Valve Train Geometry: Devil = Details

This week's Mystery Topic appears to be Valve Train Geometry, with a
total of four messages since Monday, including one today.

It's really pretty simple -- you're actuating a pivoted lever with a
push-rod. The goal is to do so with the least amount of wasted
motion. That means the mid-point of the push-rod's travel must fall
precisely upon the tangent of the arc prescribed by the pivoted lever,
with EXACTLY half of the available motion occurring above that point
and half below.

That happens to be the INPUT side of your valve train.

The output 'push-rod' is in fact the valve (!) but the rule of
tangency is still the same, meaning the point-of-contact between the
rocker-arm and the valve stem must occur along the tangent of the
rocker's arc. In fact, if you have a mono-block, water-cooled engine,
that's all you need to worry about because everything else is fixed --
no adjustment possible.

But that isn't true for a VW engine assembled from after-market
components. With a big-bore stroker you've got to deal with about
NINE variables. Get any one of them wrong and you'll give away up to
25% (!!) of the engine's potential power.

First thing you gotta do is dial in your cam. The stock 'match-the-
dots' alignment is only accurate to within a couple of degrees at the
crank, assuming you're using STOCK components and the PROPER cam
gear. VW used nine different cam gears. Dune-buggy types insist just
ONE size is good enough. They're wrong but you'd be surprised how
many people insist they know more about engines than Volkswagen :-)

The purpose of dialing in your cam is to ensure its designed amount of
lift is available precisely when needed. Failure to dial in your cam
virtually guarantees the cam timing will be off by some amount and,
sadly enough that the specified amount of lift may not be there (!).
Lotsa after-market cams are junk, poorly manufactured and improperly
marked. Dialing in your cam involves not only proper gear mesh and
cam timing, it verifies the cam's specs.

Once you know the cam's specs you can set the valve lift with a
precision of better than a thousandth of an inch and half a degree of
crankshaft rotation.

Building a BIGGER VW engine means it comes out up to an inch WIDER
than the stock engine. That means you need longer push-rods but it
also means the valve-train geometry must be completely reset.

At the beginning of this article I said it's just a push-rod and a
lever. Actually, its a pair of them... and the lever(s) use a common
fulcrum. The tricky bit here is that the input side of the system is
orientated at an angle of about +3 degrees relative to the crankshaft,
whilst the output side of the system (ie, the valves) operates at an
angle of about -9.5 degrees.

The variables in the system are the length of the push-rod(s) and the
height of the fulcrum (ie, its distance from the center-line of the
crankshaft). And don't forget that the VALVE is a de facto 'push-
rod'.

Your job is to put all that together to achieve maximum lift with
minimum losses and do so for minimum work. But keep in mind that the
two 'push-rods' (ie, the real push-rod and the valve) are angled
TOWARD each other. That puts a limit on how HIGH you can raise the
fulcrum, especially if you're using rocker-arms having an input/output
RATIO that is greater than stock. Ditto if using the Ford/Soobie
type of swivel-foot adjusters. (So what to do? That depends on the
GEOMETRY. In some cases you'll have to modify the rocker arm, in
others you'll need to shorten the valve stems. In extreme cases you
may need to do both; the engine will tell you when.)

One way to tell if you've done it right is to simply measure the push-
rods when you get done. Here's why: The stock Volkswagen cam uses
less lift for the smaller exhaust valves. (Maximum effective lift is
PROPORTIONAL to valve diameter. Any greater lift simply increases the
engine's wear whilst decreasing efficiency.) In order to use ONE SIZE
of push-rod, they used an exhaust valve that is slightly SHORTER than
the intakes, taking up the difference with the adjuster-screw. But
after-market cams typically cut all their lobes to the same lift. If
you've done the geometry correctly the push-rods for your exhaust
valves will be slightly shorter than those for your intakes -- a
simple test to tell if you've done the job correctly.

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'There's quite a bit more to it of course. 'Way back when, the editor
of 'VW Trends' magazine commissioned me to do an illustrated article
on VW valve train geometry. Not being a mechanic, it took a while to
convince the editor that he was asking for a small book -- that I'd be
lucky to get it done in less than 30,000 words and 75 or 80 photos and
illustrations. When the magazines own technical editor backed me up
we cut a contract and about three months later I shipped him the first
installment 'Dialing in Your Cam' -- which appeared as a two-parter in
the October and November issues for 2001.

The contract was for First North American Serial Rights, meaning the
magazine could print the material ONCE, after which the rights would
revert to me. Unfortunately, the editor didn't take our contract very
seriously and illustrations and side-bar material that had already
appeared in print began popping up in subsequent issues.

I sat on the subsequent articles, waiting for the lawyers to do their
thing. 'VW Trends' is no longer in business and the follow-on
articles went into the round file, along with their illustrations.
(Writing is a rotten business. But it IS a business.) So if you want
ALL the gory details of race-winning valve-train geometry on a big-
bore VW, you need another 20,000 words of text and about 53 photos &
illustrations. I uploaded the first part of the chore -- dialing in
your cam -- to my blog but without the two dozen or so illustrations
that appeared in the magazine.

-R.S.Hoover