Currently have 305 TBI LO3. Dreaming and scheming of a 350. I currently have #416 heads, ported & polished, with 1.94 in and 1.5 ex valves, and 1.6:1 rockers on my 305. I was told, and have read somewhere here, that I could put the #416 heads on a 350 block.

But I'm an idiot - I understand the smaller heads would make higher compression, but I don't know what the ramnifications of this are - please tell me why or why not I would want to use these heads, assuming I ever get the 350.

 

Compression ratio is exactly that the ratio of the full cylinder volume versus the volume of the cylinder when the piston is all the way up + the head gasket volume + the combustion chamber volume. So its full volume / combustion volume.

 

You'll make more power (with higher compression rather than lower compression)
and likley need to use high(er) octane fuel.
How much compression you'll end up with by bolting your 305 heads on a 350 depends on a number of factors.
1) The cylinder heads combustion chamber volume.
You've modified the head so it will not be the same as stock, measure it.

2) The piston clearances on the 350 motor.
(deck clearance, valve relief volume, dish volume. head gasket volume)

Plan on having to use 92 octane or better.
Other than that they work well on a 350.

 

If/when you do get the 350 block, you could have the block decked. This will allow you to run higher CR w/o the additional octane. Just have to be sure of your valve clearances between the pistons and valves like F-Bird said.

 

Always under the impression that compression is compression?

 

I have the exact same hopes and my uncle has a 70's 355 with 10:1 pistons. I'd need heads/cam/valvetrain and headers and stuff. Using my 305 heads would be nice and was the plan. My thing is... the 355 cylinders are 4.03" in diameter...won't the 305 heads be too small to fit on the 350 cylinders... kinda like the pic I've attached below?

 

No the difference is the combustion chamber size not the port or runner size. Some 305 intake runner ports are actually much larger then 350 heads.

 

Your reasoning is quite correct but the difference in diameter is small (only about a tenth of an inch all the way around) and as long as the head gasket successfully seals the combustion chamber to the block you're good to go.

 

Hmm I figured it was small, but I also figured that would leave some weird pockets in the combustion chamber (where the overlap is) that might not be good...but I do realize they are small. Then again, if people have been doing this, it might not be all that bad of an idea

 

Its no worry, I run a .40 over 350 with 305 heads.