Combustion chamber cc variability


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By Ted - 16 Years Ago
DANIEL TINDER (12/17/2008)
Ted,

Just a point of curiosity:

Are you familiar with the type of tooling/procedures used by FoMoCo factory/foundry in the 50s? I have to wonder why there was so much variation in my ECZ-G chamber volumes (3 CCs +) between cylinders on the same head. Was the final machining done in Cleveland, or were they delivered to Dearborn as raw castings? Unless done freehand, it is hard to imagine that even 50s era automated equipment could be that imprecise?

Daniel.  This answers your question that was under the heading “YBM alum head photo’s”.   It’s a new topic so I’ve taken the liberty in starting a new thread with it.

 

A 3cc variation is still normal even in the newer oem castings.  Combustion chamber size control or variation by the oem’s hasn’t changed much in the last fifty years.  A case in point is the combustion chamber volumes varied so much on the FE’s when they were first introduced in 1957 that it forced all FE heads for the first half year of 1958 car production to have the combustion chambers machined until the casting techniques became stable enough to permit the combustion chambers to be used as cast.  I just recently did a set of new 502 BBC (scrub) aluminum heads and there was a 3½ cc variation from end to end on one of the heads and a significant difference from head to head.  Needless the say, the customer was not happy but the heads were easily fixed with a little time and perserverence.  Some (not all) of the aftermarket heads are surprisingly close so the control is there if need be but many of the oem heads still exhibit a combustion chamber cc variability that would appear to be excessive.

 

In summary, the same core shifting that occurs with the cylinder walls of a block is also present when casting cylinder heads.  Where this core shift is in regards to the reference points in starting the machine work on blank head castings ultimately plays a large part in what the final combustion chamber cc’s are going to be.  On a performance build, I will cc each chamber of both heads being used and then correct as deemed necessary so all chambers are sized the same.  More often than not, the combustion chambers will be progressively larger in cc’s from one end towards the other.  In these cases the calculations are performed so that the heads are angle milled end to end so that one end is milled heavier than the other which in turn allows the combustion chamber cc’s to be more equalized over the length of the head.

By Jim - 16 Years Ago
Jeez, it all makes such perfect sense when Ted explains it! I am so very happy that Ted is a co-admin of this board along with John (Hoosier Hurricane) and I. You guys obviously know which two of the aforementioned three names are really the gearheads! To give you a hint, it ain't the guy that is making this post!
By Hoosier Hurricane - 16 Years Ago
Jim:

Thanks for blowing a little smoke up my a--, but everyone has varying degrees of various talents.  You do the computer stuff here, and Ted and I, and others will try to guide the nuts and bolts group.  And, as I have told you before, thank you for this site.

John

By DANIEL TINDER - 16 Years Ago
Thanks Ted,



Anyone with half a brain (myself excluded) should have tumbled to the obvious conclusion that cast and machined surfaces are easily discernible. (No excuse for THAT mental lapse!)



Although, I should note that milling with an end-to-end bias will likely have a minor effect on valve train geometry?

In a perfect world the rocker pad-to-deck measurements would be consistent end-to-end, and so would all the valve seat-to-deck distances. In an era devoid of CNC machining precision, hand grinding/repeated re-checking would seem the only possible recourse?
By pegleg - 16 Years Ago

  the combustion chambers will be progressively larger in cc’s from one end towards the other.  In these cases the calculations are performed so that the heads are angle milled end to end so that one end is milled heavier than the other which in turn allows the combustion chamber cc’s to be more equalized over the length of the head.

 

And, many times, but not always, the problem is in the factory machining. If the casting is not in the fixture correctly. One end, or one side, can be sitting on chips, and will be too high in the fixture. The mold assembly can be wrong, etc. When you make and cast several thousand per shift, you'll find all kinds of variables.

      Daniel, remember the casting is made in sand. Normal tolerances for that process are +/- .030". Which actually means one casting can be .060" different from the next one, and still within tolerance. Aftermarket production rates are usually much lower, more costly and the inspection processes are much more closely watched.

      Many of the engines produced by the factory run for one or two hundred thousand miles with those errors, and are scrapped without anyone ever checking or knowing how far from optimal they are. If you attempt to control tolerances in a process more closely than the process is actually capable of producing, often all you do is scrap parts that are useable. All this adds considerably to the cost of production, without gaining much. Y-Blocks were not concieved as race car motors, most were 2 bbl truck engines, or taxi cab motors, where cheap was very important. The difference between a 7 1/2 to one or a 7 3/4 to one compression ratio wasn't noticeable.   

By Ted - 16 Years Ago
pegleg (12/19/2008)
The difference between a 7 1/2 to one or a 7 3/4 to one compression ratio wasn't noticeable.


Thanks Frank.  You gave a much more detailed explanation with fewer words than I would have attempted. Also goes a long way towards explaining why one vehicle runs so much better than another when both are supposedly identical.  As far as the valve train geometry goes, it needs to be checked regardless of how the heads are milled as the same variances in the machine work in that area are still a consideration.  But as has been brought up before, the ratio variances that are present in the factory rockers will warrant checking these first as any adjustments made for the geometry itself will be a moot point if the rocker arm ratios are all over the board.
By DANIEL TINDER - 16 Years Ago
Good point Ted. While my main interest in getting the geometry correct is largely to insure minimum guide wear, I had just assumed the repro rockers' ratio would be uniform, as they are of modern manufacture. Checking actual valve lift after final assembly would seem a necessary step I had overlooked, since balancing/chamber-matching the new motor would be rather moot if the charge in each cylinder varied with lift differences.