308 heads

I like the 308's. I have a set with 2.02/1.60 valves and ported, flow numbers look like this (compared to stock 915's on the same bench)


Lift Stock 915's 308's
0.200" N/A 165.78
0.250" 154.63 184.36
0.300" 178.42 202.20
0.400" 199.97 243.84
0.450" 197.00 258.70
0.500" 193.28 263.91
0.550" 193.28 257.22
0.600" N/A 252.76


Sorry for the formatting, site keeps correcting it. Anyway, my ported 308's flow 264 cfm at .500" of lift, which should flow well enough to support over 500 hp. The design of the exhaust port is a big improvement over the J and X heads. The only issue with the 308's, like other smog heads, is that they tend to crack between the seats because of the way the seat hardening was done. Not the end of the world, and I found a set without cracks.

That said, the new RHS X heads will outflow a set of ported 308's with 2.02/1.60 valves and no porting. Plus, they're a closed chamber design. So, it depends on what you're looking for. I probably saved a little money by having the 308's ported, but not a ton. Now, if you can do your own porting, then its a different story. I also chose to go with the open chamber design of the 308's because they were going on my 340, and since it's a '68 the pistons are .018" over the deck, so closed chambers can get sketchy with valve clearance.