Monday, July 9, 2007

Crankcase Ventilation

The Volkswagen engine holds exactly 2.5 liters of oil, which ain't much. Modify the engine for full-flow oil filtration you could count on the extra quart or so but back in the Good Ol' Days, whenever that was, you could blow better than a quart an hour when you were flying low through the sage brush. It wasn't uncommon to catch & pass your competition when they pulled up to pour another quart down the spout. And for them to do the same to you a few miles later. No big deal, since everyone had the same handicap.

Except for Charlie. He flew out of San Quintin running sixth overall and managed to pass everyone in front of him by the time he reached Catavina. He was better than ten minutes up when he blew through San Ignacio and would of won for sure if he hadn't tried to plow a cow north of Villa Insurgentes.

So what was Charlie's secret? He'd stuffed his dynamo tower with copper 'Chore Girl' pot scrubbers. They created a near-perfect labyrinth separator. No blown oil meant no stops to top-up.

Every crankcase has to breathe. You gotta provide some place for air to flow in and someplace for it to flow out. If your engine's running hot a lot of oil vapor will be mixed with the out-flow. If you don't do something about it, you'll blow it overboard. Which is less of a problem running the Baja than it is flying from here to there. Running the Baja, you can pull over & park.

On the Volkswagen engine the crankcase ventilation inlet is the annular gap around the pulley hub, which was machined with an Archimedes Screw to pump air -- and oil -- into the engine. Running off pavement you've gotta seal it up or you'll suck a lot of sand & grit into the sump. The outlet is via the dynamo tower, which usta have a road-draft tube that extended below the engine so your forward motion would produce a slight negative pressure at the outlet. (Later engines plumbed the outlet to the air cleaner, using the carb to provide the negative pressure that ensured good ventilation flow.

Once you'd installed a sand seal you had to provide a new ventilation inlet to the crankcase. Most guys plumbed a filtered line to their valve covers.

Since flying Volkswagens don't have dynamo towers you see all sorts of methods used to deal with crankcase effluvium; a lot of guys don't use any kind of oil separator. But then, a lot of guys still don't believe in oil filters, mom's apple pie or that Cheney is pulling the strings :-)

My approach was to try and use what's already there, such as the little shelf just below where the dynamo tower attaches, as shown in Fig. 1. The shelf is pierced with an opening over against the wall of the crankcase. (See Fig. 2 ).

To keep the mesh in place you'll need to tap a couple of 8-24 holes along the parting line and install a couple of drilled-head bolts to serve as a fence. (Fig. 3)

You want the mesh to pretty much fill the D-shaped hole for the dynamo tower but not to bulge above it. (Fig. 4) That will allow you to use an inexpensive oil pump cover as your outlet. Nor do you want it hanging out the bottom. This is the cam-gear gallery and the last thing you need is to have your mesh get sucked into the gear train.

If you shop around you can generally find a cast aluminum oil pump cover for a full-flow oil filtration set-up. These come with a threaded outlet.

As an oil pump cover, cast aluminum is about as durabile as a politician's promise. Enormously popular of course, at least to the Kiddie Trade. But as soon as youngsters see how rapidly an aluminum cover can wear they go for a cast-iron pump cover, which is why you can often find cast aluminum covers on sale. The two shown in Fig. 5 are idential, purchased for about $5 each. The one on the left is as-received, the one on the right has been bead-blasted then treated with a thermal dispersant. (Tech Line's 'TLTD').

On the flip side of the pump covers (Fig. 6) you can see the location of the outlet hole. Also note that one of the pump covers has been treated with a solid lubricant coating. (Another Tech Line product, although I don't recall it's name at the moment.) Coated in this fashion the pump cover holds up about as well as an anodized cover and can serve as a repair part. But it also makes a dandy crankcase ventilation outlet :-)

Since dynamo tower studs are closer together than oil-pump studs you'll have to open up the bolt holes in the pump cover. (See Fig. 7) You won't need the gasket nor the deflector plate. On my engines I run the outlet down to the carb-heat box, where the carb provides the required negative pressure.

Although I originally used copper 'Chore Girl' scrubbers for this application I found they corroded pretty easily. For the last fifteen years or so I've been using stainless steel pot scrubbers, found in the paint department of the local Home Depot. Unfortunately, they are larger and springier than the copper jobbies which makes installing one like trying to shove a watermelon up a monkey's ass.

Over the past fifty years or so when converting a VW engine for flight I've found it best to keep things simple. I'm not an engineer and don't pretend to be one. But there's heaps of sound engineering out there, much of it embodied in components that are commonly available. Before resorting to something I have to buy and bolt on, I try to see if there's some feature already on the engine that can resolve the problem by simply configuring it differently. Such as the providing adequate crankcase ventilation without blowing all your oil overboard.

By devoting a bit of thought to the various problems involved in converting a car engine for use in an airplane I've managed to come up with reliable, inexpensive solutions, most of which can be easily duplicated by fellow home-builders. This approach is wildly unpopular, of course. Not because the engine's don't work but because Americans seem to have gotten out of the habit of thinking for themselves.