AMC 20 Rear

[Joey84cj7]

I have an 84 CJ7 . AMC 258 i6 / 4.2l engine. Im only getting about 65 on the highway and looking to go a little faster. Don't do much offroad. Maybe some light trails right now. This is my daily driver but I need to get it ready to make some long trips and want to get up to 75mph or so. My rear end is a AMC20 3:31 gears. 4 speed Transmission . What are your suggestions to get this ready to drive 500+ miles on highway? 33 inch tires, weber 32/36 carb. I have had vacuum leaks in the riser plates of the carb but think they are tight right now.

 

[Posi]

What is your hurry?
Slow down and enjoy life
Being positive you have no vaccum leaks would be a start.
How much vaccum is your engine pulling?
Timing, tune up?

 

[Lumpy Grits]

Re-gear your R&P is your best choice here.
Your tranny will live much longer too.
LG

 

[Posi]

The only place you can go with an axle gear change is 2.73 which I wouldn't recommend with 33" tires. There will be others that say they run 2.73 with large tires, to each their own.

First, make sure your engine is running at it's best.
If your concern is with the current 500 trip planned swap/borrow smaller tires.
Swap to an Overdrive Transmission .
Things I would do before an axle re-gear.

 

[Lumpy Grits]

The only place you can go with an axle gear change is 2.73 which I wouldn't recommend with 33" tires. There will be others that say they run 2.73 with large tires, to each their own.

First, make sure your engine is running at it's best.
If your concern is with the current 500 trip planned swap/borrow smaller tires.
Swap to an Overdrive Transmission .
Things I would do before an axle re-gear.

Different carriers, and you can go to 4:88 easy.
LG

 

[Neuner]

I had the exact same setup tuned really well and yes, 65 was about the max. These vehicles were designed at a time when max speed was 55.

I now have a 4.0 and 70-75 is easy but I choose to do only 60-65mph unless I'm holding up traffic. Ive got time management skills and cool enough to take it easy. They're not built to go fast, they're made to go over anything. Posi is right, slow the f down.

 

[Lumpy Grits]

Mine will run 85+ with ease. with 120K+ miles on it and carb'd.
Will cruise all day, at 65-70mph.
Torque curve vs. gear ratio vs. power band.
If you can't get over 65mph-you got other issues going on.

LG

 

[Kane]

Your distributer is designed for the computer controlled timing. This mean that it has very little mechanical advance. When that computer fails which most are at this point, you lose the timing advance and also alot of power. This is why most people think their engine sucks so bad. You can replace the dizzy with an aftermarket model to get full advance or you can mod yours to get this result. This will wake up your motor and give you plenty of power for highway speeds. My cj with 3.55 gears and 33 in tire with a T-176 would do 90 mph. I drove it from Utah to Connecticut at 75 mph the whole way with no issues.
Check your full advance with a timing light and I bet you will find out you don't have much.

 

[hdgapeach]

Don't know what original original tire diameter is for your CJ without checking, but if you haven't re-calibrated the speedometer for the larger tires you aren't reading speed accurately anyhow.

Just a thought .

 

[Posi]

^^^Good point
If he didn't recalibrate the tcase and the Jeep showed 65 on the speedo he would actually be traveling at 79mph compared to the base 205s. If it came with the optional 215's and the speedo read 65 he would traveling at 77mph.

Mine will run 85+ with ease. with 120K+ miles on it and carb'd.
LG

I wouldn't call 4223+RPM with ease, but that's just me.

 

[Lumpy Grits]

^^^Good point
If he didn't recalibrate the tcase and the Jeep showed 65 on the speedo he would actually be traveling at 79mph compared to the base 205s. If it came with the optional 215's and the speedo read 65 he would traveling at 77mph.


I wouldn't call 4223+RPM with ease, but that's just me.

I'm no-where near that rpm, with 33's and a 5 speed.
Come on out, and I'll take you for a ride to prove it-



Jeep CJ oem dia tires were 28-29".

LG

 

[Posi]

I'm no-where near that rpm, with 33's and a 5 speed.
Come on out, and I'll take you for a ride to prove it-



Jeep CJ oem dia tires were 28-29".

LG

33" T-5 4.88 if not 4 grand what is your tach reading at 85mph?
I'll only come out if there is beers involved after the ride.

Splitting hairs but 225, 235

 

[Lumpy Grits]

33" T-5 4.88 if not 4 grand what is your tach reading at 85mph?
I'll only come out if there is beers involved after the ride.

Splitting hairs but 225, 235

Don't recall the exact rpm now(had my hands full). BUT, it wasn't no 4K+ rpm. More like 3100-3300 IIRC.
Remember, that my 5th gear is a .86:1.
Hope you had a good 4th
LG

 

[Posi]

Yes thanks, hope you did as well.

 

[Torxhead]

Thanks for posting that number Lumpy, I do think that is still a bit on the high side for that engine though.

 

[Lumpy Grits]

I've been run'n 4:88s for over 25 yrs-LOV'em with my 5 speed.
Don't even use 1st gear on the street...........
The AMC 258 i6 / 4.2l is redlined at 4500rpm.
LG