Re: [AMC-list] T176

[Frank Swygert <farna@xxxxxxx>]

With those 28.9" tires the six is only turning 1746rpm @ 55mph. That's why it feels so lazy (plus a tired motor!). You need to turn at least 2000 to have decent power, and you'd have to be traveling 63mph in fourth gear. That's why you can't pull much in fourth gear -- it's practically overdrive with that tall gearing. My shorter 27.7" tires bring it down to 60mph @ 2000rpm. I don't run in 4th with any kind of a load unless on pretty flat ground. With the 3.31 gears and stock tires you'd be going 52 @ 2000rpm, near perfect! 2500 rpm would be 65 mph, which is a good "high cruise" speed for these trucks. I want 3.31s!! I'd be okay with 3.08s and the shorter tires I have though. Over 2500 rpm and you'll notice a sharp increase in fuel consumption, but it will still be pulling! 

Much over 60 and the power required to push these beasts through the wind goes up dramatically! Anything, really. Something like four times the hp required for lower speeds. 

Oh the six will do it -- I was running between 70 and 75 last weekend on a trip to pick up about 120 pounds of pro exercise bike. Two hours each way (freebie from bro-in-law, who got it from the gym he's a trainer at). Truck gets 15-18 mpg around town and hauling light loads at 55-60, averaged only 10 mpg running hard like that. The load was pushing it through the wind. The surprise was it didn't mind doing it at all! I averaged 17-18 pulling a trailer and with a 700 pound load in the back of the truck (800 pounds on trailer, plus 500-600 pounds OF trailer) on the 10 hour trip down to SC from DE, but didn't go over 60 due to concerns about stopping distance. Found out I had a master cylinder going out -- replaced it! 

The 4.0L has a LOT more torque than a 258. A Cherokee will run circles around my J-10 (which has a 390/4V carb, Clifford intake, good 2" exhaust, 4.0L head) and pull about the same load... easier. I had a Comanche with 4.0L and five speed. 3.08 gears, forget tire size, but probably closer to yours (it had oversize tires). Couldn't pull in fifth, but it pulled a 65 Rambler Classic V-8 on a trailer with no problems. I would have a hard time taking off without using low range in the J-10. The Comanche is a bit more aerodynamic than a J-10, but not much lighter. The later 4.0L torque rating is 230 ft/lb @ 3000 rpm, last 258s were rated 210 @ 1800. From experience, the 4.0L torque curve is as flat as the 258s and starts "coming in" about 1800 rpm. I would be surprised to find the 4.0L torque output under 200 ft/lb @ 1800 rpm. The 258 is peaking where the 4.0L is just starting, but even a 258 pulls better at 2000 rpm than 1800. The 4.0L is hands down a better motor, and feels like 
it has a lot more power than the 258. Best for torque is the 4.0L block/pistons w/258 crank/rods. That's what I'm probably going to build for my truck later. 

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Date: Mon, 06 Jul 2009 14:26:32 -0600
From: Ken Ames <ameskg@xxxxxxxxx>

Mine has the 2.73s also. I have a set of 3.31s from a Wagoneer and that was the
original plan to perk things up but the engine is getting tired already. Seems
kind of backwards to have the 2.72s as stock on 4speeds and the 3.31s as stock
on the automatics. The J120 has the stock 235/75-15s on it now that came with it. 
Didn't plan on rebuilding either engine - just check the bottom end and go.
The 4 litre doesn't have much more torque than the 258 (in new condition).

A carb and exhaust change on the 304 would give about 50% more horsepower than
the 258.

--
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
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