I agree with the previous post(LOL) But seriously, The 351W Is built similarly to 289&302 engines but has a taller deck height,larger main journals,Windsor dedicated Intake manifold,oil pan,crankshaft and connecting rods . The 351M is built similarly to the 351C also with a taller deck,larger main journals,429&460 bellhousing pattern,dedicated intake,cranks,rods,etc.... The "M" is for modified as the engine is a modified version of the "Cleveland" engine.
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Current projects: 65 Fastback, 68 Coupe Oval Track Racer, 64 Galaxie 500 XL, 54 F-100 Pickup
What they said is right on. The 351M is a basically a 400 block with a 351 crank in it and really long connecting rods. A low performance, torquey motor, in the same family as the 400, 351C and harder to find parts for.
The 351W is a 289/302 with a taller deck height, longer stroke crank and beefier rods, main caps, etc. A much better engine to start with these days because of the tons of 5.0 speed parts out there. From what I understand the weakpoint of the 351 is the big rod journals that aren't designed for high-rpm operation. Still a good motor from all accounts though, cheap and able to make plenty of power.
65 Fastback 289 4spd in storage :-(
91 CRX Si... A.K.A. my go-kart with AC!