Read­ers’ Tech Q&A

Hot Rod - - Contents - Mar­lan Davis RLM Ad­ver­tis­ing

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.

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