Dynamic or Static?

On figuring a pump gas engine, which do you use? And will the engine at rpm ever see the static compression or only see the dynamic?

Rev4Q wrote: On figuring a pump gas engine, which do you use? And will the engine at rpm ever see the static compression or only see the dynamic?

Dynamic compression is only valid at lower engine speeds or part throttle openings.
As RPM and throttle opening increase the intake inertia starts to become more, dynamic compression is no longer true.

What Randy said.

In a well executed performance combination you are going to, at some point, reach and possibly exceed 100% VE. Typically at peak torque.
At that point your static C/R becomes relevant.

When the RPM at which peak torque (highest VE) occurs moves up octane tolerance increases. Think 3K vs. 5k rpm torque peaks.
There is less time for abnormal combustion to occur as well as the piston moving closer to the quench pad(s) as RPM increases.

Octane tolerance is much higher in a modern fast burn chamber in aluminum vs. an iron cj or passenger car casting. Think Kaase heads or AFR. Eddy and TFS streets to a lesser degree imo.

Variables such as altitude, fuel quality, quench distance, engine operating temp, air temp, ignition curve, intake runner velocity, etc are additional considerations.

We calculate DCR using the @.050" intake closing point +15 degrees using the UEM calculator. 8 to 8.2 to 1 for work engines. 8.5 to 8.7 for iron headed street combos. High 8's to low 9's with modern chambers and a 6K+ hp peak.

The DCR on a full smog 70's 460 with retarded cam timing and 7.7 to 1 static is less than 7 to 1 lol.  


SJ
used 2b RHP.