Area Representative Tin Man Posted April 14, 2002 Area Representative Share Posted April 14, 2002 13" wheels with 185/60 profile tyres will give me 125MPH at 7600RPM! would it be better to go for 70 proflie tyres? At some point in the future I would like to get the diff converted to a LSD would it be better then to change it to a 3.62 diff at the same time. Any ideas on cost & the best place to get it done? Thank you Link to comment Share on other sites More sharing options...
murph7355 Posted April 14, 2002 Share Posted April 14, 2002 Keep the 3.92. Acceleration is what you need IMO, there being few places where you'll need the extra 10 mph or so, or indeed get to it if you have a screen or a lower powered engine. Wider tyres or higher profile will help but there'll be compromises elsewhere (too much grip or handling dinks maybe). Higher profile is the cheaper route. You could also get your engine upgraded to pull higher revs smile.gif THE place to get your diff. sorted is Road and Race Transmissions, and you should talk to Phil Stewart. If you search the archives you'll get his number. Getting your diff converted to LSD will cost you about 820 incl. VAT and making sure the ramp angles and preload (think that's the term) are correct. You'll then have to fit it (and get your unit to Phil in the first place). Link to comment Share on other sites More sharing options...
Chelspeed Posted April 14, 2002 Share Posted April 14, 2002 Two choices from my way of thinking: Junk the Caterham six speed box with it's ridiculously low, pottering down the shops sensible road type 1st gear(2.69:1), absolutely manic (1:1) top gear and the unnecessary number of gears between the two. Get a decent Quaife box with a proper first gear, like 2.06 and decent fifth, 0.91. Keep the 3.9 rear diff (but get it lsd'd of course). Keep the six speed box but drop the diff ratio (so that first becomes usable and you get a decent top speed). I went the former, Arnie went the latter, but I see he's now come round to my way of thinking..... There's any number of ways of skinning this cat, get hold of a rpm / mph spreadsheet and play around with the options. Link to comment Share on other sites More sharing options...
murph7355 Posted April 14, 2002 Share Posted April 14, 2002 Does "dropping" mean going down or up numerically for diffs? I could have my numbers wrong but doesn't a 3.92 on 205/60/13s give a 52 max in first? A 3.62 gives 56 and a 4.1 49?? All with 8k limits. I can see why you might want to be able to pull 150 (3.62?) if you go to the Ring a lot, have 250+bhp and an aeroscreen, but what about a more std. spec K? The car might be geared for it but you'll never make it and be blunting acceleration to boot. Going the other way you compromise top speed for want of changing gears more often... I can also see the basic advantages ( 67mph in 1st and 152 top end) of the ratios you suggest for something like a big power Vx, but I think the 6spd is well suited to the K, especially in the more stock forms - you then choose between acceleration or top end... Link to comment Share on other sites More sharing options...
Chelspeed Posted April 15, 2002 Share Posted April 15, 2002 Who knows what dropping really means. I only know what I meant. Sorry for the confusion. With the 6 speed box I think you need a lower number diff ratio, ie change from 3.9 to 3.6, this both increases top speeed and makes first more useful. I agree partly re big torquey Vauxhalls but feel the difference between a 1.8K and 2.0 Vx is relatively small, much less than the difference from a 1.4K, which after all is what the 6 speed was designed to mate with. Also as previosuly stated the revs you can pull makes a big difference, steel rods and crank plus omega psitons mean I'm on the shift light at 9000 and the rev limiter at 9050. That makes a low diff feasible, I'm running 4.4:1. SBD now do a 4.7 but I don't think the small difference justifies the expense. Link to comment Share on other sites More sharing options...
R500 CYA Posted April 15, 2002 Share Posted April 15, 2002 TM You have mail. Nick Link to comment Share on other sites More sharing options...
murph7355 Posted April 15, 2002 Share Posted April 15, 2002 Agreed about the diff. between 1.8k and Vx. I suspect, from the 7600 rev limit that this is a Supersport equipped K. Capacity I'm guessing on - 1.6 from previous posts? Maybe when I finally decide that a sequential is right for me I'll go for different ratios smile.gif Link to comment Share on other sites More sharing options...
