DIRTY STUFF

OIL is obviously essential for the smooth running of your mill, and it’s largely a fill-and-forget commodity. But what happens when this vital fluid stops circulating and the crankshaft bearings get a gutful of air? Disaster, that’s what!

If your engine has plenty of revs up, the bearing shells will be down to their steel backing plates in less than 10 seconds, which will do nasty things to the crank journals. If this starvation continues, one or more of your conrods may escape through the side of the block as well.

Yes, it can be that bad. But how can this happen?

A standard sump is virtually an open bucket, usually holding five or so litres of oil. As you drive, this lot sloshes around during braking, cornering and acceleration. As this fluid cannot escape, it climbs up the sides of the sump, away from where the pick-up screen of the oil pump sits, close to the floor of the sump. In normal road driving, the oil doesn’t rush very far away from the pick-up, as engine designers have calculated the amount of g-force needed in a violent manoeuvre to fully uncover the pick-up, and made sure that the sump is deep enough so that, unless the car is upside down with the engine still running, the pump will not suck in any air.

But competition motorsport vehicles are a different kettle of fish. Drag cars often run a wet sump, which means that there is always a set amount of oil in there. These sumps mostly need a basic internal modification to contain the oil in the area of the pump pick-up instead of it rushing up against the back of the engine. This is easily done by installing a windage tray, which is a thin steel plate that’s either bolted to extensions of the main bearing cap bolts or fixed into the sides of the sump, often using small welded-on brackets so it can be lifted out for cleaning. This tray is really a false floor, fitting closely to the sides of the full length and width of the sump, below where the conrods rotate, and has one hole to allow access for the pump pick-up tube and screen. Oil escaping from the spinning crankshaft falls onto the top of the windage tray and drains into the sump proper via the pick-up hole. As stock pick-up screens can be large and often don’t live far enough into the back of the sump to suck oil when it all rushes backwards on violent acceleration, a new pick-up and smaller screen may be needed.

The windage tray does two jobs: it keeps the oil down below where it is needed, and also stops oil wrapping around the rotating crankshaft, which would lose you vital horsepower.

Along with a wet-sump windage tray, drifters and circuit racers will need a swinging-gate baffle box in the sump. Fabricated out of thin steel plate, this square box has no bottom or top – only four vertical sides, with one hinged gate plate to each side, and the pick-up screen fits inside. The baffle box is placed so that its sides are at a 45-degree angle to the sump walls so that the gates have room to swing and close against each side. It should be a close fit against the underside of the windage tray, and the aim of all this is that the gates open when oil is rushing in their direction and close on the opposite sides to trap the oil we want in the box to feed the engine.

But for really serious circuit racers, the only way to go is to dry-sump the engine. There is a lot involved in this, as we will need at least two externally driven oil pumps, modifications to the block and sump, plus a separate oil tank and lots of flexible hose plumbing. The aim of a dry sump is to eliminate oil surge entirely by sucking oil out of the sump as soon as it squirts in there from the crankshaft, so the sump is almost completely dry. The pumps can be in tandem or separate, but the scavenge pump, which does the sucking, has to have a larger pumping capacity than the pressure pump, as it has to suck air as well. Waste oil is usually taken out of a Y-shaped sump by the scavenge pump and fed through an oil cooler before being piped into a large-capacity, externally mounted auxiliary tank. Cool oil is then drawn from this tank by the pressure pump via a feed hose and into the main oil gallery of the engine, usually through a special threaded hole in the side of the block. That way, no matter how hard the car is driven, oil surge really has no hope of happening unless the auxiliary oil tank hasn’t got enough baffling.

So, if your street machine suddenly ruins a set of bearings, don’t blame poor-quality shells or awful oil. Look for a cause of oil surge as being the obvious suspect and go from there.

THE WINDAGE TRAY KEEPS THE OIL DOWN BELOW WHERE IT IS NEEDED AND STOPS IT WRAPPING AROUND THE CRANKSHAFT