I'm swapping a 1971 350/270 horse motor into my Trans Am and was wondering if that or the LG4 has better heads? According to the book, it runs 8.5:1 compression (which doesnt make much sense since it has flat top pistons but anyways). Which motor would have better cylinder heads? I am planning on having the heads done over, and I heard the LG4 heads would be a better choice becuase they have smaller combustion chambers. Which would you guys suggest? I mean the LG4 makes like 150 horse, but that 350 makes like 220 horse (270 is gross rating). It seems to me that the 350 would have better heads, but apparently the heads on that motor aren't the greatest.

Also, after having the heads ported and polished, what type of gain would you expect? Thanks.

 

The heads on that LG4 are likely a 58cc combustion chamber, and combined with the flat tops in the 350 will raise the compression ratio. However, unless you know for a fact the 350 hasn't been rebuilt, bored or the block decked you won't know the exact compression. Is 8.5:1 the stock spec for that engine?
My point is this: As long as you have the engine out and are planning on a head swap, measure the piston deck hieght, see if the block has been bored or decked, and then figure out what your compression is going to be with the stock heads and with the 305 heads. You may end up having way more compression than you want
The LG4's heads will raise compression, but will not flow as well. See what heads are on that 350, get the casting # off them and there are a number of books and sites which can tell you "what you got" one I have used is www.chevymania.com
As far as having either set of heads ported and polished professionally, unless you know the porter and get it done real cheap, there are so many good aftermarket heads out there, not to mention the GM Vortecs which will make so much more power for the buck.