66fstbk88

I have a '66 with a built 351w, T5, 9'' rear with 3.25 gears, and king cobra clutch and making about 450hp. I have now broken 3 (90-93) T5's, not T5z. 1st tranny I broke 3rd gear. 2nd tranny I twisted the input shaft(4th gear). And last tranny, I snapped the main shaft in half. That's when I decided to go TKO!! I then spoke to Bruce from Modern Driveline who told me the reason I keep breaking T5's is because I'm running 3.25 gears and putting too much load on the T5's. He said instead of spending all that money and labor on going TKO, a good option would be to get a T5z from them which is rated 450hp and change my gear ratio to 3.55 gears. This is not a drag car, but I would like to drive it on street and still be able to play with it.
I understand the guys at Modern Driveline are very knowledgeable and know what they're talking about, but I want to know what do you guys think?

Gun Jam

Unless their Z specs are using something like the G force kits they are only rated to 330 FtLbs as I recall.

You are breaking t5s because they are not rated to handle that kind of torque not so much because of the gears. I dont know how going to 3.25 gears will "unload" anything????

Whats the cost difference between the new t5 and the tko?

-Gun

Starfury

Bruce told you that? Intersesting.

On one hand, that gearing combo would give you a shorter 1st, helping to keep from loading up the trans.

On the other hand, torque is what kills T5's, not horsepower. A T5z is only rated to 330ft-lbs, which I'm sure you exceed by a wide margin. If you hook up, you're going to break things.

Gun Jam

isnt 3.25 taller than 3.55? thus giving a longer first duration with higher trans load?

-Gun

Starfury

Hm, I thought the T5z was the one with the short 1st gear, but it's not. The T5z has a 2.95 1st, which makes the 3.25 swap make even less sense. Apparently the 2.95 1st is stronger, but that doesn't make anything else stronger.

Personally, I'd use a TKO. You won't break it. You'll have to get creative with the fit, however. JMD used a drop crossmember to cram one under his '66, but doing so can potentially cause vibration issues as it tends to screw up the combined driveline angle. JMD has no issues, but that doesn't necessarily mean you won't. Just something to keep in mind.

Gun Jam

Yeah its not really an easy choice....Im having a vibration issue with my t5 that after a year of trouble shooting and part changing has failed to be resolved.

Im down to 4 things
1) pinion angle which cant be improved without cutting the tunnel to ribbons.
2) DS is not balanced at speeds high enough for the 3.80 gears (IM hoping this is the issue)
3) bent shaft on the t5 which is unlikely
4) improper amount of free play at slip yoke.

There is a lot things that can fight you and I know the TKO would be a bigger pain in the *** than the t5 but would far more reliable with your current performance.

-Gun

66fstbk88

If I were to go TKO using my current setup, 351w, T5, 9'' rear with 3.55 gears. Which TKO would operate best? What parts can I reuse and what parts would need to be changed/modified?

*TKO-500 5 Speed Transmission 10 Spline
Ratios:
3.27 = 1st Gear
0.68 = 5th Gear

*TKO-500 5 Speed Transmission 26 Spline
Ratios:
3.27 = 1st Gear
0.68 = 5th Gear

*TKO-600 5 Speed Transmission 26 Spline
Ratios:
2.87 = 1st Gear
0.64 = 5th Gear (Fuel Efficient)

Starfury

If It were me, I'd run the TKO600. With your torquey motor, I don't think you'll have any issue accelerating with the taller gears.

You'll need to replace the clutch and figure out a way to cram the transmission under the tunnel. Easiest way would probably be a drop crossmember.

66fstbk88

What about other parts like yoke, flywheel, bellhousing, fork, and throw out bearing? Can these T5 parts be used for the TKO?

67mustang302

TKO 600 and be done. What housing do you have now? If you're upgrading the trans it'd be a good time to go to an SFI housing. Insurance for the feet, and required if you ever start going faster at the track. Not sure on the TOB etc.