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The extent of power increase to be expected on any given engine, as a result of the various modifications described, can only be approximated. However, referring to historical information from the Aquaplane Company Ltd. who have accumulated a considerable amount of data from dynamometer readings, it is possible to quote useful figures which can be taken as a yardstick. Obviously, the extra power called for, and its magnitude, will depend mainly on what the vehicle is to be used for. The owner of a standard car, for example, may wish for either just sufficient extra to give his vehicle an air of individuality, or he may be a rally enthusiast wanting the maximum power obtainable for short periods, as in special tests. Lightweight specials and sports or sports-racing cars, again, with their ability to build-up rpm very quickly, need considering in a different light. The table, showing approximate percentage power increases, will be found useful in this respect.
For the standard-bodied car used for fast touring on premium fuel, an increase in compression ratio, with no other alteration, may give up to 10 per cent power increase throughout the speed range without prejudicing flexibility; the range of rpm will of course, remain substantially as standard. The maximum compression ratio that should be used in this case is 7.5 to I, and even this may be found a little high, if so it can be reduced to a lower figure. With a relatively minor alteration such as this, there is no point in going to too much trouble in working on the engine components beyond ensuring that the engine " as standard " is in good condition.
Our Aquaplane cylinder heads give a compression ratio of approximately 8.0 to1 (on a standard 2.5" bore). This can then be reduced by removing material from around the valve area.
This modification is obviously one of the most useful in promoting more power and in extending the useful speed range. With the retention of the standard compression ratio and fitting a good manifold designed for taking a second Ford-Zenith carburettor to pair the existing one, the following figures are obtained:
If the foregoing is combined with a higher compression ratio, say, 7.5 to I, about 10 per cent can be added to the bhp figures.
The Ford-Zenith instruments have a small choke and the substitution of Zenith type VIG carburettors will provide an extra 2 bhp, or so at the upper end of the power curve. As it is feasible in this case to hold the engine at high revs for considerable periods, stronger valve springs are really essential. A similar performance will be obtained using 1 1/8-inch S. U. carburettors.
With I 1/4-inch S. U. carburettors (or equivalent sizes of other makes) it is possible to prolong the engine speed range to another 500 revolutions approximately, though there is a falling-off in power. In this case, a well-designed exhaust system and manifold is essential to deal with the extra heat; such a system is, of course, an asset with any twin-carburettor layout, but its power-adding propensities are not so apparent until the volume of extra mixture is really considerable.
Thus, equipped as stated, and with alloy head of about 8 to I compression ratio, the following are the figures:
The falling-off in power above 5,000 rpm is evident in the above example and this is, of course, due to the limitations of the standard camshaft. Apart from individual attention to engine components, however, this represents about the best that can be done, up to 1172 Formula standards. A special camshaft will extend the range of rpm to 6,000 and over; for such speeds, dynamic balancing of the rotating parts is essential, and a high pressure oil pump is also required. It is also highly probable that a water-pump will be necessary to give the required speed of water circulation. Power with special camshaft, plus equipment already detailed:
Modifications to engine components will have an advantageous effect, but obviously the more extreme measures such as relieving the block should be kept for the absolute ultimate in performance regardless of the risk of a blow-up. Acceleration is improved by the use of a light flywheel, while the general weight-to-power ratio can be decreased by fitting up the engine with light-alloy components such as tappet and timing-chain covers and water impeller.
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