I have heard this is the higher compression head so how much different then the other heads for the 55 to 62 Engines.
The information I have lists the 54-55 235 which had the pre-848 head at a 7.5:1 compression ratio. 56-57 specifications show up at 8.0:1, and for some reason, the 58-62 engines are supposed to have 8.25:1. To the best of my knowledge, the 848 head was the only higher-compression head during the 56-62 year models so I have no idea where the extra 1/4 ratio came from in 1958. Fuzzy math? Sleight of hand? Who knows?
The 54-55 head can be brought down to 848 combustion chamber volume by milling it .090", but some other tweaks have to be made to prevent valve-to piston interference of the intakes, and some sneaky geometry tricks have to be played with the rocker arms as well. Not a problem for an engine machinist who knows what he's doing, but those guys are getting as scarce as a chicken with lips!
The 54-55 5913 head had a combustion chamber volumn of 86.3 or so cc. The 848 head is 79.1. The 848 head entered service in 56, along with the neoprene rear main seal. Tom Langdon and I were on the phone one day, has to be 20 years ago or better. He was still working at gm and he had access to the specifications and looked them up. He also gave me the specs for the two different 261 heads. I have measured a few heads and they all come up to about those numbers. Some a little more, but generally pretty close. So 8.0 for the 57 with the tighter combustion chamber is in the ball park. Where the extra 1/4 point came from 58 and up, I do not know.
Next question- - - -do those numbers represent "theoretical" or actual compression? A change in cam timing could account for the extra quarter point if the figures reflect actual compression pressures. I've got a 100CC burette on order, so one day soon I'm going to be doing some "measurin' an' cypherin' " on the 216 head. I might do some checking on the 235 and 261 heads I've got as well.
The reason I ask is I recently found a 59 or 60 261 that I plan on building and it has a 850 head and wondering how it compares to the 848 head,I do have a extra 848 head on a 56 235.
848 is a casting number.
Is there a different part number for the head in different years, because of different amounts removed from the gasket surface?
848 is a Higher compression head--I have 2 on 1954 rebuilt 235 engines one in my 49 and 1 in the 51--more power-Bill
I've always wondered about that extra 1/4 point of compression. I was hoping one of you guys knew. I wondered if it was a different head gasket design. 56 and newer used the copper sandwich type.
I'm going to be doing some comparison checking soon, mostly on the 216 head, but I believe I've got all the 235 and 261 heads on hand as well. I'll be glad to post the combustion chamber volumes I get for each one. The "true" compression ratio is also dependent on when the intake valve closes as the piston is on its way up the compression stroke. That's why camshaft manufacturers recommend high compression when an engine runs a radical cam- - - -there's less cylinder left to compress as the intake valve duration increases, so "high compression" is simply an effort to get the real compression pressure back to where it used to be with a stock cam.