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Gear Compounding

Discussion in 'Intermediate CJ-5/6/7/8' started by Jon B., Apr 18, 2022.

  1. Apr 18, 2022
    Jon B.

    Jon B. Member

    SW Minnesota
    Apr 6, 2022
    I’ve read many threads here in the past week or so, and have some confusion that perhaps you old hands can help clear up…

    my T15, D20 and D44 should have a total compound reduction of 3.0 x 2.0 x 3.73: 22.38 to 1.

    I’ve seen numbers like 65, 73, and even a 98! How are these high compounds achieved? The CJ5 is kinda short to be adding gearboxes into the driveline. I’m stumped. Even with a 5.38 axle you get only 3 x 2 x 5.38: 32.28…

  2. Apr 18, 2022
    scott milliner

    scott milliner Master Fabricator

    Seattle Wa.
    Dec 17, 2002
    SM465 with a D18 and Dana 44. 6.55 X 2.46 X 4.27 = 68.80 to 1. It has to do with a Granny Low 1st Gear.
  3. Apr 18, 2022

    Warloch Did you say Flattie??? Staff Member

    Falcon, CO
    Mar 4, 2003
    Its done via the transmission swap for most of us. I have an SM465 (6.55), D18 (2.46), and 4.88s in my axles... Crawl ratio of 78.63

    With 5.38s its 86.68 - change to a Terra low kit and it keeps going.
  4. Apr 18, 2022

    Jw60 Airbender and interrupter of the light guard. 2022 Sponsor 2021 Sponsor 2020 Sponsor

    Sedalia MO.
    Apr 8, 2008
    How about sm420, jpet 5.13 gears in a d18, maybe a klug-v doubler and 5.38's in the axles...
    Oh and a low lugging diesel?
  5. Apr 18, 2022

    Jw60 Airbender and interrupter of the light guard. 2022 Sponsor 2021 Sponsor 2020 Sponsor

    Sedalia MO.
    Apr 8, 2008
    Mine is currently...
    4.3 that can lug down to about 400rpm
    T90J 2.79:1
    Stock D18 2.46:1
    5.38 axles

    My goal:
    Sm465 6.55
    D18 with 3.15
    4.27 axles
    88.1 (easily 111:1 with 5.38s)
    Last edited: Apr 18, 2022
    Fireball likes this.
  6. Apr 18, 2022

    duffer Rodent Power

    Bozeman, MT
    Sep 17, 2009
    As Warloch said, usually done with a granny geared 4 speed transmission. The question you need to ask is whether or not you need those deep ratios. Like always, it comes back to the expected use of the Jeep but even then what resides under the hood for motive power can have a big influence on optimal gearing.
    dnb5853 and Fireball like this.
  7. Apr 19, 2022

    nickmil In mothballs.

    Happy Valley, OR
    Sep 23, 2002

    Too many times people don’t match the ratios to the use of the vehicle but fall into the trap of lower is better. Sand and mud may require more wheel speed where rocks, tight trails, rutted trails, etc. may require lower wheel speed. All depends on the engine torque available, tire size, and terrain traversing. Most times it is a compromise.
    Jw60 likes this.
  8. Apr 19, 2022

    oldtime oldtime

    St. Charles,...
    Aug 14, 2006
    Jon B, There are “basically” those same 3 things that you can change to go slower from where your at.
    Engine rpm and tire diameter also come into
    Every time you change a drivetrain component it has both positive and perhaps negative influences.
    In the end the complete jeep including it’s drivetrain must work as a complete and synchronous system.

    So tell us about your jeep model and engine and how you would like to use your Jeep.

    Most of the standard old Willys jeeps will have a crawl ratio very near 37/1. When driving in large groups it can be beneficial to be in synch to other jeeps.
    Personally I suggest always having a ratio near that and also lower for more technical trail uses.
  9. Apr 19, 2022
    Jon B.

    Jon B. Member

    SW Minnesota
    Apr 6, 2022
    Thanks, all. I’m not likely to go rock crawling, since there aren’t many places for that anywhere nearby. Mud & snow are common here. I may look for an SM60 or a 465 transmission, just for grins. You don’t have to use granny gear with a 3.6 second gear (lower than first in the T15).

    Years ago - as in the mid-‘70s - I owned a ‘67 CJ5 with a Buick 225 V6. I have no idea what the driveline was, except that it had a single-stick trade and overdrive. I don’t recall whether it was a three or four speed transmission, even. It was fun to split shift when running through the gears.

    I do remember that I used low range and second gear for hill climbing. It had short, wide tires, though, so the overall gear reduction wasn’t negatively impacted.

    The engine was in bad shape when I got it, so I rebuilt it and added headers. It pulled pretty hard for a small engine.

