In this series and as touched on last week, we are really combining two things in one… the tuning of the OHV Opel engine, combined with some general tuning factoids that will likely work on most if not all classic engine builds.
Being located in South Africa there are a few issues that are specific to our environment. Two of these, namely our significant altitude variations and local fuel octane values make for interesting tuning requirements for classic car engines …that is if you want best performance…so… to start with, we touch on the fuel issue first in order to position this work correctly.
I am no fuel expert but have had sufficient exposure to understand the basics and given that we need to tune engines to the locally available elixir it is worth covering a couple of ‘rules of thumb’. It is also worth mentioning that if we are talking stock Hydrocarbon Based Fuels (no alcohol mixes or ‘octane boosters’) crude-oil refining processes worldwide are pretty similar and fuel characteristics very much the same. Here we need to remember that hydrocarbon fuel composition and function is heavily biased towards historical refining techniques. We have this rather interesting question of whether available fuels have directed engine design, or the other way round. The answer to that is straightforward… fuels are the constant, and engines have had to adapt ….and that is exactly what we are doing in tuning the ‘oldies’… we need to get our old crankers to love the available fuel.
The main difference internationally is just how the octane value of the fuel is numbered…ie is a 95 octane in SA the same as 95 octane in the USA? The answer to that is a simple… No It Is Not.
The reason for that, as already noted has little to do with differences in fuel characteristics but rather the fact that in the USA and European countries fuel octane is really an average of the two measurement criteria, RON and MON and in SA due to historical happenings we use the RON value as the communication tool.
What’s the difference between RON and MON then?
Firstly both figures are obtained by testing on a variable compression rig that measures the fuel anti knock characteristic (Octane) at two different and very specific air inlet temperature and engine speeds. This is an internationally standardised rig and test method. A typical local fuel (95 Octane RON) would have a difference in the two values of around 7 numbers…ie RON 95 and 88 MON, this difference is referred to as the octane sensitivity.
Our local fuels using the USA standard would then be in the 91-92 Octane range and truth be known the American method is probably more representative for the purposes of optimisation than our numerically inflated RON biased numbering. Either way, if we understand that, it solves the problem of differing numbers.
From this we also need to understand that the fuels we buy at our commercial pumps do not have a linear anti knock performance either. If we assume that we could have consistent cylinder pressure throughout the rpm range, knock margins will differ between low/medium vs high engine speeds. Lets keep it understandable again…as a rule of thumb the RON number tends to effect low to medium engine speed knock and the MON number is more likely to determine high speed knock at max torque and above. That statement is not directly lifted from the test method, but from practical experience.
The reason I highlight this is to identify that in tuning classic engines we have many variables wondering around aimlessly. Cylinder pressure fluctuations, octane performance and the characteristic knock resistance of the engine itself. When we add to that the altitude component and expect that by coming up with random camshaft profiles, unknown squish values and engine design variables… we then superimpose the fuel… and hope that all is going to be optimised?…unlikely.
If we do not apply some basics, it will only be ‘OK’ through two routes…the first is pure luck… or the other is through the school of hard knocks…pun intended.