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I race 4 stroke engine go kart, and a very respectful engine tuner said to me that if I run with little less oil then the engine level (80% of recommended, 0,8l) it will break sooner but will become more powerful. The engine is a honda gx390 tuned. I did some research but found nothing. Somebody has a explanation or even a tip? thx

  • important to note this advice (true or not) is specific to small splash lubricated (lawn mower) engines, We don't want anyone to even think running their car low on oil has any benefit, – agentp Jul 28 '17 at 13:19
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Running oil lower than specified may well possibility decrease back pressure and lower viscous resistance by a very small amount, but for the sake of perhaps the most miniscule increase in performance its just not worth risking damaging bearings & other internal engine parts.

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The only theory that seems plausible is to reduce rotational resistance of the crank moving through the oil. This could in theory allow the engine to reach maximum rpm sooner. These engines typically use a dip or splash lubrication method. The spinning crank dips into the sump and distributes the oil. There is no pump to circulate the oil. As your questions state you are sacrificing durability for performance, not really an uncommon feature in performance engines.

  • So, what are the bearing friction losses compared to the viscous drag losses? Are the viscous drag losses large enough to make a difference? – Solar Mike Jul 27 '17 at 11:23
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    Again an assumption on my part, the engineer designed the engine with longevity as a criteria. If adding 10% more oil than the absolute minimum increases life by 25% but sacrificed 1/4hp they would recommend the additional oil. If a racer has the option of 50% less longevity for 10% more HP more power is the choice most racers would make. – mikes Jul 27 '17 at 22:47
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The oil specified in the sump is to make sure that the oil pump does not suck air when in angled situations - the minimum mark is usually sufficient unless you are really pushing the angles.

The other function of the oil is to remove heat, and dirt obviously, and that is the other basic reason to have the level correct.

I have not heard of running lower than the minimum for power, but most racers choose an oil that performs well at high engine speed and is not too viscous etc to minimise the pumping losses while providing excellent engine protection.

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