Lotus Elan

Twincam Lowest Safe Oil Pressure?

PostPost by: type26owner » Mon Jul 19, 2004 4:38 pm

I've been slowly reducing the weight of Redline synthetic oil I've been using in the engine. Just ran out of 10-40W which when warmed (still overcooled) had 20psi at idle and 60psi at 2k rpm. Bought a case of 10-30W this time. Now it idles at 12-13psi, 2k rpm it's 40psi, 3k rpms it's 50psi and 4k it's 60psi. This engine it totally stock for the S2 but with the exception it has a 60psi high pressure pump. Is this still okay supporting the hydrodynamic film thickness and the shear strenght? I can't find any test data one way or the other. Don't think I can corner hard enough the oil starve the pump. Going to keep a sharp on the oil gauge tomorrow though. Too risky? Where is the dividing line? References? It's kinda silly to run heavier weight oil then is necessary needed to do the job.
-Keith
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PostPost by: paros » Mon Jul 19, 2004 8:17 pm

Keith
I too am intrigued by this problem - running 20/50 but in 30 degree ambient temoeratures and racing for 30 minutes. Pressure with HP and high capacity pump down to 40 [from about 60 when cold] at 4000 and back to 50 plus at 6500. When coolish then good at 60 at 6000. Chatted with red Line - very helpful and agree oil is OK, needs 200 degrees C before it suffers, so could it be a pump characteristic? I had oil cooler too far away from intake and not much air though it. By the way need 16 row as less rows restrict flow and limit pressure.
Will be intersted to see what other answers your question produces
Richard
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PostPost by: type26owner » Mon Jul 19, 2004 10:11 pm

Richard,
The general rule I've seen mentioned is a 10psi increase in oil pressure for every thousand rpm. We should be okay if that rule is actually correct. I don't know for sure. Seems that running too heavy an oil does not help slow down the destruction process, it just changes the way it happens. Forcing the oil pressure relief valve open all the time has got to be wasting hp. Freeing up hp in the engine is kinda something I'd like to do right now.
Regards,
-Keith
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PostPost by: rgh0 » Tue Jul 20, 2004 9:07 am

Keith

Supply pressure in itself is not the crical parameter for successfull lubrication of the engine plain bearings. The bearings and crank journals are kept separate from each other by hydrodynamic pressure built up due to the rotation of the crankshaft itself. For this separation to stably occur you need to satisfy the following conditions for any given bering design.

1. Have sufficient flow into the bearing to replace what leaks out from the supply point to the edges of the bearing.

2. Have an oil of the correct high speed shearing vicosity such that sufficient hydrodynamic pressure is stably built up to hold the bearings appart.

As you go to lower SAE viscosities the leakage across the bearing will increase so more supply will be required to keep the bearing full. At what point you have a problem with the Lotus bearings depends on the pump supply volume and clearances you have in your various bearings. I know of no easy way to calculate what change would be acceptable except trial testing followed by beaing inspection and listening for cattastrophic noises.

The high speed shearing viscosity is only vaguely related to SAE viscosity with synthetics such as redline having much higher high speed shearing viscosity at the same nominal SAE viscosity which is measured at relatively low shear rates. This is one of the reasons why synthetics are much better oils to use in any engine.

The old 10psi per thousand rpm I dont think has much relevance for modern engines and oils even if it was relevant at some time in the past.

My 2 cents worth, I think I would aim to ensure you maintain over 40 psi hot at above 3000 rpm as you drop the oil viscosity and I would suggest ensuring you have a high volume oil pump if you go to or below a 30 SAE viscosity rating to make sure you keep te bearings full.

Rohan
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PostPost by: type26owner » Tue Jul 20, 2004 10:56 am

My 2 cents worth, I think I would aim to ensure you maintain over 40 psi hot at above 3000 rpm as you drop the oil viscosity and I would suggest ensuring you have a high volume oil pump if you go to or below a 30 SAE viscosity rating to make sure you keep te bearings full.

My long time (35 years) parts guy that I really trust tells me I'm okay with 50psi at 3k rpm also. The 60psi high pressure pump that's doing the work must have 100k miles on it already. It's getting replaced with a high capacity/60psi one on the next overhaul which is few months away. Funny how there is tons of info related to this subject but nothing specific for the twincam.
Thanks,
-Keith
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