Changing From A T15

Hi Guys—‘72 CJ5, 304/3sp. I have another thread going about the transmission popping out of reverse. I also hear a lot of rough bearings as well. I’m fairly certain it will need a full rebuild, which Id like to do but considering what other options I have. I’ve only had it a short time, and put maybe 20-30 miles on it. One thing that’s pretty clear is that its top comfortable cruising speed is around 45mph. More than that feels uncomfortable, and engine is racing, although the bearings might exacerbating the poor feel. Just keep wanting that 4th gear. I don’t need it to run down the interstate at 75, but would like it to run highway speed comfortably.

The 304 is solid. Plenty of power, starts right up, ton of oil pressure and runs cool.

what are the common options for transmission swaps, and where are some good places to look?

I’m assuming a swap to a T18? Can you find rebuilt ready to go, or do I find one and rebuild it?

I think the engine and transmission are original, if I swapped I might even rebuild the T15 for fun, and keep it for the next guy.

 

An option I was considering—-

https://www.novak-adapt.com/catalog/transmission/transmission-packages/t18-package/

 

A T-15 third gear is 1:1 ratio. T-18 4 speed 4th gear is 1:1 ratio. There will be no top speed change.

 

Ahhh…well,…crap….

 

What are your axle ratios? Mine are 3.73. runs down the road at 55-60 no problem.

 

A '72 could have the standard 3.73 axles or optional 4.27s. Both these ratios are fine for the highway IMO, even with factory-sized tires.

My CJ-5 new from the factory in 1973 had the 304 and 4.27 axles, and I ran on the California freeways with no worries.

I suspect you just aren't used to the amount of noise that comes through the bare steel floors in these Jeeps. No insulation, unlike any modern passenger car. How tall are your tires? Do they have aggressive tread? Do you have a quiet exhaust system?

A CJ equipped like this with 30" tires and 4.27 gears turns 3100 RPM at 65 mph. No problem. You can easily go faster. With 3.73s, 2700. If you worry about this, get a tachometer and look at the actual engine speed.

 

I have 3.73 gears in my 72 with the T15 and 70mph is no problem I think that I'm around 2800 at that speed. I'm running 235/75/15's

 

Fan noise is another issue. These engines were typically equipped with a thermal fan clutch, which reduces fan noise a lot. Fans without a fan clutch, such as a flex fan, are very loud in my experience.

Have you shifted into low range? For a given speed, engine speed doubles in low range.

If you have bearing noise, that's a separate issue.

The T-15 is a good transmission. Strong. Jeep offered it as the standard transmission in J-trucks and wagons with the 350 or 360 engine, and it handled that well. 5-speeds have an overdrive gear usually, and are longer than the 3-speeds or truck 4-speeds. You don't have the room for a much longer transmission. The truck 4-speeds are a popular conversion because they are not much longer than the 3-speeds, and they have that super-low 1st gear ("granny" gear). Great on the trail, but not helping on the highway.

 

Why do you say over 45 mph seems uncomfy ?
Racing engine makes me suspect factory axle gear ratios have been modified
Excess bearing noises coming from the transmission ?
That sounds to me like a main reason why the Jeep was sold .
Transmission change will almost certainly not resolve all issues mentioned.
Please be more specific about your concerns and verify the axle gear ratios.

 

So,
Step one is to install a tach. I suggest this gauge anyway.
Couple this with a speed app on your phone (to insure your speedo is correct) and take it for a run at 60MPH. This will give us solid information on your engine RPM's versus speed.

 

THIS

 

Thanks Guys—bunch of good points. I’ll try and provide some more info. Axle is 3.73 and tires are 235/75/15. Not very aggressive tread.

Tach—agreed, going to install one this weekend so I have a better handle on the RPMs vs just feel/sound

speedo is original and I was assuming it was accurate. I’ll check it back actual with app.

Fan—I had this thought as well. I have a fixed, non flex fan. In looking through TSMs, this looks like it was OE. I would like to add a clutch. Has anyone added that can refer me to one that worked well?

I don’t believe I was in 4L. Made this mistake first day I had it and it was noticeable.

As a side note, my steering shaft couplings and bushings are completely shot. I have all of the parts to rebuild. I think this was adding to the extra “excitement” I was getting past 45 mph. Also I’m becoming convinced the T15 is completely wore out and time for a rebuild. I think that’s contributing to noise and vibration at higher speeds.

 

You could always look for the hard to find Dana 20 overdrive.

 

The ones I've seen ("Rancho" overdrive) go on the output of the Dana 20. I would predict the driveshaft is too short on a CJ-5. Ok for a CJ-6, Jeepster/Commando, wagon or J-truck.

 

The pictures in the TSM are more for illustration than chosen for perfect accuracy. Helpful, but not definitive. The parts book shows a 4-fixed-blade fan as standard, and a 7-blade fan with fan clutch for the HDC option. You'd also get a fan shroud with HDC.