Alright, I bought this 81 with 74K on it, currently as it stands needs suspension bushings (Assumed to be worn due to age; technicians working on it say the parts themselves are solid and tight, though), oil pan gasket, valve cover gasket, rear main seals, and some exhaust work so the muffler isn't heating the gas tank. Solid body, mounts, the panels have minimal rust, surprisingly unusual for having been Ziebart'ed when new...
But my real dilemma is this; I want to increase the power output on the motor, bhp wise, since the 4.2L already has plenty of torque. I know putting a different camshaft, valves, and lifters will give it that extra power... However I don't know what else would need done to support such an in-depth modification.
Any help on this would be great, thanks. :)
Depends how much of a power increase you want. For street use I would buy a header, carb and intake, cam, lifters and valve springs, in that order. Upgrade theignition if you can afford it. More (higher numerically) gear in the differential will work wonders! I'd suggest something in the mid 3's. 3.55, etc. Keep the shift point under 6000 rpm. I drag race a 72 Gremlin with a 401, so if you need any more AMC help, let me know.
The AMC six is a great engine, but does lack some HP. First step is to get a good cam. I don't recommend one of the Comp Xtreme cams because of the high lift, but one of their other cams is fine. I wouldn't put high pressure springs in as that is the main cause of premature cam wear. A good torque cam that works up to 5000 rpm (1800-5000 is a good range) is best for a good street cam. I don't recommend headers on a street engine because they tend to leak, but put a good 2" single exhaust all the way back with a generic turbo muffler. Something like a Flowmaster will likely be too loud, but it's up to you. No need to go bigger or true duals.
If the head needs work ditch it! Get a 4.0L head, valve cover, and exhaust header from a local pull-a-part yard. That head is much better than the 258 head and solves the header and leaky valve cover issues (80-86 plastic valve covers leak!). The head will bolt right on using the 258 or 4.0L push rods. The 258 carb intake will bolt on, just file the bottoms of the two intake supports to sit on TOP of the dowels instead of putting the dowels in the holes. The top of the intake should be level with the top of the intake ports (outside of intake and ports). So file a little, test fit, then file until level. You will need to make some larger washers to hold the intake and exhaust on -- that's the only tricky part. I use 1/8" thick x 1/2" wide bar stock drilled for the bolts then filed/ground to fit.
The cam and exhaust should give 20-25 hp. The 4.0L head will add about another 12-15 hp, but only with the cam and exhaust. The head alone with stock cam and exhaust system (but you must use the 4.0L header) will give about 10 hp. The head flows a lot better than the 258 head, but without parts to complement the added flow it won't do much.
You can use a better carb -- the Autolite 2100/2150 is a popular swap. Another popular swap is a Weber progressive 2V carb (preferred). There are also four barrel intakes available. The Offenhauser Dual Port is one of the best for street use with a Holley 390 or 470 Truck Avenger carb. Limit size to no more than 500 cfm. Use Google to find "Jeep 4.2L carb swap" for suggestions.
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