New Heads

To All:
Fellow blows a head. Buys a replacement, bolts it. Blows ANOTHER
head... What's wrong with this picture?

First off, the guy's original engine was a very shoddy piece of goods,
with the compression ratio on the 3/4 bank approaching 10:1 whilst the
CR on the 1/2 bank was about 8:1. Why? Several reasons: Case deck
was no equal. Head deck was not equal. Chamber volumes were unequal
by more than 5cc.

The imbalance makes the engine sound '...really kewl.' Also kinda
hard to start. But the kiddie is told that's because of the really
hot cam it uses.

First hot day, the kid hits a grade, puts his foot down... blows #3.

(Repeat the above as many times as needed.)

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If you don't know your engine's compression ratio you're going to run
into problems. To determine CR you must drop the engine, pull the
heads on both banks then measure the deck-height AND the chamber
volume. STOCK Volkswagen allows a range of 3cc across a set of four,
or a measured cc of +/-1.5cc
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After dropping the engine and pulling the heads to do the work above,
you should also measure the valve springs for total length, then for
their value at their compressed height. A bathroom scal will do; the
Real factory service manual contains the specs. (I've already
written about this. The article is in the blog. Dig it out, since it
also includes the specs.) . VW valve springs are progressively-
wound, meaning they have a top and a bottom. Make sure you test them
with the proper orientation... and install them as such.

Unlike most cars, which offer springs of uniquely different tension
for the intake and exhaust valves, the Volkswagen engine uses springs
having a single weight for both. However, the spec for the valve
spring tension DOES reflect a range. The mechanic assembling the
engine is expected to install the hi-spec springs on the larger INTAKE
valves and the lo-spec springs on the smaller intake valves.

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Valves have a spec for their length, which will change as you re-face
the intakes. (Exhausts are not normally rebuilt.) The face of the
valve stem is the first item returned to truth when refacing a valve.
This is how they shorten in service. You get down to -1.0mm, you can
install a lash-cap,there after simply replace the lash-caps to return
the valve to truth.

The rocker must contact the valve step on just one side. This causes
the valve to rotate as it is opened and serves to distribute wear on
the sealing surface. (It doesn't matter what type of adjuster you
use. The rotation is a function of the rocker's POSITION. the method
works equally well with adjusters that are stock, swivel-foot (of any
design), roller or shoe.)

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The valve guide must be within spec. See the manual but it's
something on the order of .005" at room temperature, measured in the
manner shown in the manual, It is typical for the intake valve to
show little or no wear out to about the third clutch-change ( ie,
typically about 120,000 miles ) whereas the exhaust valve will fall
out of spec within one cluth-worth of usage.

Worn valve guides are the most common cause of elevated hydrocarbon
emissions. An intake guide that is out of spec can cause the HO
emissions to be as high as 700ppm. The inspector will usually
attribute this to 'bad rings' and be wrong 90% of the time.

Due to the small diameter of the valve stems used by VW, it's
difficult to find a suitable valve stem seal; the one available from
Volkswagen hardens due to heat and will be rendered useless in less
than 10,000 miles of service, where as the Vitron stem seals used by
Toyota show no wear to beyond 100,000 miles.

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Given that VW engines converted for use in airplanes will typically
turn at lower rpms than the same engine used in a vehicle, the mass of
the valve-train components is not especially critical. But the system
DOES have mass, which must be accelerated each time the valve is
actuated. The heavier the components, the more energy needed to
actuate the valves (and the more fuel to be consumed). For that
purpose, it is standard procedure among professional engine builders
to weigh ALL of the valve train components and to install them in
matched sets according to the lobs of the cam.

-R.S..Hoover

schreef:

( ... )
To determine CR you must drop the engine, pull the
heads on both banks (...)

Bob you are confusing me. I seem to remember from the hazy long-ago days
of glory that compression was measured by turning the engine a couple of
times with some kind of measuring device or probe installed in the place
of the spark plug? Repeat for each cylinder and you have a red curve for
each cylinder on a tiny sheet of waxy paper?

What's all this with pulling the whole engine apart just to measure
compression?

On Jan 20, 10:36*am, jan olieslagers
wrote:

What's all this with pulling the whole engine apart just to measure
compression?
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You are confusing dynamic pressure with ...can't remember the term...
static or geometric ...compression.

Dynamic compression is for a running engine and is similar to a leak-
down test. It gives you a measure of merit to compare future
measurements against.

Static compression ratio is based on the mechanical dimensions of the
parts built into the engine. Racing class, fuel availability and a
few dozen other things are used to determine the optimum mechanical
compression ratio.

But when you get an engine for which the mechanical compression ratio
is unknown, you need to know it. And that calls for dropping the
engine, pulling the heads and doing a few measurements. This is
especially true when replacing head. I the customer is not sure, you
need to remove BOTH heads in order to verify the chamber volume does
not vary from one bank to the other.

Dynamic compression is supposed to reflect the chamber pressure in a
running engine, typically one that is broken in... and usually just
won a race. The fellow who came in second (or last :-) tells the
judges he thinks the other guy's engine does not match the spec for
that class or whatever. So they can do a dynamic compression test or
-- for a hefty fee -- have the entire engine torn down by the
proctors. (If nothing out-of-class is found, the protester just lost
his $2500 (or whatever). But if they find ANYTHING out of spec in the
winning engine, it's enough to disqualify the winner, who gets his
engine back in a card-board box, and the guy filing the protest gets
to keep his money.)

But if you think about it for a minute you'll see that dynamic
compression has very little to do with the engine's mechanical
compression ratio, which can only be determined ACCURATELY by tearing
it down and measuring it.

-R.S.Hoover