Workshop Manuel explains – what is a dry sump?

G'day legends, Workshop Manuel here. I was sitting down with my morning cup of coffee and skipping around the neon wonderland otherwise known as the internet when I figured I'd try and do something more constructive with my time, like explaining the intricacies of high-end oiling systems! 

If you watch much racing you may have heard of "dry sumps", which sound completely illogical. A sump (also known as an "oil pan" or "that thing I keep smashing off the bottom of my stanced E30 BMW") is basically the reservoir where all your oil runs back to in an engine or transmission. The oil pump is normally mounted nearby and sucks the oil out of the reservoir, distributing it through the engine so everything spins nicely. 

 

The problem with these conventional "wet sump" arrangements is they are often bulky as they need to hold litres and litres of oil and, particularly in older cars, they let the oil slosh around freely from side to side, and front-to-back, when you're fanging your car through corners. This is bad as it leads to aeration of the oil or, catastrophically, makes the oil pump's pick-up tube suck up air instead of oil which deprives the motor of lubrication and can destroy everything in seconds.

Below is a sump/oil pan from a typical 1960s engine, and you can see it is literally just a bucket that hangs off the bottom of your motor where oil sits.

The tube I've circled in red (with my amazing MS Paint skillz) on the MCM 240FairlazyZed's RB26DETT is the oil pick-up tube. The screen on the end stops large objects from getting sucked into the oil pump.

Upgraded oil pans are available, which feature welded or bolt-in plates (called "baffles") which stop the oil from just freely moving inside the sump like you're shaking a fruit bowl full of milk. There are also aftermarket companies which make their own upgraded sumps featuring swinging trap doors to allow excellent oil flow, without aerating the oil or allowing the pick-up tube to run dry. 

The ultimate way of ensuring your oil pump always has pressurised, cool and non-aerated oil is to fit a dry sump. This concept means the oil pan is no longer holding litres of oil on the bottom of the motor, instead holding it inside a large, tall tank mounted elsewhere in the vehicle.

When the oil runs back down into the pan after it has circled through the engine, it is the sucked out by a scavenge pump and pushed into the top of the external oil tank, where the air and gases are drawn out, before it is sucked out of the bottom of the tank by a pressure pump, drawn through a filter, and delivered back to the engine at the required pressure.

The scavenge and pressure pumps are normally externally mounted off the side of the engine on a common belt-driven crankshaft, replacing the stock oil pump and sometimes running multiple "stages". This is where you hear of "four-stage" or "three-stage" dry sump systems, where one stage is for the pressure side and the remaining "stages" used to scavenge the oil out of the motor.

A side-benefit is, because of the shallow pan on the bottom of the motor, you can mount the engine lower in the chassis thereby improving weight distribution as you shift a big lump of weight towards the centre of gravity. You also shift the weight of holding all that oil to wherever you want it in the car, also helping weight distribution, while a shallower oil pan under the motor allows for lower ride height and improved under-car aerodynamics.

 

The downside to dry sump set-ups are their complexity and expense, and they're not really practical in a street car where you shouldn't have big tank-loads of oil in your cabin or cargo area. Almost all street cars will get away with a gated and baffled wet sump as the kinds of G-forces required to cause oil pressure issues that can't be remedied with a trap door wet sump are way past what cars can generate on public roads. 

  

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