Regulating high-RPM oil pressure

[Crawdad]

I'm building a dry sump motor and using an adjustable, external bypass relief valve, on the pressure side just upstream from the filter.
My motive for using the external bypass was to capture the large volume of oil that gets bypassed to the sump in the stock system (by the oil control valve at the flywheel end) and instead send it directly to the dry sump tank, so the scavenge side of the pump doesn't have to keep circulating the excess oil out of the case. I have locked the flywheel-end piston in the up position, blocking the return path to the sump.

But now that I am using an adjustable bypass valve, the question arises: what should I set the maximum pressure to? The stock control valve is designed to open at 42 PSI. That sounds good for a stock motor with a redline of about 4500 RPM. Observing the rule of thumb of 10 psi per 1000 rpm, a motor that spins to 6500 would want 65psi. But if I set it that high, I see two problems.

First, I am using the stock doghouse oil cooler, and it may well burst at such pressures. Do you road racers who spend a lot of time at high-RPM ever use the stock cooler? If so, are you using the stock control valve, limiting pressure to 42 psi? That seems dicey at 6000 rpm. Can aftermarket coolers take more pressure?

Second, when I am driving around town, I don't want to have 65 psi at, say, 3000 rpm. That's a lot of wasted energy from too much oil pressure. I started worrying about this when Piledriver said (in some other thread) that, while cruising, about 75% of the oil is getting bypassed to the sump by the stock system, which suggests that our VW oil pumps are putting out a lot of excess volume. Presumably volume scales with RPM. That volume of oil has to go somewhere. The passages it has to pass through do NOT get bigger with RPM, so pressure scales with RPM as well. Maybe the solution is to set it at 65 psi and then select an oil viscosity that (when warm) keeps pressure in the ballpark of 10 psi per 1000 RPM? Am I thinking about this the right way?

[Bruce2]

.... when Piledriver said (in some other thread) that, while cruising, about 75% of the oil is getting bypassed to the sump by the stock system,..

I'm sure piledriver will admit that is a wild guess. Unless you have some way of measuring the flow before and after the bypass, you're just guessing.

IMO, you are worrying about nothing. How much time do you spend cruising at 6500 rpm?

" which suggests that our VW oil pumps are putting out a lot of excess volume." I wouldn't say that. When VW installed hydraulic lifters in the 94 model year engine, they also added an oil filter for the first time. If there was a lot of excess volume to begin with, they wouldn't have increased the size of the pump like they did.

[Crawdad]

Good angle, Bruce2. Pile, if you're listening in, can you clarify? It does sound counter-intuitive that the stock pump would be so over-matched to the demands of the engine.

I do plan to do track days where it will see extended time in the 4-6k range, so I'd like to hear from the circuit racers. My guess is they all use remote, aftermarket coolers that can take higher pressure. Do they in fact allow the motor to develop higher max pressure, by messing with the stock oil control valve (stiffer spring)? FJCamper, our nation turns its lonely eyes to you.

[Steve Arndt]

Not saying it is right, but this one is 80 psi
https://jayceevw.com/products/jaycee-oi ... ol-system/

[Ian Godfrey]

I work my engine at 10 psi per 1000 rpm and wind the motor to 7,500. I built the engine with fairly tight clearances and use 10-30 oil to keep the pressure in this ball park. I've got 10 at hot idle and around 70 flat out. I run a stock cooler and a remote. I feel confident of the stock cooler to at least 80psi on occasion

[Crawdad]

Ian, I'm glad to hear the stock cooler holds up. Is this drag racing, where you're spending 10-12 seconds at high RPM, or something more sustained?
Also, how did you set it up to give you higher pressure than the stock control valve allows? Stiffer spring in the stock valve, or external bypass?

[Ian Godfrey]

My car is track and road race, so sustained higher pressures. I tested a few different stock relief springs to find one on the upper side of the range. they varied a lot. The Bentley manual give a compressed length and a force. I put a cap on a piece of tube of the stated length, popped the spring in, then pressed it down on my scales until it just touched the surface of the scale and read off the Kg. Sorry, I don't have a pic
a valve spring tester would be easier.
I am rebuilding the engine next year with dry sump and I'm going to use an external valve like you have. I'll be aiming for similar pressure.

[Crawdad]

From reading around, it looks one less-remarked-upon benefit of using a big, external bypass vale is that you are not forcing the bypassed oil through that small (6mm?) hole back into the sump, which apparently does a lot to heat the oil. Instead, the excess oil goes through large chambers and (in my case) a -10AN line to the dry sump tank.

[FJCamper]

Gentlemen,

We run a very similiar oil setup on both our Historic Sportscar Racing Ghia and our LeMons Super Beetle.

