Grinding for economy

Press, 30 August 1979, Page 11

Grinding for economy

By

A. R. BATEY

It is no secret that the valve timing of a car engine determines the way that the power is developed and its amount. When people start tuning and modifying motors, the camshaft is often modified or exchanged for a “hot” camshaft. The usual “hot” or “power” camshaft can have cams of different profile and different timing, which allows valves to open earlier and remain open for longer periods. The lift or opening of the valves is also often increased. The result of all these modifications is to allow more of the pelrol-air-mix-ture to flow into the cylinders, and to allow the exhaust gases to escape earlier and easier. Camshafts of this nature alter the character of a motor so that it develops more power at a higher rate of r.p.m. The power at lower r.p.m. is usually much reduced, the motor will not idle nicely, and fuel consumption suffers heavily. With the present price ■nd state of fuel supplies, now would be an excellent time for the camshaftgrinding specialists to direct their skills towards the development of a “cool” economy camshaft. The details of the valve timing suitable for good fuel economy and torque at low speeds are not mysteries. Most cars of the early 1930 s era had very flexible motors with excellent low-speed pulling and reasonable fuel economy. The sciences of carburetion and ignition have advanced over the years, and the compression ratios employed have climbed steadily. The timing of the economy camshaft should meet certain requirements: 1. The inlet valve should close very soon after bottom dead centre to en-

sure that a cylinder full of mixture is trapped to ensure maximum compression value.

2. The exhaust valve should not open too soon before bottom dead centre, so that maximum burning takes place and pressure on the piston is utilised for as long as is usefully, possible. 3. Any valve overlap involving the inlet and exhaust valves being open togteher should be kept to a minimum so that incoming mixture is not liable to pass out through the open exhaust valve.

What is going to be the result .of a camshaft with, this liming and what will we achieve? Hopefully, a motor which formerly delivered peak brake horsepower at 5000 r.p.m. will now give its best output in the 3000 to 3500 r.p.m. range. The horsepower developed will, of course, not be as high, but should be. 75 per cent of the standard figure. Due to better cylinder filling and higher compression probabilities, the amount of torque developed should be little

reduced, if at all, and will be moved lower down the scale. A motor which formerly showed peak torque at 3000 r.p.m. could show its best at about 2200 revs and be a useful puller at as low as 1000 r.p.m.

Obviously this motor is not going to attract the younger and more sporting driver, but- that is not the object of the design. If we stay with the standard rear axle ratio giving about 30 km/h per 1000 revs, we will have reduced the comfortable top speed to about 105 km/h, but we will be rewarded by a smoothpulling motor, which will pull top gear easily at low city speeds. Its high volumetric efficiency and the very great reduction in the use of the lower gears should reduce fuel consumption by 20 per cent. The lower revs used should contribute to long engine life. In short we have a car suited to the needs of many mature motorists. For a very small financial outlay an economical and very flexible car is waiting.