swirl port heads
Can somebody show the part of the port on one of these ''swirl port'' 308 heads that makes them swirl, and how it's different than all the other x,j,587,596, heads?
Also, can anybody show me the swirl inducing portion of the 302 int port and how it's different from the 675 318 heads?
It would be nice to know the info given has some proof to back it, thanks.
The floor of the exhaust port is different (lower) on the 302/308 heads. Not sure if it really makes anything "swirl", but the gentleman that ported my 308's said that the exhaust port was very much improved compared to the earlier J heads. Most of the flow #'s I've seen for stock heads show the 308's flowing upwards of 10 cfm more on the exhaust side than the "587" 340 heads.
As far as putting 302's on a 340, I wouldn't. Especially if you have an early 340, since the pistons on the early 340's were above the deck, which makes it difficult to run closed chamber heads. And regardless, the 302's have the smaller 318 ports (and valves, although that can be fixed), so you'd be restricting a 340 with stock 302's compared to standard 340 heads.
I had a set of 308's set up for my 340 with 2.02/1.60 valves, and about a stage 2 port. They ended up flowing 264 cfm at .500" of lift, wish I had gotten the exhaust numbers. I went with the 308's because my 340's pistons are actually .018" proud of the block, just as the factory spec'd. Closed chamber heads would have to be clearanced for the pistons, so I used the open chamber 308's.
Also, be aware that the 302/308 heads have the same cracking problem as the magnum heads. It has to do with the induction hardening used for the valve seats, and the 302/308 heads are just as likely to do it as the later Magnums. The magnums are also closed chamber heads, with a smaller chamber than even the 302's. So, if you're pistons are above deck, they won't be the easiest to use either.
Here are some pictures of my 308's. Sorry, I don't have any pictures of stock X,J,587, etc heads to compare them too. But, here are the exhaust ports, before and after porting.