Crawl ratio

[Bulldog]

Right now I have about everything to get my CUCV ready for my 39" tires. All I'm waiting on are the wheels which will be in town later this week. I was hoping not to have to get my drivelines lengthened with the 6.5" of lift, but I'm pretty sure I'm wrong. I was planning on installing a 203/205 t-case doubler and getting new drivelines later.

So I was reviewing what my crawl ratio would be and found out some interesting data:

Right now I have a TH400 / 208 / 4.56 gears, so 2.48 (TH400 1st gear) x 2.61 (208 low) x 4.56 = 29.52:1 crawl ratio.

If I install the 203 / 205, it will increase my crawl ratio to 44.78:1. 2.48 (TH400 1st gear) x 1.98 (205 low) x 2.00 (203 low) x 4.56 = 44.78.

I got to thinking about my other truck which as a SM465 / 208 / 4.56 gears. 6.55 (SM465 1st gear) x 2.61 (208) low x 4.56 = 77.96 crawl ratio.

The SM465 really is geared low and makes a tremendous difference in the final ratio. If I lifted this other truck, I would not have to spend the $1,000+ for the doubler. I'd have to buy longer driveshafts in either case. Any mistakes in my math for the crawl ratios?

I understand that some people don't like a manual for crawling, but heard a hand throttle helps a lot here. I've also heard that automatics do better for mudding (mud drags in particular), but I don't expect I'll be in too much mud. I really like the trail rides we've been doing around Fairbanks and want to hit some rocks this summer. Are there any benefits from an auto that I'm missing?

[ChevyKev]

With the 4.88's, SM465 and the 205 I crawled Rainbow just fine.

Auto's can overheat going that slow yet with higher RPM's.

I really like the manual for the trails, including the rocks I have crawled on.

Just my $0.02

[DieselPower]

You have to think about the action of the torque converter in your equations. When you are calculating your ratios you are assuming that the converter input and output are 1:1 which as far as I know only happens when in lockup and a TH400 does not have that. So in reality your engine speed and the speed of the input on the converter could be spinning twice as fast as the output going to the gears. This would give you an additional 2:1 in the auto. It could really be anywere from 1:1 to 1000:1 depending on the load conditions.

Like Kevin said though the number one killer of autos is heat and low speeds and high load will yield a lot of heat. I think you would be good either way you go. The manual would be more reliable. I've blown over 5 autos and have yet to do anything harm to a manual.

Oh the great dilemmas in lifehrug:

[93hummer]

So when are we turning up the pump? I've got a 4 day weekend coming up...

Can't wait to see that CUCV on those 39's.... Boggers rule, they are like magical offroad tires that go everywhere. For calculators, my favorite is here: http://www.4lo.com/calc/gearratio.htm

FWIW, I have a 35.35 crawl ratio, 2.73 diffs. 1.98 geared hubs, 2.72 T-case, and a 2.48 first. With the 42's it does fine 98% of the time crawling, the only time I run out of power is with steep hillclimbs (like the powerline hill on FWW) where I wish I had more gear to turn the 42's. Prior to the 42's I had 38.5 Swamper SX's and had zero problems at all in the same situations. My injection pump has been turned up 1/6th. In the future I plan to turn it up a tiny bit more and install lower gears in the transmission, a 2.75 first and 1.57 second. I do not want to touch the diff gears because I like the highway RPM's at 60, it is just right, with the stock 37's it revved way too much on the highway.

I think with the 6.2 and automatic with those tires, you will want it lower than 35:1. You could accomplish that by axle gearing, SM465, or doubler, your choice. I do not see the need for 80:1 crawl ratio with an auto though...

Edit, this company makes a 3:1 first gear for the TH400 (added to my list of future mods)
http://www.crttransmissions.com/Components.html

[Bulldog]

You have to think about the action of the torque converter in your equations. When you are calculating your ratios you are assuming that the converter input and output are 1:1 which as far as I know only happens when in lockup and a TH400 does not have that. So in reality your engine speed and the speed of the input on the converter could be spinning twice as fast as the output going to the gears. This would give you an additional 2:1 in the auto. It could really be anywere from 1:1 to 1000:1 depending on the load conditions.

Like Kevin said though the number one killer of autos is heat and low speeds and high load will yield a lot of heat. I think you would be good either way you go. The manual would be more reliable. I've blown over 5 autos and have yet to do anything harm to a manual.

Oh the great dilemmas in lifehrug:

Good stuff, this is input I was looking for. I was thinking the torque converter would affect it somehow.

[SHoppe715]

Bigger trans cooler or just a 2nd one plus an electric fan for it. Lots of universal kits out there and a cheap upgrade to protect the auto trans.

