overdive or not

[CraigM]

Has anyone replaced the rear end gears to a 4.27 ratio and if so how does it pull I have been considering an overdrive which is 125% speed gain and 4.27 would be 120% speed gain over 5.38 ratio. My thoughts are the 4.27 would pull easer because it has a little lower ratio. The overdrive systems use planetary gears and planetary gears pull harder than a direct gear drive would, so adding overdrive would have the extra drag of a planetary system along with the 125% gain in speed. 

[Mark W.]

What rear end do you have? As changing a D41 is not really an option.If you have a D44 then you will find that while the hwy speed and power will be close to the same between your two options. But your bottom end trail performance will suffer with the 4.27's.

[mtnman37879]

If you want to go faster you need more engine. The little 134 only makes 60hp and it won't make it for very long at the required 4000rpm.

https://www.youtube.com/watch?v=3NHyqV4IiMM

[Mark W.]

5.38 gears 30" tall tire = 3300 rpm at 55mph
5.38 gears 30" tall tire 25% OD = 2500 mph at 55mph

At 4000 rpm in direct drive you would be doing 66.4mph with the OD you would be doing 88.5 mph

Not sure what your talking about going 4000 rpm And Roos took a box stock 134L and ran it at 4400 rpm for 100+ hours. Granted your engine has to be in proper condition to run at its peak.

[oldtime]

5.38 FDR was standard ratio for all CJ's prior to 1963.
In 1963 the 4.27 ratio became standard behind all F-134's.
If your stricktly concerned about increasing road speed then 4.27 ratio will work very well with these little engines.
However 4.27 FDR will decrease the off-road ability.

In 1964 Kaiser Jeep Corportaion designated the Warn 25% O.D. units as "Jeep Approved Special Equipment"
These unit are only approve for Jeeps having 5.38 and 4.88 Differential Ratio's.
5.38 with 25% O.D. provides a compound FDR of 4.03.
That gearing is the absolute optimum for use behind the 134 Willys engines.
I tried 4.88 DR with 25% O.D. behind a strong running F-134 in order to try gain a little more top speed.
Sufficient  acceleration was lacking and so I went back 5.38 ratio plus 25% O.D. unit within 1 week on my full time daily driver CJ-3B.

[mtnman37879]

Watch the video. https://www.youtube.com/watch?v=3NHyqV4IiMM It shows why you won't get more speed. It all boils down to Horsepower and torque and the 134 hasn't enough of either. The 134 that ran 100 hours at 4400rpm did it on a test stand not under real world use.

[aboyandhisdog]

Craig,   Is sticking with the 5:38 rear end and installing a 25% overdrive an option for you?  That is the set up I have and I could not be happier.  I'll leave the top speed question to others who know more than I do, but I can tell you it is MUCH easier for me to run 50-55 on the highway now with the overdrive.  I could not hit those speeds without the OD.  Maybe I could if pressed, but I would be really revving too high for my old L134.   I'm pretty sure that Jpet's OD tests were done before he did a complete rebuild on his engine.  I have no idea if driving his L134 60mph+ at near or over 4000rpm had anything to do with it, but I would never drive mine that fast.   And don't forget, at speed like that you have very real concerns about handling and braking in my opinion.

[CraigM]

Its not about speed 50 to 55 is ok I am more concerned about running at a lower rpm while traveling also it is hilly around here and I would like it to pull the hills without downshifting 

[aboyandhisdog]

An OD won't help on hills at all unless you are going DOWNhill.  So maybe I don't really understand what you are asking.  If you want the best performance going up hills, wouldn't the 5.38 be the obvious choice?

[athawk11]

I don't have an overdrive, but based on your hilly local terrain and your desire for top end speeds of 50-55 mph, an overdrive with standard 5.38 gears seems like a good choice for you.

[1955CJ-5]

The idea of an overdrive is to give you fewer engine rpm's for a given road speed, or, a greater road speed for a given engine rpm.
 
It does nothing for power.

You still have 60 hp, or less.

I have overdrives in both the 3A and the CJ-5.

The Cj-5 does a little better but the 3A really doesn't have the power to effectively use an OD.

Basically, the OD allows me to drive 45-50 mph with lower, more comfortable engine RPM...

The slightest uphill and you need to shift into direct.

The 3a does much better with the windshield down....lots of frontal area on a CJ and nothing in the way of aerodynamic design....

Unless you are trying to create maximum drag!

Overdrives can be added at any time.

I suggest you drive the jeep awhile with direct drive and see how you like it.

Remember, you may not go any faster with an OD, but you will go at a more comfortable engine rpm....

[CraigM]

 I was trying to find out how well OD pulled verses 4.27 ratio I do want lower RPM's. The  51 CJ3a I have has a dana 44 and so 4.27 gears are available for it so based on the input and discussion it looks like that is the option I will try. My reasoning is it should have less rolling resistance than 5.38 ratio with OD. Here is the calculation for running 2750 rpms with standard 28" tires is 55 MPH. It will not have my CJ done for a while but when I do I will update this post.

[oldtime]

The 5.38 with OD will "pull" very much better than a 4.27 geared L-134.

The 4.27 differential gears only became standard at a much later date.
That gear ratio change was due to need for increased speed occurring through out the 1960's.
The increased output of the Hurricane made that ratio change feasable.

The optimum RPM output of a Go Devil is 2000 RPM.
2000 RPM yield maximum torque output.
The Hurricane operates most efficiently  at 2200 RPM.

So the Hurricane is basically 10% better for high speed applications.
I've run both engines for hundreds of thousands of miles as full time daily drivers.
I've run them up to 4000 RPM during tests and at all day long sustained speeds of 62 MPH.

The friction resistance of the O.D. planetary unit is a negligible consideration at best.

The absolute best gearing for hilly high speed on road use and off-road applications is the  5.38 differential ratio compounded by 25% Warn O.D. unit. PERIOD.

[athawk11]

The

The absolute best gearing for hilly high speed on road use and off-road applications is the� 5.38 differential ratio compounded by 25% Warn O.D. unit. PERIOD.

I believe this is what the entire group of participants is suggesting in this thread.  Personally, I concede 100% to oldtime's knowledge on this type of stuff.  He is incapable of steering you in the wrong direction.

[RICKG]

The

The absolute best gearing for hilly high speed on road use and off-road applications is the� 5.38 differential ratio compounded by 25% Warn O.D. unit. PERIOD.

I believe this is what the entire group of participants is suggesting in this thread.� Personally, I concede 100% to oldtime's knowledge on this type of stuff.� He is incapable of steering you in the wrong direction.

Yup.. with my strong L134, 5:38's and OD I'm happy but still prepared to do much downshifting as the load increases. I can't imagine this combination w/4:27's.. not enough motor..