There is a popular view that modern petrol burns more slowly and hotter than classic petrol. This is not true.  It is a symptom of a more complex phenomenon called Cyclic Variability, introduced in Chapter 4. Chapter 6 describes Cyclic Variability in more detail why modern petrol appears to burn hotter and slower.

Cyclic Variability affects all spark-ignition engines, classic and modern. A high degree of variability is bad for an engine. It reduces power output and causes damage to the piston, crankshaft bearings and valves. The Manchester data showed, the test engine suffered from a high degree of variability particularly when being driven conservatively on the public highway.

Not something you would expect!

Why does cyclic variability cause so much damage to an engine. Why does it reduce the power output and by how much.

Yes. The tests at Manchester showed that different fuels changed the severity of Cyclic Variability. Those with ethanol in them appeared to reduce its magnitude the most.

Hello. I wonder, what differnces a sidevalve engine might have, compared to the test OHV engine,with regards to Cyclic Variability?

Cyclic Variability affects all spark ignition engines. The only difference is its magnitude. You can see the cycle on cycle variations on this video inside a running side valve engine.

The more turbulence in the cylinder, the better the mixing and the less the magnitude of the Cyclic Variability. One indicator is the number of degrees of ignition advance. More advance, means less turbulence. While this is not an exact indicator, you can plot and compare and compare your advance curve with other engines using this website.

With overhead valve engines such as the XPAG two factors work to increase turbulence. As described in the book, the position of the inlet valve creates a radial swirl (around the cylinder) and the squish increases the turbulence at the end of the stroke. Radial swirl is good because it persists over the induction and compression stroke.

In a side valve engine, the geometry will create an axial swirl (up and down the cylinder) which does not persist as well as a radial swirl. In addition there is little or no squish. Mixing will be poor and the magnitude of the Cyclic Variability will be worse than the XPAG. You can see the effects of the axial swirl during the combustion cycle in this video inside a running side valve engine.