Hey There,

I'm looking to swap my TH350 in my 1980 Vette to a TKX 5 speed. What would be the best unit behind a 550 Horsepower 406ci Chevy with Solid Roller Cam, 3.73 rear gears? Just looking for a model number.

Thanks for the help.

 

The Tremec TKX Model # I would recommend - TCET18083
This TKX has a first gear ratio of 3.27.
(The two other available GM /TKX Models have 2.87 1st gear ratios)
Assuming Your 1980 Has a 3.07 differential ratio the 2.87 1st gear ratio models would be very “SOFT / WEAK” off the line.
Your 406 will have plenty of power and torque once the clutch is let out & You are rolling over 10 miles per hour.
A great “performance cam engine” that is soft off the line because of high gearing is NOT fun or rewarding to drive!
(Unless You are on the Utah Salt Flats :-)
My experience with a comparable combination to Your proposed build, has me changing my present 3.08 differential ratio to a 3.36.
Your 1st gear ratio + rear gear ratio will have to well thought out with 550 HP, as Your aluminum differential will be the highest stressed link.

 

I have an 80 with a TKO500 (3.27 first gear), and 530hp/550tq stroker motor, 3.73 rear on drag radials. Pulls really hard through the gears. I debated 3.54, landed on 3.73 and it’s a beast.

 

I have a Tremec TKX model TCET18083A. I have a 400 HP engine with a 3.08 rear end and I am running 15 inch tires. This is what American Powertrain recommends for a 3.08 rear end. Just my two cents.

 

Someone will need to know your differential gear ratio to make any recommendation. Also, what do you prefer, a lower highway RPM, or a closer ratio between 4th and 5th?

The TKX has a great variety of options.. Which one you pick is depends on your other gears and your goals.

 

Do you have a solid roller cam and what is your max rpm? do you intend to change the rear end? I have the tko with optional shift improver and never have a problem doing every shift at 7500 rpm.

TREMEC 5-Speed and 6-Speed Kits: Silver Sport Transmissions (shiftsst.com)

 

t56 all the way .

 

You get on an online calculator. Put in your tire dia, each gear, rear gear, and rpm to get MPH Even with 4.11 rear and 2.86 first I can go 50 mph in first gear.

Where it gets ridiculous is in Over Drive. @7000 rpm I am geared with 4.11 to 221.48 mph. So to increase my top speed would be to use a 4.30 or 4.56 rear end and be doing closer to 200 mph at max hp rpm.

So choose your OD gear ratio wisely. The tko .82 od option was a good one and use 3.90 or 3.73 rear gearing.

 

Find something that matches a C6 Z06 car. This is very good for street / track driving.


So a T56 (6-speed) close ratio with a 3:36 rear is a very good match or if you like to spin tires use a 3:55 rear.

Go test drive any C6 Z06 and see how this feels to you.

My 78 I have a T56 magnum Close ratio 2.66 first with a 3:08 rear running LS3 / 480 engine (495 rated HP) mainly used for road course track days, but this is great on the street as well. 33.1 MPG Highway on my testing.

The part that helps 1st gear lauch is the twin-disc clutch with a aluminum light wieght flywheel. Moves thur the RPM Range much easier than any single disc / standard steel flywheel.

Twin disc is important to get light pedal feel. Single disc for your HP/Torque range will be very heavy pedal feel.

 

For the C6 corvette @ 198 MPH top speed's this was done using 5th gear. ~ 6800 RPM , 6th is usually design for MPG.

More gears more options on driving styles. 1st gear for drag racers/ tire spinners , 5th for top speed , 6th for MPG

 

The poster said 550 hp 406. Which would be a pretty hot rodded solid roller motor. Not comparable with a modern low duration cammed EFI My C6 project actually went faster going to a 4.10 rear end with the 6 speed modded c-6. It did several things. closed up the gear spacing and actually made 6th gear usable.

I've had the pleasure of teaching drag racing schools. C6-Z06 owners often smoke their clutches trying to do a burn out. Part of it is too tall of first gear. if you don't spin the tires something has to give. So the take off in a stock Z06 is pretty easy. turn off all traction control. rev it to 2500 and slip just enough to get rolling forward and then dump the clutch while flooring the gas.

For road racing I was open for something new. So instead of using a 4.11 rear gear and using 5th OD on high speed long straights. I figured out that 3.55 gearing and just use my TKO more like a 4 speed. In 1.00 4th gear I was good for about 164 mph at 7000.

