Red Line

[Peanut Butter]

Keep in mind that I'm not planning to go drag racing or any such thing! I'm just wondering what the red line is for the AMC 258 i6 / 4.2l and 4.0 engines. I was going through a gear calculator and got curious. My best guess was 3400 to 3500 rpm, but I'd like to hear from the experts here.

 

[Posi]

No expert here, but I would agree with 3500 on the early stock AMC 258 i6 / 4.2l with the 1 barrel carb and early intake.
The later stock AMC 258 i6 / 4.2l 2 barrel, with improved intake, and the stock 4.0 I wouldn't put any higher then 4500, after that the engine falls flat.

 

[Lumpy Grits]

The 'redline' on my tach, shows 4500 rpm for my AMC 258 i6 / 4.2l /AMC 258 i6 / 4.2l .
These are tractor engines and it's scream'n hard at 3500 rpm........
LG

 

[CJim7]

Redline for the AMC 258 i6 / 4.2l is 4500rpm, as stated.
Redline for the 4.0 is 5200rpm. The PCM wont let it go higher.

 

[Peanut Butter]

Thanks guys. Gives me an idea of what rpms it should run at normal conditions.

 

[Posi]

On the flip side my friend idles on the trail at 400-600rpm, hardly ever touched the skinny pedal, you would be amazed at what he climbs up. He makes everything look easy, just don't get stuck behind him.

 

[Peanut Butter]

On the flip side my friend idles on the trail at 400-600rpm, hardly ever touched the skinny pedal, you would be amazed at what he climbs up. He makes everything look easy, just don't get stuck behind him.

So still learning here when it comes to this stuff , he has very low axle gears and maybe a very low range Transfer Case ? My setup is a GM Turbo 400 , QuadraTrac with Low Range, and 3:54 gears in the axles, and 33x12.5x15 tires. I will be using the jeep mostly on road, dirt roads, jeep trails, and especially sand. I've debated on going to a Dana 300 with an axle change, but I may leave that till I change over to a V8 someday. The current GM Turbo 400 is a Cadillac with an original adapter to the engine. In my extra parts is a Jeep GM Turbo 400 that will be rebuilt and replace the other. That frees up the original GM Turbo 400 for a lot of different type of engines for a future rebuild. :drool: Make sense?

Of course there is the DANA 60 rebuild, before I turn 80?

 

[Lumpy Grits]

Thanks guys. Gives me an idea of what rpms it should run at normal conditions.

FWIW: On the street, I shift around 2500-2600rpm.
LG

 

[Posi]

Yes, 5.13 and atlas. However, my build has 5.13, T-18 /4.1 300 and I will use it for trail and road use (not highway).
The rig I'm currently using has a 400/T-18 5.38 and I use it for trail and road.
Your axle gearing was designed for 27" tires, if it were me, I'd focus on axle gearing first to match the tire size 33" = 4.10 or 4.27.
Side note: before my friend added the 5.13/atlas he had a 300 4.1 with 4.10 gears running 39" tires. He blew up his 300 slowly crawling up a hill. I believe the lack of axle gearing was the problem putting to much stress on the tcase. I believe deeper axle gearing would have removed the stress from the tcase.

 

[Posi]

Oh, this is the Internet, so I know there are folks out there running down the highway at 80mph with 2.73 and 37" tires without any problems.

 

[Peanut Butter]

Thanks Posi. I have 32x10.5x15 on it now (came that way). It's not a Interstate driver, but will curse at 50 to 55 nicely. I worry about too low of gearing for the sand, but that may change (comments). I think I will get it all back together, test it a little, and then decide if I need to invest in a gear change. Most likely like you suggest, I will. Right now that added expense will have to wait. Even if I just crawl to the store and hold up all the traffic behind me, I want to drive it again!

 

[Posi]

Sorry I saw this

My setup is a GM Turbo 400 , QuadraTrac with Low Range, and 3:54 gears in the axles, and 33x12.5x15 tires.

Even with 32" with 4.10s in the sand would be good, plus you always have second gear. I do agree changing gear sets is expensive and time consuming and with your 3.54 gears there is no need to rush into it. Enjoy now.

 

[CJim7]

Side note: before my friend added the 5.13/atlas he had a 300 4.1 with 4.10 gears running 39" tires. He blew up his 300 slowly crawling up a hill. I believe the lack of axle gearing was the problem putting to much stress on the tcase. I believe deeper axle gearing would have removed the stress from the tcase.

This is interesting to note. The Dana 300 has a weak point and that is the output. Lowmax and JB Conversions highly recommend changing this out when going to the 4:1. they're known to let go under low gear/high torque conditions. And probably high gear/high torque as well.

 

[Posi]

No problem with the outputs, it was the gears that got chewed up.

 

[CJim7]

No problem with the outputs, it was the gears that got chewed up.

Was it a Terra Low kit by chance?

 

[PistolDave]

Oh, this is the Internet, so I know there are folks out there running down the highway at 80mph with 2.73 and 37" tires without any problems.

Your jeep does not?? I had a -7 set up just like that and I used to pull a 26' boat at that speed no problem. For Real. Honest. Oh yeah, I got 24 mpg too.

 

[Posi]

Was it a Terra Low kit by chance?

Yes, before he installed an atlas.
The tera gears were the first thing we both thought as we knew their not as strong as lowmax, others have had issues with them as well.

After some discussion we felt the final drive, and tire may have played a bigger roll in the breakage.
He was running a 39.5 sticky tire with 4.10s which puts too much stress on the tcase.
We both wheel with another friend that had 39.5 tires, and hardly ever breaks, until he bought 39.5 sticky tires and started breaking stuff.

We all know that when push comes to shove something has to give!
One needs to have a fail safe to prevent expensive component failure.
Willys Motors had a fail safe built into their formula.
For all Jeeps the standard fail safe was realized as tire slippage.
Technically it is referred to as the co-efficient of friction.
Running large and sticky tires a normally low co-efficient of friction is drastically increased causing other components to break.
Too much tire = too much traction.
If the tires are reduced then tire slippage will occur before drivetrain destruction begins.

The M715 is closest to my specs and note the final drive.
It has the 6.398 Transmission with 1.96 tcase and 5.86 final drive mounted on 9.00 x 16 tires (35.5" tall).
This is why I pulled my 4.10s for 5.13s.