66 skylark w/340 oem auto to a "th-700r4(4sp w/o.d.)trans.

[66SKYLARK]

hi,new member,going from my present 2sp auto to a TH-700R4 W/OVERDRIVE.ANY ONE TRY IT? I`M TOLD I CAN LATER,SWAP MY 340 TO A BUICK 455,MANY PROBLEMS? I AM LOOKING FOR A TURN-KEY/CRATE 455 BUICK, ANY PLACE I CAN FIND ONE,PREFERABLY WITH HOPES,A "70SPEC.STAGE#1.THESE ARE MY GOALS+ A POSI,GEARS?? SURE COULD USE SOME HELP & ADVICE, THANKS,JIM. JAK37DOWOP@AOL.COM

[page2171]

I haven't tried it, but I think it would be easier to swap in a TH-200R4 (like Buick used behind the turbo V6) since it has the BOP bolt pattern, where the TH700R4 has a Chevy pattern.

Brian

[Guest John Chapman]

The 200-R4 would be the preferred swap. There are a lot of old wives tales about the 200-R4 being unreliable, but with a good rebuild using the correct parts, this trans would easily handle a 455. The 200-R4 is a near- bolt in swap. You'll only have to move the rear crossmember back a bit, or just buy one designed for the swap. The 200 also has better gear spacing and a numerically higher overdrive. Hot Rod Magazine did a swap on a '67 Skylark in the Aug 2003 issue.

Cheers,

JMC

[NTX5467]

I suspect the THM200R4 will take a little bit less power to "run" than the THM700, which will mean a little more "power to the ground". Seems like the THM200s had a 2.74 low gear and a 1.74 2nd gear, but the small difference in the OD ratio might not really matter, wheras the normal THM350 had 2.52 and 1.52 ratios and the THM700s had something like 3.07 low and 1.72 2nd gear. In those earlier times, the normal 3-speed automatics typically had 2.45-2.48 low gears and 1.45-1.48 2nd gear ratios (depending upon if it was a Ford, GM, or Chrysler transmission), so you can see how the THM200s relate to that. I DO like the gear spacing on the non-THM700s better as there's less rpm drop between the gears.

Using an OD transmission with anything less than (about) a 3.42 rear axle ratio will put the cruise rpm in OD too low for a carbureted engine (at 65mph) to really "like". So you might need to contemplate something like a 3.73 or 3.91 (multiply the rear axle ratio by .7, basically) to get the final OD ratio). In those earlier times, the factory ratios for the 3-speed automatics could have gone down (numerically) to about 2.56, which might be a good figure to use as a guide in deciding what your new rear axle ratio would need to be with the OD automatic.

The other thing to consider is that in the fuel economy area, you might end up with something that would accelerate better but get no better fuel economy on a highway cruise . . . than what you now have with the smaller engine and 2-speed automatic and existing rear axle ratio. THEREFORE, priorities need to be considered.

It might be "nice" to have a Buick 455 in that car, but the reality is that the stock engine, with some moderate upgrades, might be a better situation. The larger engine (and it's massively increased torque) would require (most probably) some upgraded rear axle control arms and upgraded brakes (due to the added speed potential) and a general upgrade of the complete chassis situation. Not to mention increased fuel costs. None of these things are really hard to make happen, but it all depends upon what the "real world" use might be and ultimate resale value (modified vs. upgraded stock). These would all be judgment calls on your part, but judgment calls made without looking through "rose colored glasses" or staring at the "blue sky".

A rompin', stompin' Buick hot rod might be a nice thing to have, but a little more conservatively modified Buick that starts and runs readily, doesn't overheat in city/freeway slow-moving traffic, has good performance, and gets decent fuel economy (on mid-range octane fuels) can be a much better vehicle to own, drive, and impress your friend with. And you might have pretty much that vehicle now . . . as is. Almost any car can sound "mean" and burn rubber, but those things don't really impress others as much as a car that runs well and looks great--whenever it's called upon to do so.

Your car, your money, your dreams.

Enjoy!

NTX5467