Does anyone know what the differences are between the spring used in the dynojet thunderslide kit and the stock diaphragm spring ... i.e. is one shorter than the other and are the spring rates different?
I understand that the spring constants are different, and that this will effect slide travel for a given level of load on the diaphragm. It is likely that the dynojet spring also has a different unsprung length so as to provide a different level of pre-load when installed.
I am interested in any physical traits (vice spring stiffness) that can be easily identified as differences between the springs - something like the number of coils or the unsprung length - something easily observable by placing the springs side by side and taking a gander.
I guess what you are asking does not make sense. Put the stock on in your hand compress it. First the dyno jet one and then the stock one there is no way you could not tell the difference. I have never taken the time to count coils or to measure one , because there is a simpler way to tell which is which. The kit spring is very soft ,legnth are approx the same.
As Matt hinted, I may have mixed up the springs (long story).
Anyway, I assumed (as has been pointed out) that the dynojet spring would have a lower spring constant to improve throttle response. With that, I compared the two springs to see if I could tell which one had a lower spring stiffness. To be honest, I couldn't detect much difference in spring stiffness between the two. The one I thought to be correct was a little shorter and looked a little newer, so I installed it. I've since called dynojet and they told me that the dynojet spring has an unsprung length of 5.60 inches. I can't measure the spring in the bike (without removing it of course), so I measured the one I assumed to be the stock spring and it measured 5.75 (or so) inches. Unless the tolerances are pretty loose, I think I chose the right spring. At this point, the bike's back together and seems to be running pretty strong, though maybe a bit rich. I guess what I was really looking for was validation that the stock spring is typically longer than the dynojet spring.
If you did install the thunder slide kit. You will most likely find that use of the factory spring will work better or moving the needle down a notch. The thunder slide will open too soon and on a heavy bike that will cause problems.
Good call Wrench. I took the bike in for dyno tuning, and the dyno tech did just that (moved the needle down a notch). It didn't significantly improve the performance, but eliminated a slightly rich cruise condition. I've attached the dyno results. Red is the tuning I provided, and blue is the final run with the needle repositioned.
By the way, are these results typical for a Stage I TC88B? The peak power looks alright (75.2 HP) to me, but peak torque looks a little low (81.4 lb-ft). Its a stock engine with VH Bigshot staggered exhaust, Ness Big Sucker air cleaner, and a Dynojet Thunderslide kit.