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

wrote:
VW Engine Wear Factors

The exhaust valves are the weak link in any air cooled Otto Cycle
engine using poppet valves.

For the exception that proves the rule, www.rotamax.net

On Jul 20, 12:42*pm, Ernest Christley wrote:
wrote:
VW Engine Wear Factors

The exhaust valves are the weak link in any air cooled Otto Cycle
engine using poppet valves.

For the exception that proves the rule,www.rotamax.net

************************************************* *******************
And, just where are the "poppet valves" on that engine......??????

Reggie

*And, just where are the "poppet valves" on that engine......??????
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Dear Reggie,

Perhaps you should re-read the msg. The Otto Cycle defines the
typical four-stroke combustion cycle. It has nothing to do with the
method of valving, which may use poppet valves, sleeve valves, ported
valves or a combination of the three.

-R.S.Hoover

On Jul 20, 9:04*am, " wrote:
VW Engine Wear Factors

The exhaust valves are the weak link in any air cooled Otto Cycle
engine using poppet valves. *
************************************************* **********
Is there a larger diam stem, Porsche, valve, spring, keeper,guide
assy that could
be placed in the VW head ???

Or even hot rod "kiddie" (after market) stuff ......

Reggie

On Jul 20, 1:22*pm, Reggie wrote:

*Is there a larger diam stem, Porsche, valve, spring, keeper,guide
assy that could
be placed in *the VW head *???

Or even hot rod "kiddie" (after market) stuff ......
-----------------------------------------------------------------------------------------------------

Dear Reggie,

Since money can buy anything except poverty I'll bet you already know
the answer :-)

Of course you can buy Porsche-type heads for your VW conversion. The
going price is about $5k USD per pair since they have to begin with a
new casting. But for those rolling in dough the usual procedure is to
simply buy a spare set of heads (about $125 each and you'll need two)
and simply swap heads when the engine has accumulated about 200
hours. The work will take about four hours for a first-timer, as
little as one hour for someone more experienced. You then have 200
hours to inspect & overhaul the 'down' heads, which should cost you
$28 per head [2008 prices] assuming you have the necessary tools. The
tooling and procedures are described in prior articles (with
pictures!).

The 200 hours is not carved in stone. A leak-down test, which is done
with the engine assembled, will tell you if you need to pull the head
and do the wiggle test. Some engines will soldier on in the high-
green for 500 hours or more, depending on the manner and conditions
under which they are flown. (Engines fitted with a full-flow oil
filtration system do much better than those withoiut.)

Typically, you will see the end-float opening beyond spec somewhere
between 500 and 750 houirs, indicating it is time to replace the
bearings on the lower end. A full set of bearings is about $60USD
[2008 prices]. As a general rule you can replace the mains twice
before it becomes necessary to re-machine the thrust bearing carrier's
web and when THAT falls out of spec you may buy a Universal
Replacement Crankcase [about $400 USD -2008 prices] and begin the
cycle anew.

The above is NOT a description of something new or unusual. The
Volkswagen engine was DESIGNED to be maintained in the manner
described. With regard to maintenance, the only major difference
between flying and vehicular use is that the higher output required of
an engine converted for flight dictates the need to perform such
maintenance about twice as frequenty.

While you can always buy a solution to a problem, the method described
above is based on investing in yourself, acquiring the skills and
tools needed to properly maintain your engine. Not only will you find
there's a lot of flying in 200 hours, there is a lot of satisfaction
being able to do so entirely upon your own merits as a mechanic.

-R.S.Hoover

On Jul 20, 1:22 pm, Reggie wrote:
On Jul 20, 9:04 am, " wrote: VW Engine Wear Factors

The exhaust valves are the weak link in any air cooled Otto Cycle
engine using poppet valves.
************************************************* **********

Is there a larger diam stem, Porsche, valve, spring, keeper,guide
assy that could
be placed in the VW head ???

Or even hot rod "kiddie" (after market) stuff ......

Reggie

The Fuel Injected 1600 cc engines (Type I) produced after 1975 had
larger valve stems, 9mm vice the 8mm used up to that point. The valve
itself, however, was smaller, which reduced the valve seat area, which
governs heat transfer from the valve to the head. In other words, the
smalled seats in theory offset the benefit of the larger stems. It's
not clear that this combination gave any longer valve life, and these
heads and valves, while allegedly available, are pretty rare.

I have no personal experience with them.

Veeduber has discussed in his blog using ceramic coatings to increase
exhaust valve life. Lots of potential there. I'm building an engine
with coated pistons and valves but it hasn't even run yet, let alone
flown, let alone proven itself with hundreds of flight hours.