Quote:
Originally Posted by elhigh
So, square it up.
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I don't quite get why square is magic, lets see if we can figure this one out.
Lets take a hypothetical 1 cylinder engine with 6.2832 cubic inches of displacement
3 configurations:
Code:
Bore, Stroke, Area, Circumference
1.414, 4, 1.57, 4.44
2, 2, 3.14, 6.28
2.828, 1, 6.28, 8.88
The first thing that jumps out is that the long stroke (4") version is rubbing twice as much piston wall as the 1" stroke version. the longer stroke piston is also travelling 4 times as fast for a given rpm.
So at first glance, longer stroke doesn't sound like a big efficiency winner, lets look at some winners:
The 470 mpg guy stroked an xl125 to 185cc
1985 Matsu talks
stock bore x stroke (mm)56.5 mm x 49.5 mm.
1985 Honda XL 125 R Motorcycle Repair, Maintenance & Service
so the bore and stroke he used was 56.5 x 73.26, which is well on the long side of square.
Even in the 1980 competition the long stroke harleys won toodling along at about 100mpg.
So something about the extra torque at lower rpms was helping both of them. It probably allowed lower cruising rpms for one thing.