david nelson Posted December 8, 2006 Share Posted December 8, 2006 Its winter and i am looking at what i can do on the 7 for next years sprints and hill climbs current spec 1.8SS 140 BHP 6 speed 3.62 diff What advantages or disadvantages would a 3.92 diff offer? thanks David Link to comment Share on other sites More sharing options...
Area Representative Richard Price Posted December 8, 2006 Area Representative Share Posted December 8, 2006 David, The theory goes, that a lower diff ratio will give better acceleration, and the maths looks to agree with this. With a change of ratio from 3.62 to 3.92, The force that appears at the wheels at any given revs in a given gear, will be some 8% better! (but at an 8% lower speed) However, I changed from a 3.62 to a 3.92 at the beginning of this year, and I'm not so sure its any better! For example, where at Curborough, I would have stayed in second from "Mole hill" to "Fradley", I now have to take third, and then back down to second, then along the straight, I'm up into fourth before braking for "Flag pole", where previously I would have only got to third. Previously, I would have stayed in second from "Flag pole" to "Mole Hill", but now I need to take third, and back down to second. I think that what I've gained in acceleration, I've lost again in additional gear changes. I've certainly not approached "Fradley" with the same confidence I had before, knowing that I've got a down change whilst braking heavily that may unsettle the car. So, I'm still undecided. May be I just need to try a little harder at Curborough Link to comment Share on other sites More sharing options...
Paul Bowden Posted December 9, 2006 Share Posted December 9, 2006 I have made the same change as Richard, and it certainly makes a difference, but as to quicker , I think that will depend on the course. If there is a really tight course, and a number of sprint courses have hairpins around a single bollard, then the lower ratio in second gear is a hugh bonus. But as Richard says there is the potential for more grear changes, sometimes, when you really could do with out it. Suppose that would be why race cars/ bikes change ratios to suit the individual track 😬 Link to comment Share on other sites More sharing options...
se7enmad Posted December 9, 2006 Share Posted December 9, 2006 you use lower diffs when either the track is very short and twisty or you do not have sufficient power and you need the acceleration - always at the expense of top speed. The gear changes are not usually a problem, unless the track has a combination of twisty bits and long straights, as you just ignore first and possibly second. I think you need to look at high gears, but you really need to determine if you have a problem first. Antonella 1998 Caterham Vx 1.6 Link to comment Share on other sites More sharing options...
jerrypike Posted December 9, 2006 Share Posted December 9, 2006 richard were the other conditions close enough to make your times comparable? i changed my diff. to lower ratios, but also made some other changes so the quicker times may be the result of other factors david trouble is you can't easily change them to run back-to-back comparisons, but purely for sprint times in UK i think you'd be better off with the lower ratio diff jerry Link to comment Share on other sites More sharing options...
Jason Plato Posted December 9, 2006 Share Posted December 9, 2006 I now have to take third you should be aiming to carry enough speed aroudn the molehill to stay in 3rd with a 3.92 diff ( unless you are on wood tyres) here is C7 TOP Taffia Area Rep Link to comment Share on other sites More sharing options...
Jason Plato Posted December 9, 2006 Share Posted December 9, 2006 Dave I'm taking my diff out tomorrow and I'm going to RRT in Thursday to change to a 3.62 here is C7 TOP Taffia Area Rep Link to comment Share on other sites More sharing options...
sjmmarsh Posted December 9, 2006 Share Posted December 9, 2006 Daveyou should be aiming to carry enough speed aroudn the molehill to stay in 3rd with a 3.92 diff Is that with a 5spd or 6spd box? I'll give it a go next year - I am still changing down to 2nd just after turning right before molehill (6spd). It would save me a couple of gear changes (i.e. chances to muck it up). Steve Link to comment Share on other sites More sharing options...
Area Representative Richard Price Posted December 9, 2006 Area Representative Share Posted December 9, 2006 you should be aiming to carry enough speed aroudn the molehill to stay in 3rd with a 3.92 diff ( unless you are on wood tyres) Yes, list 1a wooden tyres! I remember PowderPuff say "don't brake untill after the kerb on the right hand side before mole hill". That worked well for me untill I managed to carry more speed around "Woodside". I did keep trying to brake that late but kept ending up on the grass! I guess thats why class 3 time are quicker than class 2 times! Link to comment Share on other sites More sharing options...
Rob Walker Posted December 9, 2006 Share Posted December 9, 2006 David, IMO a 3.62 diff is too tall for a low powered car with circa 120lb ft torque . A 3.92 will be quicker especially if you are forced to use 21/22" diameter radial tyres. Its no big expense to try a 3.92 and swap back if your not happy. The 3.62 was only really suitable for superlites equipped with 20" Diameter ACB10`s. Rob Edited by - Rob Walker on 9 Dec 2006 20:40:39 Link to comment Share on other sites More sharing options...
Simon.Rogers1 Posted December 9, 2006 Share Posted December 9, 2006 Told You. Link to comment Share on other sites More sharing options...
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