Custom "FJ40" build Advice

EWheeler

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My race car has a 6L80 and a Np205 (1.96:1) and 5:38's. It does good at what it was built for (KOH), but sucks trail riding and technical stuff. It is not slow enough. I would think a 4:1 case would be better but if you have the room I would do a 4spd atlas all day long. I had a Black box-205 combo in my last rig. Never used the black box racing but did enjoy it trail riding on tech stuff.
Hmmm, that's interesting. Not accounting for the torque converter, we essentially have identical final drive ratios. You are 4.0 (1st in 6l80) x 2 (np205) x 5.38 = 43.0 I am 2.5 (1st in th350) x 3.8 (Atlas II) x 4.56 = 43.2.

If you had a 4:1 t-case or a 2:1 doubler, you would be 86.0:1 final drive ratio and with an automatic that seems very very low? I think the rule of thumb is the torque converter provides another 2:1 reduction, so almost a 172:1 effective final drive ratio? My other 40 has an H42/rock box/4spd tcase/4.10 diff drivetrain and that is right at 100:1 in low low. Unless you're building a moon buggy on stickies, I cannot see the utility of something that is geared much lower than 100:1, especially with an automatic?
 

GLTHFJ60

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Hmmm, that's interesting. Not accounting for the torque converter, we essentially have identical final drive ratios. You are 4.0 (1st in 6l80) x 2 (np205) x 5.38 = 43.0 I am 2.5 (1st in th350) x 3.8 (Atlas II) x 4.56 = 43.2.

If you had a 4:1 t-case or a 2:1 doubler, you would be 86.0:1 final drive ratio and with an automatic that seems very very low? I think the rule of thumb is the torque converter provides another 2:1 reduction, so almost a 172:1 effective final drive ratio? My other 40 has an H42/rock box/4spd tcase/4.10 diff drivetrain and that is right at 100:1 in low low. Unless you're building a moon buggy on stickies, I cannot see the utility of something that is geared much lower than 100:1, especially with an automatic?

Yoru numbers are close enough, but I disagree on the conclusion. My truck is about 150:1 or so and it's juuuust enough gearing with my diesel, because I can idle down way below normal idle speed to chug along.

Many mini trucks are 200:1 or more with doubler. You really need it with smaller engine displacements, or lower power levels.

100:1 with an auto sounds awesome to me.
 

EWheeler

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Yoru numbers are close enough, but I disagree on the conclusion. My truck is about 150:1 or so and it's juuuust enough gearing with my diesel, because I can idle down way below normal idle speed to chug along.

Many mini trucks are 200:1 or more with doubler. You really need it with smaller engine displacements, or lower power levels.

100:1 with an auto sounds awesome to me.
Is that calculation to get to 100:1 taking into account the reduction from the torque converter or ignoring it? Appreciate your input!
 

GLTHFJ60

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Is that calculation to get to 100:1 taking into account the reduction from the torque converter or ignoring it? Appreciate your input!

I'm not making any calcs, you did. You said you don't see the value of anything lower than 100:1, and I disagree with that conclusion is all.

100:1 sounds fine, but lower would be better. But no, that 100:1 I'm saying would be fine does not include the torque converter "2:1".


Fwiw, I usually use my single low at 1.96:1 bouncing around between obstacles on the trail, then double low on obstacles.

Edit for data and calcs with my truck:
Nv4500 is 5.61
Np241 doubler is 2.72
Np205 is 1.96
Diffs are 3.73

That calcs out to 112:1 rounding. f***, not as low as I thought. If it was any steeper, or if I had a different motor, it wouldn't work well in the rocks. Motor is a 4bt for reference.
 
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Hmmm, that's interesting. Not accounting for the torque converter, we essentially have identical final drive ratios. You are 4.0 (1st in 6l80) x 2 (np205) x 5.38 = 43.0 I am 2.5 (1st in th350) x 3.8 (Atlas II) x 4.56 = 43.2.

