Need a low crawl ratio

[77 K20]

I went out this last weekend and crawled up the side of a mountain (drove up to a lookout tower that was built in the 30's). This "road" wasn't much more than a goat trail for most of it.

Coming down was the problem. 4 low, 1st gear was WAY too fast. So ended up having to ride my brakes for over a mile. Stopped at one point by rolling up against a big boulder (wheel chock) and ate lunch and enjoyed the view while the brakes cooled.

I have 4.10 gears, a TH350, and a NP203.

Been kicking around the idea of putting a SM465 in it for the granny low. That being said I have manual transmissions in all my other vehicles and do like having an automatic for off roading in this one.

Been thinking to look around for one of those lomax 3:1 ratio NP205 transfer cases. This would be the option I like the best- but will be very hard to find one of these.

I don't really want to go the doubler route (NP203/NP205). Adding extra shifters and crossmembers and more weight seems awkward to me.

I even keep looking at Atlas transfer cases but they are spendy.

Am I missing other options here? I really don't like how the speed runs away when going down steep hills. Pretty scary at times too.

 

[77 K20]

Looks like lomax is making the 3:1 gearsets again:

http://www.jbconversions.com/index2.php?pc=3


And I would assume a 3:1 ratio would be low enough with an automatic trans... but I'm only guessing here. Or does the torque converter still slip too much no matter what the ratio is?

 

[bucket]

I never knew the gearsets required a new case too. But they only show a 10 spline and a 32 spline, th350 uses the 27 spline.

 

[77 K20]

I never knew the gearsets required a new case too. But they only show a 10 spline and a 32 spline, th350 uses the 27 spline.

Strange they don't show the 27 spline on that website. Offroad design shows a 27 spline version.

http://www.offroaddesign.com/catalog/transfercaseparts.htm

Something about the gears being wider... so there was no room in the original case.

 

[bucket]

Wow, they offer a lot more 205 parts than they used to.

 

[77 K20]

Found a website that might have just answered my own question:

http://www.grimmjeeper.com/gears.html

I was able to compare a 3:1 ratio NP205 vs my NP203. Not that much different really. When I was going downhill "wildly out of control" I noticed I was at 2,000 RPM. The calculator shows I was doing 8.95 mph. IF I had the 3:1 lomax then I'd be doing 5.97 mph. Not that big of a difference.

Oddly enough just earlier tonight I was asked to give someone a hand removing a SM465 out of a 1983 Suburban and put an automatic in that. Might be able to get that manual for a decent price. I compared auto vs manual on that website:

TH350: 1st gear, 2000 RPM, 8.95 mph
SM465: 1st gear, 2000 RPM, 3.71 mph

So looks like to get a nice slow crawl ratio, a manual trans is pretty much the only way to do it.

 

[MadOgre]

an NP241 is 2.8 something to 1 ratio and can readily be had in the 27 spline. 88' and 89's came with a speedo cable.

 

[DoubleDingo]

SM465 would be a good swap if you're up to it. My dad's 79 GMC with compression so low it could barely hold itself in gear would crawl down hills in granny low and 4 lo. I should ask my brother if he still has that truck and get it back from him.

 

[MadOgre]

The early NV4500s came with the granny low as well

 

[77 K20]

So in doing some more reading and re-read the spec sheet for the new engine I put in it says this:

Flywheel / Flexplate:
Like all small block V-8 engines produced since 1986, the HT383 engine has a 3.00" diameter flywheel flange bolt
pattern. Small block V-8 engines produced from 1958 through 1985 had a 3.58" diameter flywheel flange bolt
pattern. This change in bolt circle diameter was made to accommodate a leak-resistant one-piece rear main seal.

So if I was able to get that SM465 trans out of the 1983 Suburban, it wouldn't bolt in. Is it easy to get a new flywheel flange bolt so it would work?

And I thought that the NV4500 transmissions had a different bell housing bolt pattern. I had thought one of those would be great with the granny low and also an overdrive, but didn't think they fit without a lot of modifications.

 

[Stewzer55]

I think that has more to do with your flywheel and clutch than it does the transmission.

 

[MadOgre]

The difference from 3 inch 3.58 inch is where the fly wheel/ torque plate attach to the crankshaft. It has nothing to do with the clutch or torque converter attachment points.

all GM blocks(not sure about the new generation diesels) share the same bell housing bolt pattern.

Using the bell housing from an SM465 on an NV4500 may be the issue im not sure.

 

[MadOgre]

Going from a 27 spline automatic to a manual means you will end up with either a 10 or 32 spline out put. So you would need to change the input shaft in your transfer case which means a complete disassembly of the transfer case. Or a new/used one with the correct splines.

 

[MadOgre]

Also you will need to acquire the adapter with the transmission as it is the transmission that determines what adapter is required although there is a difference between the figure 8 and circle pattern 205s

Edit : and the cross member for the adapter

 

[DoubleDingo]

Yes, the nv4500 has a different bolt pattern and a bigger bearing retainer. So it would be easier to get a bellhousing for the nv4500 instead of modifying one for a sm465.

And I agree on the clutch assembly bolting up just fine to the flywheel which has the smaller 3 inch bolt pattern on the crank. It is only the crank to flywheel bolt pattern that should be different.