My current engine is 18.5:1 effective CR. Never again....
My opinion(s), for what its worth.
- The "perfect" compression cycle and what happens in our engines are not the same. We lose a stupid amount of thermal energy through the pistons, cylinder walls and head. Primarily the Pistons, then head. I measured it years ago when I was checking cylinder pressures. Cold Pistons had a noticeable impact on effective compression ratio. The cold piston affect was worse on higher compression engines. Which makes sense since thermal energy will flow faster the higher the difference between masses. The result is, the hotter the chamber, the more heat you will lose and that heat is what is making the power.
- Because of the heat loss, the "effective" compression can be a fair bit less than the volume calculated ratio. From my measurements, it ran anywhere from 2%-4% less. But the effective ratio loss was always higher the higher CR of the pistons. My 18.5 : 1 effective ratio compression on my engine for instance comes from pistons that should have put out 19.0 : 1
- The Ideal adiabatic cycle would give you ~8% pressure increase going from say 17.5 : 1 to 18.5 : 1 but effectively, it is more like 5%-6% and the loss from ideal gets higher the higher the CR.
- The biggest advantage to increased CR is the improved volumetric efficiency. That is, you are getting closer to 1 : 1 air exchange in the cylinder, so the engine is "breathing" better. Especially before the turbo comes in. Thus the snappier throttle. It also reduces the amount of dead air in the chamber to dilute the oxygen concentration. In none boosted engines, gas and diesel, this is a huge deal.
- Lastly, there is just more wear and tear on the engines lower end.
IMO, it isn't worth is. Mine is a damn rattle trap at idle, and I hate that aspect of it. Next build will be 16.0 : 1 or maybe 16.5 : 1 , but no higher.
