Since this comes up so often, I am going to post this so hopefully GregZ or one of the other moderators will put it in the "BEST OF" or the tech archives.
Some of the avantages of the magnum heads are: 8mm valve stems (lighter than the LA's 3/8 stem) "swirl port" technology in the intake runner, 63cc closed chamber, great port velocity, improved exhaust runner, stock 1.6 ratio rockers, ability to support 400HP out of the box.
to put magnum heads heads on a pre-magnum small block, here's what you need:
oil through lifters
different head bolts
magnum rocker gear
different intake (potentially)*
people say you can use the LA valve covers, some have had
problems getting them not to leak. I don't have firsthand experience on this....maybe I'll stick some LA covers on my ram and try it out...
now a little discussion about the components in order I listed them:
any prices I mention are from www.mopartsracing.com
a rough estimate of total cost, since that's an easy place on line to find part numbers and pricing. any MP dealer should be able to get you these MP parts.
you can custom measure and get custom length ones, or MP offers these:
Special hydraulic pushrods
Designed to allow the use of magnum heads and
standard magnum rockers on standard 1967-91
"A" short block/engines.
MP Part number: 5007477
Magnum head on standard "A" block-set of 16 7.625"
if you're running a lot of lift (small base circle on the cam) with standard non-adjustible rockers, or adjustible rockers (dependign on how much lift), you may have to get custom length ones. my guess is you might be able to find some off the shelf ford/chevy parts to work.
they need provisions for pushrod oiling. if you have stock lifters (stock cam) they probably don't. if you have an aftermarket cam, you probably do. the way to check: find out your lifter manufacturer (i.e. are they summit lifters? comp? crane? etc) and see if the part number for a mopar is the same part number for an AMC motor. if they are, then you have pushrod oiling capable lifters. most do. I know for sure comp & summit do.
the bolts are different length than old LA ones. luckily they're cheap:
MP part number: 4876759 Magnum engine $15.95
any '92-up 318 or '93 up 360 will have them. you might have issues finding cracked heads, or you might not find junkyards to sell you heads, they may want to sell the whole motor. if you get them from a Junkyard, make sure you get the rockers and valvecovers. if you can't fund them in a yard, what are you to do? well, mopar performance sells them
as brand new castings:
MP PN: 4876256 $139.95 each (1.92/1.625 production castings)
or you can purchase R/T heads, which feature stock port locations, and a W2 style bowl, and improved exhaust port. arguably the best iron small block head you can find.
MP PN 5007140 $375/ea 1.92/1.625 valve
MP PN 5007140 $435/ea 2.02/1.625 valve
OR if you want, you can purchase assembled heads. I've heard that the assembled production heads have the crate motor valvesprings. hopefully someone can confirm. these should be good for cams up to .51" lift. CAVEAT: magnum heads start running into seal to retainer clearance at valve lifts much above .525 lift, so if you want to go higher you'll have to machine down the valve guide for clearance.
assembled magnum head:
MP PN 5007086 $310.50 each
assembled R/T heads:
P70005 $595 (1.92/1.625)
P70005 $650 (2.02/1.625)
stock magnum rocker arms are actually the same rocker arms as the old AMC 290-401 engine series, and are 1.6 ratio instead of the LA 1.5 ratio. to find out the valve lift, take your cam's valve lift and multiply by 1.067 (1.6/1.5)
aftermarket roller rockers are usually the same pieces as SB chevy pieces, and may require different rocker arm studs. some kits may include them. most aftermarket rockers are available in 1.6 or 1.7 ratio
if you're running a moderate hydraulic cam, stock rockers are fine. if you don't have junkyard ones new ones are available from MP:
MAGNUM V8 ROCKER ARM SET
Consists of 2 rocker arms,2 pivots,2attaching
boltsand 1 guide.for use with hydraulic cams
only,production 1.6 design ratio. NOT for
MP PN 4876050 $8.95 (one cyl, need qty 8 )
currently only MP makes intakes
MP PN 5007380 dual plane $239.95
MP PN 5007381 single plane $239.95
MP PN 5007398AB 2BBL MPI W/EGR $335 (92-96 truck replacement)
MP PN 5007852 2BBL MPI W/O EGR $335 (97-02 truck replacement)
Edelbrock is coming out with a magnum Air Gap this year, and Professional Products is coming out with the Crosswind, a copy of the eddie airgap that will bolt to both an LA or a magnum head. (http://www.professional-products.com/sbchrysler8.htm#)
*Also, fellow board member 349magnumRT has come up with a simple yet ingenious jig to redrill your magnum heads for an LA bolt pattern, so any LA intake can be used. PM him for details. He's loaning it out for the cost of shipping to board members.
That's basically the gist of adding Magnum heads to an LA motor. for a 318, the 1.92 valve is about the perfect size. any larger (to say 2.02's) will physically fit, but the shrouding caused by the smaller bore on the larger valve will negate any flow increase. besides, ported, the stock valve will flow over 260 CFM, enough to support 500+ HP. look in the Best Of section for some of ryan J's flow results from magnum heads.
I'd like to thank the board members who have over the years contributed information, and to Mopar Muscle, car craft, and Steve Dulcich for the articles they've written/published on the matter who I have gained some of the info from. This is a great forum with a lot of knowledgable guys, hopefully I've been able to contribute a little to help others out.
Steve D also had a great article in building a non-ported head 318 using magnums here: The Little Mopar that Could
which is a great reference article.
Moderators, please make sure this makes it's way into the best of section so I don't have to type this again