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Compression Ratio:
The Silvertop has a static compression ratio of 10.5:1 compared to the 11:1 of the black. Both the combustion chamber volumes and the piston shapes cause the change in the compression, see under "Pistons & Gudgeon pins-" and "Head (combustion chamber)" for more information.
Head (combustion chambers):
The combustion chambers between the engines are quite different in terms of their squish area. While their valve size and position are identical the Silvertop features a chamber with a squish area in every gap between the valves. The blacktop on the other hand features all but the smallest flat section behind the exhaust ports. This led to a large difference between the combustion chamber volumes where the Silvertop is 35cc (reported by Arias) and the black at 37.8cc (measured by myself).
Pistons & Gudgeon pins:
Even though the Blacktop head features a more open combustion chamber (see "Head combustion- chambers") it still has a higher static compression ratio (see "compression ratio"), this is purely because of the piston design. The Silvertop piston features a very small dome with two deeply angled exhaust flycuts, two moderate inner flycuts and a moderate flat middle flycut for the middle intake valve. The Blacktop piston is very different, to accomodate for the larger chamber the dome is considerably higher with reduced flycuts. The exhaust flycuts are still present but more shallow and only the middle intake flycuts remains but even that is just a tiny flat circular dish machined out. A blacktop engine is an interference engine unlike all other A series engines. Although the pistons are initially interchangeable physically they aren't practically as a Silvertop with Blacktop pistons has a very high compression and quite low when the combination is reversed. Also of note is that the Silvertop piston is lighter at 308 grams without rings or a gudgeon pin installed compared to the 315.5g of the blacktop. The Blacktop does have a superior gudgeon pin design in that it has countersunk ends which give it a small 3g advantage bringing the weight difference slightly closer. The gudgeon pins them self are readily interchangeable.
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safer for boost 