Overdive for a 65`

Thinking about installing an overdrive unit in my 1965 CJ6 with the F134 engine any thoughts on what this might accomplish for me.

I am currently running 33x1250 but will be swapping those tires our for 31x1050

any advise on this would be great....

Signed
Overdriven

 

The purpose of an overdrive is to reduce engine rpm at a given speed. Without knowing what axle gear ratio you have and the intended use we have no way of determining whether it will be useful to you or not.

 

Thank you I have 4.27 gears and right now my top end is 50 mph I would like to get up to the speed limit for trips to the otterbanks

 

i have a 55 M38A1 with 5:38 gears, also a 350 sbc for power. i have the warn overdrive, all range. it will lower my rpm about 500rpms. with 33" tires i can do 80 mph with no problem, other than mental. i think thats why they came up with the overdrive, the older jeeps and the low gear ratio.

 

My two cents...you should keep the 33's and do an engine swap instead. The O/D will drop your effective gear ratio into the mid 3's...I don't think the f head has enough oompf to move the Jeep effectively at hiway speed with that gearing.

 

Had your gear set-up in my 60 cj5. Rebuilt F-134, stock T-90 & T-18, 16" wheels with military NDT's and 4:27 gearing. I was able to use Over Drive on completely flat road with no "Head Wind !!! " Any thing other than that I just lost RPM's. Remember, you are driving a vehicle with the aerodynamics of a BRICK. The stock F-134 just doesn't have the power to push thru the wind resistance. You need to decide what your primary use will be and opt for an engine swap to V-6 or gear swap to 4:88 or 5:38. I chose to go back to 5:38 to stay stock.

 

This!

With 4.27's and the F head, an overdrive will only marginally increase the potential top speed. If you have 33" tires, you may gain close to nothing. With the 31's you will get a little more top end but only on the flats or downhill. Forget about freeway speed. It ain't gonna happen with a 134.

 

This topic has been discussed a lot in the past - search the archives using Google - "site:earlycj5.net <your search terms>"

Everyone agrees that you can't expect highway speeds out of a F134 in a Jeep except under ideal conditions (flat, no wind, ideal gearing and tire height, long time to run up to speed). You are power limited. I would guess that 4.27s are pretty close to ideal now. As duffer points out, with 33s I would expect the OD to make you slower, not faster.

Realize that wind resistance is a mighty barrier that goes up with the square of speed. You just don't have the power to push the CJ with an F134 much faster, even with ideal gearing.

 

Exactly.
The O.D. unit simply compounds the final drive ratio. (FDR )

So the real question is what is the optimum FDR for the Hurricane engine in the jeep ?

Everyone seems to agree that 5.38 FDR does not produce enough speed.
Your 1965 was standard with 4.27 FDR and that is much better than 5.38 FDR for attaining top usable speed from the Hurricane.

But much better is not the optimum.

I have tried 4.88 FDR with 25% O.D. which yields a compounded FDR of 3.66.
3.66 FDR is much too high for the Hurricane.

4.03 compound FDR or 4.09 FDR is optimum for the Hurricane engine CJ's when attempting top cruising speed.
The optimum compound FDR of 4.03 is attained by installing 25% OD with a 5.38 FDR.

I should also mention that your 4.27 CJ is already at optimum cruising speed FDR without O.D. unit because your tires are taller than standard.

 

I tried driving mine with the F-head and 5.38 gears. Was no fun. Bought a core a couple days later. I am building a 225 odd-fire and am looking at a 4 speed with OD (3+1).

 

If I had your near original 65 Tuxedo cj6 I would not change anything except possibly go to electronic ignition. What ever you do be sure to keep all the parts you take off (change) so the next owner can go back to original if desired. Just me.

 

I thought about an engine swap but this jeep is almost all original and they only made 459 CJ6a

 

Thanks I should have looked deeper in the archives thanks for all the input it has really help me alot

 

I agree and that is what i am going to do...thanks Walt you have been great

 

What Walt said