You hot rodded a 4 cylinder Vega? That’s making the most of what you’ve got right there! Good job.
One other thing to consider with the shorter stroke engine is that the combustion chamber is more flattened around TDC.
Thus more surface area than the long stroke chamber, which is closer to a cube or sphere.
More surface area during combustion means more heat loss from the burning gases.
This is what kills the rotary (Wankel) engines fuel efficiency.
Very oblong chamber at time of combustion.
This heat loss is worse at low RPM.
Another thing that is seldom considered, mostly because it’s almost never included on spec sheets for engines, is the stroke to connecting rod length ratio.
The image above is a diagram of the Wartsila Sulzer ship engine which is one of the most fuel efficient reciprocating engines in the world. Not only does it have a super long stroke to bore ratio, 38 inch bore, 98 inch stroke, (8.2 feet) but the crosshead bearings allow for super short connecting rods. Having short connecting rods means that there is a shorter dwell time near top dead center and a longer dwell time near bottom dead center. This means that after the fuel is combusted, the piston wastes no time going down minimizing thermal losses by getting the adiabatic expansion over with before the air has time to transfer its heat to the combustion chamber. Then it takes its sweet time near bottom dead center giving the two stroke engine more time to scavenge the cylinder with fresh air for the next firing, meaning that less pressure is needed for scavenge air, thus reducing pumping losses.
On a four stroke engine, a short rod ratio actually can increase pumping losses because the piston accelerates so rapidly on its way down that the airflow can’t keep up with it then sits at the bottom waiting for the cylinder to fill, the energy used to pull the piston down against a vacuum is never recovered.
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