Join Date: Dec 2019
Location: West Boston suburb
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 21 Post(s)
There are many variables. But... A rough rule of thumb is you need around 90 psi to start without a lot of assistance.
For comparison, a new engine could have as little as 130 psi, or as much as 200 or more. The higher compression equates to higher combustion temperature and, thus, more power.
Note that an engine with very low compression can still run somewhat productivity.
But for reasonable starting there needs to be a certain minimum of compression and I believe that 90 psi is getting pretty low.