Tuesday, April 21, 2009

Harvesting Data

Most guys call it blueprinting. You measure the part then check the measurement against the 'blueprint.' Which hasn't been a real blueprint since the 1930's. Nowadays it's just a file of dimensions and tolerances provided by the manufacturer, usually in their Factory Service Manual. Alas, the task isn't as simple as it seems. In many cases the factory manual cites only metric dimensions, leaving the conversion up to you. In other cases they cite both Metric and English units of measurement... and sometimes gets them wrong, requiring you to keep on your toes when they say one millimeter equals 0.3937" ...when they should have said 0.03937".

An even trickier bit is what happens when you depart from those stock dimensions. When you build a big-bore stroker you are literally designing a new engine. Cutting the heads to accept larger jugs requires you to measure the diameter of your new jugs and then add the anticipated thermal expansion to that dimension, and finally to add some small amount for your tooling wear.

This sort of thing -- the designing of a new engine -- is largely a desk job. No greasy fingernails. And yes, your computer comes in very handy. Not so much for the computation of things but simply for storing them, ideally with drawings.

So where do you come up with those figures that are not listed? Standard engineering manuals, such as Machinery's Handbook will give you the accepted standards for the thermal expansion of your parts but the particular alloy plays a role I'll mention in a moment.

Thermal expansion takes place in all three dimensions. The engineering manuals say that the intra-molecular space will INCREASE as the temperature of the part increases. Some call these dimensions -- the ones not listed for our particular engine -- the experience factor when in fact we've simply memorized the values of interest from tables found in various manuals. Pratt-Whitney and Machinery's Handbook sez 0.003" per inch for cast iron and .007" for cast aluminum... which works well enough for the alloys used by VW in their heads and barrels.

But if you don't know what alloy is being used, you'll be safer if you put the part(s) in an oven and raise them to their anticipated normal temperature. And learn to measure them quick like a bunny, before they can cool off and before they can heat-up your micrometer. That's enough to give you some idea of the problem, which is the fact your heads and barrels will expand at different rates.

Not everyone does it that way. In fact, a lot of 'experts' don't blueprint anything. Their engines usually end up in dune buggies but there's a few who build engines for airplanes. In doing so they follow the same procedures -- and use the same tolerances -- they have used for dune buggies. This entails considerable risk because there are significant differences between the VW engine in a vehicle and one that has been converted for flight. The most evident of these differences is the higher temperatures experienced in engines converted for flight. This higher temperature reflects the fact that a flying Volkswagen is operating at a continuous level of output at least fifty percent greater than that found in any vehicular engine.

The builders of dune insist there is no difference between their engines and those used for flight and that is substantially correct. But there is a profound difference in how the engine is USED and that is easily illustrated by the fact an airplane must sustain itself in the air. To do so demands a higher level of output on a continuous basis.

-R.S.Hoover
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