ive got a 69 307. i just pulled the heads off, and noticed that the "bumps" like what identify the type of head is different on one then the other. im confused at this. one says in the middle GM 54 and the other GM 59. here is a pic ill attach. whats this mean?
I don't know what the casting marks mean but look at where the heads are almost touching...thats the straight edge I was talking about. these are not the lightened heads they have good metal in them.
350/327 is a 4" bore
350 3.48" stroke
327 3.25" stroke
305 3.736" bore x 3.48" stroke
307 3.875" bore x 3.25" stroke
The 307 is a 283 block with the longer stroke crank used in the 327.
The 302 chevy was a 283 crank in a 327 block
If you want a 5 liter engine for putt putt pulling go with a 305 for the long stroke
If you want a 5 liter engine for high winding performance go with the 307
If you want more just build a 350
the larger the cylinder bore is... the larger the valves you can put in the heads and the better it will breath at high rpm's.
The 307 heads had lower compression... than a 283... that's not necessarily a bad thing. and remember high octane gas in an engine that will run on low octane gas actually hurts performance.
the lower the compression ratio...the more space you can fill with air/fuel mixture prior to combustion and again the cheaper the gas you can run.
Octane was high good back in the old leaded gas days...
307's...they had small valves...that is a bad thing if you want power.
If these heads have the valve rotators on the exhaust valves you want to eliminate them...
They were invented to keep the seats clean from the residue of leaded gas. with unleaded they will grind the valves into the head.
Manufacturers induction harden the cast iron seats in the heads. This is true...
I have read that they didn't do this after lead was added to the fuel because it wasn't needed. I can't say if that is true.
Fact is that gas wasn't always made with lead in it and the manufacturers used to harden the seats in pre-leaded fuel engines.
Many modern engines after the removal of lead from the fuel also had induction hardened seats or hardened seat inserts.
So I'm not sure if the valve recession that occurred running engines designed for leaded fuel on unleaded was due to non hardened seats or just the valve rotators but either way you look at it... rotators are bad.
I have used and had heads designed for leaded fuel like these last a very long time on unleaded just by removing the rotators and getting a stock valve grind. a 3 angle valve job opens up the flow for more power but the contact area of the valve with the head is thinner. It might not last as long as a stock grind unless you install new seats.