    My ‘73 I had later was a 304, three speed and not nearly as much fun. It was better on the road, though…

  10. Apr 19, 2022

    Norcal69 Out of the box thinker 2022 Sponsor 2021 Sponsor 2020 Sponsor

    Northern California
    Sep 6, 2014
    What Duffer and Nickmil said above is what you need to know.
    Driver skill is probably the biggest factor in a jeep's performance when compound gears are needed.
    I have seen jeeps with a 43:1 crawl ratio outperform jeeps with a 90:1 crawl ratio. The drivers ability to see the line while working the throttle, brake, clutch and steering wheel as they traverse the obstacle is key. I don't claim to be a good 4 wheeler but I learn quick and ended up leading our forum group through the Rubicon last year.
    Extreme crawl ratios, lets say more than 80:1 (with 31" or 33" tires), require more traction than lesser crawl ratios. Think of it being similar to starting a vehicle out on ice or snow. It is easy to over torque an off road situation by attempting it in too low of a gear which may lead to wheel spin. Your crawl ratio needs to match your momentum as you cross an obstacle. If you are having to get a "run" at an obstacle to bounce your way through it to prevent the engine from bogging down, you likely would benefit from more crawl ratio. If you slowly creep into an obstacle at high RPM due to very high crawl ratio, and your tires spin, forward momentum is reduced, driver control is greatly reduced and you likely are attempting it with too much crawl ratio.
    I have witnessed both situations and while they are fun to watch and laugh about in camp later, they could just as easily turn your fun day into a nightmare of broken parts or bodily injury.
    When on a trail like the Rubicon you do not need your lowest gear all the time. For a good 75% of the trail I run the transmission second gear and transfer case in 4 low, which happens to be 32:1 crawl in my jeep. When I get to a harder obstacle I shift the transmission into first gear which lowers my crawl ratio to 66:1.
    My drive train consists of a Dauntless V6, T18 with 6.32 first gear, D20 with Terra low 3.15, 3.31 axle gears and 33" tires. I may be cruising the freeway at 70mph for a couple hours on the way to rock crawl all in the same day. You have to choose what combination works best for the majority of your jeep use. The only time that my drive train combination has ever disappointed me was at the Sand Lake Dunes, last summer's ECJ5 forum Tillamook trip. We were running up and down the largest sand dune in the park. SocalNickG and Bigjohn were ripping up and down the dune with ease. I was able to climb the biggest dune but it was harder for me with the 3.31 rear gear. 1st gear was too low and I ran out of RPM before the top, second gear was too high and I bogged down near the top. Eventually I found a combination of like 3rd gear and 4low that allowed me to pull the dune, not as comfortably as I would have liked, but I made it to the top a half dozen times. Would a rear axle gear like 3.73 or 4.10 be better? Yes but my freeway cruising RPM would be much higher and I cruise on the freeway much more than I climb sand dunes.
    You have to find a compromise that works for you.
    Improving your crawl ratio can be done in stages. Minimal jeep modification would be to install a Terra low gear set in your D20. This modification would make zero external changes to your jeep and improve your crawl ratio. The granny 4 speed will be more involved as there is sheet metal work, clutch release work, drive line length differences, cross member changes and possibly more. You may also want to consider a T18 with a 4:1 first gear as they are native to your intermediate Jeep and offer a more driver friendly gear split for mild trail use.
  11. Apr 19, 2022

    FinoCJ 1970 CJ5 Staff Member 2021 Sponsor 2020 Sponsor

    Bozeman, MT
    Jul 18, 2013
    Thanks for saving me a lot of typing....if the OP decides to modify and increase crawl ratio, I'd also suggest starting with the 3.15 low gears for the D20. That alone increases your CR to 35 or so, with no external mods needed (everything bolts back together as is), keeps high range exactly the same as now, and you may very well find it's plenty of crawl for your usage. But...if you know you want 60:1 type crawl, then look into the mods needed for a granny 4speed transmission swap.

    PS...I have survived with only the transfer case low gear mod (albeit with 4.88 gears and OD for highway), but am intending to add the 4:1 T18 to eventually get my CR to 60.
    Norcal69 likes this.
  12. Apr 19, 2022

    oldtime oldtime

    St. Charles,...
    Aug 14, 2006
    I certainly agree that putting Tera Low gears in would definitely be my first choice. No other modifications will be required and you get rid of that very meager 2.01 reduction ratio.
    A 5 gear set of Tera Lows is probably the cheapest fix too.
    You’ll end up with a much improved crawl at 35/1.
    So you can easily run in unison with the Willys jeeps.
    You get to keep your T-15 which is a much better transmission if you get stuck in mud or snow ruts.
    The smooth T-15 shift pattern allows you to much easier rock the Jeep foreword/ reverse.
    Then if it’s still not slow enough, you can looking into other changes like transmission vs. final drive gearing.
    Norcal69 likes this.
  13. Apr 19, 2022

