351 C
Having raced 351 C's on dirt track with both 2 bbl and 4 bbl heads, I would recommend you go with the 2 bbl heads, as they are more tractable for street. The effectiveness of the 4 bbl head doesnt come into play until you are all out of rpm with the 2 bbl head. ( well over 7500 rpm). Save your money and put in some one piece valves in your 2 bbl heads. If you are wanting to increase compression, you can safely mill the 2 bbl head up to .080". The ports in the 4 bbl head dont lend them selves to well to stump pulling torque like you will want for the street. There are many aftermarket 4 bbl intakes available for the 2 bbl head. The one I used in racing was the Weiand Excellerator. I would go with a dual plenum unit, like an Edelbrock Performer, and a 600 CFM carb. That carb will get you down the road in rather fine style and give you ample breathing up to and over 6500 rpm. As for appearance sake, you cant tell the 2 bbl head from the 4 bbl head unless you know what to look for when the engine is all buttoned up. ( A 4 cast into the end of the head near the exhaust port).
A .030" cut off the 2 bbl. head is approximately one full ratio in compression.
OOPS! The reason I mentioned putting one piece valves in your 2 bbl heads is because the factory valves are a 2 piece unit and with a valve spring with serious pressure, they do have a tendency to come apart, making expensive noises and damage to your entire engine. Some minor bowl work and a good 3 angle vavle job will have your 2 bbl heads flowing like niagara falls.
I used to run the 4 bbl headers on my 2 bbl heads so I could take advantage of the 2 inch primary tubes. This required a spacer plate between the exhaust ports and the header flange as the 4 bbl headers wont seal to the 2 bbl port otherwise. What I did was contacted Hedman, and he made me a set of 4 bbl flanges with the 2 bbl port in them. They worked great. All I did was take my handy dandy die grinder and match the opening to my exhaust ports, as the roof had been raised a tad.
Besides the difference in valve sizes, 2.19 intake/1.73 exhaust for the 4 bbl head vs 2.04 intake/ 1.65 exhaust valves in the 2 bbl head, the 4 bbl head has greatly larger ports and a a lot smaller combustion chambers. The 2 bbl head had a polispherical or open dome type chamber while the 4 bbl head had a closed chamber , which shrouded the valves to some degree. The open chamber was used on 72 and later 4 bbl heads but the valves and passages remained the same sizes.
The 2 bbl heads all had 76.2 CC combustion chamgers, while the early 4 bbl heads had 62.8 CC chambers.
If you run short of 2 bbl heads, they can always be pirated from a 351 or 400 M block as they are the same head.
There were some minor CC differences between the of the 4 bbl heads even, depending on which version of the engine you had but the chamber design was the same until 72 when they made the first drop in compression on the 4 bbl head. Since there are so many variations of the 4 bbl head, and finding a matching pair could be difficult, as well as costly, stick with the 2 bbl jobs.