Well when racing was over and driving around with .64 OD the jump from 4-5 was bad and forced me to go about 80 - 85 before even using 5th. So back in went my spare 4.11

 

I agree if your goal is 100% performance only. And as your stated you need to match the RPM of what the engine likes to match what you want from the car. eg engine likes 3000-6000 RPM and you want to drive in 1st or 2nd gear.

T56 6-speed
1st gear
with 3.08 this would be 29MPH - 58 MPH
with 4.11 this would be 22MPH - 44 MPH

2nd gear
with 3.08 this would be 43MPH - 87 MPH
with 4.11 this would be 32MPH - 65 MPH

If your goal is 2-3sec of acceleration the 4.11 gear for performance works great on the street. Keeps you well below speed limits in both cases. The 3.08 would allow only 1st gear rush.

But the idea behind a 6 speed was to give you a balance of good performance and good gas mileage. The current setup of T56 mag with ~ 3:42 from the C6 gives a great balance between performance / street driving.

C6 with lauch control does very well and without practice its hard to beat 60ft times without lauch control. If you give people 3 tries without practice no lauch control, and then 1 try with lauch control, the last attempt is 99% the best time.

Its comes down to what do you want from your car. I believe most corvette people like 1st gear lauch, 2-3 sec of acceleration lauch 1st or 2nd gear only.

People that use their corvette for track days look at different things.

eg. At my road course using a 4:11 gear would increase the laptimes. More shifting and less optimized torque curve on the straights. Goal is to reach top MPH at the straights. My track is 2nd & 3rd gear.

When driving out at SpringMountain 2.4 (c7 Z06) east layout the only time I use 4th is on the long straight again to reach 140 MPH and ~ 6000 RPM.

 

I updated my first post with more info. Cam info, 3.73 rear gears.

 

With 3.70 gears you're better off with the 2.87 first gear. The 3.27 first gear is way too deep, especially with big power.

 

I would think you want the .81 OD version. The .68 OD version has a big 900rpm drop from 4-5. And your high revving small block will not like that.
You need to keep the revs up.

My buddy with a ZZ427 and WR TKO could not stand his with 3.70s. He changed it to 4.11s and is much happier now. But he was mostly concerned with 50-60 mph and much lower rpm than you, and he had the big 4-5 drop, and bad sidepipe resonance at 55.

Here is my gear ratio spreadsheet. The CR (.81 OD) version should work well for you with 3.73s. (see column GB)
1st gear = 10.7 to 1 good for a high revving SBC. 411 might be even better.
4th gear @ 3000rpm is 65mph and gives you a 600rpm drop into 5th. 411 would drop that a little.
70 mph in 5th would give you 2674rpm. Probably great with a high reving SBC. No lower. 411 would be OK too at 2950rpm.

I just received my TKX this summer and got the WR one. At the time it was the only TKX being manufactured, and the TKOs were also out of production. The CR
TKX CR .81 OD TCET17722 model had an idefinate date of manufacturing / availability at that time, as of 6 months ago. So you will want to call both Tremec and Silversport and see what is available now. Or when they anticipate it will be available.

FWIW Silversport manufactures there own C3 specific offset shifter, and attaches it to the basic TKX, all the Tremec distributors do this. but theirs is the best in the business, light years ahead of Modern Driveline's, or other distributors. I would consider no other.

Let us know if you find out if the CR trans is being manufacured yet.

 

So we're looking at the TCET17722 2.87 First Gear, .81 OD version?

 

this is correct and you could never give me a Trans with. 050 overdrive.

 

Yes. If you like the rpms it generates. Best to call Tremec and see if it is even in production yet. It was not last May. They did not even have an anticipated date.

 

That's where I got my two Tremac TK-500's. 3.73:1 rear end. Overdrive 5th gear is great for highway cruising. Really like the shifting feel of the Tremacs. I think the TKX's are upgraded replacements with slightly higher torque ratings. Don't know much about them.

 

If you have not bought one yet, let me put in a plug for SilverSport. There are several different distributors of the Tremec transmissions, but they all make up their own installation kits, and the C3 requires a custom modified shifter, not a normal Tremec one. IMO the Slversport STX shifter is the best Tremec C3 shifter available!

Silversport STX shifter uses all hefty brackets, linkage, bolts and a custom housing just for C3 to give a perfect fit with no offset shifter brackets.


Competitor's shifter with a weaker design shifter using a single bolt attaching an offset bracket. I know of one person who sold his Tremec specifically because this bolt came loose so often.