“SUN” SERVICE STATION

Sun (Auckland), Volume II, Issue 474, 2 October 1928, Page 6

“SUN” SERVICE STATION

“Motordom” section of The Sun includes in its service to readers a Queries and Answers Department, the object of which is to give accurate information regarding mechanical and touring subjects. Whatever the particular puzzle may be in either diagnosing irritating troubles, or motoring information of any kind, “Headlight'’ will endeavour to help, or refer the point to men who are recognised automobile experts in their respective lines. All communications must be accompanied by the inquirer’s name and address (not for publication), and sent to THE SUN “Service Station, Motordom,” THE SUN, Auckland. Answers will appear weekly in this column.

A.L.M.—There is a persistent knock or heavy rattle under the floorboards of my car—a Ford —that I cannot locate except when the car is travelling at fairly high speeds. I experienced this annoyance, and traced the cause to the portion of the brake pedal which comes through the floorboard being slightly bent, knocking on one side of the slot when the car was in motion. The knocking ceased immediately the pedal was held rigid against one side of the slot in floor. I straightened the pedal, cut away a small portion of the metal plate which is attached to the floor board, giving it greater clearance. The knocking ceased. The clutch pedal could doubtless cause the same trouble. H.F.F.—As I am a “Hup.” (1924 model) owner, I am interested in that make of car. Therefore, would you tell me what is the meaning of hyper-ex-pansion, such as they advertise in their new models? What is the principle of the idea, also its effect or? engine performance; does it effect economy in running, or has it any ultimate deleterious effect on the engine, etc.? A short reply in Tuesay’s S.S.S. column will suffice. Answer: The principle of “hyper-ex-pansion” is more or less explained by the coined phrase itself, as “hyper” means “extreme,” or “carried to the extreme limit.” So that, to cause anything to expand to its extreme limit, it is first necessary to compress it into the meanest possible space. The matter to be expanded in a motor engine is fuel, and expansion takes place on explosion. This boils down to an increase in the compression ratio, which is 16 per cent, greater in the 1928 Hupmobile than it was in any previous model of that make. With the increase in compression, a new cylinder-head was necessary, and this is a high turbulence head, as spherical as possible, and devoid of “pockets” which store unburnt fuel. If all the fuel, instead of, say, 90 per cent, of it, can .be made to explode, the expansion is greater, even without higher compression. The result should be more power, better acceleration, and greater economy, for each charge of fuel should impart more driving force to the crankshaft if it is wholly consumed and fully expanded. The only possible deleterious effect would be caused by a use of metals not sufficiently strong to stand the strain of greater compression and a fiercer punch on pistons and bearings. The cylinder head construction helps to minimise this punch by progressively stepping the force down on to the piston, and it can be safely assumed that — i Has Mother joined the users of j Radium Polishes —for Boots, Floors, i Metals? Save the Coupons—win a j prize. I- ,

the metals are sound, for tensile strength, pressures and breaking points are elementary knowledge in any design department. T.C.R.—When I dismantled my engine I discovered that the valves had become so deeply pitted that grinding is not sufficient to smooth them again. What can I do? If your valves are so deeply pitted that grinding will not smooth them, take them to a machine shop and have them shaved on a cutter. A good mechanic will tell you whether there is enough metal on them to risk shaving some off. If they are old valves they may have been ground so much that cutting would be inadvisable, in' which case the best thing to do is buy new ones. The cause of pitting may be poor metal. Lots of valves in old cars were made of the wrong steel, but present-day alloys seldom pit seriously. A mixture either too rich or too lean, which makes the exhaust valves too hot, may ’>ave something to do with the trouble, because it causes local corrosion. Little bits of hard carbon also catch between the valve and its seating, and are aam - mered into the valve every time it lifts and falls. The best thing to do • lock them over pretty frequently and touch them up if they show small pitmarks. “Puzzled.”—A second-hand car I bought some time ago used to get very hot, and I was told the mixture was too rich, so adjusted the carburetter several times. As it still got _oo hot, I inquired again, and was told the mixture was too lean. Now, if the engine can get too hot, because it burns too much petrol in each charge, how can it get too hot when it has not enough petrol? The reason a weak mixture causes the engine to overheat is this: —Being weak, the mixture does not burn quickly enough, so that instead of exploding in one flash it burns all the time the piston is falling in the firing stroke, and sometimes while it is rising on the following, or exhaust, stroke. The top of the cylinder is coled by water jackets, but the lower cylinderwalls are not so well cooled, and cannot quickly get rid of the heat they receive from the slow-burning mixture which is hot all the way down, so to speak, instead of merely at the top of the cylinder. A HJ NT ON TYRES “Being in the tyre business, I get the usual number of complaints from people who say their tyres are not standing up to the mileage, ana so few of them know how to treat their tyres, I would be glad if you would broadcast this. Tyres will not wear well if they are kept at low pressures when tho car is being driven fast. Pressures recommended by the makers are all right for running around the city, and a pound low might not hurt, but people who go out of town during week-ends with the same pressure in their tyres and drive at 50 and 60 miles an hour, will get quicker tyre-wear. For high speeds they must use high pressures. Three pounds above the normal pressure should at least be used, and in some cases 51b.”