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So here's another thing I've been reading up on.

When I finally got my C5Z to a point where oil pressure rose to the top of the "to do" list, several people said they were concerned about the low oil pressure showing on my gauge. I don't remember exactly, but seemed to me when the 10w-30 oil I was using was running at operating temperature, the pressure was only around 22 to 24 psi at idle. My engine builder (LME) said it was nothing to worry about and was perfectly OK. But I asked him about going to a ticker oil to bring the pressure up. He suggested Joe Gibbs 15W-50, which is what I have been using for over a year now. Yes, the pressure is up to around 35 psi at idle. And so far I haven't seen anything remotely like a problem using this heavier weight oil.

But I'm rethinking this now after the reading I've been doing.

The oil pump I have in the engine is a Melling HP Ported pump (model 10295). The "HP" stand for "High Pressure". Actually according to Melling's website, it means "Standard Volume, High Pressure". It's supposed to be a high pressure performance upgrade to their model 295 oil pump, supposedly offering 10 percent higher pressure at stock volume. Supposedly "providing additional flow to performance engines with wider bearing clearances."

But what does that MEAN, actually? Is there a target PRESSURE that this oil pump is striving to meet? Or is it FLOW that really matters, and this pump just delivers the pressure needed to meet that needed flow? Logic seems to tell me that if you raise the pressure 10 percent, then wouldn't volume increase over stock? Does this mean the pump moves MORE volume so that stock pressure readings are met?

Just how much oil pressure is really NEEDED to provide adequate flow for the engine?

Well, I heard from quite a few engine builders (I emailed a BUNCH of them when I was experiencing that above mentioned "low" oil pressure) and every one of them (I mean EVERY ONE of them) said that as long as I was getting at least 10 psi per 1,000 rpm, I was fine. I'll be perfectly honest, I never tried to monitor oil pressure while running up the RPMs. Is that oil pump delivering at least 60 psi at 6,000 rpm? Beats me. I always had something else I felt I needed to be looking at during that time. If I were seeing a pressure reading of 120 psi at 6,000 rpm, is that good or is that bad?

So what about that 15W-50 oil I am now using? Well, I have read that it can cause a small reduction in engine output power, but that really wasn't a concern for me. At least not as a "what power is getting to the rear wheels" kind of issue. And supposedly the thicker oil tends to cling better to engine parts and doesn't seem to "drip off" as readily nor easily as a thinner oil could do. So perhaps one negative is balanced out by that one positive.

But maybe I've just been looking at this wrong. What, after all, IS oil pressure? Well, push come to shove, it's the RESTRICTION that the oil system in the engine is giving AGAINST flow from the oil pump. Without any restriction at all, you would see zero pressure. With a full blockage, well, I don't know what would happen. Does the oil pump break, or does the pump just churn the oil via a bypass that stops trying to send oil into the block. Certainly not a good thing, I would imagine. What good is saving the oil pump if the engine self destructs from oil starvation?

Anyway basically, now with the 15W-50 it is taking 35 psi of pressure that the oil pump is able to deliver to push the same volume of oil that it could at 22 psi with the 10W-30 weight oil. That is because the thicker oil is just tougher to move than a thinner oil. Well, that sounds OK, doesn't it? As long as the engine is getting the oil flow it needs and the pump is able to provide the needed pressure, we're good aren't we?

Or are we?

This could be looked at from a different perspective. Now, with the thicker oil, the oil pump (and what drives the oil pump?) is working much harder to be able to push the volume of oil the engine was designed to receive. Hmm... What is it that we WANT from an engine? Power? Is using more power to drive the oil pump such a good idea? Does having the oil pressure higher than what is actually NEEDED going to cause MORE wear and tear on the engine and especially the oil pump itself?

If you read up about oil pumps, everyone is going to be telling you that oil pumps don't deliver pressure, they deliver flow. Pressure is being caused by the restrictions in the engine. Well, yeah, maybe this is how it's explained on paper, but in the real world, PRESSURE is how we are measuring oil delivery to the engine. THAT is what is important to us. We have no sensors to detect oil flow, so that's the best we have available to use as a guide.

If you start your car up one morning and notice you have zero (0) oil pressure, are you going to think, "Wow, man, that's GREAT! My engine is able to flow so much oil that there is no longer any restriction to my oil pump!"? Hell no! You are going to say "Holy crap!" and shut down your engine in a hurry, because you SHOULD be thinking, "Damn, my oil pump might have failed!". The logical assumption here is that zero oil pressure means zero oil flow.

Oil flow is needed for an engine to be properly lubricated. Oil PRESSURE is what tells us that the flow is taking place. Probably the first sign you notice when you are having an oiling problem (lost pressure or lost volume) is that your hydraulic valve lifters will start kicking up like you threw a bunch of steel ball bearings into the engine. Suddenly they are not pumping up from the expected oil pressure and now you have play between the lifters, camshaft, rocker arms, and the push rods making the valve train clatter. When you hear that noise SHUT THE ENGINE OFF.

Of course, you have to wonder what your first sign of an oil system failure would be if you had an engine with solid lifters. Smoke, perhaps? :shrug01:

Speaking of which, I read that too low oil pressure can cause partial temporary failure of the lifters during high RPM. Not sure that this would be something audible, but surely it would cause a drop in power. Could it cause an actual drivetrain failure? Beats me, but I would think collapsing hydraulic lifters at the top of the rpm range really wouldn't be a good idea.

So again, how much oil pressure at PEAK RPM is actually needed? Does the 10 psi per 1K rpm still apply then?

Anyway, as I mentioned, I'm rethinking the 15W-50 oil I'm now using. Am I just working the oil pump more than it needs to be worked if the 22 to 24 psi I had with thinner oil is perfectly acceptable? Is my engine working harder than it should just to run the oil pump and deliver that higher pressure in the oil system? Are the negatives now outweighing the positives in this equation? Am I just overthinking this and it really doesn't matter all that much? :shrug01:

Heck, I don't know. I've still got another oil change worth of 15W-50 oil on the shelf, and I just did an oil change a couple of weeks ago. Perhaps compromise to a 15W-40 weight oil?