The Excelsior Project, 27: Engine Design
It is now Oct 31 / 06 and I have been back from Davenport for two months. These eight weeks have been so very different from the eight leading up to Davenport when the shop was humming at warp speed and the chips were flying.
Autocad is now in constant use, along with internet, sketch pads, catalogs, emails, phone calls, visiting people, and more phone calls. I have fabricated almost nothing.
The mockup cylinder heads (below) were completed by Lance Hayward (www.haywardperformance.com) by finalizing the port shapes. I really believe Lance is one of the best. The tan bondo is mine, and the pale blue filler is his. The heads and barrels will be cast in 356 aluminum. I had originally wanted to use ductile iron to be true to the original, but the casting industry has changed greatly, and no local foundry was willing to take on this complex job. They want big jobs that aren't too fussy... lots of "tonnage". Casting in aluminum means lighter weight, better cooling, and a hi-tech Nikasil liner...
The valve train design came together in August when the mockup heads were built. The focus now has shifted to the bottom end, and it has been much more of a challenge. Originally, these engines produced about 8HP for the street models, and the 1919 OHC racer may have had 20-25HP... A lot has changed in 85 years to say the least, and I'm told to expect 50HP with a more modern design, materials, and fuels.
Finding space for the larger bearings required by the increase in HP has been difficult. In some ways it would have been a lot easier to start with the proverbial "blank sheet of paper", but I had to stay true to the original form on the outside.
And so, the bevel drive had to be redone yet again; this will be version #3. Pinion side bearing will now be a 62mm double row ball bearing, exactly the same as a Norton Manx. It took weeks to come to this decision... because of the bevel gears, the flywheels have to be located on the right side, and there was no narrower bearing that would do the job without having them custom made, and that didn't make sense at all. This process of engine design calls for a multitude of correct choices before embarking on the expense of pattern making and foundry work. I'm very aware that one poor decision can mean a weakness in the motor that could be VERY expensive to re-engineer. You know how when you do something for the first time you can ALWAYS do it better the second time? In a sense I'm trying to do the impossible by getting it right the first time.
Using Autocad to define the bevel drive. The bevel gears are 47.5 degrees, and I started with 17 teeth on each. This placed the gears too low, so I tried 18 and 19 before settling on 20 teeth. To design a bevel gear is a full page of formulas from Machinery's Handbook. If you get one equation wrong, it affects formulas down the page. Took me right back to Physics 12 that I wasn't very good at ... thank you Scott Taylor for correcting my errors. At the bottom of the bevel drive case are the two oil pumps. The yellow lines show where the case needs to be enlarged slightly to make room for gears and bearings >
The bevel drive itself is a modular system. The three gears -- with no teeth -- are aluminum blanks machined along the pitch angle as a mockup. The bearing holders will be CNC machined from 7075. The two arcs at the top are for location of the assembly inside the case >
Here it sits in the case. The lower bearing support is missing in this photo...