Need advice for new transmission
Thread Starter
Instructor
Joined: Sep 2018
Posts: 165
Likes: 64
From: Lakeville MN
Need advice for new transmission
I have a 1979 L82 with a factory 4 speed, the transmission is whiney, old and feels like its coming to the end of its life. I'm seriously looking at a Tremec TKX or go all out with the magnum 6 speed. The car has a little north of 300HP and I have no idea what rear end it has, the rpm's are at 3100 @65mph in 4th gear if that tells you anything. My goal from the very beginning is to make this a comfortable short road trip car, the suspension is all new, the interior is new and it has a new A/C system so a quiet overdrive transmission is the last piece of the puzzle. I've had a few people tell me the the car is going to bog down with a six speed (not enough horsepower/torque), but Im thinking isn't that what down shifting is for? Anyway, I'd prefer the double overdrive of the 6 speed since most of my driving is highway but if the TKX 5speed is a better fit for this car I wouldn't mind going that route. I have a limited understanding of gearing and all that so Im turning to the forum to give me the pro's and con's of the 6speed vs 5speed. (Im going to have the new transmission professionally installed)
Racer
Joined: Dec 2003
Posts: 487
Likes: 160
From: Two Lane Black Top
What tires are on the car?
3100 rpm @65mph in 4th gear - with a ~26.7 inch tall tire is about a 3.73 rear gear. Definitely no issues with a T-56 or Magnum with a 2.66 1st gear. There also a wide ratio gear set for the transmission with a 2.97 1st gear.
I briefly considered a Magnum for my 72 vette but the shop said it was a do able but ugly install because a lot of cutting was required to fit the Magnum.
3100 rpm @65mph in 4th gear - with a ~26.7 inch tall tire is about a 3.73 rear gear. Definitely no issues with a T-56 or Magnum with a 2.66 1st gear. There also a wide ratio gear set for the transmission with a 2.97 1st gear.
I briefly considered a Magnum for my 72 vette but the shop said it was a do able but ugly install because a lot of cutting was required to fit the Magnum.
Last edited by 99 Black Bird TA; Oct 29, 2021 at 04:56 PM.
Race Director
Joined: Mar 2017
Posts: 11,953
Likes: 4,510
From: Virginia
I'm looking at the TKX as well. It seems that getting the bellhousing square is the key consideration.
There are several videos on YouTube now, and I just found this Hemmings article today.
https://www.hemmings.com/stories/202...ompact-gearbox
There are several videos on YouTube now, and I just found this Hemmings article today.
https://www.hemmings.com/stories/202...ompact-gearbox
Melting Slicks
Joined: Nov 2006
Posts: 3,416
Likes: 1,824
From: Apple Valley, MN
Your differential, if original, is either a 3:36 (standard) or optional 3:55 or 3:70 ratio. The “sweet spot” ratio for running a TKO is 3:70 which gives you the benefits of faster acceleration and a comfortable cruising RPM, thanks to the over drive ratios. As 99 black bird TA states, your report of 3100 RPM at 65, with the tire size you stated would suggest you have a 3:70 but you should confirm this by jacking up the car, ( note you need to get the trailing arms supported to keep the halfshafts from binding) and measuring how many driveshaft revolutions are observed per 1 rotation of a wheel.
If your car’s transmission is original, it is a Borg-Warner. The M20 was standard, with a 2.64 1st gear but if you have a M21 close ratio, 1st is 2.43.
Assuming you have an M20, the Tremec won’t make any difference off the line as it has the same 2.64 1st gear ratio. The overdrive ratios will, of course, drop cruising RPMs but with your stock engine, IMO the 6 speed is moving past the point of diminishing returns.
With the TKX recently released, there are more and more TKO-600s showing up for sale so you may want to consider that route and save a bunch.
If your car’s transmission is original, it is a Borg-Warner. The M20 was standard, with a 2.64 1st gear but if you have a M21 close ratio, 1st is 2.43.
Assuming you have an M20, the Tremec won’t make any difference off the line as it has the same 2.64 1st gear ratio. The overdrive ratios will, of course, drop cruising RPMs but with your stock engine, IMO the 6 speed is moving past the point of diminishing returns.
With the TKX recently released, there are more and more TKO-600s showing up for sale so you may want to consider that route and save a bunch.
Thread Starter
Instructor
Joined: Sep 2018
Posts: 165
Likes: 64
From: Lakeville MN
Good info, thanks so much. The engine block may be original but overall it is not stock. From other late 70's C3's I've driven mine has substantially more hp, previous owner said about 325. I've never had it dyno'd but it sure feels like it. So would the 6speed still be too much for this engine? Im mostly looking for a very low cruise rpm
Melting Slicks
Joined: Aug 2016
Posts: 2,347
Likes: 1,180
From: CLEVELAND STUCK IN OHIO
2024 C3 of the Year Finalist- Modified
What RPM do YOU want to be running at 70mph???
