Snapshot 21-06-2021 05:18

Wet sump and dry sump systems

Guess this dry sump

Good day everybody. What is the difference between a wet sump and a dry sump? We’ll find out. I’m Aaron and welcome to another Tech Bit episode here on YouTube, and if you’re listening on the podcast, Mod Tune Win Repeat. Before we can get into the content, consider checking out my website, builtonpurpose.co. And if you’re not subscribed, consider subscribing. 

And if you do subscribe, smash that notification bell. Okay guys, so what I have in front of me, the podcast listeners are just going to have to imagine it. What I’ve actually got is a dry sump out of an Audi R8, V8 manual. The last of it. This was manufactured in February 2007. And I will attempt to show you the serial number.

So you don’t think I’m lying. So, if you could go Google that you’ll… I’m hoping that it’ll validate what are my claims because this is just online. And it’s a great piece of German engineering. But this is vastly different to this dry sump is vastly different. You can see it, it’s really only as thick as the fittings that go into it for the hoses and whatnot, is vastly different to a wet sump, which hangs down under the motor. 

Wet and dry sump oiling strategies

So if we’re getting too much further, I’ll discuss the primary differences between the two, which will help give some context to the comparison later on, between wet sump and dry sump. 

The wet sump oiling system

A wet stamp collects oil under the engine in a reservoir, which has got to be contained wholly. And in that is an oil pickup, which sits in the oil and it is connected to a pump which is normally mechanically driven by the crank shaft.

The dry sump oiling system

A dry sump doesn’t collect oil under the engine. It just captures it… And the pumping system, it has multiple pumps. The nature of a different purpose. You have two kinds of pumps. One is a scavenging, which grabs the oil from the oil pan and drops it up in the reservoir. So you have a remote reservoir somewhere else in the vehicle. And then other pumps attached to the assembly draw from that reservoir and put it back in the engine. 

So you have multiple pumps and you can tailor and configure the amount of pumps depending on the oiling that you need. The scavenging pump also when it picks up the oil from the sump, also serves to regulate the temperature and deaerate the wheel. Because if you could imagine on the inside of a crankshaft, you’ve got all these bits moving around, things flying at great velocities, everything’s getting churned and whipped up. You’re going to get air mixed in with the oil.

And if you get air mixed in oil, even in microscopic form is less than ideal, but it does occur, but a scavenging pump, ultimately, as it picks it up the way the pump works will deaerate most of the air out of the oil. The pumps on the dry sump system that draw from the reservoir and put it back in the engine and normally regulated. And they… Again, their pressure and volume is often specific to its need for the engine. And you might have more than one pump. You might have two pumps. One from pushing in oil in the front of the engine, one in the back.

Same with the scavenging pump, you might have one at the back of the engine for acceleration. You might have one on the side with a slight tilt. And particularly like if you’re in an aircraft, you might have multiple pumps around it, if it’s an aircraft that inverts. So the oil is not always falling down in a sense, it’ll always be under the effect of gravity. So you might need multiple pickup points and multiple scavenging pumps. The poor old wet sump pump is less refined. It’s very agricultural and it’s just one pump that pushes oil all the way through the engine.

Pros and cons

So what are the pros and cons of each? I’ve separated these into the following discussion points, which is crank case conditions and emissions, size and weight distribution, maintenance, oil quality, performance effects, and costs. 

1. Crankcase conditions and emission

The crank case of a wet sump is almost always positively pressurized, primarily due to gas escaping past the piston rings and most engines from, well in about the 1960s emission started to become a real big thing. Normally this was vented to the atmosphere which would push oil mist and contaminants into the atmosphere. And when environmental measures started coming in, that’s when they invented the PCV valve, which is a positive crankcase ventilation valve, which draws that oil mist that’s positively charged back into the intake to improve emissions. By comparison, a dry sump is almost always in vacuum as a result of the scavenger pump, pulling oil through.

And different from the wet sump, a scavenger pump doesn’t need to be submersed in water to work properly. It’s a different kind of pump. So it doesn’t matter if it sucks in a bit of air, but essentially a scavenging pumps pull all that oil mist out of the crank case and circulate back into the oil reservoir. And this vacuum condition that’s created in the crank case does have some role on effects too, which I’ll discuss primarily in the performance effects.

2. Size and weight distribution

So a dry sump essentially enables… Well again, you look how thin the dry sump is, you take the wet sump, which hangs down considerably low, but if you put a dry sump on, you’ve instantly changed the height of your engine. And why is that important? Because you can actually lower the location of the engine and also affect the centre of gravity. It also takes up less space around the engine I should say. Dry sump requires a reservoir. 

