The Arnott "Depinker"

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IT has long been known that, in theory, the admixture of a small amount of water to the charge defeats “pinking.” Water will not pass through the jets of a normal petrol carburetter, so alternative means must be found of blending it with the petrol-air mixture after the petrol has left the jet. At various times, devices have appeared on the market which, taking their supply from the radiator or from a separate tank, have supplied water or steam to the induction system.

So far as we know, once they were turned on, they all maintained a supply at a more or less constant, or at best manually controlled, rate. This led to an excess of water being supplied when the engine was not in a position to dispose of it, and to water remaining in the combustion spaces after the engine was switched off. The result was the generation of carbonic acid, which leads to rapid corrosion (and wear) of the bores.

Now it so happens that the conditions which lead to pinking are precisely those which enable an engine to dispose of any water which may be present in the charge. As the engine only needs water when it is in the “pink-producing” condition, it follows that, if the water can be so controlled as only to arrive at those moments, the corrosion snag should be overcome.

That is what the new Arnott device sets out to do. Water is fed to a float-chamber of normal design, and thence, via a control valve which governs the maximum flow, to a submerged jet. Controlling the outlet is a suction-operated valve which is the crux of the whole affair. When the engine is idling or on the over-run, there is maximum induction-depression, the valve shuts, and no water flows. Stamp on the accelerator and you produce the most complete breakdown of induction-depression: the valve opens fully and the full flow of water takes place. Intermediate throttle openings, and maximum revs, at the wider openings, produce a less complete breakdown of depression, and the flow of water is correspondingly less. There is a second pre-set control beneath the suction valve which governs the degree of opening for a given suction, in exactly the same way as the hexagon nut beneath an S.U. carburetter controls mixture.

On the test car, the 1 gallon water tank is over the driver’s feet, feeding by gravity to the “Depinker” situated between the two S.U. carburetters. The consumption, incidentally, is approximately 1 gallon of water to 10 of petrol.

On this particular car it was an easy matter to make up a little bracket to attach the “Depinker ” to the square-sectioned aluminium balance pipe between the two carburetters.

On other cars, a little ingenuity may be needed to devise a suitable mounting. The aforementioned mounting is convenient because it enables a very short pipeline to pick up induction-depression from the balance pipe. From the front of the “Depinker ” a forked pipe leads water to a small nipple tapped (2 B.A.) into the carburetters between the piston and the throttle butterfly. On “fixed choke” carburetters, this tapping would be made at the point of greatest constriction, i.e., where the airstream would be moving quickest.

Now for the “Depinker” in use. Actually, on the car in question, it does not entirely stop pinking, but it does reduce it by at least 80 per cent. We should add that this car has a compression ratio in the region of 9 to 1, and having done nearly 8,000 miles since it was last taken down, was in a desperate state of “pinkage” when water was applied.

Pinking was immediately reduced from the intolerable to a degree comparable with that of the same car with a clean engine running on Discol. No alteration in petrol consumption has been noticed, but it must be remembered that the vastly improved tractability enables the car to be driven much harder. Also, finality of mixture setting under the new conditions has not yet been achieved.

After about 300 miles, the car showed a tendency to oil its plugs, and we began to wonder, remembering the claim that this water-treatment puts an end to coke formation, whether it might have washed away the precious layer on the piston crowns! Actually, we suppose that the factors which cure pinking may also call for a softer plug: at any rate, a softer grade now gives results which are excellent.

Whether or not the principles governing the Arnott “Depinker” do successfully get over the bore-wear snag altogether, we cannot say. It will be up to Arnott’s to conduct a test with two identical cars, one with and the other without the “Depinker,” and measure their bore-wear after a given mileage. Meanwhile, we feel that, to anyone whose car suffers really badly from cesspool pinking, this “Depinker” may be a Godsend.

Makers are: Carburetters Ltd., Grange Road, Willesden, N.W.10.