Comments by "John Luetjen" (@jehl1963) on "Bore vs Stroke - What Makes More Power?" video.

  1. An interesting example is the air-cooled 911 engine, which was made in a number of different bores and strokes, and the HP and BMEP tested by the same people using the DIN method. It winds up being a bit of a multi-variable set of experiments. Of specific interest to this subject is the 2.5 liter entine from the 911 ST of the early 1970s (the immediate predecessor to the 2.7 RS). Porsche originally made the 2.5 liter engine by lengthening the stroke of the 2.2 liter engine with an 86.7 mm bore and 70.4 stroke (2.45 liters). The engine made 266 HP at 8000 RPM and piston speeds of 3696 ft-per-minute and had a BMEP of 173 lbs/sq-in at 8000 RPM. The problem was the Porsche wound up with vibration problems on the crank which resulted in the flywheel bolts backing out. So they went back to the parts bin and made the engine (same head, valves and cam) with the earlier 66 mm stroke and bumped the bore up 89 mm. This created other problems with cylinder wall thickness which they ultimately resolved by going to a nickasil coating on the cylinders, but I digress. This 2.42 liter engine made 263 HP at 8000 RPM with piston speeds of 3465 ft-per-minute with the same BMEP at the same engine speed. In the end, the two engines performed virtually the same, and produced the same torque. Porsche than created the 2.7 RS by essentially combining the 89 mm bore with the 70.4 mm stroke. Note that in meantime they had also solved the flywheel bolt issue.
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