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.

-------------------------------------------------------------------------------------------------------------------

'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

As I've mentioned several times over the past twelve years, when three
or more people ask the same general question in a relatively short
period of time, I take it as justification to post a comprehensive
covering not only the questions asked but the subject in general.
That's how the so-called 'sermon' files came into being.

The individual questions still get answered, if they've provided a
valid address. I know that sounds crazy but an awful lot of people
don't. Two of the valve-train questioners provided addresses that
bounced my reply. Of the other two, I told one that his push-rods
appeared to be too short, which got me an ear-full of 'that can't be
right.' The other fellow had worked out the input-side of the
equation okay but was hung-up on achieving tangency at the output,
with several indications that he needed to shorten his valve stems OR
the arc of the rocker-arm, meaning he'd need to reduce the length of
his adjusters -- possibly even grinding away about sixty-thou from the
under-side of the threaded portion or the rocker-arms so as to allow
his elephant-foot adjusters to be screwed-in a bit more.

This really is pretty basic stuff, the sort of thing you can figure
out for yourself if you'll just think about it for a while. But it's
a topic that starts BEFORE you assemble the engine and has little in
common with the same task on a water-cooled engine, where only the
OUTPUT side of the equation is addressed. (Why? Because the heads --
and the rockers -- retain the same distance to the center-line of the
cam, which is to say the factors on the input-side of the equation
don't change.)

The major stumbling block is all the 'experts' telling the newbies
that they don't need the correct cam gear, that dialing in their cam
isn't necessary, and they only need concern themselves with the output-
side of the equation. That horribly wrong advice guarantees the
newbie will NEVER achieve proper valve-train geometry.

When addressed in its entirety the Volkswagen's valve train geometry
isn't something that lends itself to a quickie message via email.
Fortunately there's plenty of information out there, from such experts
as Lawrence, Ricardo and Taylor. Most of it dates from the 1920's but
the problem -- and the solution -- has not changed. There's even lots
of VW-specific valve-train information out there -- there's at least
two comprehensive articles in my blog (Dialing in Your Cam and Valve
Train Geometry) plus several others on basic head work. That's the Up
Side.

The Down Side is that the most common reward for answering questions,
pointing people toward sources of valid information and telling them
to THINK FOR THEMSELVES... is a file full of Snotty-Grams from the
idiots..

-R.S.Hoover

Some more resources for setting up Valve Train Geometry on VW engines:

Simurda, Bruce. "Setting Rocker Geometry." Dune Buggies and Hot VWs
Oct.2005 58-59. This article DOES NOT discuss sizing the cam gear,
and it uses a rather uncommon type of valve adjuster, but this a
help. And since this magazine still exists, back issues are not out
of the question. http://www.hotvws.com

Hoover, R.S. "Dialing In Your Cam. Part One: Organizing Your Engine
Buildup." VW Trends October 2001 66-73.

Hoover, R.S. "Dialing In Your Cam. Part Two: Finding the TDC." VW
Trends November 2001 24-34.

These article DO discuss sizing the cam gear, as well as fine tuning
the cam timing. Not so much about setting the valve train. Good luck
finding them. I've got them, but no, you can't have copies. Even
though VW Trends did not honor their commitment to Veeduber, I'm
strong on honoring HIS commitment to copyrights and all they imply,
and no way I'm distributing this material.

http://www.webcamshafts.com/pages/degreeing.html Generic article on
degreeing your cam.

http://www.hotrodder.com/kwkride/degree.html Another generic article
on degreeing or clocking cams, oriented to V-8s.

http://www.shoptalkforums.com/viewtopic.php?t=97118 LONG discussion
with pictures on valve train geometry. Read the whole thing.

http://www.shoptalkforums.com/viewtopic.php?t=103707 Another such
discussion, although
http://www.shoptalkforums.com/viewtopic.php?t=45778 More

It's easy to get lost in these, they discuss a lot of non-standard
parts. Try and grasp the PRINCIPLE. The details in YOUR
circumstances may differ.

There is one more excellent thread in ShopTalk Forums I'll find later
today.

wrote ...
The Down Side is that the most common reward for answering
questions, pointing people toward sources of valid information
and telling them to THINK FOR THEMSELVES... is a file full
of Snotty-Grams from the idiots...

Heavy is the head that wears the Crown. ;-)

Mike Isaksen wrote:

wrote ...

The Down Side is that the most common reward for answering
questions, pointing people toward sources of valid information
and telling them to THINK FOR THEMSELVES... is a file full
of Snotty-Grams from the idiots...


Heavy is the head that wears the Crown. ;-)




I remember it well...


Richard

On May 31, 9:57 am, flybynightkarmarepair wrote:

There is one more excellent thread in ShopTalk Forums I'll find later
today.

Well, that forum is out right now, so I'll leave finding it as an
Exercise For The Reader. Here are some more good threads with
pictures from a different forum:

http://www.thesamba.com/vw/forum/vie...lift#130498 1
http://www.thesamba.com/vw/forum/vie...train&start=20

If you don't understand Cam Gear Sizing, you can read about it in Tom
Wilson's book, or seach either TheSamba or ShopTalk forums for it.
You need a bunch of cam gears to get it right - good, OEM ones that
are actually marked. Vendors in TheSamba can getcha. If you're only
building one engine, sell the ones you don't use back to somebody else
via the same means.