As it became clear that the owner of Nourish Engineering, a certain Mr. Chris Bushell, was never going to deliver an engine to me, nor anybody else I assume, it meant I had to go back to the drawing board and come up with a replacement engine. This was a simple decision. It had to be a NORTON, again with a displacement of 750 cc. This engine was originally designed in 1947, as a 500 cc called “Dominator”, by Bert Hopwood. Over the ensuing years the basic design was kept but new models introduced with increasing cylinder volumes. In 1968, Norton introduced the Commando model and this engine is what forms the basis of my new motor. If you have read my “MY STORY”, at the first page, you realize that I am now back to where it all began – with a Norton Commando engine.
The old NORTON company ceased to exist many years ago but luckily a number of small firms have taken up the manufacture of practically all parts needed, and several also specialized in the development of the original design with one single object – extracting more power.
In fact I am not at all sorry having had to switch engines. With the Norton I will have the opportunity to use whatever is left of my engineering skills, in the process of deciding on main parameters as well as the selection of components. As basically everything is possible to buy ready-made I don’t see myself designing anything myself. Maybe just as well.
My first Norton engine had a bore and stroke of 73 x 89 mm and produced, at best, 73 horse power at a red line of 7800 rpm, which translates to a mean piston speed of 23,14 meter per second. As mentioned elsewhere on this site, as a rule of thumb one should keep the piston speed below 20 meter / second in order to safeguard reliability. On the other hand, this is a racing engine so reliability has to take the back seat while priority is horsepower. As horsepower is a function of cylinder volume, volumetric efficiency, compression ratio and rotating speed, one of the things we have to push is rotating speed. Cutting the stroke from 89 to 80,4 will allow the red line to be increased to 8500 and still keeping the piston speed below 23 m/s. A shorter stroke, 80,4 mm, means that the bore can be increased to 77 mm and still be within the maximum 750 cc total cylinder volume. No problem, I can get a cylinder with 77 mm bores and a crankshaft with 80,4 mm stroke. This combination gives a little more than 748 cc so we are very close to the 750 limit.
The next variable is compression ratio. This is simple, the higher the ratio the more power. However,
the higher the ratio the higher is also the mechanical and thermal stresses. “Luckily” we have another factor also which will limit how high we can go. This is something called detonation of the fuel/air mixture. Detonation means that the normal controlled burning of the fuel is replaced by explosion, which in turn will quickly burn a hole in the piston top. For any given combustion chamber design, the compression ratio where detonation will occur is determined by the detonation resistance capability of the fuel. Experience with this engine says that we can go to a maximum of 11,5:1 in compression ratio, running on high octane pump fuel. I have selected pistons that will give me about 11,2 to 11,3.
This leaves us with the most difficult parameter – volumetric efficiency, or in short: how much air/fuel mixture can we trap in the cylinder at the point when valves are closed and the spark plug fires its lightning? The more air/fuel the more power. Simple but how to do it?
It is a function of many things. Cylinder intake tract size, shape and length, exhaust tract size, shape and length, camshaft profile, valve sizes, carburettor efficiency, exhaust system layout and so forth.
I will take one step at a time and leave the trickiest part of this work to a couple of the best Norton Commando engine tuners on this planet. You will be able to follow this in my LOG BOOK.
Another deviation from the standard Norton layout is the crankshaft itself. The original is a standard 360 degree ( see under IGNITION what this means ) while mine will be a 270 degree crank. This means that the pistons will not run up and down at the same time but that the left, drive side, piston will be trailing the right side piston by 90 degrees. The crankshaft is specially made by taking a Triumph T140 crank, cutting it in two pieces, offsetting the two crankpins by 90 degrees and then welding it together again. This coupled with a couple of trick light weight pistons and connecting rods is supposed to reduce vibration to a very minimum. Vibration has always been the curse of 360 degree crankshaft parallel twin cylinder engines, no matter what make, and have always caused all nuts and bolts not secured by safety wire or Loctite to separate from the motorcycle. We´ll see if the theory here will work in reality. If not we will need to change the reality, as Einstein said.
Having said all that, my target for this engine is to make it produce top horsepower at around 8500 rpm and to have a redline at 9000. This should put it up with the best Norton 750 engines which are said to produce 80-85 horsepower at the rear wheel. This is a lot more than I can ever hope to take full advantage of when riding the bike, but we are not here to make something mediocre. It is supposed to be able to run with the fastest and best.
The main difference between the Norton and the NRE engine is that the latter has 4 valves per cylinder while the Norton has only 2. This is a power disadvantage that is practically impossible to compensate for but we shall be doing our best.
Here is a nice picture that I found in Google, showing the Commando engine mated to a gearbox.