If you had a 4:1 t-case or a 2:1 doubler, you would be 86.0:1 final drive ratio and with an automatic that seems very very low? I think the rule of thumb is the torque converter provides another 2:1 reduction, so almost a 172:1 effective final drive ratio? My other 40 has an H42/rock box/4spd tcase/4.10 diff drivetrain and that is right at 100:1 in low low. Unless you're building a moon buggy on stickies, I cannot see the utility of something that is geared much lower than 100:1, especially with an automatic?
I think the biggest issue is most of the people I trail ride with have real low gearing. Last time I had it out, I would wait back about 50 yards and then catch them quickly. They wheel too slow. I am at 80:1 in my small tire FJ40 and out pace the guys I wheel with too! I always seem to end up leading and have to sit back and wait for the rest to "show up" haha. I like to cover a lot of ground when I wheel. It does rock crawling at the Hammers but we are racing when we do that too. I have hit 78 mph in low range and still had 6th gear to go!
 
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GLTHFJ60

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I think the biggest issue is most of the people I trail ride with have real low gearing. Last time I had it out, I would wait back about 50 yards and then catch them quickly. They wheel too slow. I am at 80:1 in my small tire FJ40 and out pace the guys I wheel with too! I always seem to end up leading and have to sit back and wait for the rest to "show up" haha. I like to cover a lot of ground when I wheel. It does rock crawling at the Hammers but we are racing when we do that too. I have hit 78 mph in low range and still had 6th gear to go!

For a data point, what is your crawl ratio not counting the torque converter? I recall you telling me it was way too fast for crawling, but it would be good to have that number for comparison.

On an unrelated note, next time I see you with that truck, I might con you into giving me a ride :hillbilly:
 
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For a data point, what is your crawl ratio not counting the torque converter? I recall you telling me it was way too fast for crawling, but it would be good to have that number for comparison.

On an unrelated note, next time I see you with that truck, I might con you into giving me a ride :hillbilly:
It is about 42:1 running 37' and/or 40's.
 
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I'm at 120:1 in low low, manual, ls1, on 38 and it is great, could be a smidge lower at times but most trail/tech stuff the 120:1 is ok.
I will say that my friends who go with and have ls with autos, 40's, atlas at 3.8 tell me they are just on the brakes all day following me and they arent at 100:1 so IMO an auto at 100:1 might be good for not riding brakes all day.
 
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I'm at 120:1 in low low, manual, ls1, on 38 and it is great, could be a smidge lower at times but most trail/tech stuff the 120:1 is ok.
I will say that my friends who go with and have ls with autos, 40's, atlas at 3.8 tell me they are just on the brakes all day following me and they arent at 100:1 so IMO an auto at 100:1 might be good for not riding brakes all day.
That is the way I am, on the brakes all day long... and they are non-power! sucks!
 

EWheeler

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That is the way I am, on the brakes all day long... and they are non-power! sucks!
Wouldn't a lower crawl ratio just make this problem worse? More torque multiplication in the drivetrain would require more clamping force from the brakes to resist that torque. If you wanted to decrease the amount that you are on the brakes, wouldn't that be solved through the torque converter and the appropriate stall speed? I have read that too low of a stall speed in the converter with an auto and you are driving through the brakes at idle in low gear. I'm just posting this all as a discussion, not criticism. I am always wanting to learn more about setups and how rigs work.
 
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Wouldn't a lower crawl ratio just make this problem worse? More torque multiplication in the drivetrain would require more clamping force from the brakes to resist that torque. If you wanted to decrease the amount that you are on the brakes, wouldn't that be solved through the torque converter and the appropriate stall speed? I have read that too low of a stall speed in the converter with an auto and you are driving through the brakes at idle in low gear. I'm just posting this all as a discussion, not criticism. I am always wanting to learn more about setups and how rigs work.
No worries, I did install a higher stall, about 500 over stock but the speed itself is fast and when going down hill you can not lug. I even have Wilwood 6 piston calipers and 14" rotors. You are right a higher stall should help the driving through the brakes issue. I would rather have a lower crawl speed for downhill so I am not riding the brakes soo much. Uphill is not so much the problem, the HP I am not willing to give up!
 
