Convert TH350 to O/D Manual.

Discussion in 'High Tech Retrofits' started by wpatterson9, Jan 20, 2020.

  1. wpatterson9

    wpatterson9 New Member

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    Man. I've been reading through forums, google searches, online articles, and have reached the point of being overwhelmed and stuck spinning my wheels.

    To offer some context and background about me, this is my first resto project. I have a novice/intermediate level of mechanical knowledge and am blessed/cursed with an overly-analytical engineer's mind. That said...

    I'm looking to swap the TH350 in my 73 LT for a manual with O/D, but with so many options I can't figure out where to start. At some point, I know for sure that I want to replace the stock 2-bolt 350 block with a 4-bolt or an LS. I'll also be upgrading the rear gears from the stock 2.73s to a posi unit but again, what to choose? After playing with the Transmission/RPM calculator, it seems I need to figure out the transmission gearing before the rear and from reading other's forum posts, highway RPMs shouldn't be the only criteria for making a decision. For rear tires, 335/30x18's.

    What do you guys think? My goal is to build a car that I can take on the highways around town and have fun taking laps on track days. I need help narrowing down a direction to work towards. Thanks all in advance for your advice and for taking the time to read this and helping out.
     
  2. flht99b

    flht99b Veteran Member

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    I use the Tremec calculator. The TKO600 5 speed with the .64 fifth gear will net you 72mph at 2200 rpm with a 3.73 gear and a 26 inch rear tire. That is very close to my car with 4.10 gears, .64 fifth gear and TKO600 with a 28" rear tire. That sounds good on paper but the reality of a carbed engine with even a moderate cam will be happier with a bit more rpm at 70mph. Drove my 71 Z28 on the 2019 HRPT and I wished I had gone to a 27" rear tire to give a little more rpm at highway speeds. LS engine and fuel injection changes all that. Going to a Tremec 6 speed trans gives you 2 overdrive ratios but will require some trans tunnel work. Best to nail down the engine and mods along with carb or FI before selecting a transmission and rear gear.
     
  3. wpatterson9

    wpatterson9 New Member

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  4. tom3

    tom3 Veteran Member

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    I'm running a TKO five speed with a 2:73 posi rear. Perfect for my torque and mid range built 355. But I'd repeat the above, map out your engine build and rear end etc., then select a transmission.
     
  5. biker

    biker Veteran Member

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    TKO 600 with 3.73. Very nice combo. I have a carbed 383 with a fairly lumpity cam and no problems with cruise RPM @70mph. It is taller than I thought it would be, but still pulls cleanly in 5th from 2000. If I want to make a quick top speed run, I just hit 4th or 3rd and hit redline in each gear. Can't pull redline in 5th just yet. My tires might explode and at some point around 160, I figure wind might be an issue.
     
  6. camarochevy1970

    camarochevy1970 Veteran Member

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    TKO 600 with .64 5th gear and 3.55 rear end here, on a 383 stroker and love it. Fun to drive and gets the best MPG of anything we own, no joke
     
  7. rchydzik

    rchydzik Veteran Member Lifetime Gold Member

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    I agree with tom3 on the tko500. First gear (3.27) with 2.73 rear gives you 8.93 in first gear, and the overdrive will give low rpm cruise on the highway (70-80 mph). TKO needs a crossmember, driveshaft yoke, shorter driveshaft, NSS, and reverse wires, but usually doesn't require surgery to the tunnel (mine did not). Stock clutch, linkage, bellhousing, all work. I have a tko600 in my 76, and t56 in my 2002 Z28, both with 3.42 gears. Honestly, i prefer the five speed over the six speed. In hindsight, I would have saved my money on the 3.42 gears and gone with a tko500 and 2.73. I rarely use sixth gear except on long highway drives. You would have to weigh the benefits of six gears to the complexity of the t56 install (bigger, non-stock linkage, bellhousing, etc.).
     
  8. biker

    biker Veteran Member

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    Yes. The TKO 600 fit into my car like it was made for it, which it was, since I bought it as a kit. I think the TKO 500 is the same dimensions, so it should fit just as easily. One nice thing about going with the TKO is that it eliminates all the backdrive linkage for the reverse lights/key switch lockout. Much tighter feel to the shifter.
     
  9. flht99b

    flht99b Veteran Member

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    Also, the TKO600 comes in 2 versions. Both utilize the same first through fourth ratios but fifth gear can be a .64 overdrive or a .82 ratio. IMO, .64 ratio fifth gear is better suited to the shorter gear ratio diffs (higher numerical number) where as the .82 fifth gear ratio is better suited to the taller gear ratio diffs (lower numerical number). With the .64 fifth gear, there is about a nine hundred rpm drop from the one to one ratio of fourth gear. Again, tire diameter, engine model/displacement, mods and fuel delivery system play into the selection. The Tremec calculator will give you speeds for all theirs and other transmissions in each gear for a given rpm, rear gear ratio and tire diameter so the combinations are many. Time spent determining the car usage, desired cruise rpm and speed for a given tire diameter will direct you to the desired results.
     
  10. czizza

    czizza Veteran Member

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    I had a TH350 with 3.73 and 26in tires, then went to ST10 which was really fun to drive, then I got lucky and found a TKO500 for 1k on Craiglist and I can tell you this is the best thing that ever did.

    The deeper first gear is almost too much, but 2nd to 4th is a blast and overdrive is so cool to cruise and not kill the engine. Don't get me wrong, I still love being on the highway at 4K doing 70 and on a long run, just dropping to 5th gear and then just cruising.
     

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