Peter Dohm wrote:


Some pretty experienced people (in VW engines) have said that the valves

are

the weak link, and that much more that 45 HP will melt the valves down, if
run at that level for more than a few minutes.
--
Jim in NC


For what it's worth, I'd have to agree, with the thought that you can
operate at higher power - until - the heads are heat soaked.
Then it's 45 HP or bust.

I think the thinking is that the engine needs to turn up fast to make
maximum engine power. Which may be true drag racing dune buggies.
Grab a gear and spin that puppy up!

Fixed pitch props don't do that.

Think one speed automatic transmission - with a lot of slip.
Pure torque converter.

IIRC, and it's been a while, the VW factory specs said max rpm ws 4200?
The torque curve peaked about 3000 (?)

Swinging a propeller at 2500 to 3000 RPM puts us on the rising part of
the curve - approaching peak.
The faster it turns - the more torque it makes - the faster it turns.
That's fun!

I think that's why VW powered parasols and biplanes tend to be faster
than their two-stroke powered brothers.

For a VW to turn a propeller ~~3000 rpm means...
1600cc 54-56" prop diameter
1835cc 56-56"
2180cc 58-60"

Our little low and slow airframes lean toward the long end.
Swing as big a stick as you can.
It pays off in prop efficiency, disk area, and tip circumference.
(compared to a 66"~68" prop on a Rotax 503, VW's climb like sea slugs)

Faster airframes tend to use shorter props and throw the excess torque
into pitch to go faster! But at high cost in prop efficiency (again).

Well, like I said, for what it's worth...


Richard