I have a '78 360 four-barrel engine from a truck, which I have been gathering parts for to build an LA/Magnum hybrid. So far I have purchased the windage tray kit, passenger-car oil-pan kit, high-volume oil pump, Magnum heads, and porting templates, Magnum head bolts and conversion pushrods from Mopar. What I need to know is what compression ratio I will end up with by simply dropping the Magnum heads on the stock short-block and using a .028-inch head gasket.
This motor will go into an M-Body cop car with 2:94 sure grip and 904 with shift kit. I would also like your opinion on cam choice. I would like to retain the stock valvetrain. however, I am looking more for low- and mid-range torque than high-end horsepower. I would also like to limit rpm to 5,500. Small tube 151/48-inch headers, a 211/42-inch exhaust, and dual mufflers will be used. My cam choices would be Mopar's .430/.450-lift cam or Comp Cams' .447/.455 Xtreme Energy. What do you think?PaulVia e-mail
Overall, for a low-buck but effective combo, I think you have a pretty good combination. The Magnum heads have a much smaller combustion chamber at around 60 cc, compared to about 70 cc for the original 360 heads. It's pretty hard to give an exact compression ratio number without measuring everything, since these engines varied some in piston deck height, but for a basic 360 of that vintage, the compression ratio was around 8.2:1. Making the changes you plan to would raise that to around 9.3:1, which is a very useful gain. This alone makes the Magnum heads a worthy swap, but there are other advantages. When comparing stock heads, the Magnums will also flow more, and the Magnum valvetrain will give you 1.6:1 rockers for more lift. Overall, if you are sticking to stock, unmodified pieces, your plan is definitely a good one.
As far as the cams, these cams are pretty close, though the Comp will make a little more power and torque because of the higher lift and faster valve action. You should double-check the retainer-to-guide clearance, but generally the Magnum heads will work fine when using just a little over .500-inch lift, and you will be at a maximum of .485 inch with the Magnum's 1.6:1 ratio rocker arms. Remember, you will need a Magnum-style intake manifold because of the different bolt pattern on the heads. Edelbrock's RPM AirGap is a great manifold for this application. Your M-Body will be an animal on the highways with that gear ratio and this 360 combination.