i have a 74 454 and discovered my heads have been changed. the casting number is 3917215. information i found shows 68-69 396, 427 closed chamber 101 cc. can anyone verify this and give any additional info, are they good heads or not. also what if any benefits with this head installed on my motor. what is the differance between open and closed chamber. what plugs should i use, just get 68-69 396-427??? 

The information I have is that casting number is a late '67 and '68 model year head for the 390 and 400 horsepower 427 big blocks...at least in a Corvette. I don't have the information pertaining to a 396, but there's no question it will fit them. It also is an oval exhaust port head.

Production of 215 casting heads was from May of '67 through May of '68.

With these heads on your car you likely have a somewhat higher compression ratio than is factory stock, assuming nothing else, such as the pistons have been changed.

As far as spark plugs, I think you need to ask someone more knowledgeable than I. While the plugs will interchange fit wise, the difference in compression rato and resulting burn characteristics may change the application you need. The plugs for a '68 are probably a colder heat range due to higher compression than a stock '74 454 would use.

Hopefully someone else here can better answer that.

gunslinger

I don't know if there are more reasons for the change to open chamber heads, but the major reason was due to emission requirements. The open chamber head exposed more of the combustion chamber to the air/fuel mixture for more complete combustion. It tended to reduce compression ratio but it looks like the more complete combustion made up the difference in power production. I'm sure there's a point of diminishing returns on that, particularly after about '71 compression ratios dropped considerably.

To find out what heat range spark plug you need, you'll likely have to know the engines exact compression ratio, proper timing specs. etc. A too hot heat range can lead to premature spark plug wear, detonation and damaged pistons. Too cold a heat range and you'll get fouled plugs.

If your engine has never been torn down, you can use the specs for the lower end and the specs for the heads you have to come up with the compression ratio. There is a formula for that, but I don't have it. Once you have that, someone can recommend the proper spark plug. Maybe AC or Champion's websites may have a tech service that can do that for you.    

A/C R43, or 44, should work fine. Just be sure to get the correct type: Gasketed, or tapered seat. The early heads like that should be a gasketed plug.

You normally wouldn't be able to put a closed chamber head on an open chambered engine, due to the height of the piston domes on the open heads. With a '74 engine, tho, the piston domes are almost non-existent! This combo might have raised the comp. ratio a half a point.