Gasflow of VW engine

Has anyone had any experience with the "gasflow" of VW engine to improve
horsepower at lower RPM?

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If by 'gasflow' you mean volumetric efficiency, I've some experience in that
area.

I'll also assume your question hinges on the fact that maximum torque always
occurs at the point of maximum volumetric efficiency while maximum propeller
efficiency comes from the longest prop that amount of torque can swing at an
rpm that will keep the tip speed below about 880 feet per second.

Toward that end I believe you will find it is fairly easy to emulate the torque
& rpm curves typically found in aircraft engines by simply grinding a cam that
approximates the timing & duration found in the small Continental engines (ie,
a-65, for example). Unfortunately, following that path I found that the small
displacement (1584cc or 96cid) of the stock engine leads to a rather
unreliable powerplant unless you keep the manifold pressure fairly low, which
of course limits the output, usually to something to small to be considered
practical as an aircraft powerplant (ie, power to weight is poor). The obvious
next step (at least, for me) was to increase the displacement of the engine.
This gets you back into the A/C powerplant ballpark but the next limitation you
will encounter, if your experience matches mine, is the thermal barrier imposed
by the fin area of the VW heads. When the continuous output exceeded about
45bhp I found cylinder head temperatures in the vicinity of the exhaust valves
exceeded the safe maximum level (about 450 degrees on the Fahrenheit scale).
Operating at or above that level for any length of time lead to a drastic
shortening of the useful life of the exhaust valves (as little as ten hours, on
some experiments).

On a similar note, efforts to improve volumetric efficiency through the use of
a tuned exhaust system were more amusing than effective. At anything below
about 3200 rpm the weight of the exhaust manifolding begins to approach the
weight of the engine, since the minimum runner length is on the order of sixty
inches :-)

I found the best compromise to be alternate-pair exhaust stacks (ie, cylinder 1
coupled with cylinder 3 and 2 coupled with 4) each having a total length of 27"


As a point of interest, I began trying to improve the output of the VW engine
in the late 1950's, starting by applying all of the typical hot-rod techniques
that have been used to improve volumetric efficiency. Some of them work but
most do not, largely because of the nature of propellers; what you gain in
apparent output at the prop flange is lost in the reduced efficiency of the
propeller at high rpm.

Experiments with larger valves and ports having a mirror-like finish gave
remarkably better rates of flow on the flow-bench. But on a real engine there
was no significant improvement in output, probably because of the relatively
low flow rate of even the largest VW engine when running at prop speeds.
(Typically, the engine would have to operate at 3500 or above to see any
improvement in flow-smoothing.)

Experiments with different combustion chamber shapes proved to be an expensive
waste of time. Altering the shape of the VW's wedge-type combustion chamber
appears to upset internal flow during the process of combustion. At prop
speeds the typical result was less power, apparently do to improper combustion.
This is partially confirmed by a marked increase in hydrocarbons in the
exhaust gas, evidence of after-burning in the stacks and so forth.

I'm presently working with thermal barrier coatings, having spent a couple of
years learning how to appy them :-) Preliminary results on one-cylinder
engines have been positive although difficult to quantify due to the relatively
small scale of the improvement and lack of laboratory facilities (you have to
adjust your temperature & torque observations for the local temperature &
atmosphere, etc.).

Finally, none of the above is new information. I've previously posted this
information, including some engine modification drawings, to mailing lists
devoted to flying Volkswagens (which this Newsgroup is not). If you'd care to
provide a valid email address I'll be happy to point you toward the various
archives.

-R.S.Hoover