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Heads are horsepower, which heads are we comparing?



a 273's 3.625" bore really limits valve size to 1.78/1.5...you'd have to do a lot of work on a set of 273/318 heads to get them to flow comparable to even a stock magnum head. IIRC 1.88" valves need cylinder notching for clearance, I know 1.92" magnum valves do on a 273....

the 318's bore will fit more valve area, and IMHO the magnum sized valves are about ideal for that bore, as a trade off between curtain area and valve shrouding. a 318's bore will fit 2.05 intakes without notching for clearance.


build the 2 engines identical (cam, compression, heads, intake, etc) and the 318 should make ~16% more power, because of 16% more displacement. that would put the 'teen at about 320-325 HP.

the engine I built for my car, I'm guessing, is making about 320-330 HP conservatively. it's a stock '86 roller short block, pistons .055" in the hole, roller cam reground with bullet's HR259/316 lobe (259 adv, 208 at .050, 127 at .2" lobe lift) for both the intake & exhaust, OEM mag heads ported by me following dulcich's magnum porting article from the may '05 (IIRC) mopar muscle, thin head gaskets (results in 9:1 comp), headers, eddie RPM air gap, and an eddie 600 carb.

I can make as much power with a MUCH smaller cam than the theoretical D-dart 318 for a few reasons:

1) HEADS. low lift flow is a function of the curtain area of the valve. a larger valve should have better low lift flow. peak flow of the magnum heads ported by dulcich was ~235intake/185 exhaust in the .5" lift range. OEM 273 heads ('920 castings) are probably in the 150/120 int/ex with peak flow in the .450" lift range.

2) cam. the newer hydraulic rollers like mine can be pretty aggresive, especially compared to the ramps of the 60's. comparing my roller cam to a "modern" aggressive flat tappet hydraulic, an XE262, while my .050 dur is 10 degrees smaller, my .2" dur is only 3 degrees smaller, and my lobe lift is .008" higher, coupled with the magnum's rockers, I have .506" lift, with only 259 degrees of adv duration.

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