Thursday, November 23, 2006

VW - Chamber Volume


Holding chamber volume to .1cc is like running a four minute mile . . . not everyone can do it and even those who can often choose to take the bus.

Unless you’re going for the gold, the added time and expense of nailing down the chamber volume to a perfect, repeatable figure within .1cc across all four jugs is really gilding the lily. It has taken me as long as a week to match a set of heads to 0.1cc, whereas getting them within 1.0cc ( plus or minus 0.5cc) is pretty easy.

Normally, when making small adjustments to your chamber volume you do so by sinking one of the valves. If you’ve got a lot of volume to make up, you sink the intake, but never very much. You want the intake valve to keep its head up. On the other hand, the exhaust valve can be down in a well and still work, thanks to the pressure during the exhaust cycle. But adjusting valve depth is not a trivial task. You need a set of stones and the tools and the skills. And you must be accustomed to working to very close tolerances. Go a bit too far and you’ll end up having to do the other three combustion chambers over again, to match your mistake.

You’ll encounter the same difficulties when you try to keep your deck height to some small value. Deck height determination starts before the engine is assembled. You need to determine the precise center-to-center length of each rod . . . they vary according to how the big end is honed . . . and on each piston, the distance between the wrist-pin bore and the head of the piston. You also must measure each of your cylinders using a surface plate and surface gauge. You do your first pre-assembly by matching ‘short’ rods to ‘long’ pistons in an effort to make all of equal length but you don’t really know anything until the crankcase is torqued and the cylinders seated with a dummy head, a plate with spacers that allows you to torque the cylinder studs to spec. Deck height is measured through a hole in the plate.

The trick here is to arrive at a consistent height for all of the cylinders while you maintain the same deck height. It is virtually impossible to do this without having a lathe, since the final dimension is usually achieved by shaving a few thou off the heads of the ‘fat’ pistons, but you can shuffle components to arrive at the best average.

Deck height is the most critical of your volumetric balance dimensions because a small dimensional change results in a large change in volume.

If you’re building a plain vanilla engine, measure everything but do not attempt to balance chamber volume until you’ve done at least the first pre-assembly to determine deck-height. Often times you’ll be able to balance a big chamber with a small deck-height and arrive at a good average volume across all four jugs.

A key point to keep in mind is that your engine will run with some amazing imbalances in both mass and volume, but it does so at an equally amazing cost. An out of balance engine is destined to live a short, expensive life whereas one that is balanced with respect to both mass and volume uses less fuel to produce the same power and has a useful life considerably longer than it’s mass-produced cousins. And these are improvements that are built-in for the life of the engine, repaying their cost many times over.

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