OD Split Shift

I know this has somewhat been covered in other threads in the past, but I was doing the math last night and figured I'd post these findings.

I'm (still ) putting in a SM420 in my 3.73-geared :v6: ECJ5

As we all (may) know, SM420 ratios are:
7.05 - 1st
3.58 - 2nd
1.71 - 3rd
1.00 - 4th

In my case, I'm coming from a T14, so my old ratios were:
3.10 - 1st
2.61 - 2nd
1.00 - 3rd

I always felt that first gear was too high, and that the 2-3 shift was a really big jump when running the T14, especially on the trail.

Since 1st gear is really short on a lightweight vehicle like a CJ5, and since really I'd only be using it on the trail, I've been planning all along that I'd start off in 2nd on the street. That brought into question the big gap between the SM420's 2nd and 3rd gears, which is an even bigger gap than between 2-3 on the T14, and even worse since that gap on the SM420 2-3 would be almost 4 times the gap between the old 1-2 on the T14.

Last night I ran the numbers on what my ratios would be if I had Herm's overdrive installed along with the SM420... Here's my results:

7.05 - 1st Direct ---- "1st"
5.29 - 1st Overdrive- "2nd"
3.58 - 2nd Direct --- "3rd"
2.69 - 2nd Overdrive "4th"
1.71 - 3rd Direct ----"5th"
1.28 - 3rd Overdrive "6th"
1.00 - 4th Direct ----"7th"
0.75 - 4th Overdrive "8th"

I still think "2nd" would be a bit short for street starting, but I get to close the gap but the step between "3rd" and "4th" is very nice, making the gap between "3rd" and "5th" even nicer... after that it's probably a moot point up until "8th" when you really a "true overdrive" between the engine speed and the input to the tcase.

How many of you guys "split shift" with your overdrive?

If you do, you might want to see this video:

http://youtube.com/watch?v=G-aDqAWkZHQ

Left stick is his "LoLo-Lo-Direct-OD" splitter, and the right stick is his "1-2-3-4-5-R" transmission. He only uses Lo-Direct-OD in this video.

Can't wait to get my jeep back under its own power...

 

http://youtube.com/watch?v=G-aDqAWkZHQ
I'm absolutely worn out after watching this.....

 

I have a V6 and the T-14 tranny with one of Herms OD's backed up with 4.88's. I experimented a lot last summer and have found I like to only use my OD as a "4th" gear when I am on the hiway.

 

Same setup here but never thought of doing that. I usually just leave it in OD. I'll have to give Mike's way a try.

 

Really, I just laid out what all the ratios would be.

I think actual use would provide more of a seat-of-the-pants kind of experience factor, but I was always displeased with the T14 + 3.73 gears, and don't want to end up even more displeased because of the SM420.

In actual practice, just looking at the numbers, I expect my daily driving habits would be along these lines:

2-Direct
2-Overdrive
3-Direct
4-Direct
4-Overdrive

Which essentially means it would be a 5-speed with two handles. The gaps between 3-Direct-to-3-OD-to-4-Direct are so small they wouldn't be necessary unless maybe I was pulling a heavy trailer, and my trailer hitch was removed years ago so that's not likely

 

You said it I do not like the overdriven 1st gear at all, as it is like your 3.73 without the OD. 2nd gear split is almost usless on the street. So that is why I use it as a "4th". I have, on a real fast trail used the OD in low but only found one occasion that suited that. I think your "5" speed idea will work well for you.

 

Holy Shift !!!!! Man, that's when truck drivers were truck drivers. We had an old Mack we used on the jobsite but I never figured out how to shift it. Sure makes realize why they learn to shift without the clutch.

 

First, I have to say that I appreciate the old-school truck drivers that would shift a Brownie box like that. I drove a dump truck for a short period when I was in high school that had a five speed manual tranny with a pneumatic thumb controller that shifted between low, direct, and high. I learned very quickly to shift off of rpms rather than using the clutch. In fact, when I first started, I used the clutch every shift and would always miss a shift at around 3rd-direct, which would cause me to get stuck in neutral and would require me to pull to the side of the road to start all over again - major bummer.

O.K. now about splitting with the overdrive. I am responding since I feel our set ups are almost identical. I have a 225 v6 (rebuilt with stock cam, hei, 2 barrel carb, and headers), sm420, warn od, 3.73 gears, and 31-inch BFGs. As you, I thought the jump between 2nd and 3rd on the sm420 would be an issue. I experimented with splitting gears with the overdrive, but soon realized that the Buick v6 didn't care about the "jump" one bit. It pulls third gear in the sm420 strong from about 1200 rpm to about 2200 when I shift into 4th. I have found that the only time I "need" the overdrive is on long grades to split 3rd and 4th when towing my Jeep trailer, or on near flat ground to cruise at 65 mph.

So that you know, and it is something that I have heard repeated on this forum, I think the 3.73s are not the way to go if you want to use an overdrive. I am currently looking to put 4.88s in my axles, which I think will fit my style of driving the most (i.e. 50% around town and 50% off road).

I hope this info helps.

 

The whole crux of the problems I've had on and off-road as far as performance have been related to the 3.73's, I know this...

I haven't put new gears on the list because the list is full of other things, and the 3.73's aren't broken.

Plus, I've thought about changing axles at some point, so maybe I'll just get some matching lower geared axles if/when that time comes, otherwise I'll just rebuild the diffs with new gears.

 

Yep! I'm in the same situation. I currently have 5:38 ratio'ed axles to swap in; however, I think that ratio is too low for me. Unfortunately, my powerlock'ed and tapered rear end just won't break - giving me the excuse to spend money upgrading. With my luck, I will be buried with the 3:73's in the jeep.

 

When I went to 35" tires, even with 5.38 axles, I lost the use of 4th OD unless I'm going downhill or have the wind at my back. Around town I use 2nd, 3rd, and 3rd OD. I use the OD in 1st and
2nd a lot offroad. With the Ford T18 tranny, sometimes 1st is too low and 2nd is too fast. 1st OD can be right where I want to be on some steep rock climbs. Enough inertia to make it up but slow enough to have good control.