Lynn E. Hanover

Since too slow a cranking speed gets you a no start, it is a safe bet that cranking speed affects "Dynamic" rather than "Calculated" compression readings.

Compression testing is done with the throttle held wide open. So inlet vacuum numbers would be about zero and very nearly unaffected by cranking speed.

The rotor can be compared to a piston with one ring with 21 end gaps. So sealing is dismal. That is where a high cranking speed becomes critical. The faster the cranking speed the less time to leak down. So many tired worn out engines, and fresh engines built with too much clearance in the gaps will not start on the starter but will bump start quite redily. It is just a matter of cranking speed.

Lynn E. Hanover

MazdaMike02

Aha yes sorry about that I didnt think hard enough before I posted haha. Yes, extending the port (increasing intake duration) Would indeed mean that the port timing is retarded due to the port still being open during part of the compression cycle. But if you were to just modify the port to make it open earlier than you would be advancing the timing.