TOPIC: Desaxe engine

Thought some of you guys might want to know. From the typical accessnorton forum thread

dynodave wrote:
SNIP
750 and 850 engines are a desaxe type engine.

robs ss wrote:
Dave
Apologies for the off-thread question - though it is related...
I have noticed the crank is offset in the fore/aft direction previously - never heard the term "desaxe" before, but looked it up. Interesting!
Have all Norton heavyweight twins (500 to 850) had this feature?
Was it a specific design intent, i.e. to lengthen the induction and power strokes?
...or was in a convenient solution to some geometrical problem Norton was facing?
Cheers
Rob


dynodave wrote
An understandable question. 500-600-650 all grew to the practical limit using a symetrical bore increase, considering the limited clearance between the valve/push rod tunnel positioning. These engines (sorry for NON commando content) are all small bolt pattern barrels.
Therefore the growing process to 750-850 made the bore...grow only to the rear, hence the BIG bolt pattern barrels which in many places I have mentioned that the "growing" was only by moving the rear set of barrel bolts.
This general principle carried over in two stages for the 750 (medium) and 828 (large) bolt pattern heads.
Finally, since the crankshaft/camshaft/pushrod/rocker gear were not moved it turned the engine into a Desaxe designation...... (if I am correct) for the last two sizes only.

I had never heard the term Desaxe before, but after Wikipedia-ing the term it makes a lot of sense. More leverage on the crank journal in the power stroke means more power at the flywheel, and less wear on the thrust side of the piston. And in the case of Norton, cost savings Vs. re-designing the engine to achieve larger displacement. I doubt they had anything other than cost in mind, but it was a fortuitous oversight. And now a whole variety of modern engines use it both for performance and packaging reasons. Even Henry ford used it in the 239.4 CID Flat head V8, so there is a lot of history here.

An engineering question for Dyno Dave - since the length of the power stroke is disproportionately longer than the intake stroke, does the reduced cylinder filling offset the power gained, especially on a normally aspirated engine?

The timing differential of cam to piston is probably quite small. If norton did not change even one chain or gear sprocket timing device from the symmetrical 650,( based on the part numbers I don't think they changed a thing) It is likely again the difference is quite small and more than made up by the displacement increase. It would effectively make the cam timing late from the .078" lagging crank. I'll assume piston TDC is always king and everything else is measured from there, despite it is not straight up in line with the block/cylinders.
IIRC a normal performance move would be to advance the cam a few degrees.
Otherwise I have not looked at desaxe that closely from a mathematical stand points.
I don't see power and intake length being different from each other.