Interesting article from the economist

[jpu471]

The Difference Engine: All change, oil change

Nov 5th 2010, 7:26 by N.V. | LOS ANGELES

WITH a trip planned up the back roads to the High Sierras and another 5,000 miles on the odometer since his last oil change, your correspondent spent a recent morning beneath his superannuated runabout, draining the sump and wondering what precisely to re-fill it with. It is no good looking at the workshop manual. The lubricants recommended by the manufacturer when the car was built back in the 1980s live on in name only. Their formulations have changed out of all recognition as a consequence of the motor industry’s quest for ever better fuel economy and lower exhaust emissions.

All of a sudden, a lot of well-maintained old cars, especially sportier models, have begun to fail in surprising numbers—as if struck down by some virulent new disease. Engine builders say they have seen more broken motors suffering from the same camshaft damage over the past three years than in the previous 30. Reports have circulated within both the classic-car and hot-rod communities of priceless performance engines failing after only 300 miles since a rebuild.

Modern car engines that use rolling or sliding tappets for transmitting the rapid up-down lifting motion of the spinning camshafts—to open and close the engine’s inlet and exhaust valves—seem immune. But those that use the older flat-faced tappets to trace the camshaft’s profile are particularly prone to the disease.

The first sign of illness is a sudden loss of power which, if not rectified immediately, can cause the engine to self-destruct. On stripping down a damaged motor, the tappets are invariably found to be heavily pitted and bits of metal broken off the camshaft lobes. With metal fragments lodged in the oil galleries downstream of the oil filter, the engine block has to be flushed to prevent the loose bits from causing further damage to bearings and cylinder walls.

Some point a finger at the “aggressiveness” of the cam profiles used on elderly performance cars as a possible source of the problem. Indeed, it was because similarly aggressive camshafts started being used on modern family cars—to get more power out of smaller engines and hence better fuel economy—that flat tappets were eventually abandoned for less stressful rolling or sliding camshaft followers.

In any event, the sharply pointed shapes of the lobes on aggressively profiled camshafts exert pressures in excess of 200,000lb per square inch (14,000 kg/cm²) at the contact point between the tappet and the lobe. Some believe that the hardened gray iron used to make the components simply “fatigues” with age—as metal parts that go through millions of stress reversals eventually do. But that does not explain the short life of many engines after a rebuild using new components.

Others put the outbreak of camshaft failures down to the recent influx of poorly made components from abroad. Once roller tappets started being used on push-rod engines and slider followers on engines with overhead camshafts, half the automotive component firms in America making flat tappets either ceased production or went out of business. Today, only GM and Stanadyne produce flat-head tappets—and, even then, in nothing like the volumes they used to do.

The shortage has created an opening for Asian suppliers. Unfortunately, though flat tappets from China are cheap, their metallurgy is suspect—and the same goes for the quality of their machining, heat treatment and surface-hardening. Nevertheless, while such tappets are unlikely to last anywhere near as long as the original parts, they are hardly going to disintegrate after a few hundred miles. Shoddy manufacturing is unlikely to explain the sudden failures observed.

That leaves only one other suspect—the oil used to lubricate the engine. To prevent excessive wear of the valve gear, motor oils have traditionally contained a fair amount of a zinc and phosphorus in the form of zinc dialkyldithiophosphate (ZDDP)—with typically 1,700 parts per million (ppm) of zinc and 1,600 ppm of phosphorus. The ZDDP additive works by getting the zinc to react with the iron of the tappet surface and cam lobe, creating a glassy sacrificial barrier between the two. As the barrier wears away, it is replenished constantly by the ZDDP in the oil. The phosphorus plays a similar, though lesser, role in protecting the engine’s valve gear.

When it left the factory in Britain 23 years ago, the 20W-50 blend of mineral oil in your correspondent’s car contained ZDDP in the above proportions. He has tried to use something similar ever since. Unfortunately, that has become increasingly difficult of late. Carmakers everywhere have pressed suppliers into reducing the amount of ZDDP in their motor oils. Having abandoned flat tappets, their modern cars no longer need the zinc and phosphorus in such proportions.

More importantly, however, as engines age, their cylinder bores wear slightly and develop “blow-by”, allowing traces of oil to creep past the piston rings and into the combustion chambers—and thence into the exhaust. Apart from poisoning the catalytic converter and reducing its useful life, zinc and phosphorus mixed in with the exhaust gases add to the motor industry’s emissions woes.

As a result, the ZDDP in motor oils has been steadily reduced. Lately, however, the concentration has fallen precipitously in America—in preparation for the introduction of catalytic converters on big trucks. Under the latest standards for motor oil (SM and GF-5) recommended by bodies such as the American Petroleum Institute and the International Lubricant Standardisation and Approval Committee, motor oils can have zinc and phosphorus concentrations of less than 800 ppm. That is nowhere near enough to protect the flat tappets in an old Chevrolet Corvette, Ford Mustang or your correspondent’s beloved Lotus (“Lots Of Trouble Usually Serious”) Esprit.

What to do? Some suggest using motor oils formulated for old diesel-engined trucks and buses. For the time being, these still have the full complement of anti-wear additives. Shell’s Rotella T diesel oil is available with either a mineral or synthetic oil base and a traditional package of anti-wear additives. The oil’s higher viscosity—while a detriment to modern petrol-engined cars with their tighter tolerances and need for lighter oils—is a positive advantage in older engines with their looser-fitting parts and lifetimes of wear.

Diesel oils also have superior detergents to keep the engine’s innards clean. That translates into lower oil consumption as well as better mileage. Even so, your correspondent remains unconvinced. He worries, in particular, about the blow-by in his elderly Lotus engine allowing dollops of detergent to build up inside the combustion chambers. That can cause the air-fuel mixture to detonate prematurely and destructively, rather than burn in an orderly manner.

He has looked at off-the-shelf racing oils, like the fully synthetic 0W-50 version of Mobil 1. While this has all the ZDDP, and more, for preserving flat tappets (1,850 ppm of zinc and 1,750 ppm of phosphorus), it would make short shrift of his car’s catalytic converter. Also, you can have too much of a good thing. Excessive amounts of ZDDP can corrode the engine.

As an experiment, he has now filled his car with a synthetic 20W-50 oil that has the viscosity and anti-wear additives identical to the blend Lotus specified back in 1988. The only difference is that the oil in question has been designed not for aging motor cars, but for modern vee-twin motor bikes. He now awaits with interest to see whether his rejuvenated sportscar develops a propensity to do wheelies down the street.

[Chris@Adams]

Interesting read!

[Adam]

Excellent and interesting read Ken!

[jpu471]

Morning Adam, sorry I missed you at ACC. So does this mean you will start running diesel oil in your Corvette?

[whiterp199]

The corvette has a modern ls motor I believe. So I would not. Great article though.