I am looking to swap a new tranny into my '70 FJ40. I have been thinking about an automatic, maybe a TH350. On the other hand, I like the sound of a first gear ratio of 7.05 with an SM420. With the stock 3 spd tranny I have now, I tend to heat the clutch up pretty good when rock crawling, and just is getting to be a hassle but not sure which would be better in the long run, a manual or an automatic. Just thought I would toss this out, and see what any of you think, maybe some experiences, suggestions or ideas. My cruiser does have a Chevy 283 V-8 already in it.
transmission debate
- Thread starter toyman
- Start date
goin4xn
Jeepaholic
- Location
- Eagle Mountain, UT
Man - the jury is still out on the auto vs manual debate.
With an auto don't have to do the three foot dance on steep stuff. One foot on the brake and one on the gas give you GREAT control.
You get a 2:1 reduction at really low speeds (where we all crawl at) just because of the torque converter. (Don't ask me to explain it - it has something to do with the flow of the fluid at low speeds that doubles the torque of the tranny).
Autos are longer than the equivalent manual tranny. They can be a pain to try and fit into your rig (but this has been done so many times with a CHevy 350 conversion I'm sure it's not much of a problem).
The TH350 is a good stout tranny and there are lots of them around. This means that they are well supported in the aftermarked. There will be plenty of parts available for them and there should be plenty of shops/people who are knowledgable and capable of working on them.
With the SM420 you get a shorter drivetrain so you get a longer rear driveshaft (a plus in a swb vehicle). It's bullet proof so you don't have to worry about breaking it and with the proper gearing you can also avoid the three foot shuffle but your on road ability will suffer because of the lower gearing as well.
I believe that the SM420 is starting to get harder to find. Parts are harder to find aswell but it's still a tough tranny and if you find a good one you shouldn't have to worry about rebuilding it.
So you see - it really boils down more to personal preference than anything. I've got a T18 in my CJ right now that I really love but I've been very tempted to swap in the TH400 that I've got sitting in an old Wagoneer. I'm just trying to decide if it's worth it in the adapter dept.
Scott
With an auto don't have to do the three foot dance on steep stuff. One foot on the brake and one on the gas give you GREAT control.
You get a 2:1 reduction at really low speeds (where we all crawl at) just because of the torque converter. (Don't ask me to explain it - it has something to do with the flow of the fluid at low speeds that doubles the torque of the tranny).
Autos are longer than the equivalent manual tranny. They can be a pain to try and fit into your rig (but this has been done so many times with a CHevy 350 conversion I'm sure it's not much of a problem).
The TH350 is a good stout tranny and there are lots of them around. This means that they are well supported in the aftermarked. There will be plenty of parts available for them and there should be plenty of shops/people who are knowledgable and capable of working on them.
With the SM420 you get a shorter drivetrain so you get a longer rear driveshaft (a plus in a swb vehicle). It's bullet proof so you don't have to worry about breaking it and with the proper gearing you can also avoid the three foot shuffle but your on road ability will suffer because of the lower gearing as well.
I believe that the SM420 is starting to get harder to find. Parts are harder to find aswell but it's still a tough tranny and if you find a good one you shouldn't have to worry about rebuilding it.
So you see - it really boils down more to personal preference than anything. I've got a T18 in my CJ right now that I really love but I've been very tempted to swap in the TH400 that I've got sitting in an old Wagoneer. I'm just trying to decide if it's worth it in the adapter dept.
Scott
I have done some wheeling in both types. I think that I like the auto trannies better. They are not quite as bullet proof as a stout manual, but they can come close. Ultra low gears are more important with a manual, so in some cases an auto can be cheaper to wheel than a manual because you dont have to spend lots of money on lower t-case gearing.
The new adapter that AA is selling for manual fj's is pretty cool. It is a low range gear box that has a 3.44 "I think" reduction. This box adapts the sm 420 to the toy t-case. This looks like a cool way to go. I'm sure its big bucks though. I think this info is correct, I read it in a SEMA report somewhere.
The new adapter that AA is selling for manual fj's is pretty cool. It is a low range gear box that has a 3.44 "I think" reduction. This box adapts the sm 420 to the toy t-case. This looks like a cool way to go. I'm sure its big bucks though. I think this info is correct, I read it in a SEMA report somewhere.
goin4xn
Jeepaholic
- Location
- Eagle Mountain, UT
Originally posted by toyman
Hey thanks Scott for the input...a lot to think about. I'll let you know what I decide when I figure it out.
No problem Toy!
