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Th350 Vs Sm420

Discussion in 'Intermediate CJ-5/6/7/8' started by Steve Ryan, Nov 9, 2021.

  1. Nov 9, 2021
    Steve Ryan

    Steve Ryan New Member

    Foresthill,ca
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    Jan 22, 2021
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    10
    Hey guys ☕️
    Which of these transmissions would give
    a lower crawl speed ( d20 and 304) ?
    Im leaning towards the auto due to bum knees.
    However if its a major difference I could
    man up i suppose.
    The sm465? Is also an option.
     
  2. Nov 9, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    The Novak page will show you the ratios. They also have a crawl ratio calculator that will do the arithmetic for you.

    IMO it's not a valid comparison. The automatic is a different style of driving than the manual. With the manual, you keep your foot off the clutch and use the throttle to creep over obstacles. With an automatic, it's more of two-footed style where you control forward progress with the brake and keep the engine speed constant. Different. You don't need such low gearing. You're not going to stall if you stop entirely.

    This makes a lot of heat in the transmission; extra cooling helps. You also need good brakes.

    Have you driven a truck 4-speed much? It can be work in stop-and-go traffic. They are slow, heavy shifters.

    The TH350 is a good option. They can be built strong. IMO I would skip the Muncie transmissions in a 304 CJ and run a T-18, T-19 or NP435. You can find T-18s from Jeeps that are already adapted to your transfer case. Lots has been written here about this. site:earlycj5.com truck 4-speed swap - Search
     
    Last edited: Nov 9, 2021
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  3. Nov 9, 2021
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
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    If you have bum knees, go auto. Comparing numbers on paper between manual and auto isn’t really fair. Anything you come up with for the auto, you can pretty much double it due to the torque converter.

    If you’re just starting to source all your parts, a Dana 300 coupled to the TH350 (rather than the stock Dana 20) will gain you an extra 0.6:1 of low range gearing.
     
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  4. Nov 9, 2021
    RATTYFLATTY

    RATTYFLATTY I think you need a little more throttle

    Central MN
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    Oct 23, 2007
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    I assume you are keeping your 304. If so putting an auto in a 5 is always a challenge but doable, I ran a V8/auto in a flatty for years. With that being said have you explored using the TF999/Dana 300 combo out of the 80-86 CJ's? They have the lowest first gear, (2.74 vs 2.52 in a th350) of any non-overdrive auto and the 300 is .62 lower in low range.
     
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  5. Nov 9, 2021
    Boyink

    Boyink Super Moderator Staff Member

    Tulsa, OK
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    Sep 20, 2002
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    6,193
    Agreed. Crawl ratio means nothing if you don't enjoy driving the rig.

    My TJ is the first automatic Jeep I've had and I really like it. Simplifies offroading - two pedals, two feet.
     
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  6. Nov 9, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    Just a comment - the 999/300 combo only appeared in the CJ-7 and CJ-8(?) - Jeep considered it too long for the CJ-5.

    If you are really concerned about length, how about dropping the V8 and going with a 4.3L TH350 combo, and moving things forward? You'd save the ca $600 for the TH350 to AMC adapter. I was going to suggest a SBC earlier... there should be room, and you'd gain a modern MPI engine without needing an engine to transmission adapter. Lots of donors out there.
     
  7. Nov 9, 2021
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
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    I’m a hardcore manual guy. And I’m looking forward to my 3b being an auto. :shrug:
     
  8. Nov 9, 2021
    Lockman

    Lockman OK.....Now I Get It . 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Silver Hill,...
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    The Buick 231 mates up to a TH350 just fine in a 5. Build it & have some fun wheelin' it . What do you plan on doin'n for trails?
     
  9. Nov 9, 2021
    Lockman

    Lockman OK.....Now I Get It . 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Love my Auto in my 5 . Great on any of the trails I do & the Daily . I , however , don't do the Rubicon every week, so crawl ratio isn't a concern.
     
