This is a subject that has interested me even before I bought my XB, but probably understood it even less then. After riding both the 984cc and 1200cc versions I always thought that there was another engine lurking within those Sportster XL cases.
I even wrote to Erik several months before I got my City X with my ideas. He replied eventually to wish me luck modifying my own. Buell were probably already contemplating the Helicon twin!
As many on here may already be aware that con rod length and the length to piston stroke ratio has some impact on how a motor makes it’s power. Briefly a longer rod to stroke ratio reduces piston acceleration at the top of the stroke and gives longer dwell time increasing efficiency of the motor at higher rpm, also reducing angular momentum of the rod and sideways loading on the piston walls. While a shorter rod to stroke ratio gives faster acceleration of the piston away from TDC and improving intake gas draw down and mixing and therefore better torque at lower revs. Obviously many other factors like cam timing, compression ratios, etc, etc can complicate the equation.
The XB12 appears to have an optimum rod/stroke ratio for its operating range of 1.84 while the XB9 has a somewhat F1 like 2.33 ratio. But spare a thought for the poor little Blast which was blessed with a ratio of 2.38 I think. F1 motors like the Ford DFV had high ratios for their day of 2.25 if memory serves, but later 19,000 F1 motors were at a heady 2.7!
According to some stories the Blast begat the XB9, and the XB9 begat the XB12, but the truth is probably more mundane and a case of convenience. The architecture of the 12 was already in existence and the Blast was designed to be made with economies in mind to be assembled on the Sportster production line with as many common parts as were available. I read that the Blast uses a con rod unlike any of the Evo twins being the same length as the iron barrelled XL61 (1000cc). Maybe HD had crates full of unused pre Evo rods gathering dust so they suggested using the front inner fork rod from that stock?
Given that the design would use a standard Sportster barrel this meant that a standard Sportster piston would reach too high at TDC. Then I read that the ‘deck’ of the Blast’s crankcases was raised 0.168 inches to allow for the longer rod. Those with more accurate knowledge may wish to comment? The Blast crank was heralded as being new and innovative in one article, going on to influence future crank design…..does that simply mean that it was to no longer be a serviceable item and needed to be replaced complete when it wears like my XB9?
The XB9 would appear to have been given its own unique rod length so that it would match the rest of the ‘X’ twin production. Although the design gave higher revs from a allegedly less stressed motor that rod ratio of 2.33 was probably made out of convenience and cost constraints as well to utilise a 1200 set of barrels and cases. Didn't stop the 9 lunching its pistons though. I am conscious of the fact that my old Honda 250RSA used a 'forged' piston back in 1982!
My Yamaha SRX400E ‘Mono’ has the same architecture as the bigger SRX600E (the motors are interchangeable) as the distance between the crank centre line and head gasket is identical. The 400 therefore uses a longer rod and higher ratio to stroke. If you look at the torque curves of the 4 and 6 you see the same sort of differences as between the XB9 and XB12.
The 600 motor has a fat hump or constant torque curve, while the 400 has a flatter curve with a pronounced step in the mid range just like the XB9. Long rod characteristics?
You can feel these differences in the power characteristics of the different sized XB motors and that is quite apart from one engine being 22% bigger, it fills its boots better in the midrange!
I'll be quite honest and admit that I am not a very sporty rider. In my teens I had a 12,000 rpm 125 twin and that was enough for me. A few years later I rode a friends newly acquired Combat Commando 750 and ever since I've wanted torque, but with an edge. I wanted to see a sort of hybrid motored XB with a stroke of say 3.41 inches which would reduce the rod/stroke ratio giving something of the best of both the 984 and 1200…..a 1074cc! One day maybe?
I guess my constant fettling with a venture has sort of tricked my 9 into thinking that it has a shorter rod/stroke ratio. Maybe I'll try it on the dyno when my MOT man recovers from his injuries, to see what's actually happening at the rear wheel.