Does a modified car need a different oil from that specified by the manufacturer?

If you modify your engine, should you change your oil from that specified by the manufacturer?

First, let’s look at the criteria that the engine/car manufacturers take into consideration when selecting an oil for the engine:
• Cost
• Quality

A manufacturer will select an oil that just meets but does not exceed the standard engine specification. If the car is petrol fuelled, this is usually API SJ. Almost all cars leave the factory with a low-cost mineral based oil in the engine, and dealers use a similar grade for servicing. This is because it costs more to produce a high quality oil than a lower quality oil. Consequently this usually means that the oil the manufacturer chooses is operating somewhere near the top end of its capability.

What happens when you modify your car/engine?
First and foremost you invalidate the manufacturer’s warranty. (This is your problem, not anyone else’s.)
Second, you cause many of the engine’s normal running parameters to change.

What happens?
To get more power, an engine must be modified so that it converts more fuel per minute into power than it did in standard form.

For example to produce 6.6 million foot-pounds per minute of power (approx. 200 BHP) a modern engine will burn about 0.5 litres of fuel per minute (equivalent to 18mpg at 120mph.) To increase this output to 300BHP or 9.9 million foot-pounds per minute it must be modified to burn (in theory) 0.75 litres. However, fuel efficiency goes out of the window when power is the only consideration, so the true fuel burn will be rather more than 0.75 litres/min.

Note: a modern turbo engined rally car such as the Subaru WRC will use approx 1.0 litre of fuel every mile (4.5 mpg) when used in anger.

The fundamental problem is that less than 30% of the fuel (assuming it is petrol) is converted to all those foot-pounds. The rest is thrown away as waste heat. True, most of it goes down the exhaust, but over 10% has to be eliminated from the engine internals, and the first line of defence is the oil.

More power means a bigger heat elimination problem. Every component runs hotter; for instance, piston crowns and rings will be running at 280-300deg C instead of a more normal 240-260C, so it is essential that the oil films on the cylinder walls provide an efficient heat path to the block casting, and finally to the coolant. Any breakdown or carbonisation of the oil will restrict the heat transfer area, leading to serious overheating.

A modern synthetic lubricant based on true temperature-resistant synthetics is essential for long-term reliability. At 250C+, a mineral or hydro cracked mineral oil (particularly a 5W/X or 10W/X grade) is surprisingly volatile, and an oil film around this temperature will be severely depleted by evaporation loss.

The 1970’s solution was to use a thick oil, typically 20W/50; in the late1980s even 10W/60 grades were used, but wide-ranging multigrades of this type tend to be vulnerable to shear effects, and a 10W/60 oil based on an average VI improver will soon thin down in service. But in modern very high RPM engines, with efficient high-delivery oil pumps, thick oils waste power, and impede heat transfer in some situations. They are also more prone to aeration, which can lead to rapid component wear, such as big end and main bearings.

A light viscosity synthetic PAO/Ester oil, formulated for severe competition use, is the logical and intelligent choice for the 21st century.

It should also be remembered that most highly modified engines are not fuel efficient, and so the oil suffers to some degree or other from fuel dilution and bore wash, so the oil should be changed more frequently depending on the rate of dilution.

Remember - power costs money, both in terms of modification, and running costs. You can’t spend umpteen thousands of pounds modifying your car and then expecting a £12.00 can of psuedo synthetic oil to take good care of your pride and joy.

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