10-09-2017, 11:52 AM
(This post was last modified: 10-09-2017, 02:01 PM by Steve Jones.)
(10-09-2017, 10:07 AM)Alan Wrote: The problem you have is not with top gear, but the intermediate gears. 3.50x17 wheels with a 4.9 diff will give more or less the same gearing in top as 3.50x19s and a 5.25 diff. The problem is that second gear (3speed box) which was already too low, becomes even lower. The same is true for a four speed box. Making Austin 7s go fast is as much about gearing as it is engine tuning - you have to get it right. It matters not whether you use 17in wheels or a different axle ratio, the change to top gear speed will be the same,but it's the spacing of the intermediate gears you really need to sort. So, what I'd do - 5.25 or better, a 5.625 and stick with 19 in wheels (why? - they look nicer), but then swap the box for one with Ulster ratios. This will be ok for most places, but then if I was getting serious I'd have 17 and 16 inch wheels as well, because it really is all about gearing, and the more options you have, the better
Unsurprisingly, I agree with what Alan says. My car which does OK but hasn't the most highly tuned engine out there has 3.50 x 19 tyres with a 5.25:1 CWP and 'fake' Ulster C/R 3 speed gearbox ratios. It's OK for most places but better for hillclimbs than sprints (and we do more of the former than the latter). Second gear is excellent and is the one used for most of the time on most of the hills we do. Only time I use top is on some sections of the longer hills on the Isle of Man. For sprints it's a bit different and I do have to use top. I don't, really, like to as gear changes cost time but it's either that, rolling off the throttle a little or revving the engine to places I don't want it to go to!! Snetterton Sprint next weekend will be something of a challenge in this respect!
Steve