Car appreciation... the aesthetics, the engineering, etc

Crank is forged as stock

An insight into what it takes to get an M96 to spin to what your Mazda can do out of the box:

1. 100mm bore, 72mm stroke, giving 3.4 litres, utilising a standard Porsche crank and rods. This gives the maximum overlap between the big end pin and main bearing journal, leading to what would be most suited to very high rpm.
   *RPM@ 20.148 m/s, 8395
   
2. 100 bore, 76 stroke, giving 3.6 litres. This would require a bespoke crank and rod set up, but a very good engine size to achieve Dammit’s goals. In EN40B, this crank would be very strong, and along with having the rods made from 300M material, would be happy to rev hard.
   * RPM, 7976
   
3. 100 bore, 77 stroke, giving 3.63 litres. As above but with a fraction less pin overlap.
   * RPM, 7842
   
4. 100 bore, 78 stroke, giving 3,675 litres. This is the stock 3.4 stroke, so could utilise standard components. The feeling is that the stock parts would not be up pulling 8k rpm, at least, not for long. A billet crank and decent rods would put that right. This was our initial favoured configuration.
   * RPM, 7745
   
5. 100 bore, 80mm stroke, giving 3.77 litres. Again a bespoke set up required.
   *RPM, 7576
   
6. 100 bore, 82.8 stroke, giving 3.9 litres. This is the stock 3.6 stroke, so a standard crank and rods could be used, but are unlikely to take kindly to the increased rpm. A steel version would survive, but there will be considerably less pin overlap than with the 3.6/3.7 configuration.
   *Base RPM, 7300
   
7. 100 bore, 84.5 stroke, giving 3.98 litres. Potentially tricky to assemble, Baz has said that the clearance between the rod bolts and the bottom of the cylinders would require attention, also the window for gudgeon pin fitting could become an issue. As a one off experiment, we feel that an engine of this capacity represents a risky proposition at this juncture. Besides, it would lift torque at the low end and most probably run out of breath at the top, the exact opposite of the end goal here.
   * RPM, 7153
   
   

*In terms of hitting the required rpm, I have looked at the stock 3.6 996.2 stroke and calculated the mean piston speed. This works out to 20.148 m/s at 7300 rpm. Translating this number to the various configurations, a 4 litre would have a redline at 7153 rpm, whereas at the opposite end of the scale, the short stroke 3.4 could be pulled to 8395 rpm. These numbers are shown above for reference vs. the various stroke lengths. So keeping the standard Porsche mean piston speeds as a reference, Neil’s target of 8k is met with a stroke of 76mm, at 7976 rpm. Did anyone say Mezger?! With the components being fitted to this engine, these numbers are very safe in terms of bottom end reliability. For reference, a well-developed race motor would happily stand 24.5 m/s .