And normally that has to be relocated or it’s located in some other part of the vehicle. So you lose volume there. But having said that you can locate it in a dead space in the car, which is not lost to anything because it’s a dead space. In comparison the wet sump has a big sump underneath and it often limits… It’s normally the lowest, well, generally one of the lowest parts of the car, just to get the engine down by default.

3. Maintenance

It’s a bit hard to tell on maintenance, because it depends on the aspect you’re trying to talk about or the aspect of maintenance you’re looking at. If it’s in relation to replacing a pump that’s failed, a wet sump loses that, because there’s a lot more effort to remove everything to get to that pump. And the pump is normally integrated into the block. Whereas, if you’re thinking, well, excluding the Audi which might have the pumps actually integrated into the motor. If it’s a retrofitted system, the are pumps normally external, so you can only reach down and access it along with all the hoses.

As far as draining the oil. I’ve never really drained oil from a dry sump, but it really depends on where the reservoir is. An advantage of the wet sump is that you just pull it to some plug and it drains it under the car, whereas a dry sump, the reservoir might be an inconvenient location that results in oil splashed on the interior or other parts of the engine, just because it’s difficult to drain properly. But having said that, I’m sure a well thought out system will be just as easy to drain the oil out as it would a wet sump.

4. Oil quality

The dry sump wins again here because of the positive effect the scavenging pumps have on regulating oil temperature, which makes it last longer. If an oil stays hot for a long period of time, it breaks down faster. Hydrocarbon oils are always, as soon as they’re refined are slowly breaking again, but heat really accelerates it.

Deaerating also improves the maintenance or the lifespan of the oil. So again, that’s a function of the dry sump. Another advantage of the dry sump is they normally have significantly larger volumes of oil, which means… Again, better temperature regulation, whereas a domestic vehicle might have five litres capacity, dry sump might have 20, so that’s a lot more, it’s more easier. It’s less of a challenge to regulate the temperature of the oil.

5. Performance effect

I mentioned earlier that the vacuum condition created in a crank case can improve performance and how it does that is, that vacuum pulls the piston rings down and outwards to seal them better. When the crank cases in vacuum without exposure to oil whipping up, and you lose the effect of parasitic oil or air drag, which is commonly referred to as windage and just the viscous oil drag of oil splashing around, that would normally be associated with a wet sump, with pool of oil splashing around hitting things and slowing things down that way.

Having said that there’s one risk of a dry sump. Again, referring back to the retrofitted system. If it’s on the outside of the engine, there is the risk of exposure to damage. Particularly most retrofitted oil pumps are belt driven, if a belt comes off in the middle of a race. Well, you want to turn that engine off pretty quick. Otherwise you’re going to lose, you’re going to be throwing wallet trying to fix it. And particularly if you got external hoses, they can be damaged through heat or physical damage or whatnot. So, that’s a risk from a dry sump perspective in a retrofitted car.

6. Cost

So really the last point is the big point. This is the only point that really doesn’t favor the dry sump and it’s cost. While many motor vehicles like this Audi R8 sump here, it’s incorporated into the purchase price of the vehicle. We’re having to that. An R8 is rather a high end car and most high end cars, I think even Mercedes SLSs have been using dry sumps for a long time, and it wouldn’t surprise me if there are many other high-performance supercar manufacturers that had dry sumps in it. But essentially it does add cost to the production of a motor vehicle. And particularly if you’re retrofitting a dry sump to what was originally a wet sump, there’s an added cost to build there too. That doesn’t mean it’s impossible though.

I guess you could take your favourite engine code. RB26, Nissan RB26, dry sump, punch that into Google and you’ll probably get heaps and heaps of products, dry sump products, and you’ll probably get heaps and heaps of cool photos that you could go drool over. So again, for the cost, a wet sump wins in that category. So these points where a comparison between wet and dry sumps, not a recommendation. But I highly doubt there would be a reason to convert, if your car came out with a factory dry sump, I doubt there’d be a reason to convert it to a wet sump. So I would think it’d be most likely that the decision you’ll either be deciding you have a wet sump and you’re deciding to convert it to dry sump.

Closing words on dry and wet sump systems

this is not a recommendation, this is just comparison to help you make, well, just to inform because every engine build is very different. And when we’re talking, if you’re thinking of a dry sump, that normally means you’ve got a serious application, if you’re converting from a wet sump to a dry sump, and it’s really important that you consult the appropriate experienced people to make that decision. 

For me, particularly for my car, probably it’d be nice to have a dry sump system, but it’s really unnecessary. So I would just be sticking with a wet sump system with just an improved pump, which is much more cost effective than putting a dry sump on it, on my 240Z. So I hope you feel informed about the difference between a wet and dry sump oiling systems.

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