Topic: Tera 4:1 or SM420

[neale_rs]

Looking at a bunch of options this weekend, I think my best options for lower gearing are either swapping in an SM420 or a Tera 4:1.  This would be done some time next year.


The 4:1 would give me a crawl ratio of 72:1 and the SM420 would give a crawl ratio of 87:1.  Using junkyard parts for the SM420 swap, I think it would cost more than the 4:1, but maybe at the most 50% more.  The SM420 would really change the shifting of gears on the street but since I only drive the Jeep for a short distance once a week and about once a month out to a trail (fourth gear most of the way), I think it would be tolerable.

Which would you recommend?  Any pitfalls?

[jcsanders79]

Don't know anything about the SM.  I am pretty satisfied with my Tera Low 4:1.  Easy to install, doesn't affect driving around town.

[Jeffy]

Looking at a bunch of options this weekend, I think my best options for lower gearing are either swapping in an SM420 or a Tera 4:1.  This would be done some time next year.


The 4:1 would give me a crawl ratio of 72:1 and the SM420 would give a crawl ratio of 87:1.  Using junkyard parts for the SM420 swap, I think it would cost more than the 4:1, but maybe at the most 50% more.  The SM420 would really change the shifting of gears on the street but since I only drive the Jeep for a short distance once a week and about once a month out to a trail (fourth gear most of the way), I think it would be tolerable.

Which would you recommend?  Any pitfalls?

It really depends on how your Jeep is set up.  Do you need 5th gear?  Keep in mind that the granny 1st isn't used when on road, so you'll be driving it as a 3-speed.  You'll want to rebuild the SM as they were used in commercial vehicles like dump trucks.  Also, you might find the SM465 as well since they were the following transmission.  Have you located a SM already?  I think that is going to be the first task.  Then you're going to have to rebuild it.  Then locate a Chevy bell housing and have someone mill out a spot for the crank sensor.  You'll still need to mix and match for clutch parts as well.  Then you'll need an adapter to get it to mate to the NV231.  Oh and you'll need to get a new pilot bushing as well as a 23 spline input shaft.

I still say Klune.  Or maybe go with the 4to1 and then do a mad rooster or whoever is still making that kit.

[neale_rs]

Haven't used fifth for quite a while now.

The Klune + super short SYE is actually pretty hard to justify, an Atlas 4sp is just a bit more expensive and with the short output option I would end up with a longer rear DS than I have now while the Klune + super short SYE would result in a shorter rear DS and just a bit lower crawl ratio.  Also, the Atlas 4sp is stronger than the Klune + NP231.

The 4:1 is the lowest cost and easiest to install alternative.  That might be the way to go until the AX-5 eventually fails.


What is the mad rooster?

Thanks

[aw12345]

What it comes down to is this, spend 800 bucks and about 4 hours of wrenching and then go wheeling and be happy. Or do a heck of a lot of work end up with an old transmission that yes it works but pretty much miss most of the summer wheeling and spend a lot more money. A lot of these old transmissions don't shift all that great. They are tough though, I rebuilt quite a few sm465's their biggest drawback is that the shift covers/ shift forks can wear really quickly.
U have ran the snot out of my terra low and it has held up great, just don't run it over 10 mph in low range and they seem to hold up just fine. Them little boxes make crawling so much easier.

[neale_rs]

That is true, the difference in the cost and work involved is pretty big. I'm leaning towards the Tera 4:1 now. 
My Dad has a 1985 2wd Chevy pickup with an SM465, I always liked it, has a good solid feeling.

Thanks

[Bounty Hunter]

You say you haven't used 5th gear in a while, are you satisfied with your current axle gear ratio?

[neale_rs]

I think 4.56 is good enough.  I always had it in the back of mind that a 4 sp tranny would be swapped in eventually so I did not want to need 5th.  Any crawl capability now will need to come from tranny or transfer case upgrades.  4.88 might be an option someday if I get 35's.

[Bounty Hunter]

That's not very low for a 4banger, and you see no on-road improvement with either the 4:1 or the granny tranny.  Improve the axle ratio and you improve both off-road and on-road performance.  You could install a pair of d44's with 5.38 for about the same money.

I run 5.38 and the AX5/231 are geared just right for all-around trail use, and can still get out on the interstate and run 65-70 in 5th and even accelerate without downshifting.

To me it just sounds like you're throwing mods at the wrong problem.

[neale_rs]

It could be but even 5.38s (a front Dana would be really nice to have) will only get me a 58:1 crawl ratio, better than my current 49:1, but only 18% better. I want something really low, around 50% to 100% lower, low enough not to need a clutch.  The Jeep's on-road performance is a non-issue. It gets driven to work once a week, about 5 blocks, just so it gets run at least once a week, and then driven on the highway to a trail about once a month if I'm lucky.  The highways here are not high speed interstates, we rarely get over 55 mph and only for short stretches between small towns.

This is a pretty hard decision.  Maybe I'll flip a coin and let my budget force me to be happy with whatever gets done!

Thanks

[aw12345]

a 4:1 and 4.56 gears gives close to 80;1 crawl ratio depending on your first gear ratio with my 6 speed and the terra lowmy crawl ratio is 83:1
4.88 will help it a little more 5.38 would be nice for crawling but would suck on the freeway. 4,56 or 4.88 is not bad if the engine has some power mind you the ax5 has a much higher overdrive than the 6 speed trans

[Bounty Hunter]

5.38 doesn't suck on the highway, puts my 2.5L in the perfect power band with 35" tires.  33" would be fine.

[jfrabat]

5.38 doesn't suck on the highway, puts my 2.5L in the perfect power band with 35" tires.  33" would be fine.

I run 4.88 on mine with 31 tires, and even at 65mph, I am barely reacing 2,500 rpm's, so I think 5.38 and 33's will put you somewhere in the 3,000 rpm range, which is fine for the little 4 banger...

[neale_rs]

.

Thanks to everyone for their comments.  It gave me several alternative points of view.  I won't actually be changing anything until next year (well, maybe a few small details)...got to save up for a while!

[Jeepn95yj]

I agree with the tcase mod.  Too much work and not enough benefit from the SM420.

I run 5.13s in my '01 with an AX-5 and 33s.  Works out to be about 3000 rpm at 70 mph.  About perfect really