I saw this email/post on one of the yahoo lists I monitor. Very interesting and somewhat applicable to VW engine use on aircraft.
“A VW Type 1 1835 engine built as followers; Scat C25 cam, 041 performance heads with 40mm intake and 35mm exhaust, 7.5 CR , 300CFM carb, four tube into one headers and otherwise blueprinted should give the following HP.
2000 RPM 38 HP 99 TQ
2500 RPM 48 HP 101 TQ
3000 RPM 61 HP 106 TQ
3500 RPM 78 HP 117 TQ
Give or take a couple horses depending on your assembly accuracy . You will also need a good disturber or mag in the dist. hole. The flywheel in mag isn’t as good, no spark advance.
Now for a little of my background, I raced SCCA Formula VW in the late 70 and early 80. If you don’t know what that is it’s a 1200cc VW that we could get a little over 50HP out of in mostly stock form, that would send me down the straight at 7000 RPM and about 125 MPH. In addition to that I have been building VW for Dune Buggy’s and Baha bugs for 40 years.
I also had a 1835 in a Volksplane in the 90′s swinging Sterba 54-34 prop, takeoff was about 3300 and cruse at 2900-3000 and 90-95 mph. It would take off in 200 -300 feet.Later I put a Tennessee prop same size with about the same numbers.
I thank your problem is the prop. The RPM up to 3500 is OK, much faster and you loose efficiency.”