5 vs 6 speed

[Cleve]

Thank you for all your replies. You all have been very helpful. Have a great day.
Cleve

[BradCraig]

TBF, you didn't actually ask a detailed question and just posted this am. We all have jobs so exercise a bit of patience.

IMHO, 5-speed TKX is all you need and simply bolts in with no mods. Hope this helps.

[juddster]

I went with the MT82 6-speed because I bought an engine/transmission package. If you go this route there will be the $900 adapter kit from Forma (great kit just $$$) and then you will need a custom driveline that will cost $900-1,000. As far as drivability I've only driven the 6 speed and I'm happy, but you hear a lot about how good the 5 speeds run.

[CraigS]

Generally a 6 spd needs IRS rear suspension. The trans is longer which would make the driveshaft way too short to work w/ a 3 link or 4 link. I would look very carefully at the torque and HP curves of you engine. If it won't run comfortably at 1500 rpm a 6 speed would be worthless.

[GoDadGo]

Generally a 6 spd needs IRS rear suspension. The trans is longer which would make the driveshaft way too short to work w/ a 3 link or 4 link. I would look very carefully at the torque and HP curves of you engine. If it won't run comfortably at 1500 rpm a 6 speed would be worthless.

Double Ditto Mr.Craig!

I turn 1,725 RPM at 70 MPH with my combo and I love it.
If you run a 6-Speed be sure to gear the rear appropriately.
There isn't a speed that my car is unhappy until I lift my right foot.

https://youtu.be/9WEe6-wdNtA

11:30 ET at 124 MPH with a really bad 2.57 60-foot time on regular Nitto 555's.

[JohnK]

Generally a 6 spd needs IRS rear suspension. The trans is longer which would make the driveshaft way too short to work w/ a 3 link or 4 link. I would look very carefully at the torque and HP curves of you engine. If it won't run comfortably at 1500 rpm a 6 speed would be worthless.

Not sure I understand.

The TKX is available with a few different OD ratio's but the most commonly selected one is the 0.68 OD.
The T56 is available in two varieties, but I think the most commonly selected one has a 0.80 5th gear and 0.63 6th gear.

Playing with their gear ratio calculator (https://www.tremec.com/calculadora.php), with 315/30R18's on the rear and a 3.55 IRS you'd be running just over 2,000 RPM's with the T56 and 0.63 6th gear, and you'd be running just over 2,200 RPM with the TKX and 0.68 OD.

The main benefit of the T56 over the TKX is not that it has a significantly taller final OD ratio, but that it has an intermediate OD that bridges between 4th gear and the final OD ratio. This may be useful with lower-torque engines in heavier cars (not really the case with the roadster) or for track use.

Bottom line, play with the gear ratio calculator with different tire sizes, final drive ratios and and transmissions and it should become clearer what the right choice is for you.

[Jim1855]

Just my thoughts on the 5/6 speed debate.
If you follow the big question of which OD is best with the 5 speed transmissions (cruising (0.6x) or roadrace (0.8x) then the 6 speed answers all the questions by having both options.
The downsides are cost, length and weight and the associated issues that come with each.
Jim

[GoDadGo]

The length of the T-56 really makes for a tough install in a MK-4 or FIA car.
In my case, the ZF S6-40 has a tail shaft length similar to the TKO and TKX transmissions.
If I were building either of the open top cars, I'd install TKX because of shorter length.
I'd imagine stuffing a T-56 in an IRS car would be interesting to say the least.
While I love my 6-Speed, the final gear is only used on the Interstate.

ZF S6-40 Technical Data:
http://www.zfdoc.com/techinfo.htm

Good Luck With Your Build & Remember That You Are Building Your Dream!