It appears that I may be getting closer to getting my Voigt T9 conversion (waiting since Dec 2018).
Alan Voigt has proposed three options for the gearset:
- Long 1st @ 2.9(7?). Gives the 5 speed box similar ratios (1 - 4) to the semi-close stock gearbox I have now but with an O/D.
- A set with a 3.3 1st. This is of no interest to me as I think this is too low to be practical
- A close ratio box with 2.66, 1.75, 1.28, 1.0, 0.84
Options 2 and 3 come with a heavy duty layshaft bearing. Not with #1. Also a significant premium (additional ~1000 pounds) for #3 over #1.
I like the idea of the HD bearing since I have a high output Twink (181 hp, 143 lb-ft).
I also have a wide and flat torque band from 4500 to 7000 such that the existing semi-close ratios stay within the peak band, (1700 cc, stroked) when shifting at redline (I use 6900). I also have a 3.77 diff. As well as 185/60 low profile tires that equate to an equivalent 3.94 diff. I will say that I am happy with the current ratios (except for lack of an overdrive 5th). I don’t find myself burning out in 1st gear with the sticky tires. Car is quick.
The CR ratios also stay in the torque band through shifts but, from calculation, a large portion of the lower rpm peak torque band (4500 - 5500) is missed with the shift to 3rd and to 4th. The rpms stay too high which is perhaps better for a peaky engine.
My question is whether to stay with the 2.9 first (as stock SCR) or opt for the close ratio set. Given my wide torque band it seems that I don’t have to sacrifice the potential better acceleration of the SCR ratios while still maintaining peak torque. It seems that the CR ratios are better suited to a more peaky engine, once you get off the line, where you have to stay in a narrower torque band.
Be interested to hear from some of the gurus on here if my “analysis” makes sense. I have been focused on maximizing acceleration but perhaps there are other issues to be considered. The car is for the road and not raced. So not tuning for a particular race course.
Right now, I am inclined to go with the 2.9 SCR but without the HD bearing. I tried to get Alan Voigt to do a SCR with the BGH HD bearing - which they offer on the BGH website but Alan said only the 3 options presented are available for his 5-speed T9 conversion.
Here is the plot of the SCR ratios shift points (stays in torque band and utilizes lower rpm torque):
Here is the Voigt CR ratios (misses lower portion of torque band in 3rd and 4th, and presumably slower off the line):
Twincam Dyno: