2cyl Specs & Limits
In this discussion we will explore the reasonable limits of 1/2 VW power output. An explanation of the 1/2 VW conversion options and limits will also be given.
Specifications and Limits
Max RPM: 3400 or 400°F CHT
Static rpm: 3000-3100**
Prop: 50x32 to 60x20 (55x28-30 typical)
Firing Order: 1-4-3-2
O2 sensor: .45v ideal (lean .3-.4v / rich .5-.9v)
Idle speed: 700-800 rpm
Point gap: .016" (0.4mm)
Plug gap: .024" - .028" (0.6mm - 0.7mm)
Plugs: Bosch W8AC, W175T1 or Champion L87Y
Oil Capacity 2cyl: 1.5 Quarts (1.4 Liters)
Valve Clearance (cold)
Before 1965, and 1200 & 1500 w/long rocker arm studs
Intake = .004" (0.10mm)
Exhaust = .004" (0.10mm).
Intake = .008" (0.2mm)
Exhaust = .012" (0.3mm)
Ignition Timing (static)
1100/25hp bug: 5° BTDC
1200 bus: 10° BTDC
1200/36hp bug: 7.5° BTDC
1200/42hp bug: 10° BTDC
1300 bug: 7.5° BTDC
1500/51hp: 'G' case: 10° BTDC
1500/53hp: 'H' case: 7.5° BTDC
1500S/66hp: 'R' case: 10° BTDC
1600/57hp: 'B' case: 0° ATDC
1600/60hp: 'AD' case: 5° ATDC
1600/60hp: 'AE' case: 5° ATDC
1600/65hp: 'T' case: 7.5° BTDC
1600/65hp; 'U' case: 0° ATDC
1200 bug: 50°
1300 bug: 42° - 58°
1500/53hp: 'H' case: 42° - 58°
1500/54hp: 'K' case: 50°
1500S/66hp: 'R' case: 50°
1600/60hp: 'AD' & 'AE' cases: 44° - 50°
1600/57hp: 'B' case: 47° - 53°
1600/65hp: 'T' case: 50°
1600/65hp: 'U' case: 47° - 53°
Oil pressure ~10psi for each 1000rpm
Green: 180°-220°F (at pump inlet)
CHT sensor in head
(for plug sensor add 50°F - see note)
Red: 400°F (takeoff up to 400°F 5min)
Red: 1500°F (Takeoff up to 1450°F 5min)
1100/25hp Solex 28 PCI
1200/36hp Solex 28 PCI
1200/40hp Solex 28 PICT
1200/42hp Solex 28 PICT-1
1300/50hp Solex 30 PICT-1
1500/51hp Solex 28 PICT / PICT-1 / PICT-2
1500/53hp Solex 30 PICT-1
1500 Type-3 1 carb Solex 32 PHN
1500 Type-3 2 carb Solex 32 PDSIT 2 & 3
1600/57hp Solex 30 PICT-2 / PICT-3
1600/60hp Dual Port Solex 34 PICT-3
**It is recomended that static rpm on the ground not be less than 3000 rpm. There is a big difference in thrust between 2800 and 3000 rpm perhaps as much as 20-30%.
Why is RPM & Power Limited?
- inadequate cooling and the resultant over heating is the main reason VW engines do not commonly use redrives, turbo-chargers, or higher rpm's - it is primarly a heat problem. VW auto engines have more efficient cooling (verticle flow with dedicated fan) than aviation installations.
- VW cranks can not handle the higher rpm prop loads and vibration. Anything over 3400rpm in a 1/2 VW has generally been met with crank failures. The crankshaft bends at higher RPMS. Ferry Porsche designed it to run sweet at max 3600 on the road. If you strip a VW down that has been run at higher than normal RPMs, check the centre bearing mains, this where you can see the condition of the crank.If the brearing numbers from the back of the shell are imprinted on the case then odds are it has been thrashed. You can line bore the case and elimanate temparary mechanical issues, but high rpms will thrash it again.
- Another problem is valve bounce caused by weaker valve springs. Fitting harder springs will reduce the bounce but increase the wear on the cam and followers due to the higher pressure caused by the valve springs.
- Stock VW rods fail quickly at high rpm due to the extra loads and viberation from the propeller. Steel rods are the most dependable in terms of fatigue and chevy H beam is considered the best (and very expensive). Billet aluminum rods are an upgrade over the die cast aluminum rods used in the VW industrial engine and are used in auto drag racing engines where they are changed regularly.
- Wood props are less efficient at high rpm than metal and can not handle the stress of breaking the sound barrier. Metal will hold together near the speed of sound and are about 10% more efficiant than wood props, but they still loose efficiency dramaticaly after 90% speed of sound. When Formula One racers where allowed to use metal propellers, it was common to run prop tips past the speed of sound. However, the VW engines used for Formula One were only designed to hold together for a few hours, not hundreds. Unfortunately, VW engines come apart quickly with metal props (crank and bearing failure).
There is nothing preventing a 4cy VW from operating at 4000rpm other than prop efficiency and heat. Since most folks are using Force One pulley end bearings, the Force One is not designed for anything over 80hp. However a designer could successfully use a flywheel end prop and hub as long as the engine did not overheat. I rate my 4cy at 3900rpm for 5min or heat, whichever comes first. Max cruise is 3400rpm.
1/2 VW engines are generally limited to max rpm of 3200-3400 because of vibration and that's something that you can never get away from with a two holer. A reduction drive solves all the stress and vibration problems of a 4cy (making a metal prop possible). However, belt reduction drives have not worked on 1/2 VW. Many style of belts have been tried but will not hold up to the firing impulses of 1/2 VW. For the same reason chain drives have also been unsuccessful.
So, are we left with out any possible solutions? No, that is not the case. There are some things that can help control the heat issues. They include...
- Adding a large external oil cooler (usually below engine).
- VW’s do not have enough finning for continuous high hp operation and so ther has been some experimentation with welding larger fins on near the heads.
- Adding full flow oil filter
- Drilling the oil / tappet galley larger
- Adding oil grooves to rocker threads, cam follower and cam web (HVX mods)
- Optimizing cooling baffle design
- Improved rocker foot design (elephant foot, ball foot or roller)
Aviation experimenters are always willing to try new things. There has even been some experimentation with water cooled heads. Whatever, extremes designers may try, we would be wise to always remember that despite the head overheat issues and limited oil supply, the humble VW engine remains the least expensive path for budget minded aviation enthusiasts.