  10. Nov 9, 2021
    Lockman

    Lockman OK.....Now I Get It . 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Silver Hill,...
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    Here's the biggest problem with running an Auto & a D20 T case . I've got to modify my tranny pan somehow?
    Front Shaft hits Pan.jpg Front shaft hits pan.2.jpg
    Front shaft hits pan.3.JPG
     
  11. Nov 9, 2021
    johneyboy03

    johneyboy03 The green beast

    Quebec, Canada
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    Yes the pan can be modified. You can gain about1" of side clearance. I try to take a picture of mine tonight.
     
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  12. Nov 9, 2021
    johneyboy03

    johneyboy03 The green beast

    Quebec, Canada
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    I went to a 360/t18/dana 20 (t18 sm465 are similar in gearing) to a 5.3 th350 and dana 20 during the last year. i got a good low range torque converter and i'm mising some gear when trying to crawl. I was 100% about manual transmission but after a complete season of wheeling i wont come back to a manual.

    You better check with herm the overdrive guy to get a 3.15 kit for the dana 20 it's the easiest way to go. Other than that you can go with a dana 300 but you will add about 2" to the lenght of transmission and transfert case, and re gear the axle.
     
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  13. Nov 9, 2021
    Lockman

    Lockman OK.....Now I Get It . 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    I'd appreciate that.....johneyboy :) ( + any plan of attack or instruction would be nice ! )
     
  14. Nov 9, 2021
    Steve Ryan

    Steve Ryan New Member

    Foresthill,ca
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    My goal is Rubicon and Fordyce before im dead
    (He says while looking around for the grim reaper lol )
    Especially since i live on top of the next divide from Rubicon
     
  15. Nov 9, 2021
    Steve Ryan

    Steve Ryan New Member

    Foresthill,ca
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    I think i have a plan just dont tell the wife hehe
    Ill get some tera low gears from Herm , a TH350 from Novak. Now all i gotta do is pay for it. Lol

    Thank guys
    Great responses
     
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  16. Nov 10, 2021
    johneyboy03

    johneyboy03 The green beast

    Quebec, Canada
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    i just hope mine's wont find this forum....ahahah
     
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  17. Nov 10, 2021
    dnb71R2

    dnb71R2 SuperDave 2023 Sponsor

    Grand Mesa, CO
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    What axle ratio do you have? With an automatic, you may not need the Tera-low kit. ;)
    Remember, a torque converter gives you an approximate 2:1 reduction at low speed.
     
  18. Nov 10, 2021
    fesser

    fesser Napa, CA 2024 Sponsor 2022 Sponsor

    Napa, CA
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    I can't see any way the answer isn't the SM420 to your initial question. As for which one you will like to drive better, I am siding with the pro-auto crowd. The main reason is your knees. They will thank you.
     
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  19. Nov 10, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    The SM420 has the lowest first gear of the truck 4-speeds, at 7.0:1. The wide ratio T-18 is 6.32:1, which is about 10% off from the SM420. JMO - the practical advantages of the T-18 outweigh the modestly better crawl gear of the SM420.

    Any of the wide ratio truck 4-speeds will basically double your crawl ratio. All of them are tough enough for use in medium sized trucks like dump trucks and buses. Pointless to compare strength between them for use in a Jeep. The Ford transmissions will bolt up to a Jeep factory AMC bellhousing for a T-150 or T-176. They are not huge like the SM465. This has been discussed a lot if you read old threads. If you are serious about a truck 4-speed, take some time to be informed.
     
    Last edited: Nov 10, 2021
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  20. Nov 10, 2021
    Lockman

    Lockman OK.....Now I Get It . 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Silver Hill,...
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    Autos are the way to go....... We can't afford the gas to run our rigs any way . :steamed:
    ( or........ At least, I can't )
     
    Last edited: Nov 10, 2021
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