Make My 318 A Magnum
My friend's son has bought his first car: a '73 Dart with a 318 two-barrel. His father is a Pontiac guy and really doesn't know anything about Mopars. I have been asked to help since I am the Mopar guy of the group. So far, I have found him a set of headers and a stock four-barrel cast intake. He has a 600-cfm Holley carburetor to top it off. His best quarter- mile to date is a 15.6 at 86 mph, with a 904 transmission and "spectacular" 2.76 gears in the wimpy 711/44 rear. We are going to replace the rearend with an 811/44 spinning 3.23 or maybe 3.55 gears.
His plans are to rebuild the 318 this winter. I have suggested Magnum heads and the Edelbrock Magnum AirGap intake. My question is, what cam should we consider for the buildup? We're aiming for 9.0:1 or slightly higher compression. I'm assuming the deck will have to be milled to take advantage of the heads. He will probably keep the stock converter for now. I have read in past articles that a single pattern cam is OK with the favorable intake/exhaust flow of the Magnum heads. Also, we're considering some dual-pattern cams. I'm thinking somewhere in the 0.210/0.220-degree duration at 0.050-inch lift range. The Comp Cams I have considered are the high-energy 268H, the Xtreme energy 262H or 268H (although I think the 268 might be a little large), and possibly the original Magnum Muscle grind for the 340, 0.222/0.226 at .050-inch lift.
Any suggestions will be greatly appreciated. His father has a '70 Lemans with a 400 poncho in it. His son would really like to beat him in the quarter-mile with this Dart. I figure we have a considerable weight advantage, but we just need a little more power to hold him at bay.
Brad Churchill
London, Ontario, Canada
Your plan sounds pretty good to me. I would recommend machining the block to achieve a piston deck height of zero. This, combined with a FelPro 0.040-inch thick head gasket will put the piston-to-head clearance at 0.040-inch, which will really help the torque production and detonation tolerance with an active squish/quench. This setup would give a 10 to 10.2:1 compression ratio in the 318, which may seem a little high, but that's where I'd set it. I like either of those Comp XE cams, but the 262 may be a little too low in duration if you go with the higher compression. Don't forget the single pattern Magnum-series cams. The 270 Magnum might be just right.
I would also recommend a little custom head work. Rather than just bolt the heads on, go in and perform a mild bowl blend, smoothing the transition from the machined bottom cut of the valve job into the as-cast port and around the short turn. You don't have to remove a lot of metal, just cut down the peaked ridge. This will only amount to a few hours work and doesn't take a lot of special skill, but it will be worth it with nicely enhanced head flow. With this combo, the Dart should have no trouble handling any stock Pontiac.
Sickening Smoke
I find your tech articles very useful, but I have not seen any addressing the issue I have. I recently built a 371ci (340 block, 4.060-inch bore, Mopar Performance forged crank, 3.58-inch stroke), small-block for my '68 Barracuda. I am running a four-speed. The motor runs great at partial and wide-open throttle, pulling very strongly to 6,800 rpm. My issue is at idle. The motor seems to smoke excessively (equally from both tailpipes).