From what I've seen as an ARCA (now RCAA) judge the trend has been to move toward Auto trannys. The Mud boys have been running auto's for years now as a loss in momentum with a gear change (manual clutch) would kill them in the gooey stuff.
Like I've said - I've got a T18 (6.32 1st gear) that I just love but I've been tempted to go to a TH400 just for the convience of an auto transmisson.
With the auto I'd make sure I had a good tranny cooler installed to help keep the fluid happy on the long slow and HOT trail rides. Depending on the auto tranny you may be able to get a deeper fluid pan that (hopefully) won't impact your ground clearance.
Just some more stuff for ya to think about.
Scott
there are about as many tricks of the trade on either side of the debate. like autos you have to pay attention to the stall on the convertor and the cooling as previously mentioned. getting a tranny built to handle the horsies and the constant abuse is important. but i am of the opinion that when you pay attention to details in a transmission, you cant go wrong with either manual or auto!!
Andy
aka. Hollywood
- Location
- Here and there....
Autos rule! I'm a convert from the clutch, and I couldn't be happier. It did come at a price though, two tip overs in one weekend. But worth the price.
Box Rocket
bored
- Location
- Syracuse, Utah
Since I drive a FJ40 I'll chime in and give my opinion. But before I do that I'll preface it with what's been said already, find something that meets your needs, no what someone else says is cool.
My 40 is a '76 and had a 4speed from the factory which is understandable a bit different from your 3speed. I swapped to a SM420 a couple years ago. I also ditched my 4speed t-case and went to an early 3speed style for the slightly lower ratio. I've also got 4.88's. An auto might be nice in a lot of situations but I am TOTALLY happy with my 420. I don't wheel as extreme of stuff as some of the guys on here, but I think I've got the gearing to do if I wanted to. There is definitely a difference in how you drive an auto vs a manual as you know, but I dig the low gears of the 420 on steep downhills and ledges. It's nice to let the gearing ease you down an obstacle without having to give as much braking input.
As for on-road-ability.....I still drive my 40 almost every day. 35 mile commute. It's not fast (I top out around 65mph) but I don't have the truck to be fast. With my setup, I think I'm at about 82:1 crawl ratio. The nice thing about the 420 is that you'll have room to add the AA rocbox if you felt like you needed more gearing and that would put you well over 100:1.
The 420 is way simple and straight forward and you won't have the need for tranny coolers etc and you can use your existing tcase rather than having to source a new one although you'll need an adapter. I suggest the plate style from Danny Warden.
SO, plain and simple, I think its a good move to go with the 420. You'll love it. BTW, you should come out with Wasatch Cruisers sometime.
My 40 is a '76 and had a 4speed from the factory which is understandable a bit different from your 3speed. I swapped to a SM420 a couple years ago. I also ditched my 4speed t-case and went to an early 3speed style for the slightly lower ratio. I've also got 4.88's. An auto might be nice in a lot of situations but I am TOTALLY happy with my 420. I don't wheel as extreme of stuff as some of the guys on here, but I think I've got the gearing to do if I wanted to. There is definitely a difference in how you drive an auto vs a manual as you know, but I dig the low gears of the 420 on steep downhills and ledges. It's nice to let the gearing ease you down an obstacle without having to give as much braking input.
As for on-road-ability.....I still drive my 40 almost every day. 35 mile commute. It's not fast (I top out around 65mph) but I don't have the truck to be fast. With my setup, I think I'm at about 82:1 crawl ratio. The nice thing about the 420 is that you'll have room to add the AA rocbox if you felt like you needed more gearing and that would put you well over 100:1.
The 420 is way simple and straight forward and you won't have the need for tranny coolers etc and you can use your existing tcase rather than having to source a new one although you'll need an adapter. I suggest the plate style from Danny Warden.
SO, plain and simple, I think its a good move to go with the 420. You'll love it. BTW, you should come out with Wasatch Cruisers sometime.
goin4xn
Jeepaholic
- Location
- Eagle Mountain, UT
Wasatch made some good comments and gave some good input. I'm a big fan of run what you've already got so if you've already got a SM420 it's a good move to make. You'll love the low gearing. Auto's are very nice but there are some more issues involved to make them bulletproof. Not saying that it can't be done by any means (and I'm looking to switch over to a TH400) but it's a lot simpler to bolt a gearbox in and go vs. setting up a slushbox.
Scott
Scott
goin4xn
Jeepaholic
- Location
- Eagle Mountain, UT
I think you'll be very happy with the 420 and '300 setup.
Scott
Scott