Sounds like thats your only concern.
In your case, just design it for that goal, in 6th gear. If it doesn't work out by the rear gears and tires you have, you may have to consider a 5spd.
Math doesn't lie,,,,
Sounds like thats your only concern.
In your case, just design it for that goal, in 6th gear. If it doesn't work out by the rear gears and tires you have, you may have to consider a 5spd.
Math doesn't lie,,,,
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Racer
Joined: Dec 2003
Posts: 487
Likes: 160
From: Two Lane Black Top
The six speed T-56's were in 1994-1997 (3400 to 3600 pound) 275 hp LT1 Fbody Z28's & Trans Am's with 3.42 rear gears stock.
It was also in the 1998-2002 (3400 to 3600 pound) 305 hp (actually more like 335hp) LS1 Fbody Z28's & Trans Am's with 3.42 rear gears stock.
Sounds like your vette make more than a LT1 and a little less than a stock LS1 or similar so it seems to me with the better 3.70 rear gear it should be good to go with the T56/Magnum.
If the "surgery" isn't an issue go for it 😀😃🙂
It was also in the 1998-2002 (3400 to 3600 pound) 305 hp (actually more like 335hp) LS1 Fbody Z28's & Trans Am's with 3.42 rear gears stock.
Sounds like your vette make more than a LT1 and a little less than a stock LS1 or similar so it seems to me with the better 3.70 rear gear it should be good to go with the T56/Magnum.
If the "surgery" isn't an issue go for it 😀😃🙂
Last edited by 99 Black Bird TA; Oct 29, 2021 at 09:56 PM.
Melting Slicks
Joined: Aug 2016
Posts: 2,347
Likes: 1,180
From: CLEVELAND STUCK IN OHIO
2024 C3 of the Year Finalist- Modified
Six speed will work. IMHO, your eng won't be happy at that RPM @70mph.
So,,, there ya go,,, thats what down shifting is for,, and you'll have 5 other gears to pick from..
.
Eather way,, 5spd or 6spd is a great up-grade for a C3.
. . .Get-er-dun!!!,,
So,,, there ya go,,, thats what down shifting is for,, and you'll have 5 other gears to pick from..
. Eather way,, 5spd or 6spd is a great up-grade for a C3.
. . .Get-er-dun!!!,,
Racer
Joined: Nov 2020
Posts: 265
Likes: 76
I just finished swap on my 1980 with 3.07 rear end from TH350 to TKO600 and I was looking for the sports care feel that in my opinion is not there with auto as well as better cruising RPM's on the highway since I do drive quite a bit on florida highways. I was a bit concerned as others have mentioned that my stock L48 would not do well on the hightway with the gearing expecially a .64 5th gear. After trying it for a few days I have to say that I love the way the care drives. I'm doing 2000 rpm at 80 mph with my old setup i'd be at about 3700 RPM and that just feels like the car is screaming down the highway. Like you said if I want to drive 60-65 I can just stay in 4th gear and it's fine. Like someone else mentioned you can get deals on the TKO's and that's what I did paying $1500 for a nice unit that included bellhousing, flywheel and hydralic clutch. It didn't have the shitfer case modication that you get when you buy the kit for SST or american powertrain but I made a shifter offset and it turned out perfect. The guys at American Powertrain and SST are great but the full kit was like $6k and I did this all in for like $2500. Good Luck.
Melting Slicks
Joined: Aug 2016
Posts: 2,347
Likes: 1,180
From: CLEVELAND STUCK IN OHIO
2024 C3 of the Year Finalist- Modified
Happy for you. That's great. If you keep your cruzein' RPMs 2000-21000, in that old L-48 it should be happy. Any lower, grab another gear....
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
I have owned a few IROC 5 speeds and a 98 4.6l Mustang GT Vert......both with T5's.....and they all responded the same way, it was a 4 speed with an overdrive gear......they would pull 5th gear no problem but you never used it unless you were on the interstate (or Texas Farm Road).....so to me, a 5 speed is enough. The 6 speed is a 5 speed with an OD......so even less drop between gears which is beneficial but how often do you want to shift?
Whether or not it bogs in top gear is directly effected by your final drive......if the rear end is, say, a 3.08, then the OD on either will have to be more in the .80 to .75 range than a .64 which is fairly common.....so do the math and research your setup before you buy.
I put a TKO 5 speed in an unmolested 69' Camaro 327 with mods.....it was just perfect.....you could not improve on the ratios of this particular car if you tried.....it did everthing right on the street and strip, completely transformed the car from the bullshit Saginaw that was in it......
