Above: 2nd gen right side view
Below: original right side view

Below: 2nd gen carb detail


ENGINE NO. SIX DATA (2nd Generation)

Cylinders: 1
Type: Vertical DOHC
Bore: 1.062”
Stroke: 0.875”
Displacement: .778 cu in
Compression Ratio: 8:1
Ignition: Glow plug
Cooling: Air
Fuel: 15% nitromethane/methanol mix
Max. RPM: 13,000
Est. Horsepower: 1.0


Background -- This engine was originally designed with simplicity (and a low count of ball bearings) in mind. The original SOHC version used timing belt cam drive, cup/bucket type valve actuation and a variable venturi carburetor (a small lever connected to the carburetor slide served as the throttle control). It also had a one piece head and cylinder like the old Offenhausers. The basic idea was, to quote William B. Stout (designer if the old Ford Trimotor airplane), ”Simplicate and add lightness.” The simplication worked out okay, but since this was not an airplane engine, I didn't worry much about lightness.

The 2nd Generation -- Not being able to leave well enough alone, ol' No. 6 has now gotten both more complicated and simpler. I cut off the head and replaced it with a dual overhead cam hemi head (more complicated) using my new cam follower system which replaces the cup/bucket system (simpler). This seems to work well so far. It also uses an improved variable venturi carburetor.

Construction -- In the original SOHC version, the head and upper part of the cylinder were hogged out of one piece of 6061-T5 aluminum. This eliminated any problem of head leakage and also removed the neccesity of finding room for head screws. The lower half of the engine was really the crankcase.

One experimental feature was the use of twin glow plugs. I used a flywheel for smoothness, although an engine like this will run without one -- it just won't idle very well. The early Offenhauser engines didn’t even use crankshaft counerweights. This gave them almost instant acceleration which was useful in the old dirt tracks.

Operation -- The original engine turned out to be one of the best performing singles I’ve built to date. The twin glow plugs however, turned out to be of no advantage. Apparently the combustion is pretty good with a single plug. Also the location of the plug did not seem to have much effect as it ran the same on either plug, so the twin glow plugs were eliminated in the 2nd generation DOHC version. Idle of the original turned out to be around 2,500 r.p.m. and max. speed was around 13,000 to 14,000 r.p.m. Performance of the second generation is still a work in progress (see below).

*THE LATEST (February 24, 2008) -- While the engine runs OK, it showed what I originally thought were symptoms of valve float. Took me a while to figure out that what was actually happening was that the timing belt was slipping cogs at high RPM and thus changing the valve timing. This hadn't been a problem with the SOHC design, I suspect because the belt was so much shorter. At this point I'm still working out a solution that involves gears instead of a belt for at least part of the cam drive mechanism. Stay tuned.




Detail showing intake side of new DOHC head. The colored assembly drawing shows how the new blade-type cam follower works. A bucket type actuator was used in the original SOHC design.

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