Curbside What If: The Turbo Tempest Twin-Drive

Tempest DrivelineLook at the back of the transaxle: see the torque converter?

While researching last week’s write-up on a 1962 Buick Special, I read Paul’s earlier write-up on a Y-body Pontiac Tempest. In the article, he showed a Tempest transaxle which has a torque converter hanging off the back side. This image remained in my head for several days, hinting of an unexplored path. To my mind, this converter looked incomplete, naked, somehow missing an important element.

Corvair Engine Bay

Then I suddenly realized what was missing: an air-cooled flat six! Due to several unique engineering decisions made by both Pontiac and Chevrolet, adding an entire second engine onto the Tempest driveline is “simply” a matter of adding a bell housing onto the back of the Tempest transaxle, and then sourcing a Corvair engine for use in the Tempest trunk.  I immediately began considering ways to build a twin-engined Tempest and while this may seem like an unlikely project, consider the following:

thebeastfromtheeast_comImage: thebeastfromtheeast.com

As this transaxle image shows, the Pontiac Flex-Drive driveshaft always spins the torque converter at engine RPM, and then the power moves back into the Powerglide. Because stopping the vehicle does not stop the driveshaft, we can mate a second engine to the torque converter, and it will turn in perfect synchronization with the existing (front) engine.

Starter Explanation

Since the two engines would turn together, the front engine starter could theoretically be used to turn over both motors, eliminating the need for a second starter circuit.

By using an air-cooled Corvair engine, there’s would be no need to increase the Tempest’s cooling capacity, or plumb addition cooling lines to the rear engine.  And because the Corvair engine bolts to the back of the torque converter, the Tempest Driveshaft carries no additional power, preventing additional stress from damaging this unique component.

More goodies

Finally, the Tempest transaxle uses the same basic layout as the Corvair transaxle, so it can accommodate the additional weight of the Corvair engine. While the two transaxles are not interchangeable, the Tempest does share rear axles and suspension arms with the Corvair, so the basic design includes room for a flat six wrapped around the torque converter.

Tempest-Twin-Drive.1

This image shows the basic concept.  At top is a Tempest driveline cutaway, in the middle is the Corvair, and finally our Tempest Twin drive, with two engines mated at the transaxle. While the Powerglide and rear axle have to handle increased horsepower, GM’s two-speed automatic has a well deserved reputation for toughness, and the Tempest uses a beefed up transaxle, giving us the potential to use the best components available.

Cut hereCut Corvair bell housing, paste onto transaxle

To create our twin motor beast, we’d first have to cut a bell housing off a Corvair Transaxle, machine a mounting pad on the face of the housing, and then bolt it onto the back of a Tempest transaxle. Rather finicky work, but the only challenging fabrication work in the project. This step looks a bit tricky, but the bell housing driveshaft bore provides a common reference point for the machinist, assuring us the parts will line up.

Trunk FloorDon’t forget to remove the trunk floor!

Of course, the trunk floor would have to be cut out, and a mounting pad for the Corvair engine welded in, but the body structure matches that of the Corvair and these modifications are fairly routine in the world of hot rodding. The only other challenge is the gas tank: the Corvair engine would displace the Tempest’s tank, and the Tempest engine resides in the location used in the Corvair. But there are plenty of other locations available, starting with the rear seat area.

Engine options

Once we build the car, we then have a dizzying list of available engines, all bolt in options. During the three years it was built, the Y-body Tempest offered three distinct engines, starting with the Trophy Four (110 or 140 horsepower), followed by the Buick aluminum V-8 (155 HP in the Tempest) and topped off with the 326 cubic inch, 260 horsepower Pontiac V-8.

In addition to those engine choices, the Buick Special V-6 could bolt into the Tempest engine bay, offering yet another option. Out back, the Corvair Flat Six was built over an ten year span, offered in three different displacements and eight different horsepower ratings (pick one: 80 to 180 horsepower). Using all these options, let’s talk about possible powertrain combinations, along with my thoughts on each one.

Four & Six

Corvair H-6 and the Tempest Trophy Four (the I-H-10)

This engine combination offers the smallest cylinder count, and the smallest total displacement (by 2.5 cubic inches). Combined horsepower would be in the range of 230 to 250 horses, depending on the tune of each motor. However, the Trophy Four was a big displacement four, offering big shaking in a small package, while the Corvair’s boxer engine offered a perfectly balanced engine layout. To my mind they’re a poor match. Unless you’re enamored with the Trophy Four’s big torque curve, I’d avoid this combination.

Fireball & Corvair

Corvair and Buick V-6 (the V-H-12)

I like the symmetry of this engine combination. It just seems natural to place a six cylinder at each end of the Tempest. While the Buick V-6 may not match the Corvair for smoothness, the overall performance is very similar, offering a horsepower rating right in the center of the Corvair’s power range. If you’re looking for balanced power from your driveline, this combination works best, delivering about 140 horsepower from each end, for a total of 280 horses.

215 & six

Corvair and Buick V-8 (the V-H-14 type 1)

This engine package is interesting for several reasons. First, thanks to the Buick’s aluminum engine block, it provides the lightest package of any listed here. Second, both these engines were exclusively Y-body power plants. Other manufacturers may have used the Buick V-8, but GM only mounted the motor in this platform. Of course, only about two percent of the Tempests came with the Buick V-8, so finding one so equipped presents a challenge. Still, the combination delivers up about 320 horses, nothing to sneeze at.

326 & six

Corvair and Pontiac 326 (the V-H-14 type 2)

Pontiac only offered the 326 in the Y-body for one year, but that’s long enough to qualify. Of all the options listed here, this one is the most prosaic, substituting cubic inches for technology. I’d avoid building this option for three reasons: First, a 326 Tempest is a pretty special Y-body, an I’d just as soon not chop one up. Second, this engine combination would tax the Powerglide to the max, beyond my comfort level. Third, while it’s easy to get 400 horses out of this package, it’s more sledgehammer than scalpel.

Twin Turbos

Corvair Turbo and Olds Jetfire V-8 (the Turbo Tempest Twin Drive)

Here we go: the ultimate engine combination! If we’re going to build a fantasy Tempest, let’s go with the biggest power options offered in the two Y-body motors. These motors net 395 horsepower, while pegging the cool meter. While this driveline may grenade Powerglides on a weekly basis, sometimes the ends justify the means.

Wow, I’m starting to run out of breath! There’s still many topics to discuss but I think I’ll just throw out a couple, and see what evolves in the comments section:

-How would the various torque curves match up from engine to engine?

-How would one clock the crankshaft TDCs of each engine to get the smoothest power pulses?

-What about front to rear weight distribution? Which combination would work best for overall balance?

Craiglist Tempest

Right now, I’m thinking about where I’d find the garage space to take on such a project (it could easily cost as much as 15 or 20,000 bucks).  After a quick search on Craigslist, I also found this excellent starter car.  This Texas car’s Trophy Four ran supposedly when parked, and it’s available for a very reasonable $2,000. What do you think: should I make the call?