Just my opinion......early morning coffee.....
Jebby
Whether or not it bogs in top gear is directly effected by your final drive......if the rear end is, say, a 3.08, then the OD on either will have to be more in the .80 to .75 range than a .64 which is fairly common.....so do the math and research your setup before you buy.
I put a TKO 5 speed in an unmolested 69' Camaro 327 with mods.....it was just perfect.....you could not improve on the ratios of this particular car if you tried.....it did everthing right on the street and strip, completely transformed the car from the bullshit Saginaw that was in it......
Just my opinion......early morning coffee.....
Jebby
Last edited by Jebbysan; Nov 4, 2021 at 07:25 AM.
Team Owner
Joined: Jan 2006
Posts: 39,391
Likes: 171
From: Into the Mystic And yet, despite the look on my face, you're still talking TN
St. Jude Donor '09 thru '25
I put a TKO 600 in my 79 L82 and couldn't be happier. 3.70 rear end before at 80 mph turned 4000 rpm, now it's 2600. Huge difference.
Give Silver Sport Transmission in Knoxville a call. Very knowledgeable and helpful and they stand behind their products.
https://shiftsst.com/
Give Silver Sport Transmission in Knoxville a call. Very knowledgeable and helpful and they stand behind their products.
https://shiftsst.com/
Team Owner
Joined: Mar 2002
Posts: 44,686
Likes: 1,833
From: Huntsville AL
Just shot an inquire to Silver Sport Transmission in Knoxville on a Tremec TKX 5 Speed kit cost. I like that this comes in a turnkey kit with no crossmember modification required. While my Muncie is fine and works great I do feel at cruise speed that I'm looking for another gear.
Team Owner
Joined: Jan 2006
Posts: 39,391
Likes: 171
From: Into the Mystic And yet, despite the look on my face, you're still talking TN
St. Jude Donor '09 thru '25
Just shot an inquire to Silver Sport Transmission in Knoxville on a Tremec TKX 5 Speed kit cost. I like that this comes in a turnkey kit with no crossmember modification required. While my Muncie is fine and works great I do feel at cruise speed that I'm looking for another gear.
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
gear ratios depend on what you ultimately want to allow your car to.
I look at it from performance I want.
I like tight turns and I want a gear available
to allow power thru that turn.
like hi-speed turns, need a gear for that.
want to save gas, yet another one.
crucial to match the 5 or 6 speed available models ratios to your rear gear ratio to get the power when you want it.
good luck
edit
forgot the important part,
where the motor makes power and ends.
"power band"
I look at it from performance I want.
I like tight turns and I want a gear available
to allow power thru that turn.
like hi-speed turns, need a gear for that.
want to save gas, yet another one.
crucial to match the 5 or 6 speed available models ratios to your rear gear ratio to get the power when you want it.
good luck
edit
forgot the important part,
where the motor makes power and ends.
"power band"
Last edited by calwldlife; Nov 4, 2021 at 05:47 PM.
Melting Slicks
Joined: Jun 2011
Posts: 3,116
Likes: 598
I have a 1979 L82 with a factory 4 speed, the transmission is whiney, old and feels like its coming to the end of its life. I'm seriously looking at a Tremec TKX or go all out with the magnum 6 speed. The car has a little north of 300HP and I have no idea what rear end it has, the rpm's are at 3100 @65mph in 4th gear if that tells you anything. My goal from the very beginning is to make this a comfortable short road trip car, the suspension is all new, the interior is new and it has a new A/C system so a quiet overdrive transmission is the last piece of the puzzle. I've had a few people tell me the the car is going to bog down with a six speed (not enough horsepower/torque), but Im thinking isn't that what down shifting is for? Anyway, I'd prefer the double overdrive of the 6 speed since most of my driving is highway but if the TKX 5speed is a better fit for this car I wouldn't mind going that route. I have a limited understanding of gearing and all that so Im turning to the forum to give me the pro's and con's of the 6speed vs 5speed. (Im going to have the new transmission professionally installed)
T56 Magnum Install 78 Corvette. - CorvetteForum - Chevrolet Corvette Forum Discussion (this installed was before silversport existed) no complete kit, but still a doable project.
My car has the close ratio T56 2.66 first gear with a 3.08 rear axle. This is very driveable on the street. If you like to burn tires then higher gears are prefered.
With my ZZ383 engine I was able to get 27.8 MPH on the highway, then when I upgraded the Engine to a LS3 / 480 I now get 33 MPH Highway.
Either way you will be happy vs a 4 speed. Both 5 or 6 speeds shift much better than the factory 4 speed.
Last edited by cagotzmann; Nov 4, 2021 at 08:22 PM.
