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XPAG Questions–Piston Speed

The odd chap occasionally recognises me at M.G. functions, and most say they enjoy some of the articles I write. A few even say they like them, but do not understand half of it! So here is one explanation for you.


Q. “What do you mean by piston speed? Surely it is the same in every engine for the same rpm?” (Revolutions per minute).


A. A modern engine has a bore and stroke ratio almost 1:1. That is, the diameter of the cylinder is almost the same distance as the piston goes up and down. This is termed ‘square’, and if the bore is wider ’under-square’ or ‘short-stroke’; if the stroke is longer ‘over-square’ or ‘long-stroke’. The XPAG has a bore/stroke ratio of approximately 2:3. The piston goes up and down 90mm in its 66.5mm bore. The XPAG is called a ‘long-stroke’ engine.


Now, one has to imagine the XPAG turning over at 3000rpm, and a modern MGF engine turning over at the same 3000rpm. The XPAG piston will travel 180mm per revolution (once up and once down the bore). The MGF’s K series piston will have travelled just 160mm with its 80mm stroke in its short 75mm bore. Also, you need to remember that the piston STOPS at both the top and bottom of its stroke, so it really needs to be moving pretty fast in the middle of its stroke to keep up with the crankshaft. A quick bit of maths shows that the XPAG piston travels 540 meters a minute at 3000rpm, and the MGF’s 480 meters, so the XPAG piston is going a lot faster; almost 25% faster. To be able to run at 3000 rpm the XPAG piston is travelling much faster in its mid-stroke than the MGF. If one assumes that 770mpm (meters per minute, about 2500 feet per minute, an arbitrary safe continuous piston speed) is a safe piston speed it will be obvious that the XPAG cannot rev as high as the MGF. The higher an engine can rev the more brake horse power it can develop.


Remember that the 540 meters the piston travels at 3000 revs per minute is a ‘distance’ only, not its speed. The piston stops at the top and bottom of its stroke, so in the middle it might be doing twice that. The ‘safe’ piston speed of 770 mpm in mid-stroke equates to about 390 meters travelled. At a guess the XPAG will be doing about 45mph where as the K series will be doing 60mph at this ‘safe’ piston speed.


If you were to travel a long journey with both engines, the XPAG’s pistons will have run in their bores 25% further than the K series as well as having travelling much faster mid-stroke. The inertial stresses involved with first accelerating the piston up the bore, then arresting it at the top till it stops; only to repeat the acceleration again down to mid way and then arresting it again at the bottom are huge. So the mid-bore piston speed is a serious restriction on how fast your engine can go without suffering high bore wear or eventually blowing up.


The long-stroke XPAG type engine does have a very wide torque band, with its greater ‘leverage’ over the crankshaft. The short-stroke engine can rev a lot higher, but its torque band is narrower and towards the top-end of the rpm range. This means your XPAG can probably pull away from 20mph in top gear with its four-speed gearbox. The shorter stroke K series requires a five-speed gearbox to get the best from its narrower power band and will not be able to cope with 20mph in its fourth let alone its fifth gear. But the K series will run much happier at motorway speeds with its almost ‘overdrive’ fifth.


The 1938 XPAG 1250cc has a ‘long stroke’ of 90mm with a narrow bore of 66.5mm. The 1961 Ford ‘Kent’ 1200cc engine has a ‘short-stroke’ of 58.17mm and a bore of 80.97mm. At every stroke the XPAG piston travels 180mm where as the Ford’s only travels 116.34mm, hence the Ford’s ability to rev very freely and its engine’s use in racing.


Q. “Why did we have long-stroke engines when the short-stroke is better?”


A. Mostly because of the old RAC road-tax formulae used by the Inland Revenue. This RAC system used ONLY the engine’s cylinder bore and took no notice of the stroke. It classified the engine’s bore only in relation to its horse-power. This was probably correct when it was first used, but quickly became a farce as it took no notice of the stroke. To keep their engines in the cheap 8, 10 and 12 hp tax bracket, manufacturers made engines with narrow bores and long strokes. By 1938 when the XPAG arrived it was classified as an 11hp engine, but actually produced 54 ‘brake’ horse-power in twin carburettor form. The RAC horse-power rating is now called the ‘nominal’ hp. The road-tax restriction on the cylinder bore restricted the development of pre-war British engines. But after WW2 when the old HP tax was abolished and cars were all taxed the same, manufacturers soon developed more efficient short-stroke engines.


Neil Cairns.