    52M38-73CJ5 Member

    Renton WA
    Nov 11, 2020
    My 73 CJ has
    T18 6.32
    Dana 20 Tera low 3.15
    Dana 44= 4.10
    81.6 to 1

    On trails I run in 3rd gear low range or 21.8-1
    Climbing if it gets rough in 2nd.
    First is just for crawling
    If I did it all again I would keep the 3.73s with the Tera low for running around the streets and have gearing for trails.
  14. Apr 20, 2022

    dvader Member

    Northern Nevada
    Jul 20, 2006
    Yes, Tera Low has a lot of bang for the buck. I have a NP435, with a Tera low Dana 20, and 3.73 diffs. That puts me at 78.5 in 1st, and 39.3 in 2nd in low range.
    Another advantage of the NP435, besides the granny gear, is that it has a lower 2nd gear than I had with my original T15. 3.34 compared to 3.00. This really helps my street driving in high range. I have 32" tires, and starting out in 2nd on the street is now just about right.
    ITLKSEZ likes this.
  15. Apr 21, 2022

    CJ51973 Member

    Versailles, KY
    Nov 6, 2009
    I did this dance in the early 2000s. I had T14, D20, 3.73 which ended up in the 20s crawl. Swap to a T15, did the 3.15 Teralow and continued with the 3.73 got me in the low 30:1s. Later I put in a T18 at 6.32 granny first, low 70s to 1. Ended up crawling around most of the time in second gear. If you need more gear and have the T15 I'd do the Teralow. The drive shafts don't have to change, seamless.
  16. Apr 21, 2022

    Dan240 New Member

    San Diego, Ca
    Dec 29, 2019
    I believe the D20 Teraflex kit is discontinued. At least 4wheelparts believes it is and I have not been able to find it anywhere online.
  17. Apr 21, 2022

    steve1973 Member

    Nolensville, TN
    Jul 27, 2011
    Herm the overdrive guy bought the remaining stock from Tera-flex. I received a set from him last fall. Works great with the swapped in T-18 with the 6.32 first and 3.73 gears
  18. Apr 21, 2022
    Rich M.

    Rich M. Shoe salesman 2022 Sponsor

    Mar 23, 2008
    I'm like 83:1. T18 (6.32 first)2.46 low range and 5:38 gears. I run 2-3 low when group running. The complaining behind you gets annoyingly loud on day 3 so I usually run drag position. It's about as deep as I think I'll ever need for my uses. Low/low is really handy for staying off the brakes on steep downhill descents (again turn the radio up due to the shouting behind you) I have a locking throttle so going up pick a path stuff (like Iron Chest Mine) is really nice without needing the pedals.
    I'm going to be the dissenter. Granny 4 speeds are still relatively economical and generally easy to adapt ($ wise too) and provide a lot of bang for the buck without spending around $1k for a low gear set (assuming you already possess something 4.27-5.38 range diffs)
    Fireball and ITLKSEZ like this.
  19. Apr 21, 2022

    jpflat2a what's that noise?

    Riverside CA
    Jul 30, 2003
    I'm going to be the dissenter. Granny 4 speeds are still relatively economical and generally easy to adapt ($ wise too) and provide a lot of bang for the buck without spending around $1k for a low gear set (assuming you already possess something 4.27-5.38 range diffs)[/QUOTE]

    I spent around 2k for my SM420 conversion when it was all said and done.
    Novak adapter, drive shafts modified, new clutch assy. etc etc.
    That doesn't include modifying the floor for the shift tunnel, reconfiguring the t/case shift levers etc etc.
    Yes I did a lot of the work myself.
    Where is this mythical $1k trans swap your comparing to the cost of low gear set ?
  20. Apr 22, 2022

    Fireball Well-Known Member 2022 Sponsor 2021 Sponsor 2020 Sponsor

    Pullman, WA
    Feb 9, 2018
    Yeah, I'm at least $1400 into my just finished T18 swap. The transmission itself was cheap at $150, but the font/rear adapters, rebuild kit, and driveshafts are not cheap when all added up. Not to mention all the labor moving/fabbing crossframes and rebuilding the floor. I my case I also rebuilt the transfer case and rebuily an original Warn overdrive that needed all the gears replaced. I'm probably $2500 into that whole project.

    If Tera-lows can get you where you want to be with no other mods, there is realistically nothing cheaper.

    That said I like my granny gear four speed and overdrive. I probably should have done Tera-lows while I was at it but had already spent enough money. I'm at a 58:1 crawl gear with 3.73 axles. It's on the marginal side for off roading but I have the advantage of a very torquey Buick 350 that will happily lug down to 400rpm. I was planning on changing axle gears to 4.27 or 4.56, but having driven it a bit now, I think the 350 will be fine with the 3.73s.
    Rubicloak likes this.
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