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Oil Viscosity In Hot Lap
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Toying around with going to a higher split in the oil viscosity grades.

I run a low HP lightweight 6.50 car and subject myself to short turn around times frequently. The water and trans temps are brought back in line but oil temp (presumably) has not. All aluminum engine with a 7 quart steel pan and a diaper.

Currently running conventional 10W30, looking at a 5W40 synthetic.

My thought is that the heavier weight oil at hot temp will slightly increase the windage losses in this situation. It seems to throw a fast run occasionally in this situation and it's caught me off guard in later rounds.

For clarity water/trans temp is 150/150 at burnout, water flashes to 170-175 on the way back and is cooled back to 150 in 5 minutes. Trans goes to 175 and I can generally get it back to under 160 in that situation. I've run the trans hotter and colder without seeing this affect. My last two doubled runs were less than 10 minutes apart.

The temps were 150/150 on the first run and 150/158 on second. Second run was .01 faster at the 330, 60 was within .001.
Posts: 89 | Location: Inverness, Fl | Registered: November 14, 2021Reply With QuoteReport This Post
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Run the transmission at 180 degrees when you pull around the corner into the water. From your first time run to the final.

It'll manage the engine oil temp.

It's one of the good things about a spragless torque converters / methanol.
Posts: 9398 | Location: Madeira Beach Fl. | Registered: June 12, 2018Reply With QuoteReport This Post
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You will likely notice better hot pressure with a 5W40 or 10W40 oil, but you will also likely see more stable oil pressure just by switching to synthetic. With synthetic your oil pressure may be a touch less when its cold, but you will usually see less of a drop when it gets hot too, less of a swing in pressure from cold to hot... If you pay attention you will also likely see less of a swing in pressure from when the oil is fresh to after it has a lot of runs on it. I don't know why anybody would run conventional oil in a race engine (unless maybe it's a solid lifter & not a roller) but that's a different discussion! The small number in an oil viscosity is how it behaves in freezing temps so I wouldn't waste too much effort deciding between 10W40, 5W40 or 15W40. I'm sure one of our board-certified scientists will be along shortly with an in-depth lesson on the composition of motor oil, but that is my experience after 30+ years of racing with synthetic oil.

Billy Duhs - BD104X@gmail.com
Posts: 634 | Location: New Jersey | Registered: February 26, 2000Reply With QuoteReport This Post
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My observations of trying many different oils and of different viscosity is exactly as Billy posted.

I have found that I prefer a semi synthetic oil running Methanol though but that is a different animal than the question. If I was on gas I would always run full synthetic relating to the original question for decreasing swing.

I have no idea how running the trans at 180° will manage engine oil temps though.

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Posts: 2959 | Location: KIEFER, OK. | Registered: August 17, 2007Reply With QuoteReport This Post
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Lighter oil will be faster and also for me safer on the engine. When I ran the 20-50 if I stood on the brakes to kill some it would lose oil pressure until the turn off. While it was never a problem and my bearings always looked good I did not like it. When I went to 10-30 never an issue. My thinking there is the heavy oil does not drain back into pan as quickly.

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