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Its a balance, higher stall for lower brake input gets you on the climbs & ledges, you gotta goose it a bit then hold it back from launching, makes for a really on off type style whereas the lower overall gear and low stall overall is a smoother day but in the end I think there is alot of braking with the ls, auto, atlas, big tire set up, play with torque converters and see where the balance is for you. I have heard many KOH drivers say that when the race car is competetive it is not driveable on the trail, just too much and a different set up overall.
 

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I was 150:1 doubled with NV4500, toybox, and split case, 5.29s, and it was really nice. I'm now around 180:1 (with torque converter slip) with 8l90 (4.56 1st), and an atlas 3.8 and 5.29s.

I haven't gotten a chance to run the 8l90 yet, but I'm excited to try it.

As long as you have a full manual mode to hold gears for downhill, I would run the 3.8:1 atlas all day.
 

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I think the biggest issue is most of the people I trail ride with have real low gearing. Last time I had it out, I would wait back about 50 yards and then catch them quickly. They wheel too slow. I am at 80:1 in my small tire FJ40 and out pace the guys I wheel with too! I always seem to end up leading and have to sit back and wait for the rest to "show up" haha. I like to cover a lot of ground when I wheel. It does rock crawling at the Hammers but we are racing when we do that too. I have hit 78 mph in low range and still had 6th gear to go!

I always like to go fast when wheeling. I'm used to dirt biking and really covering ground, and I can't stand first gear crawling around unless it is really in the rocks. I guess it helps to have a comfy suspension too 🙃
 

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I was 150:1 doubled with NV4500, toybox, and split case, 5.29s, and it was really nice. I'm now around 180:1 (with torque converter slip) with 8l90 (4.56 1st), and an atlas 3.8 and 5.29s.

I haven't gotten a chance to run the 8l90 yet, but I'm excited to try it.

As long as you have a full manual mode to hold gears for downhill, I would run the 3.8:1 atlas all day.
Does an atlas mate up to the 8l90?
 

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Does an atlas mate up to the 8l90?

Yep. 32 spline GM mate. Needs to be a 4x4 case, but you can convert a 2wd to 4wd pretty easily.
 

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Yep. 32 spline GM mate. Needs to be a 4x4 case, but you can convert a 2wd to 4wd pretty easily.
Right on, same bell housing as the predecessors? 4l80e/6l80e? Going to make a sweet setup
 

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Right on, same bell housing as the predecessors? 4l80e/6l80e? Going to make a sweet setup

I believe the bellhousings are a bit different, mine has around 14 bolts iirc. Cases are wildly different though, and there is no standalone controllers for it yet, so it needs to be mated with a engine that knows how to run it.
 
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I am going to get drug over the coals for this, but for your desired finished product, I think a Jeep is a better starting point. Buy a 2 or 4 door JK depending on your space needs. Then go to MotoBilt and spend about $10k-15k on link subframes, axle brackets, armor, steering and other neat stuff. This stuff is literally like an erector set to build a highly capable trail rig. Pretty cool the products they develop, I know they aren't the only ones doing this kind of thing, but I like their product.



Get rid of the front clip of the jeep and graft an FJ40 front end onto it, like this:



I'd also spend $25k on 74 weld portals and run nearly zero lift and have a ton of axle ground clearance:


Lots of resources for dropping the LS drivetrain with an atlas right into a Jeep frame, so all you have to do is buy the parts and drop them in. Harness integration is no problem and you don't have to build stuff from scratch.

Jeep - $20k
Fabrication items from Motobilt - $10-15k
Built 1 ton Axles - $10-15k
Portals - $25k
Coilover Shocks - $5k
Drivetrain (engine, trans, atlas, harness, plumbing, etc) - $15k
Other stuff - $10k

Build Cost - $105k

Even if you do start with a $5k FJ40, add in the cost of the Aqualu tub and you're still spending close to $100k.
no comment needed. I’m subscribing, @Chris Carrera . I will have to block @EWheeler for a bit 😀. I didn’t know that there were options alternative to the Land Cruiser
 
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This guy knows his stuff. He designed the 6 spds for GM. He does my tuning and helped supply the parts to beef up my 6L80. Other racers use him as well.
 

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