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Oil advice for modifiers!

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Oil advice for modifiers!

Postby oilman » Fri Jul 03, 2009 11:38 am

If you are modding your car and adding BHP or using it off road then consider your oil choice carefully as the stock manufacturers recommended oil will not give you the protection that your engine requires.

A standard oil will not be thermally stable enough to cope with higher temperatures without "shearing" meaning that the oil will not give the same protection after a couple of thousand miles as it it when it was new.

Let’s start with the fundamentals. An engine is a device for converting fuel into motive power. Car enthusiasts get so deep into the details they lose sight of this!

To get more power, an engine must be modified such that it converts more fuel per minute into power than it did in standard form. To produce 6.6 million foot-pounds per minute of power (ie 200 BHP) a modern engine will burn about 0.5 litres of fuel per minute.(Equivalent to 18mpg at 120mph). So, 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 often 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.

That’s the fundamental point, here’s the fundamental problem:

Less than 30% of the fuel (assuming it’s 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 280-300C instead of a more normal 240-260C, so it is essential that the oil films on 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 hydrocracked 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.

Back in the 1970s the solution was to use a thick oil, typically 20W/50; in the late 1980s even 10W/60 grades were used. But in modern very high RPM engines with efficient high-delivery oil pumps thick oils waste power, and impede heat transfer in some situations.

A light viscosity good synthetic formulated for severe competition use is the logical and intelligent choice for the 21st century.

You must seriously consider a "true" synthetic for "shear stability" and the right level of protection.

Petroleum oils tend to have low resistance to “shearingâ€
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Postby Ferret WR1 » Thu Jul 09, 2009 5:02 pm

Excellent advice, although it's put doubt in me as I've just had my WR1 serviced at a Subaru approved service centre and they used Shell Helix Ultra 5W-40. What you view on this oilman?
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Postby oilman » Thu Jul 09, 2009 9:09 pm

Should be fine for a standard road car

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Postby John Mc » Fri Jul 10, 2009 7:51 am

I've just had mine serviced at a local Subaru dealer, not sure which oil they used but I'm assuming standard manufacturers recommended??!

When I drive normally (as in not driving hard) the oil temperature is stable (on Defi's) at 80ish deg.... if I drive it fairly hard (short bursts in 2nd & 3rd @ 1.4bar) for 1-2 minutes, this shoots up to 90-95 very quickly. I am not surprised that the oil temperature rises but I didn't know whether to expect it to rise this quickly?

I was expecting that you'd have to drive it hard for 5-10 minutes for the oil temp to rise this much. Is this "normal" or should I have requested a higher spec oil?

The car is totally standard.

Cheers.
[Just incase this gets lost in translation, I am not slandering you, I know people can be sensitive to these harsh words.]
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Postby oilman » Fri Jul 10, 2009 9:14 am

According to the handbook, if I recall this car needs 5w-30 and would recommend a synthetic, especially if driven hard.

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