So I am cc my chamber on the Thomas head to start planning for the banger build. The numbers I am getting seem to be off. I have read that these heads are higher compression and new ones are 6.7:1. When I do the test as outlined in Jim Brierley’s book I have been getting 155cc plus 20cc for gasket that is 175cc... if referencing the chart that puts the compression at 5.5-5.75. Seems too low for this type of head. I know it changes slightly with cylinder size, but am I missing something??? Sent from my iPhone using The H.A.M.B. mobile app
How much deck height do you have? That will affect compression a whole lot it the piston are down in the cylinders very much.
Well, I guess I don’t really know. Already took motor apart... not much ridge to see where piston was topping out. Sent from my iPhone using The H.A.M.B. mobile app
I can’t tell if block was ever decked... pistons were +.045. I guess according to literature that I have read I want to shoot for 6.5-7:1 compression.
Just like measuring and knowing the cylinder head volume, the deck height is also needed to calculate the compression accurately. Just like the bore size, stroke, head gasket thickness and volume. They are all pieces of the puzzle that need to be known to calculate compression.
Ok thanks... good to know. So how do I measure deck height? Assemble engine again and somehow measure the space above piston at end of stroke? I thought I read somewhere that a stock model a piston protruded 1/32 above the deck. So a lower deck would raise compression and a shorter stroke lowers??? Is this right? Sent from my iPhone using The H.A.M.B. mobile app
Yes, you will need to verify by mocking the rotating assembly up and checking the piston height at their TDC position. And yes, the higher the piston travels upward in the bore the higher the compression with everything else remaining the same.
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