[Angler]

when I was running 35's on 3.07 gears in my YJ I hated the manual I was forever stalling and just looking stupid on the rocks, I even bought an automatic trans and was going to swap it out, before I did the trans swap I did an axle swap and geared my jeep down to 5.38 gears, I went out and found all my stalling while crawling problems went away and I actually enjoyed having the reliability of a manual trans and no longer felt the need for an auto and sold the TF999. I did have a hand throttle but rarely used it after I geared down. Now words of caution, I loved the low gears off road but HATED i say again HATED the low gears on the street, to the point of not even wanting to drive it anymore on the road, 55mph at 3500 rpm sucks.
So on my new project I am gearing it down in the Tcase, I am installing 4.7:1 gears in the case which will give me the crawl I want in the hills and rocks but will still give me a 1:1 in 4hi which will make street driving more of a pleasure than arse pain

[Bulldog]

So when are we turning up the pump? I've got a 4 day weekend coming up...

I forgot about that, I'll have to get with you.

Can't wait to see that CUCV on those 39's.... Boggers rule, they are like magical offroad tires that go everywhere. For calculators, my favorite is here: http://www.4lo.com/calc/gearratio.htm

My 39's are the XMLs and if I go with the 465 setup, I'm putting this truck on a second CUCV frame.

I do not want to touch the diff gears because I like the highway RPM's at 60, it is just right, with the stock 37's it revved way too much on the highway.

I don't want to change my gears either. With 33's right now I'm pretty high in the RPM's at highway speeds. I'm hoping the taller tires will drop cruising RPM's and the low crawl ratio will help on the trail/rocks.

[Bulldog]

The nice thing with the SM465 is the 6.55:1 first gear and the 3.58:1 second gear. I will still have 1:1 in fourth, just no overdrive.

With the 3.58 second gear x 2.61 low in the 208 x 4.56 gears, I will have a 42.61:1 crawl ratio in 2nd gear. This should be very streetable and only use 1st gear while crawling. I can always do a gear change if it doesn't work, but I think it should.

[93hummer]

Good stuff, this is input I was looking for. I was thinking the torque converter would affect it somehow.

Torque converter effect is huge.... For a manual I would want no lower than 80:1, but that would be way overkill for an automatic. For an Automatic I would go no more than 50:1.

I used to have a YJ with a 3.8 atlas, T-18, 3.73's, 38 boggers and the crawl ratio was around 89:1; more gear than I ever needed. I actually stalled it in 4 Low 3rd one day in the mud, tried to take off in second, stalled again, put her down in first and tried it again and it snapped a front D44 shaft and smoked the centerforce clutch.... too much gear will break things. Just for laughs one day I went across the parking lot at 4 wheel parts in Atlanta in RWD low, 1st gear on the T-18, doing about 5 MPH at about 5000 RPM..... lot's of people looking wide eyed.

[SHoppe715]

... too much gear will break things...

I hear that. I'm afraid to get on the gas in 1st & Lo. 90:1 crawl and 1310 u-joints under a heavy truck...:whistle: I pretty much only use 1st gear in Hi range and almost never on the street.

[Angler]

I'm afraid to get on the gas in 1st & Lo. 90:1 crawl

with a crawl ratio like that u should not have to get on the gas, it should walk over anything, I loved to put my jeep in 4 low and point it at a steep hill and take my foot off the gas and just put up and over while lots of other guys had to back up and floor it and break things trying to get up. (kinda like I have to do in the TuRD now)

[mousee]

Don't go auto and be like all the non drivers out there. I am sure I will get a lot of hate mail for that, but that is my opinion. The sm465 is a stout tranny and will live behind just about anything you throw at it. If you wanted to click off a few more crawl points, go with the sm420. That would drop you down to 83.4-1. Pretty close to that mystical/magical 90-1. With the wide ratio trannies like the old muncies, you get a lot of options on gearing. The 420 has a 7 first, something like a 3.5-1 second and 1.7 third, lots of options. I have been running the 420 for a few years now with a final drive of 68.something and am always wishing I could go deeper, but keep my existing set of gears, thats why I am working on a crawl box/adapter...
If I was doing your rig to run, I would run the 420 with the 203/205 doubler and call it good, that would give you 126.5-1 and a whole crap load of options...

just my thaughts...

[Bulldog]

I got to thinking about this tonight.

The 208 is a chain driven case, where as a 203/205 doubler would be all gear. Would I break a 208 if I was running either a 383 or a 454 in this truck? I've heard of 203 chains breaking, but I personally haven't heard of 208's breaking chains often. Would I be better off spending $1,100 or so to get the doubler setup or run the 208 (free, I already have it)?

[sevenslats]

You'll never be sorry you went with the 203/205. You may often hate the fact that you kept the 208.
It just may take you longer to save $$$.

[Angler]

208= chain=
205+203=gears=

[AK20]

And doubler with SM465 running 4.56's is 118:1, IIRC.

Now if I could only get around to install the doubler I have sitting in the garage...