[Jay S]

Definitely some things wrong in those figures using Bulter’s compression calculator. The piston dish or dome volume would not be zero. I assume the engine had valve reliefs, likely 4.5 cc or more. You may be assuming the engine has zero piston deck to height clearance. Unless you mill the block deck, the piston to deck distance is usually .025-.015 below the deck on most longer stroke aftermarket rotating assemblies.

You don’t want a narrow LSA like a 108. It needs to be wider, preferably at least 114. It is certainly not ideal, but with a well chosen cam, you can use a thick head gasket to drop the compression.

Your compression is likely mid 11s with stock parts and SM’s. I helped someone do a similar combo with speedmasters, I think he compression was just over 12:1. I think it needed 94 or 95 octane with the parts he had. He was going to run 93 and spike the fuel some. I don’t think he has it running yet though, so I don’t know how he got along. He was making short trips to the track running more of a drag car.

You can likely make a flat top stroker speedmaster combo work, but a lot more details are needed if you want more help.

My guess is your better off rebuilding the iron heads. The SM’s don’t fit the combo very well.