Sunday, January 11, 2009

It's the Little Things

I once watched a fellow assemble a VW engine. It took him twenty-two minutes. Then he did another. Took him about the same. Then he started on a third and shoved it across the metal-surfaced work-bench in exactly twenty minutes. He was sweating pretty good. It was a hot day in Los Angeles and he was working in a metal-roofed building, along with three other assemblers. A woman was doing the parts-pulling: making up a tray with exactly one engine's-worth of parts. She was keeping all three of them supplied. there were a couple of other guys there, tearing down junk engines, checking the parts for condition. Not by spec but by eye

The engines got a new set of jugs and a new set of bearings. Everything else was taken from the trays of parts kept filled by tearing down the junked engines. The baskets of junked parts went into the washer and a couple of guys on the other side of the shop were doing heads, which is why I was there. I needed some cheap heads. But after watching them split the valve guide boss by driving in new guides with a chatter gun, I wandered back out the door and drove home.

I don't know exactly how long it takes me to assemble an engine. Probably not very long. But then I usually take it apart again. I generally put an engine together five or six times before I'm satisfied. Of course, before arriving at the point where I can do any assembling at all, I've had to ensure the parts mate properly; that the bearings are a good match to the rods. Doing heads, I can spend all day to produce a set of heads that tells me they're ready to run.

Then too, I assemble my engines using only hand tools. The Mexicans working in the shop where all using pneumatic tools. Just installing the heads, I can spend an hour flipping the engine from one side to the other, rotating the crank with my hand each time I increased the torque, taking maybe five steps to arrive at the final value. I've got a hunch I'm no longer strong enough to turn the crankshaft by hand; I'll probably have to use a wrench.

The reason for flipping the engine from side to side -- working on first one bank then the other, is to equalize the stresses in the crankcase. Or at least, that's what I was told by the German mechanic who showed me how to do it. The engine is in a fixture, hanging off the edge of the bench. It's easy to flip it back & forth.

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Up at the top of the page you'll see a picture of some valves. They're both new; never been run. The one on the right displays a small lip on the underside of the angle. The valve on the left has had the lip removed. The reason for removing the lip is because the lip can reduce the flow through the valve by as much as half.

No one ever believes that. You have to set-up a head on the flow-bench, get things air-tight and pull some numbers for that chamber. Then you do it all over again using a stock head fitted with new, stock valves, and repeat the same 'input numbers' -- that is, showing the air density & temperature hasn't changed and the 3hp electric motor is turning the same rpm. Then you watch the 'juice-tube,' and record the numbers.

If you want to see what's going on you'll have to substitute a clear glass tube for the black sewer pipe which forms the mythical 'cylinder wall' because the blower is actually a sucker, located outside of the shop. By sucking air down through a four-inch diameter hole in the top of the work bench, you can determine a 'measure of merit' for heads and manifolds and stuff like that. I've never bothered to quantify the numbers. I'm not interested in x - cubic feet vs y - cubic feet, I only want to know what's happening when I make a change. Is it better or worse? Most times it's worse :-) ...because VW had some pretty sharp engineers.

But sometimes things are better, especially when you're fiddling with valves, opening up the combustion chamber, and things like that.

Getting rid of the lip is better. Surprisingly so. In fact, it's so much better that you do it four or five times, using both inlet and exhuast valves. That is, large valves as well as small ones. And you grab a full-trick head that you know flows like Niagra and pop its valves and insert the ones you've smoothed.

With the Pyrex barrel in place of the black sewer pipe, and a metal mirror proped up, and a flashlight and a smoke wand, you get to SEE what's happening, which is pretty neat.

Ever shoot rapids? You can tell where there's rocks and ledges under the water because the water will sort of stand-up where it flows over the rock. That little-bitty lip is doing the same sort of thing. It's causing the smoke from you wand to stand up. And when it does, it causes the air above it to stand up and so on. In effect, that little-bitty lip is reducing the EFFECTIVE size of the opening by about half!

Change the rate of flow and the numbers change, so that you only see the maximum effect when you're trying to get get the maximum flow; that is, with the blower running full blast.

Then comes the fiddling around. And about the first thing you discover is that someone has already discovered this, because you can buy specially contoured valves that don't have the lip. The late Bill Fisher, in his excellent book on how to hotrod VW engines has a nice drawing of a valve showing the lip has been removed.

But now I know why they do it.

I grind off the lip. Chuck the valve into the lathe using a copper bushing, then feed a fine grind stone into it, back & forth, changing the angle after ever few passes, about five or six passes at each angle is all it takes. Then I polish them up with a hard wheel. Most of the gain will appear in the intake valves but I do it to the exhaust valves as well. Another of those 'unimportant' details that so many 'experts' insist aren't needed.

Open up the combustion chamber to accept a bigger cylinder, you should move the wall of the combustion chamber back away from the edge of the valve. You do that, your bigger chamber will fill faster and more uniformly. Anything that disrupts the in-flow of the fuel-air mix contributes to stratification of the charge because of the enourmous difference between the mass of air when compare to the mass of vaporized fuel. When you ignite the compressed mixture you want that mixture to be as homogenious as possible so it will burn at the same rate. And you want those conditions to be as perfectly matched across all four jugs as you can make it, because the homogenious mix results in a homogenious burn which results in homogenious pressure in each of your cylinders.

All unimportant, of course :-) Too much trouble or something. Schlock shops cranking out three engines per hour, making LOTS of money selling those junk engines to youngsters.

When we start using MONEY as our measure of merit, we've lost the fight.

-R.S.Hoover

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