Stock oil pressure relief springs and pistons
CB Performance dry sump pump (unmodified)
3-gallon oil tank
Dual external oil coolers fed by engine case top adapter
Fram HP-1 filter
10AN steel braided hose
Lucas 10w30 and 20-40 Classic Hot Rod oil.

The Ghia is for sprint races 7000+ 10.5:1 cr, the Bug is 6000rpm endurance engine, 9:1 cr.

[Crawdad]

My NEW worry (I worry a lot - worry twice, build once) is about the other valve – the relief valve at the pulley end – which controls the pressure at which oil gets sent to the cooler. Using pressure as a proxy for temperature isn't ideal, especially if using modern, light-viscosity synthetic oil, which will trick the system into thinking the oil is warm when it isn't. Making matters worse, the stock-strength spring doesn't seem to be available anywhere, only the "pressure boost" springs that have the effect of making oil go to your cooler at a lower temperature (compared to a stock spring). For a daily driver, that seems like a really bad idea, especially if you've got a couple gallons of oil to heat up. The obvious solution is to use a real thermostat. But I'm clinging to the idea of using the stock cooler, rather than adding yet more plumbing and expense. If my oil isn't getting up to temp, I'll have to rethink.

[Crawdad]

Thanks for that, FJ. Sounds like KISS is working for you.

[Bruce2]

From reading around, it looks one less-remarked-upon benefit of using a big, external bypass vale is that you are not forcing the bypassed oil through that small (6mm?) hole back into the sump, which apparently does a lot to heat the oil.

Not everything you read online is true. This one's 100% false. It doesn't make any difference what size the return hole is to the sump, the oil will heat up exactly the same.

When the oil is under pressure, it has potential energy. In other words, you can do work with it, like drive a hydraulic motor. When you release that energy by dumping it back to the sump, all of the potential energy it had is then converted to heat.

[FJCamper]

I concur with Bruce here. I know a few guys have relieved the oil dump hole a little, but in my recollection, they were fighting some oil pressure problem. I myself have experimented with the flywheel-end relief valve spring tension by shimming the short spring with a single 1mm washer. I blew two stock doghouse coolers doing this, in moderate to warm weather. No high revving, just on cold start.

My reason for the shim was hoping it would raise overall oil pressure just slightly. Wow. What it did was prevent the overpressure dump to the point that the overall oil pressure did in fact rise and blew my coolers. I thought the first cooler was just happenstance, but the second cooler proved it was my fault.

BTW, I am an advocate of the stock doghouse cooler for all street use, and doghouse plus external for hard road racing.

Having said that, I have to explain our current dual externals fed from the case top adapter (no doghouse) are configured as they are (case top stock oil in and out) so as to leave the engine case oil management intent alone. No full-flow oil systems, no external oil pressure dump returns.

FJC

[Crawdad]

Putting the practicalities of VW engines aside for a moment, there's an interesting bit of physics here...

"When the oil is under pressure, it has potential energy." It would if it were a compressible fluid (like air). For such a fluid, the amount of energy stored can be expressed simply: PV=nRT where P is pressure, V is volume, n is the number of gas molecules, R is a constant and T is temperature. But because oil (like water) is incompressible, putting it under pressure doesn't do any "work" on the oil itself. It can TRANSMIT force (for example, run a hydraulic motor), but it hasn't stored any energy. In an air conditioner, by contrast, the "working fluid" is a gas (freon) that gets compressed over here, and expands over there, moving heat energy from one location to another through its compression and expansion. When it's compressed, it is storing potential energy. You could use that energy to do all sorts of work, for example to shoot a projectile.

Based on this logic, I'm starting to doubt the internet wisdom I cited earlier, about the oil getting heated by passing through a restrictive hole. IF that is true, it would be due to viscous friction, not compression.

[FJCamper]

For clarity, forcing oil through a hole under pressure would not (in and of itself) heat the oil except in some very small way depending on pressure and resistance. And that small amount of heat (if any) would be lost in the greater volume of oil based on our popular example of a "drop in the ocean."

To get technical about it, circulating oil is less efficient than circulating water in a cooling system. Oil absorbs and releases heat more slowly than water, even to the point of there being very pronounced thermal layers in oil circulation systems, the coolest oil being on the outside nearest the pipe or container walls. This is one reason you can fill a VW engine with synthetic oil and with an external infrared temp gun get lower external temps on the valve covers.

This means circulating oil faster and under higher pressures does not automatically gain us more cooling.

Sorry for going down that rabbit hole, but one of our team was a NASA hydraulic engineer. (RIP Paul Ray the Rocket Scientist).