VW Engine Wear Factors

VW Engine Wear Factors

The exhaust valves are the weak link in any air cooled Otto Cycle
engine using poppet valves. The service manuals for Pratt-Whitney and
Wright provide the best examples of how the exhaust valves should be
designed for optimum wear. Valve stems of large diameter and sealing
surfaces of generous width provide the path for heat to flow out of
the head of the valve and into the body of the cylinder head.

The Volkswagen engine compromises most of these design factors in
order to keep the cost as low as possible although some features, such
as fatter valve stems, may be seen on Porsche engines.

The VW factory service manual cites the specs for the valves. Prior
to 1971 when the oil cooler was moved outside the blower housing, in
normal operation the exhaust valves the exhaust valves would fall out
of spec after about 500 hours of service. The first valve to fail
was always #3 exhaust due to the obstruction of airflow at which time
the dealer would replace the left-hand head under Block 10 of the
standard VW Work Order (ie, Other Service As Required) and schedule
replacement of the right-hand head for the next service interval. The
charge was about $40 1979-era dollars. The pulled heads were normally
overhauled in-house and the overhaul included replacement of the
exhaust valves and the guides.

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It is important to understand that equal rpm does NOT mean equal
work.

Engine wear reflects the LOAD placed on the engine and must
incorporate manifold pressure as well as rpm.

When a VW engine is converted to flight the load of the propeller
increases the load by a factor of about four. The exhaust valves of
such engines may be expected to fall out of spec at about 200 hours.
If the engine has been fitted with after-market valves of larger
diameter, the service interval may be as short as 100 hours.

VW uses the standard ‘wiggle’ test to determine axial wear of the
exhaust valve/guide, which is the same test used by Continental,
Lycoming, etc. The test jig is described in the Factory Service
Manual. Prior articles (see my blog) show the test jig I use.

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It is interesting to note that the service interval of the Continental
A-40 engine was 200 hours and determined by the wiggle test performed
on the exhaust valves.

It is also of interest to compare the bearing surface areas of the
Volkswagen to the Continental, with the latter providing more than 4x
the bearing area for approximately the same amount of power.

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Post-1971 VW engines in which the oil cooler was moved outside of the
blower housing have a slightly better wear factor but existing
obstructions to the air flow still put the #3 cylinder at a
disadvantage. When installed in an airplane and fitted with a
suitable ram-air cooling system, wear of the rear-most exhaust valves
is about equal and there are examples which have enjoyed as much as
500 hours of service before falling out of spec.

It should be understood that ’falling out of spec’ does not mean an
immediate, catastrophic failure. I have seen some VW conversions
struggling on with thousand-hour valves… and for which a atrophic
failure WAS imminent.
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Proper cooling of the VW engine MUST ensure that the cooling air flow
passes through the fins of the cylinder head in a DOWNWARD direction.
Simply leaving the heads exposed to the prop blast ensures the forward
cylinders will run too cold whilst the exhaust valves on the rear
cylinders will run too hot. Adequate pressure and flow-distribution
is controlled through the use of shrouding on the UNDER-SIDE of the
heads.

The heat-flow capability of VW heads may be significantly improved if
all casting flash is removed from between the fins, that the
passageway immediately adjacent to the exhaust valve GUIDES are
cleared, and that the EXTERNAL surface of the head is abraded using
COARSE media, in the process pioneered by Continental.

The stock Volkswagen head casting typically uses a softer alloy
containing a higher percentage of aluminum than does the typical after-
market head-casting. As a result of using THICKER sections around the
combustion chamber, after-market heads provide LESS cooling fin area
than stock heads.

The stock, SINGLE-PORT head is less prone to thermal fracture than the
later DUAL-PORT head. For engines of less than 2000cc displacement,
when operating at rpm suitable for a directly driven propeller, the
flow-rate of SINGLE-PORT heads is more than sufficient.

-R.S.Hoover