Readers’ Tech Q&A
I’m in the middle of rebuilding my street 1968 Pontiac Tempest convertible and have a problem that most people would look forward to. Kaufmann Racing has built a stroker 455 for me that dyno’d at 692 hp on pump gas and 932 hp with the nitrous shot.
The car currently has a Tremec fivespeed with a max rating of only 650 hp. I want to know if there is an H-pattern–style five-speed (with Fifth as the overdrive) that could handle the horsepower for weekend trips to the dragstrip when the car will have sticky slicks on it.
With the car’s 3.73:1-geared rearend, the overdrive will be easy on the motor for highway cruising, and gears 1 through 4 will be all I’ll use at the track. I want to have the interior look close to stock, so a trans with levers to shift gears would not only take away from the look I’m after but also be a pain to drive on the street.
Welcome to 2018, when a “street” trans rated at “only 650 hp” is no longer adequate. When yours truly grew up in the early 1970s, a street motor making 500+ hp was considered a pretty big deal. The classic-era, muscle-car four-speeds like the so-called “rock crusher” Muncie M22, Super T10, or Ford Top Loader were officially rated no more than 400 lb-ft in stock form. But there is a solution to your problem.
G-Force Transmissions (part of Long’s Machine & Tool, manufacturers of the famed competition Long manual-trans shifters) has the five-speed you want, but it will set you back around $4,250. Rated at 1,000 hp/1,000 lb-ft, its custom GF3650 is based on a Tremec 3650 core, as used behind 2001–2005 Mustang V8s. In those model years, this trans still had a conventional, externally splined mainshaft for a slip-yoke. The basic design continued in use through 2010, but with a flange on the output shaft instead of the slip-yoke, plus a less desirable semi-remote shifter anchored to the vehicle body. All the 3650s have a “semi-integral” bellhousing where a front-bellhousing component bolts up to the trans using multiple bolts that attach it to the main case from the bellhousing’s interior side. The 2001–2005 bells were set up for cable-actuated clutch forks; the 2006–2010 versions for an integral hydraulic throwout bearing.
In stock form, these trannys are very smooth shifters (particularly the 2001–2005 version with the directly attached shifter) and, as a mainstream production unit, there are hundreds of thousands of cores available, but the stockers are not even close to being strong enough to withstand the numbers your Poncho is making. However, the basic design leaves plenty of room for growth, serving as a point of departure for G-Force’s ultimate street/strip five-speed that still retains conventional synchronizers as well as a normal “H” shift pattern.
G-Force replaces the weird production bellhousing section with an adapter plate that adapts the trans to a conventional bellhousing of your choice. Nearly all the internal gears are replaced with aerospace-spec, ruggedized gears. “We’ve been offering this build for five years now, and we’ve never broken one,” maintains G-Force’s Mike “Bubba” Masteller. “The gear ratios are not the original factory. Our box has a 2.9–1.91–1.37–1.00 gearset, with overdrive options of either 0.58 or 0.69. More overdrive options are in the pipeline. We recommend 3.55:1 or steeper rearend ratios.
“We believe this trans is stronger than a T56 [six-speed], it’s quieter, and the gears we put in are slightly wider. An equivalent T56 to our version of a 3650 would set you back at least $6,500—but, yes, we offer those, too.”
The G-Force custom 3650 packages retrofit into old chassis a lot better than the rotund T56. It even fits better than the popular Tremec TKO fivespeed. “A TKO is big and square, with a flat top,” Masteller explains. “But the 3650 is very smooth overall, with a more rounded top. It fits Camaros and Chevelles very well; 90 percent of these installations require no tunnel mods other than possibly
enlarging the floor-shifter hole.” (Your Tempest is the same basic GM A-body intermediate chassis as a Chevelle, just as a Firebird is a Camaro sister.)
Crossmember repositioning and modification may be needed to accept the Ford-style trans mount, as well as position the trans for proper driveshaft phasing. The shifter location—21 inches back from the front of the as-delivered trans with adapter plate—is about the same as the old Muncie, but the 3650’s internal-rail shifter is centered in the tunnel, not offset left as it is on the Muncie and similar external shift-linkage transmissions.
Even with G-Force’s new bulletproof internal gearset, Masteller says the custom trans “still retains the 3650’s smooth shift characteristics.
It’s not notchy like an original TKO, which still uses ancient T10 brass synchro technology and heavy gears with small teeth. That old tech makes it hard for the synchro to slow down and match the gear speeds. A T10 cluster had about 20 pounds of rotating mass, but a TKO—with similar synchros— hefts out about 38 pounds. That’s why the old transmissions and even newer ones with similar technology tend to be hard shifters.”
By contrast, “The 3650 uses triple-cone synchros consisting of sintered metal on the inner rings and steel outer rings for the 1–2 gears; the 3–4 gear synchros are brass but with a carbon lining like a T5. We’ve also designed in a wider H-gate. Our modified 3650 trans will feel like you’re driving a brand-new Mustang, shift as well as a T5, but withstand just anything you throw at it. We even have some behind hot rodded diesels that make massive torque at much lower rpm than gasolinefueled engines.”
One potential downside is a standard 3650 lacks mechanical (cable-driven) speedometer provisions. Like other late-model transmissions, it drives electronic speedometers through a vehicle speed sensor (VSS). That’s not a dealbreaker for old-school cars: Major aftermarket gauge-makers such as AutoMeter offer calibratable electronic speedometers, VSS-compatible speedometers, or even GPS-based solutions. Want to keep your original stock speedo? Abbott Enterprises offers a “Cable-X” box that converts VSS signals to mechanical speedocable-compatible output. Alternatively, G-Force does offer a case modification to accept oldschool mechanical speedo cables directly, but Masteller says, “It’s kind of a pain, because the tailhousing tends to distort when we machine it and weld in a plug for the speedometer hookup, which means we have to rebore the tailhousing and install a new sleeve and bushing. We charge another $250 for this extra service.”
[ Rated at 1,000 hp/1,000 lb-ft, G-Force’s GF3650 five-speed is based on a production 2001–2004 Ford V8 Mustang five-speed manual trans core, but the internals are massively beefed up. G-Force removes the stock semi-integral bellhousing, replacing it with a billet-aluminum adapter plate that conformsto the conventional bellhousing of your choice.
Tremec [ This is a stock 2001–2004 Mustang Tremec TR3650 five-speed as used behind Ford modular V8 motors. Note its semi-integral front bellhousing that’s set up for the stock cable-operated shift fork, short rear extension housing with internal rail directly attached shifter, and beefy 31-spline output shaft that mates with a conventional slip yoke.