EFA Posted April 15, 2002 Share Posted April 15, 2002 Graham, I haven't come round to your way of thinking, it just the box I bought came with the hi 1st and overdrive 6th. I'm looking for a 4.44:1 CWP set to restore things to their former glory. So when you sell your's can I be 1st in line..... The best solution is big rear wheels for the road, 13's for the track. Fat Arn Visit the K2 RUM siteid=red> See the Lotus Seven Club 4 Counties Area Website hereid=green> Edited by - fat Arnie on 15 Apr 2002 22:31:31 Link to comment Share on other sites More sharing options...
Gary-Cornwall Posted April 15, 2002 Share Posted April 15, 2002 John, briefly off topic...How did the footage at Coombe look? Link to comment Share on other sites More sharing options...
CHRIS CLARK Posted April 16, 2002 Share Posted April 16, 2002 Phil Stewart (Road & Race Transmissions) always thinks that this spec is best suited to a 3.62 diff if the car is to be used for fast road work as the gearing gives better top end (albeit at the loss of a little acceleration) & better fast cruising fuel consumption. Link to comment Share on other sites More sharing options...
murph7355 Posted April 16, 2002 Share Posted April 16, 2002 Does Phil wear those dresses you make? Big girl's blouse. Fuel consumption? Pah. I'll have to give him a ring. And he looks like another 6ft plus, less than 6st er too. Link to comment Share on other sites More sharing options...
Area Representative Tin Man Posted April 16, 2002 Author Area Representative Share Posted April 16, 2002 Gary, I have put all your footage on to a VHS tape. It has come out quite well. May see you tomorrow. I will bring the tape with me. Thanks for all the replies. I will probably try the 13" 185/60 profiles & see how it goes. Now the best place to buy the wheelsteeth.gif Link to comment Share on other sites More sharing options...
old captain slow Posted April 16, 2002 Share Posted April 16, 2002 Arnie, Do you run nose down on the road then? The car I mean with big wheels on the back and little-uns on the front? C7 CDW Link to comment Share on other sites More sharing options...
Peter Carmichael Posted April 17, 2002 Share Posted April 17, 2002 IMO talk about the 3.92 diff giving *better acceleration* are misleading. That is only true for speeds where you are in the same gear and it means that the 3.62 gives better acceleration when you have had to change up with the 3.92 and still have a few revs to go with the 3.62. The appropriateness of the diff is determined by how well the selection of gears suits the driving you are doing. If you are hardly ever dropping to second you aren't makign best use of the box. I *really* like the 3.62/13inch wheels/six speed combination. It worked well with 7600 revs and 130ish horsepower. It works well with 9200revs and 250bhp. Extra revs are like longer gearing; they make each gear appropriate for higher road speeds. I now use first quite a lot, but only in situations where a shorter gear would embarrass my traction. The ratios mean that at any road speed between 50 and 146 mph I can deploy over 235 bhp by selecting the appropriate gear. Link to comment Share on other sites More sharing options...
murph7355 Posted April 18, 2002 Share Posted April 18, 2002 Who wants your O anyway teeth.gif Isn't what you said the whole point? Perhaps the only time your factor will come in is when you're doing 130+....or for small fractions of a second through each gear. Link to comment Share on other sites More sharing options...
Robin Renton Posted April 21, 2002 Share Posted April 21, 2002 I,ve changed my diff from 3.92 to 3.62 LSD (6 Sp. VVC) and put 205 x 13 60s on the rear. Phil from Road Race supplied the new LSD, the change to 205s on the rear was his suggestion and I agree with him that it improves the handling balance - it also has the added benefit of a slightly larger profile than 185 x 60. My car (160 bhp) will happily pull all the revs in top and the car is significantly better with the new ratio IMHO. Link to comment Share on other sites More sharing options...
EFA Posted April 21, 2002 Share Posted April 21, 2002 Peter, IMO your theory is flawed. You are overlooking 1 key factor, without which your theory makes perfect sense. I'll even tell you what it is..... - after Curborough! Fat Arn Visit the K2 RUM siteid=red> See the Lotus Seven Club 4 Counties Area Website hereid=green> Link to comment Share on other sites More sharing options...
Peter Carmichael Posted April 21, 2002 Share Posted April 21, 2002 Theoryquestion.gif I merely expressed an opinion, to stand with its peers; other opinions offered by others. I stand to be corrected, but in theory (sorry "in my opinion") you will be soundly beaten at Curborough; who will listen to your key factors then? Link to comment Share on other sites More sharing options...
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