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Discussion Starter #1
I have a TH 400 in my 68 behind a 454 and a 2500 stall conveter. When left in drive, the shift point from 1st to 2nd happens very fast, just barely out of idle speed (1500 RPM) unless I really hop on it fully. I find if I want to drive at all aggresively (god forbid) I have to shift gears by hand. Why would this be. I have a shift kit installed (B and M I think).
 

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Get an adjustable vacuum modulator if you don't already have one and play around with the adjustment until you find a happy medium.

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'69 RS Camaro
355 5-speed 4.11
 

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It is desirable to have a transmission shift automatically when in Drive. It reduces ware and improves fuel economy. Most quality shift kits drop the shift points - particularly the 2nd - 3rd.

When performance is desired, use the manual gears. Manual shift raises critical hydraulic pressures and engages bands in 1st and 2nd gear to lessen the chance of slipping.

Sounds like your transmission is working like it should. An adjustable modulator will help with minor adjustment of shift points ( you can go too far with modulator adjustments). If major changes are required, there are modifications that can be done to the govenor to raise all shift points proportionally.

-dnult
 

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I might be wrong here, but from what I remember the TH400 doesn't use a vacum modulator, the TH350 does though. Someone please correct me if I'm wrong here.

I've run nothing but 4 speeds for the last several years, but I've had a few TH400 behind big and small blocks. The thing I least like about that trans is it shift way too soon at wide open throttle. Even if left in low gear, shifts into second on it's own at around 6,000 rpm. Most of my motors were just starting to make good power with a couple thousand rpm to go and bang second gear. Manual valve body will solve that though.
 

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Discussion Starter #5
Is the vacum modulator the external canister that the vacum line hooks directly onto?
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by J early:
Is the vacum modulator the external canister that the vacum line hooks directly onto?<HR></BLOCKQUOTE>

Yes, and THM400 does use a vacuum modulator. If you pull the vacuum line off, you'll be able to insert a small standard screwdriver. Make adjustments about 1 or 2 turns at a time. Turn in to shift later and harder, turn out to shift sooner and softer. Don't over do it though. The adjustment is no substitute for a shift kit. I know several people who tweek the adjustment to get a harder shift which is asking for trouble.

If you can't get the shift adjusted via the modulator within a few turns, you may need to consider modifying the govenor. Also, if tranny fluid drips out when you pull the vacuum line off, replace the modulator.

-dnult
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by dnult:
When performance is desired, use the manual gears. Manual shift raises critical hydraulic pressures and engages bands in 1st and 2nd gear to lessen the chance of slipping.<HR></BLOCKQUOTE>

I have always heard that manual shifting an auto is hard on the tranny. What is your opinion on this?

-Funk
 

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You certainly can abuse a transmission more when shifting manually. As examples, you can rev higher before shifting and downshift. Both cause wear on the friction surfaces and bands. However if you want performance, shift it manually. Doing so will raise shift pressures, engage additional bands / clutches, and activate additional clutch pistons (sort of) to reduce slipping. By comparison, if you were to do a full throttle accelleration through all the gears in drive verses shifting manually, you would do more damage in drive. Also, holding a gear manually in slow traffic (like we have here in Austin) reduces wear since the transmission won't be shifting up and down as much when you're crusing between 20 and 30 miles per hour on a 4 LANE HYWAY FOR CHRIST SAKE
. Automatics do a pretty good job of shifting under normal conditions, but they can't look ahead and see the hill comming, traffic slowing, or know that you want to beat the guy in the next lane. Thats when it's time to grab the stick.

-dnult
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by CFunK:
Guess you haven't seen Atlanta traffic at rush hour.

One word: Brutal

I would think the tranny would take more abuse when manually downshifting.

-Funk
<HR></BLOCKQUOTE>


You're right to a point. I don't think that downshifting in rush hour traffic is all that harmful. On the other hand, when I drove my dad's 76 chevy truck at age 18, I often downshifted to low at 40mph and waited for the transmission to downshift once I slowed down enough. Made a nice screaching sound like an airplane touching down. The transmission was still going strong at 80,000 miles when we got rid of the truck. However, if I caught my son doing that I'd choke him. I'm old enough to know better now and realize I'd have to pay for the repair myself.

I'd recommend downshifting in slow traffic or on a steep grade. Usually downshifting isn't necessary though and brake pads are easier to replace than transmission parts. I guess it's like most things - if taken in moderation usually no harm will come.
 
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