Onefast V

iTrader: (3)

pressure is a measure of resistance (I agree). Therefore creating a greater resistance to flow (smaller lines) should increase the pressure and bigger lines should result in a pressure drop. what am I missing?

MN_V

iTrader: (5)

Sidebar - what are you guys seeing for oil temps after a 16(ish) mile cruise on the highway? That is my daily commute each way and my oil typically does not get above 155. Then again it has been cold here for a long time so I can't remember what it settled at during the summer.

Onefast V

iTrader: (3)

depends on how you drive it and if that's all freeway or stop and go. But I generally get over 200F by then.

01_SuperSlow

iTrader: (7)

I normally see about 210-220 oil temps at full operating temperature.

garrettg

iTrader: (1)

I rarely get oil above 200.

AAIIIC

iTrader: (19)

What are you missing? To be blunt, you are missing a basic understanding of fluid dynamics, because what you've said is completely backwards. I'm not trying to be a dick, just telling it like it is. You're far from the only ones who get this mixed up. Honestly, I think what throws everyone off is that we've probably all at some point in our life put our thumb over the end of a garden hose to make the water spray out harder - "Ahhh, smaller area makes higher pressure!" Nope, smaller area makes higher velocity, because the same amount of water is now trying to get through a smaller area. Velocity goes up, but pressure goes down (see Bernoulli's Law).

I'll try. The pressure drop through a length of tubing/pipe/hose is due to friction against the walls of the tubing and friction in the fluid itself. The latter changes with the fluid's viscosity - a "thicker" fluid will have more internal friction against itself, a "thinner" fluid will have less. Since we're dealing with the same fluid regardless of what tubing diameter we're looking at, that internal friction is the same, so we can ignore it.

So, now we look at the friction against the walls of the tubing. Let's say we put 1 cu.in. of fluid into each hose. If you go through the math, you'll see that 1 cu.in. of oil in a -8AN hose (assume 1/2in inner diameter, although it's actually a bit smaller than that) would wet 8 sq.in. of surface area. If you go through the math for a -10AN hose (5/8in ID), you'll end up with 6.4 sq.in. of wetted surface area. For a given amount of fluid, more of the fluid in the smaller tubing is in contact with the walls and subject to friction, thus there is more pressure drop in the smaller tubing.

You can find all sorts of online calculators that will determine pressure drop through a hose. Here's one example (oil pump is ~8-10gpm, specific gravity of oil is ~0.9, and viscosity of a typical motor oil is ~185). Once you click CALCULATE, the bottom table on the page that comes up shows the diameter you selected compared to some other diameters.

Now we get to another part that is counter-intuitive to many - Well, if there's more resistance to flow through the smaller diameter tubing, then the flow rate will be reduced, right? In the case of an engine oiling system, the answer is "yes, but not all the time". The oil pump is a positive displacement pump, so at a given RPM it is going to pump out a given volumetric flow rate. If the system has more restrictions in it (smaller diameter tubing in the oil cooler circuit), then there will be a greater pressure drop, ΔP, across the system. The ΔP = Ppump - Ppan (pressure at the outlet of the pump - pressure at the pan, which is where all the oil is going to return to). Well, Ppan is the same whether I have little oil cooler lines or big oil cooler lines, so if the ΔP is bigger, then Ppump has to go up.

As RPMs go up, the flow rate the positive displacement oil pump is pushing out goes up, ΔP goes up, and that means the pressure at the outlet of the pump goes up. With a positive displacement pump, that pressure will just go up and up until something breaks, which obviously would be bad, so instead we put a relief/bypass valve on the outlet of the pump to protect the system. So, at some RPM you'll get to the relief valve setpoint, and some of the flow will start bypassing the engine's lubrication system and just go right back to the suction of the pump (or to the pan - same thing, effectively). A system with more restriction to flow (smaller oil cooler lines) will reach the relief setpoint at a lower RPM, at which point a smaller portion of the oil pump's flow rate is actually getting to the bearings.

AAIIIC

iTrader: (19)

I'm not sure what you mean by "raises relative pressure", but smaller lines will experience a greater pressure drop, as discussed above. Thicker oil has more internal friction (as discussed above), which will make Ppump go up. That also means you'll reach the relief valve setpoint at a lower RPM, as with the case of using smaller oil cooler lines.

By "flow volume" I assume you mean volumetric flow rate, which will be the same in either case until you reach the relief valve setpoint. Flow velocity will be greater through the smaller oil cooler lines, but again due to Bernoulli's Law that means the pressure will be lower.

AAIIIC

iTrader: (19)

That seems reasonable. The oil takes a surprisingly long time to warm up, particularly under light load (highway driving).

garrettg

iTrader: (1)

I appreciate the detailed explanation, going with the larger lines sounds like the ticket.