Re: [AMC-list] 300 6cyl

[Frank Swygert <farna@xxxxxxx>]

Best way to do this is get a 4.0L block and head then use a 258 crank and rods. If he's using a carb stick with stock replacement 4.0L pistons and get roughly 9.7:1 compression. If keeping the EFI he will either need to use a cam that bleeds off a little cylinder pressure, a lot of machine work + custom pistons to very closely control quench, or dish stock replacement pistons (only a couple brands are thick enough -- the dish depth needs to be increased by 0.06"). The EFI doesn't like more than 9:1 compression, stock is 8.7:1. If he keeps the 9.7:1 compression with the EFI and stock or aftermarket EFI cam he'll have to run premium fuel all the time. Cheapest way is to use a cam with a bit of overlap (no extra cost if you were replacing the cam anyway). Second is to dish the pistons (should be $20-25 each, $120-$150). Most expensive is the machining to control quench and retain high compression -- deck block, clean up chamber, custom pistons... around $1200-$1500 total over th
e rebuild. You gain about 20-25 hp over the other two options though. Not worth it to me, might be to him.

I presented ALL the popular options. Of course there are some "inbetweens" -- like custom pistons instead of the dished ones. The main point with the EFI is that it is prgrammed for stock compression of 8.7:1. It won't take much variance from that, but 9:1 is safe -- I wouldn't go over and expect to run regular fuel. When going with custom pistons the slightly longer 4.0L rods are usually retained, so you only need a crank kit, not rods. 258 core motors are pretty easy to find though.

I've been running a "budget" stroker (258 crank and rods, replacement 4.0L pistons) since 2000. Reliability isn't an issue. I built mine before the compression issue was known. Running regular resulted in pinging under moderate loads (pulling up hills, accelerating) without VERY careful driving (downshift more than usual). Mid grade only pinged under relatively heavy loads, premium only under VERY heavy loads (fully loaded station wagon or pulling a light trailer). When I rebuilt the car in 2003 due to a careless driver totaling my baby (rebuilt via "body replacement"!) I took the opportunity to dish the pistons. Now I can run on regular with no power loss even though losing a full pint of compression. I have a Renix computer with knock sensor which was keeping the timing retarded as much as it could. Note that an HO computer wouldn't have been able to run on regular due to no knock sensor -- move everything up a notch! Regular would run like mid, mid like premium -- would ha
ve to use octane booster with premium to get ping to only heaviest of loads.

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Date: Fri, 9 Jul 2010 22:02:11 -0500
From: "Michael C Canada"<gcanada@xxxxxxxxx>

Hi guys I have a guy locally that is obsessed about getting a 300 CID 6 that sapposidly was used in Mexican AMC cars, for his Jeep. Does this motor realy exist and is it worth getting and does any body have one or know how to get one?

--
Frank Swygert
Publisher, "American Motors Cars"
Magazine (AMC)
For all AMC enthusiasts
http://www.amc-mag.com
(free download available!)


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