Rocker Geometry
This article discusses setting up rockers on the VW Type 1 engine.
The rockers discussed here are cup adjuster/wiper style high ratio
rockers. Though similar principles apply to the stock style
rockers, different methods must be used.
In this article, I give my experience with setting up rockers on my
2332 engine with a Webcam 86b camshaft. Three different rocker
sets were used to test results.
Rocker set #1 - Scat 1.4
Starting with these, the first thing I did was check the alignment of
the rockers with visual observation and measurement.
In the picture above, a dial indicator is used on the spring
retainer. Light springs were used to make measurements. Note
the lash cap on the valve stem. Although in this article, you will
only see one valve being checked, checking all of them is important.
Once you think you have one set correctly, check them all. Any
differences may indicate a problem or may dictate what you choose for
pushrod length. Valves are not always set in the head exactly the
same, though in a perfect world they would be.
What we want to do is measure lift range and check conditions at
zero, half and full lift.
Dial indicator at valve fully closed : 0.8340
Dial indicator at valve full lift : 0.3055
Total lift is 0.8340-0.3055 = 0.5285
Lift of my cam is 0.575 @ 1.5 rockers (86b)
Doing the math, the lift of the lobe is 0.3833
Using 0.5285/0.3833, the lift on MY rockers gives a ratio of 1.38 using
my Scat 1.25's. That works out, since that's what they really measure.
So.. half lift = 0.3055 + (0.5285/2) = 0.5698
When I rotate to that point, my adjuster is nearly in line with the
pushrod, using no shims and a 0.080 lash cap.
Here are the rockers at zero, half, and full lift.
Some things to notice - at zero lift look at the wiper pad on the
lash cap. It is basically centered. At half lift it is not,
it rides up the lash cap, and at full lift it is even more. Also
the red line represents the surface of the lash cap extended down at
half lift. To reduce the amount of the wiper pad's vertical
translation during the range of operation, this would ideally be passing
through the center of the rocker shaft axis. But as you can see it
is not.
What would be ideal is for the wiper pad to be centered at half lift,
and rock above and below this slightly.
So what can be done here?
Well, non centered axis is not the worst thing that can happen, but
what's important is that the wiper needs to be centered on the lash cap
so that the net action of the wiper is across the center. But to
get there, the valve stem would have to protrude more, or the rockers
would have to sit further into the head, which would require milling the
rocker stands or the blocks and doing some clearancing. Ew.
Well, after simulating a 0.180" lash cap, or a longer stem, or
milling the stands or blocks down 0.100", and using 0.080 lash caps,
this picture shows the rocker at half lift. The axis is on, but although
closer, the centering on the stem is not. AND I have determined that I
would have to clearance the head bosses for the rockers to clear, since
they are a little wider than the rocker blocks. In this case, I feel the
problem lies with the rocker design.
So what else can a person do here?
Well, fortunately these are not the only rockers on the market, and
though these might work well for some applications, they're not going to
for mine. Moving on.
Next up is the Berg 1.4 rockers. I scored a set lightly used,
and put them on. This is how it fit for me:
The geometry is looking pretty good! See the axis and the
engagement at the cap at half lift is nearly perfect!
However, the other issue is the angle of the adjuster to the pushrod.
At half lift I would like to see it parallel with the pushrod, because
that basically means that the pushrod cup in the adjuster will see
minimal angles with the pushrod, as minimal as is possible.
Looking at what I have now:
You can see that they are not in line, but it's not that bad.. and my
pushrod had screwed in a little and made it look even worse here than it
really is. However, there is one more set I wanted to try, the CB
rockers. So let's see what happened with those.
Update: 01/05/2012
The CB rockers worked out about like the Berg rockers. I ended up using them.
