TH 350 w/ lockup or 200R4? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 10-25-2007, 10:26 AM
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TH 350 w/ lockup or 200R4?

For some time I have been considering replacing my perfectly good TH350 with a 200R4 pro tranny to get the 4th gear for improved milage on the highway. The other day I was discussing this idea with a fellow rodder who suggested that I could add a lockup feature to my 350 and achieve the same results. I suppose that is true to some extent. I'm wondering what happens to the ride comfort at highway speed with a lockup. Will the car lurch or shake or vibrate because the dampening effect of th torque converter is eliminated? How does a lockup work? Would an engaged lockup at highway speeds disengage automatically in an emergency situation? What do you suggest or advise?

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Old 10-25-2007, 10:44 AM
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I'm no tranny expert, but I'm pretty sure you can't use a lock-up converter on a non-lock-up tranny. You would need a TH350C tranny. The pressure to apply the lock-up clutch in the converter comes from the transmission and is activated by a solenoid in the tranny.
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Old 10-25-2007, 12:11 PM
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You need to get a later (post 82) th350 to get the lockup. You can put a non-locking converter in a lockup 350, but you can't go the other way like you suggest. Its a whole different thing that is engineered into the later trannys.

There is no lurching or anything different in the operation. The only thing you'll notice is that as you accelerate up through the gears, after you get to third, the converter will lock up. It will almost feel like a tiny extra gear change since the RPMs will drop slightly. It just stops the slipping and directly connects the engine to the tranny.

A torque converter sorta works like a manual clutch that's half engaged. A lock up converter just gives you the added benefit of becoming fully engaged at higher speeds.

The 200-4r is a great tranny, and (if your 350 is a short tail version) the 200-4r is almost a direct bolt-in. Its the same length with the same output shaft, so you don't need to change driveshafts. The tranny mount is further back on the case, so you'll need to move your crossmember back. most GMs of that era have other holes drilled in the frame for just that purpose, but if they don't you can drill it yourself. There is an electrical modification to make to get the converter to lockup, but its a pretty easy swap.

Give us more info... what is the car, engine, current rear gears and tire size? If your th350 is backed with 2.73 gears, you don't need an overdrive as it will just lug the engine too slow.
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Old 10-27-2007, 10:39 PM
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One thing to keep in mind, the cost of buying a new 2004R (In my case, $2000), would take quite awhile to make up in fuel savings. Thinking long term, how often will the car be driven at highway speeds? Id rather have my motor cruising at 1500 rather than 2700.

I have a 2.73 rear gear, with a 2004r, and I dont have any problem with a lugging engine in overdrive. It actually cruised along very nicely.
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