Quote:
Originally Posted by Nickmysta
Hi Henry - this may be a stupid question, but what are the key benefits or characteristics of short stroke vs. non-short stoke? Does it rev faster, feel any different?
Thanks
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Covering this topic in general will require a semester of classes so I'll just cover the different between 3.2 Carrera and the Short stroke 3.2. The main difference is the relationship between the crank and rods.
In the 911 engine, Porsche found themselves stuck with a dilemma. Because they were a tiny car company, each engine development was an attempt to improve the previous engine changing as little as possible.
That meant every time they made the engine bigger, they compromised something.
In trying to keep the engine's external dimension consistent (to eliminate chassis changes) they were forced to keep the distance from the head to the crank consistent.
That meant every time they increased the stroke they had to shorten the rod (compromise the rod to stroke ration).
Starting with a 2.0, Porsche rod length to stroke ratio was close to 1.97:1 (2:1 being theoretically desirable) . As the engine grew that ration decreased. Finally in the 3.6 it was 1.67:1
The short stroke engine provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of engine will produce very good mid to upper RPM torque.
The longer rod will reduce friction within the engine, due to the reduced angle (see illustration) which will place less stress at the thrust surface of the piston during combustion. These specifications work well with numerically high gear ratios and lighter vehicles.
There is also the issue of journal size. The 3.0 journal is smaller 53mm vs 55mm, making the crank lighter and allowing for a larger rod bolt that will still clear the oil pump.
This larger rod bolt increases the reliability of the engine at higher RPMs.
After the 2.0 (906 type), few if any, Porsche factory race engines were ever built using the 55mm, 3.2 Carrera rod journal configuration.
Some version of the 53mm journal was used in the 917, 935, 962, 956, 936 and on.
Even the recent versions of the GT3 uses the longer version of the 3.0 rod .
This rod length to stroke ratio was a major player in the design of the Supertec SuperCrank.
