Thanks Supercuda, that was a good explanation, and I did not have the cc data on the 354 car head.

Truck, I have had the same experience when measuring various 50s and 60 engines (from all of the big 3) as your builder. Generally speaking advertised factory compression ratios and HP & TQ numbers were optimistic at best. I'm not saying the factory was making the numbers up, but rather what they (all) were delivering for testing was a truly blueprinted engine.

The blocks would have been square decked, line honed, crankshaft indexed, and heads CCed then equalized and taken to the minimum volume allowed. While the heads would not have been ported you can rest assured they would have been aggressively cleaned up. Pistons would be selected for the correct compression height. The rotating assembly would have been lightened as much as possible in accordance with the engine blueprint and then balanced.
Clearances on the bearings and pistons would have been set on the loose side of the specs to cut down friction.

Out on the production floor things were quite a were quite a bit different. Deck heights were generally taller and more often than not one was taller than the other. Combustion chambers were generally always larger than advertised and varied from mold to mold. A good example of this are the 301 heads i pulled. Both pair were 1555768 casting numbers but one pair was suffixed as -1 (the ones I'm using) and CCed at 102-104. The other pair were -2s and measured in at only 95-97 CCs.

The 331/354 Hemi heads I've measured over the years have been all over the place. I had a 354 truck head come in at 112 CCs and the other at around 105 CCs on the same engine.

As far as the projected 9:1 compression with the pistons I picked up. These are factory 1956 354 pistons with an "advertised" 9:1 compression ratio. Like the factory preparing a test engine, the first steps the block will undergo will be line honing and square decking. Provided we get the deck heights close to the factory specs it "should" be close to advertised compression. I'm also keeping in mind that the factory used a steel shim head gasket with compressed thickness of .026 and all that are currently available are composite gaskets with a compressed height that is .015-.025 thicker (resulting in about a 1/4 point lower compression). Milling the heads to compensate is not out of the question.

Now admittedly this all sounds good on paper but the only way to really find out will be to mock everything together and get real world measurements. This is something I have to do anyway so I can determine if the valve notches need to be modified for the Poly heads or worse case have a set of custom pistons made. The mock up is also critical to determine how much cam I get away with.

One of the purposes of this exercise was in part to find out if the blanket statement about converting a 301/331/354 Poly to a Hemi or converting a 392 Hemi to a big inch Poly by swapping on 354 Poly heads were true. So far some of the results have been surprising.....BUT ONLY APPLY TO MY SPECIFIC BUILD WITH THE SPECIFIC PARTS I'M USING.

For instance if I were to start with a 55 301 with an advertised compression ratio of 8:1 (and realistically truly only being in the 7-7.5:1 range) and throw a pair of 112 cc truck heads on with the thicker composite head gasket I would end up with an engine that would run, but performance wise be very disappointing. While the blanket statement that going from Poly to Hemi heads results in a compression drop would hold true in this case, it does not apply to mine as the Poly and Hemi heads I am using have virtually identical volumes.

Sorry about being so long winded but hopefully this provides some insight about where I'm going with the project. I'm really looking forward to comparing the test results in the 2 configurations (provided I don't run into any show stoppers along the way). My initial thoughts are the Hemi heads will make more HP, but I wouldn't be surprised to see better Torque from the Poly heads.