A Salutory Warning

“Tuned” Engines Running On Current Fuels

by Ken Lea and Harvey Hine
We are all aware that both legislators and oil companies continue to make changes to available fuels. Thus we have all learned to cope with unleaded fuels and the all but complete demise of leaded fuels to the previous standard.
                More recently, some members have reported either rough running at light throttle at engine revolutions above about 45% of the maximum permitted or of more serious problems. In one case with which I was associated, an uprated
Derby 4¼ litre engine suffered piston failure by pre-ignition, which burnt out the piston crown. This failure phenomenon is usually associated with the part load mixtures being too weak and well above the Stoichiometric value of 14 parts of air to one of fuel. It can also be associated with incomplete mixing at the point of fuel ignition where the ratio is richer overall but burning regimes are uncontrolled. This engine had been uprated some years ago to produce about 40 % more power and had run without any problems and accommodated the change from leaded to unleaded fuel without problems. The engine suffered complete failure of one piston and damage to another. The engine was carefully rebuilt with all common aspects of pre-ignition failure being checked, but nothing significant was found. Disappointingly, the engine suffered a second failure on the same cylinder within a short mileage and the engine was rebuilt again. It seemed sensible at this juncture to recheck the mixture regimes on a rolling road, although no change from the original had been made since it was tuned originally after its uprating. Astonishingly, on fairly light throttle at engine speeds above 2500rpm with a normal maximum of 4200rpm, the mixture ratio became progressively weaker as revolutions increased. This engine had a compression ratio increase as well as attention to manifolding when it was modified and has run for thousands of miles on 97 Octane fuel and, in its early life, was raced successfully. Attention to needle specification cured the problem and, since the check and adjustment, the engine has already passed the miles to failure of the second failure.
                The cause of the problem was not fully understood but, in the meantime, a second uprated Derby engine similar to the first but with different but non-standard inlet manifolds but with a common free flowing exhaust system, exhibited rough running characteristics of misfiring and power loss above 2500rpm and typical of a weak mixture as all ignition components tested correctly. Again this engine had been tuned successfully some years ago on a rolling road, has been raced extensively and successfully without an engine rebuild. In view of the problem with the first engine, the car was taken to the same rolling road facility as the first, and although the weak mixture problem was less pronounced, the air/fuel ratio measurements showed exactly the same trend. Retuning of the needle specifications restored the engine to full power. In both cases, the increase in fuel flow mid range and above was significant. Discussion of the problem amongst a few members revealed that WO engines that had been uprated had also suffered a similar piston failure.
                It was not clear if all engines were being affected and hence a standard 4½ litre WO engine with modest compression and a standard build 4¼ litre
Derby engine were subjected to a rolling road test. In the case of the 4½ litre WO, only the CO reading was taken, but it did exhibit a reading of 0.3% instead of its usual 3.0%. Slight enrichment of the needle cured the problem although the engine exhibited no overt problems. The 4¼ litre Derby engine had air to fuel ratios as standard. Both engines operate on 95 Octane fuel.
                The situation is worrying, particularly as there is no complete explanation of the change. One change that has been forced upon us is the mandatory introduction of Ethanol into fuel and it is clear that larger quantities of Ethanol are associated with the higher octane fuels as it is apparently used as an octane enhancer, but has a lower calorific value.
                There is also another problem that has appeared recently and that is for the B60 engine fitted to MkVI, R and S types. There is a discernable reduction in the thermal capacity of clean coolant systems, especially in heavy traffic conditions, which I too have noticed on my own MkVI and has been confirmed across the industry. Again, there is no complete explanation but suspicion is centred on fuel specification.
                All of very mysterious concern and members are recommended to check air fuel ratios on WO and
Derby engines, especially if they have been uprated. As far as the B60 is concerned, the recommendation is to fit a “pusher” electric fan, thermostatically controlled, in front of the radiator matrix.
                We would welcome news of any similar problems experienced by members.


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