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700R for a 455

Guest LBarnes944

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Guest wildcat465


455's with automatics has 3 speed Turbo 400's in all years of production.

The 700r4 has been a popular conversion, but BOP bell housings are scarce.

the 200r4 is a better trans to put behind 455. BOP bell housings are more common, have lower numerically first gear. You do not need deeper first gear with 455 torque.

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BOP bell housings are scarce.

the 200r4 is a better trans to put behind 455. BOP bell housings are more common, have lower numerically first gear. You do not need deeper first gear with 455 torque.

Scarce as in unobtainium. 700-R4's were only made with Chevy bellhousings.

200-4R's come with dual pattern bellhousings - BOP and Chevy. If you look for one, try to find one that came from a Buick, Olds, or Pontiac. The Chevy 200-4R's used a different style flexplate and converter. The BOP's will bolt up as is. What ever you do, do not mix and match Chevy and BOP parts.

If that's the route you're looking to go, try to find, at the lowest, a 3.42 rear gear. A 3.73 or 3.90 would be better. Hell of a launch plus a nice highway gear.


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A lower (numerically) rear axle ratio than 3.42 is not too good with a carbureted motor. Fuel depends upon air flow through the carb to pull it into the venturi cluster. With fuel injection, the instant your throttle foot moves, fuel delivery is automatically adjusted via the computer. ONE thing to remember . . . if you go "deeper" on the rear axle ratio so you can use OD below about 75mph, and you started with a 2.7 (approx rear axle ratio), you'll end up spending a LOT of money to end up with a similar OD gear ratio (as the prior "high gear" ratio) and NO fuel economy improvement . . . but with a much better "launch feel".

THM400 >> 2.48 - 1.48 - 1.00 THM700 >> 3.06 - 1.72 - 1.00 - .70 (approx) THM200R4 >> 2.75 - 1.75 - 1.00 - .70 (approx)

The THM200 family of automatics is WELL supported in the aftermarket for guts which will take additional power, PLUS it absorbs LESS power than the THM400 and THM700 units. PLUS . . . with that wide "chasm" of gear ratio spreads between 1st and 2nd in the THM700, NOT too good for a moderate power motor in certain acceleration situations (i.e., short on-ramps that need a gear between 1st and 2nd in ratio for best performance.

The first THM200 3-speeds were designed for the first fuel economy-era 4cyl and 6cyl engines of the later 1970s. The earleir ones didn't have a very good "survival" rate, by observation. When they put that trans behind the GN Turbos, I thought they were crazy, from what I'd seen these transmissions do behind less powerful engines. BUT the "crazy" didn't seem to materialize like I suspected it would. Plus, anybody that was serious about performance had the trans upgraded anyway! As the trans obviously takes less power to run than other OD automatics, the aftermarket jumped in with upgraded "guts" so that the same trans, when built correctly, can be reliably put behind a Chevy 454 or other similar "upgraded" high-torque V-8 in street rods and such.

With throttle body fuel injection, the '87 Chevy 5.7L pickups used a 3.08 rear axle with P235/75R-15 tires. With the THM700R4, it worked well. Yet when you might try that same axle/tire/trans combination without any kind of fuel injection, you'll need approx 80mph to be able to use OD, no matter what. Even if you could get it engaged at slower speeds, throttle response would be in the "lugg" orientation, rather than otherwise. Hence, the need for the 3.42 rear axle ratio so that you can use OD at lowre road speeds (with carburetion).

3.42 x .70 = 2.39, which is close to the 2.41 ratio which many cars USED to have normally. As a comparison . . . my '77 Camaro with factory 2.56 rear axle ratio and P225/70R-15 tires runs about 31mph/1000 rpm in high gear. A 2.41 with that tire size would be a little more mph/rpm, with cruising speed being a little less rpm . . . with a 3.73 being the same overall gear ratio in OD as a 2.62 rear axle ratio in my Camaro. With the 3.42 axle ratio and carburetion, this could well mean that OD would not engage at less than 60mph.

In any event, the 4-spd OD automatic can make the car nicer to drive (even race?) IF it currently has a three-speed automatic and a 3.42 rear axle ratio. The "financial justification" of the swap will take years to happen, from decreased fuel use and expense. I've seen some guys who felt they could make their swaps work with carburetors and 3.08 gears, BUT they all ended up chasing a 3.42 or 3.73 gear set when they discovered they couldn't use that "fuel-saving overdrive gear" at less than 75+mph . . . AFTER they'd done the trans swap.

Figure out where your priorities are and go from there. In some cases, with the 3.42 rear axle ratio, you might do some other changes (a little more camshaft, possibly) which will build more cylinder pressure in the 2500-3500 rpm range, with a better exhaust system, etc. which can also improve cruise fuel economy and be more cost-effective than the trans swap and such. One of the popular self-learning EFI kits costs close to $3k, but "First Born" did one using salvage yard/reman parts for less (check that thread on EFI on a Straight 8 Buick).

From what some have posted in these Buick forums about the mpg they got from their Buick 430 V-8s in Electras, THAT might be the best alternative!

Just some thoughts . . .


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  • 2 years later...

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