Octane Rating of Canadian WWII vehicles

[Tony Smith]

Another point to consider is the physical design of Automotive engines.

Britain was lumbered from the introduction of internal combustion vehicles with the peculiar notion of "Taxable Horsepower" or RAC Hp. This meant that a certain car or truck was taxed annually for the road based on it's "taxable" Hp. But this figure bore no resemblance to the actual horsepower of the engine!

Taxable Horsepower was a notional value derived solely from the total area of the piston crowns. It was not concerned with the stroke or the total capacity of the engine, and certainly not the output of the engine. So two comparable 4 cyl engines, one of a bore of 3" and a stroke of 3" (cap of 85ci or 1.4l), and another with a bore of 3" and a stroke of 3 1/2" (cap of 99ci or 1.62l) would both have an identical "Taxable Hp" of 14.4Hp. But plainly the 3.5" stroke would be a larger capacity and make more actual power and torque.

So the tendency for British motor manufacturers was to design engines of an "Undersquare" design where the Stroke was proportionately longer than the Bore. This was an inherently inefficient restriction to best design practise (a point that was not evident to the UK Govt, who continued to encourage inefficient products for far too long!). The US auto industry was not hampered by this inefficiency, and refined engine designs to produce better Hp/ci ratios from "Square" (ie equal Bore/Stroke) and "Oversquare" (Bore larger than Stroke) engines that were able to achieve higher RPMs, and therefore more power.

However, the stroke of an engine directly relates to the speed of the piston as it moves up and down the bore as the crank rotates. As the piston in a short stroke motor does not move far in 1 revolution, it's speed is lower. A long stroke piston, in that same 1 revolution the piston moves a further distance, the piston speed is higher. Material properties of the pistons, (initially steel or iron, later aluminum), lubricants and ring material all contributed to what the actual maximum speed of those pistons could be, but for any material, the short stroke engine was ALWAYS capable of higher Rpms than the long stroke engine.

The one benefit of of the slower speeds imposed on long stroke engines is that the compressed fuel mixture had longer time to burn as the piston travelled down the bore. Low Octane fuel is a slower burning fuel and works best in a long stroke engine. In fact, the US move to find Higher Octane fuels was a direct result of their development of oversquare engine designs. A higher rpm engine has less time to burn the fuel mixture, and a higher CR was needed to increase burn time within a much shorter duration "Power Stroke", and this in turn required a higher Octane rating of fuel. Conversely, using quick burning High Octane fuel in a long stroke motor results in lower power output as the fuel has completed burning well before the piston has completed the Power Stroke.

[Lynn Eades]

In saying all that, for Dan's benefit, The relatively low compression ratios of most war time engines was in the area of 5 or 6 to 1. This in most cases would have been pretty forgiving except that (as per Tony's post) most British vehicles had a very low power to weight ratio.(iets say a conservative approach to horse power)
Nowadays Dan, most European and Asian built cars are running about a 10 to1 compression ratio (often, on top of which goes a turbo)
It's all about volumetric efficiency which means stuffing as much as you possibly can, down the hole (the air fuel mix, that is)
I hope this all makes sense and I am sorry I've not yet found any info on the standard octane ratings of British fuel, during WWII.

[Alastair Thomas]

After the war was over, my Father mounted an expedition to visit Ain Dalla. This uninhabited oasis is NE of Farafra oasis to the East of the Great Sand Sea. It was a jumping off base for the LRDG and there were reputed to be stores dumps still there. And so it proved. There were boxes of flimseys and some jerry cans. Some of the flimseys still had petrol in them and my Father drove back to Cairo on it in his Jeep, some 370 miles. When he got the chance he sent a sample to the Shell offices and they analysed it for him. They pronounced it to be 53 octane and "should not be used in a motor vehicle"!
Having said that he also fitted a second fuel tank to his Jeep and would run on paraffin when the Police were not in evidence.
We still have his desert equipment including his bubble sextant and Cole sun compass. In a burst of enthusiasm I have just finished making a small batch of replica Bagnold sun compasses.
Alastair
F60S
Lynx I MkIII*

[Lynn Eades]

I love the (9.00?) x 16 sand tyre on the jeep. Does that equal low range starts?

53 octane. That is pretty low. Had all the petrol evaporated leaving the bunker oil?