Engine oil viscosity change?

[Bomber604]

I'm looking to eak a couple more horses from my 1.8ltr 4 pot dohc 16v engine which is already built to the max spec allowed in my championship.

Currently running 10w50 Morris's X-RPM fully synth competition oil as recommended by the engine builder.

Engine has covered 400 miles over roughly 8 hours since build under testing & race conditions without missing a beat.

My question is this, will running a thinner oil i.e. 5w40 (fully synth) reduce internal drag/frictional losses sufficiantly to release any more power without putting the engine at risk of damage?

[GORDON STREETER]

Running an engine on a thinner oil will produce more power through reducing frictional losses and using less power to drive the oil pump. It normally drops the oil pressure as well.
However as to "how much" you won't know unless it's tested on a test bed as a rolling road isn't accurate enough, and you won't notice any "massive" difference.
I would have thought that there is no problem in running a fully synthetic oil that is recomended by the original engine manufacturers regardless of the state of tune.
The only problem that could arise is if it has a failure then your engine builder will not have anything to do with it regardless of if the "problem" has anything to do with the oil at all.

[Notso Swift]

Originally Posted by GORDON STREETER

Running an engine on a thinner oil will produce more power through reducing frictional losses and using less power to drive the oil pump. It normally drops the oil pressure as well.
However as to "how much" you won't know unless it's tested on a test bed as a rolling road isn't accurate enough, and you won't notice any "massive" difference.
I would have thought that there is no problem in running a fully synthetic oil that is recomended by the original engine manufacturers regardless of the state of tune.
The only problem that could arise is if it has a failure then your engine builder will not have anything to do with it regardless of if the "problem" has anything to do with the oil at all.

Yes, but only a the specific rpm before the relief valve opens, because the if the valve is set at (say) 60psi and was opening at 3000 rpm (remembering the oil pump is 99% chance going to be a positive displacement pump so it is pretty much proportional in output to speed) it may now open at 3500 rpm.
The thinner oil has a higher flow, so you have increased volume for the same pressure (same volume at same speed, though)

I was once told by a powerplant engineer I race with to "Always use the thinnest oil you can, but not too thin!"

Other things to consider, the "rule of thumb is you need 10psi per 1000 rpm", but of course like all of these rules the exceptions will kill your motor! (I reckon this rule was built around old 50/60's tolerance pushrod motors, not modern stuff)

I guess that doesn't help much but it may give you more to think about

[Al Weyman]

10psi per 1000rpm sounds like the old Chevy Small Block advice. I like to see a bit more at the lower revs as ticking over at 7 or 8 psi would be a tad worrying to me!

[Notso Swift]

Most likely it is SBC (and probably Mopar/Ford as well), a lot of "Race-lore" starts with that stuff

I think the peak as more what they are talking about, but don't discount the requirement for low load being significantly less, especially if the volume is good. Most of the pressure a bearing sees is created by the crank and wave of oil , not really that much to do with the system pressure, the system pressure is required to fill up the area behind the crank and replace the oil that has been lost out the sides.

[wnut]

Originally Posted by Bomber604

I'm looking to eak a couple more horses from my 1.8ltr 4 pot dohc 16v engine which is already built to the max spec allowed in my championship.

Currently running 10w50 Morris's X-RPM fully synth competition oil as recommended by the engine builder.

Engine has covered 400 miles over roughly 8 hours since build under testing & race conditions without missing a beat.

My question is this, will running a thinner oil i.e. 5w40 (fully synth) reduce internal drag/frictional losses sufficiantly to release any more power without putting the engine at risk of damage?

Speak to your chosen oil company's technical department, they will advise you what might help!

If they won't help you change your oil supplier!