I mentioned to Andrew at Vetech a while back that I was a little concerned about how the Vetech Busa would drive on a motorway cruise. It would likely be revving quite highly at 70mph and I feared the noise might become a bit tiresome on a long journey, as well as high revs putting more stress on the engine. Andrew came up with a couple of options for reducing the engine speed whilst cruising; one is to use a higher final drive ratio and the other is to swap the 5th & 6th gears for a custom pair with taller ratios.
I initially favoured the diff option, as having a taller final drive might make for better traction in lower gears too. Having driven the test cars though, traction is nowhere near as much of a problem as I expected it to be, with both the normally aspirated and supercharged version able to put the power down remarkably well, even on a damp track. I ran some numbers to see what difference the taller final drive and the different ratios for 5th & 6th would make. Here’s what the standard ratios look like (the tyre size is based on 10″ wheels with 70 profile tyres, by the way):
| Engine RPM: | 5986 |
| Gear Shaft RPM: | 3750 |
| Final Drive: | 2.84:1 |
| Tyre Height (inches): | 19.094 |
| Gear | Ratio | Road Speed (mph) |
| 1st | 2.615 | 28.68 |
| 2nd | 1.937 | 38.72 |
| 3rd | 1.526 | 49.15 |
| 4th | 1.285 | 58.37 |
| 5th | 1.136 | 66.03 |
| 6th | 1.043 | 71.92 |
So you can see that at an engine speed of around 6000rpm, you’d be doing around 72mph. I’d like to bring that RPM figure down a bit, to make cruising a bit more comfortable.
Andrew pointed out that the gearbox doesn’t work quite like a regular car gearbox in that the gear shaft does not spin at the same speed as the engine. It uses a reduction gear to spin at around 1.596:1, so you can see the gear shaft spins at 3750rpm when the engine is spinning at 6000rpm.
If the final drive ratio is made a bit taller, at 2.52:1, the figures change to those below (which are based around a cruise speed of around 70mph):
| Engine RPM: | 5188 |
| Gear Shaft RPM: | 3250 |
| Final Drive: | 2.52:1 |
| Tyre Height (inches): | 19.094 |
| Gear | Ratio | Road Speed (mph) |
| 1st | 2.615 | 28.02 |
| 2nd | 1.937 | 37.82 |
| 3rd | 1.526 | 48.01 |
| 4th | 1.285 | 57.01 |
| 5th | 1.136 | 64.49 |
| 6th | 1.043 | 70.24 |
So with the taller final drive ratio, a cruise at 70mph brings the engine speed down to around 5200rpm. That’s certainly an improvement.
If we go back to the original final drive ratio and instead swap the 5th & 6th gear ratios to 1.042 for 5th and 0.81 for 6th, it basically makes 5th gear what 6th used to be and makes 6th gear an overdrive. With those ratios, 70mph in 6th gear brings the engine speed right down to 4550rpm. That’s about what I was looking for and should make a motorway cruise much more comfortable.
As a side effect, it will mean that on accelerating, I’ll drop out of the power band when changing up to 6th gear and to a lesser extent when changing up to 5th. I’m not too bothered about that, as if I was accelerating hard in 4th gear, I’d be doing well over 100mph by the time I needed to change up.
Based on the figures above, I’ve asked Andrew to fit the extended 5th & 6th gear ratios.