford Probe LX 3.0 V6 1986 Ford Tempo GL 2.3L 4 cyl. 
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#2
05-05-2010, 01:44 PM
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Re: Shifting Down - What does it tell you?
Good question that one. I've done it in my GP to slow the car when passing a cop running radar without throwing on the brakes.
 I'd be interested to know as well.
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#3
05-05-2010, 10:55 PM
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Re: Shifting Down - What does it tell you?
Not so much to test compression in your motor because if your motor is running it has compression.
Downshifting is sometimes used to diagnose trans issues. If you downshift and don't feel your car lug down, you could hav an issue with a one way clutch in your trans. From there you would need to use a clutch/band application chart specific to your trans in order to find out what one way clutch is bad.
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-2000 Grand Prix GTP 170,000mi (daily driver) -2000 Olds Alero 100,000mi (soon to be DD with gas at $3.45/gal) -1997 Chev K1500 4x4 115,000mi (Natalie's truck [nans_grandprix]) AF "2.0" Community Guidelines Conservative Victory 2012!!! "I'll Keep my Guns, Freedom, and Money. You can Keep the Change!" ----->>>>> Did You Know? <<<<<-----
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#4
05-06-2010, 08:17 AM
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Re: Shifting Down - What does it tell you?
when suddenly downshifting but off the throttle, the torque converter will unlock causing slippage, reducing compression braking. It may not be an "apples to apples" comparison, as I'm not sure if the Jeep has an all electronic or vacuum/electronic shift system. Plus, your jeep has a lot more driveline drag, which would increase downshift/compression braking.
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 Still waiting for the "good old days" I'll get to bore my future grandchildren with!
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#5
05-06-2010, 10:33 AM
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Re: Shifting Down - What does it tell you?
Yeah, the Jeep probably isn't the greatest to compare with the car. I more think I've noticed over the past four years that the car doesn't slow down when I shift down like it used to you know? Of course I have very little confidence in the life span of these transmissions anyway and I am planning on having to replace the trans before I get rid of the car.
But so far it doesn't sound like this down shifting thing really has a whole lot to do with the actual engine, is that right?
__________________
2000 Pontiac Grand Prix GTP 3.8L SII V6 S/C 2000 Jeep Grand Cherokee Limited 4.7L V8 ---------------------------------------------------- 1996 Chevrolet Camaro Z28 5.7L V8 1993 Chevrolet Suburban K1500 5.7L V8 1991 Ford Probe LX 3.0 V6 1986 Ford Tempo GL 2.3L 4 cyl.
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