In 64-65, the 273s used closed chamber heads (part #2465315) which had 57 cc chambers. What are the thoughts on using these smaller closed chamber heads on a 1971 318 in order to bump compression? Being a low compression motor, these heads alone could get it closer to 9:1 which would make for a respectable increase and open up other cam opportunities.
I read somewhere else that these early heads had different intake bolt angles though so I don't want to go down a rabbit hole.
Yes: You would need the matching 273 intake, and they aren't great.
I did that on the 318 that came with my '69 Barracuda. Made a noticeable difference over the open chamber 318 heads. Also nice if you get the adjustable rockers with those heads. I don't recall any difference on bolt patterns, I kept the same intake and exhaust manifolds.
I've been doing more internet research and it seems the early era 273 heads would be more trouble than their worth. It's a 71 stock motor and I'm guessing the open chamber heads are probably 65cc? Maybe more?
I suppose another easier route might be to mill the original heads heavily (like .050 or more) and also use a thinner head gasket. It has the thick fel pros so those things are probably.042 compressed. I think there are other brands that compress to .026. Those two changes would probably get me 3/4 point extra?
At some point down the road, I'd like to add dual exhaust, a small 4 barrel and perhaps a slightly stouter cam for low end torque. I figured the compression bump would go nicely with these modest changes yet still leave it idling nice and smooth.
Just remember that when you mill the heads, you've got to mill either the intake surface of them as well or the intake manifold. Machining the intake surface of the heads would allow you to use standard intakes. 273's weren't known for high flow heads and there's several threads on here about using the later 318 head that's casting # ends 308. As per Neil, the yflow better than the XX or J 340 heads as is. I believe they're also set up with a squish zone, too. If you go that route take a very good look at the valve seat areas as they're also known to crack between the seats. Caveat Emptor! :cheers:
I've read the threads on those 308 heads. They are about as hard to find locally as unicorns. The ones I've seen that aren't local, people are asking insane prices so I guess I'm back to the milling option.
I believe the #308 heads are designed for 360, the #302's came on 318's and might be a better match. But all these heads (inc. 273 closed chamber heads) are getting hard to find.
I looked for the 302 ones also with no luck