Simon.seven Posted July 1, 2008 Share Posted July 1, 2008 I have the new R400 duratec and the gearing leaves the engine reving its b***s off. 80mph is approx 5000rpm top gear (6th). I do not know how they can rate this @ 140mph top speed. I need to improve the gearing some how the question is should I do this through the diff. ? Would 15" wheels make much difference to the gearing over the 13" ones that are on? Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted July 1, 2008 Share Posted July 1, 2008 Depends what tyres are fitted to the current 13" wheels and what you can fit under the arches. It is the outside (rolling) circumference that matters. Glad I have the 5-speed ... I think your 6th is like my 4th. Edited to make sense !!!! 7 related photos Edited by - Stationary M25 Traveller on 1 Jul 2008 23:12:51 Link to comment Share on other sites More sharing options...
Graham King Posted July 1, 2008 Share Posted July 1, 2008 Simon, there are plenty of gearing calcs around, but assuming you are on 185/60 R13 and change to 195/45 R15 (which I think are Caterham standard) then your RPM would reduce by about 7.5% for any given road speed assuming everything else stays the same. Do you know what Diff ratio you have 3.62 or 3.92? Graham. R7 GPK Superlight #85 Link to comment Share on other sites More sharing options...
Simon.seven Posted July 1, 2008 Author Share Posted July 1, 2008 I must have the 3.62 diff if I have my understanding the right way round. It is certainly geared for accelaration but too much at the expece of top speed. How difficult is it to regear my diff. It is an LSD. Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted July 1, 2008 Share Posted July 1, 2008 Standard Roadsport is 3.92:1, Superlight is 3.62:1 according to my old Handbook. 7 related photos Link to comment Share on other sites More sharing options...
Graham King Posted July 1, 2008 Share Posted July 1, 2008 Simon, I believe all LSD were 3.62, a 3.92 would make it accelerate faster but be greater revs for given road speed. About the only factor you have to really play with is wheels/tyres, I am not sure if there are any realistic options below 3.62 from CC and the gearbox final drive can't be changed. You could try 205/60 R13 on the rear this will have a greater impact than switching to 195/45 R15, but is probably cheaper. Graham. R7 GPK Superlight #85 Link to comment Share on other sites More sharing options...
Graham King Posted July 1, 2008 Share Posted July 1, 2008 Simon, I made a mistake on my 1st post 195/45 R15 are virtually same rolling radius as 185/60 R13, 195/50 R15 will give rough 7.5% increase (actually less). On the other hand if you are on CR500 which I think are 175/55 R13 then changing to 205/60 R13 at rear will reduce RPM by about 10% for given speed. Hope calcs are clear. Graham. R7 GPK Superlight #85 Link to comment Share on other sites More sharing options...
Support Team Shaun_E Posted July 1, 2008 Support Team Share Posted July 1, 2008 Using gearcalc with 13" wheels and 205/55 tyres (Assuming CR500s) With a 3.92 diff 80mph = 4818rpm With a 3.62 diff 80mph = 4449rpm This would suggest you have a 3.92 diff which could be changed to a 3.62. I think there was a thread somewhere about early Duratec R400s having the 3.92 diff fitted due to no 3.62s being available. Yellow SL #32 - member of Drowned Rat Racing Link to comment Share on other sites More sharing options...
Pezky Posted July 1, 2008 Share Posted July 1, 2008 I thought 6th gear in the Caterham box was the equivilant of 4th (ie 1:1) in a normal 'box? Either fit a 5-sp or change dif ratio. I would have thought a Duratec would have enough torque to cope with a 5-sp after all 6-sp was developed for the high reving k-series Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted July 1, 2008 Share Posted July 1, 2008 Oops - you are right - I meant 6th is like 4th in a 5-speed (at 1.0) My 5th is 0.82 So many numbers !! 7 related photos Link to comment Share on other sites More sharing options...
rj Posted July 2, 2008 Share Posted July 2, 2008 SM25T This is why standard fitment is 3.92 if 5 speed box and 3.62 if 6 speed box Link to comment Share on other sites More sharing options...
Leadership Team SLR No.77 Posted July 3, 2008 Leadership Team Share Posted July 3, 2008 Simon, with 200+ bhp I find even t he 3.62 diff to be too low geared for the 6 speed box and I'm considering swapping to a 3.38. Bear in mind that the revs/speed you see on the gauges will not be affected by a tyre change if you have the standard setup of speed sensor on the gearbox. Only a wheel speed sensor (as in the Stack setup) would notice a speed change with a change of tyres. Might be worth using a GPS/Sat-Nav to find out what the actual road speed is before changing anything. Stu. Joint Area Representative MAD Sevens (Merseyside And District) www.superse7ens.co.uk..........the rebuild 😬 Link to comment Share on other sites More sharing options...
John Vine Posted July 3, 2008 Share Posted July 3, 2008 Simon, Are you sure your road speed is accurate? I ask as my (non-Stack) speedo initially under-read due, I suspect, to poor signal pick-up from the speed sensor (on the RH driveshaft). Re the diff ratio, you can check it easily by jacking up the rear, and counting how many wheel revolutions you get to four whole turns of the propshaft. A 3.62 will give almost exactly 1.1, whereas the 3.92 will give just over 1. JV Link to comment Share on other sites More sharing options...
Simon.seven Posted July 4, 2008 Author Share Posted July 4, 2008 Having just used gearcalc, I believe I have the wrong diff. The technical spec for a superlight says 3.62 but according to gearcalc my R400 is running a 3.92.. Time to ring CC. Thanks everyone for your posts, I am a wiser man now. Link to comment Share on other sites More sharing options...
patneale Posted July 4, 2008 Share Posted July 4, 2008 Simon, Have you got or got a friend with a Sat Nav. Most have a speed facility which is normally very accurate at steady speeds. Has your car got the "normal" instruments or is it a Stack display. The "normal" instruments are made by Caerbont in South Wales and are widely adjustable via the rubber plug at the back. Check your speed is right first. I am willing to bet that CC did not calibrate the rev counter or Speedo to the car ! I had this trouble with my SV with 6 speed. Speedo was way out. Indicated 70 was an actual 64 MPH. Most garages will be able to check out if Rev Counter needs adjustment and it reading the same as theirs. Hugh Robinson. Link to comment Share on other sites More sharing options...
Simon.seven Posted July 4, 2008 Author Share Posted July 4, 2008 Hugh, I have used a tom tom to guage speed and you are right the speedo is out reding more than actual. This just means my revs are even higher than I thought. Very close to 5k for 80mph. I do think I have been given the wrong diff. Link to comment Share on other sites More sharing options...
John Vine Posted July 4, 2008 Share Posted July 4, 2008 I do think I have been given the wrong diff.Well, as I said, to be sure you've only to jack up the rear, turn the prop and count.... JV Link to comment Share on other sites More sharing options...
patneale Posted July 4, 2008 Share Posted July 4, 2008 Simon, It does sound as if you have got the wrong diff. I think there are some indentifying marks on the diff casing, which no doubt CC will advise about. ( Expect there is a notch cut into one of the webs on the casing as an indicator) You can hardly just pop down to the factory like i can within 45 minutes! Final check could be your local friendly garage who can check the rev counter is correct. My local Rover man was able to plug his laptop into the multi plug under the dash and access all the engine functions on my K series. But rev check should be possible without access to multiplugs or access codes....on your engine. My 6 speed is 19.2 MPH per 1000 revs in 6th on 16" 205/45 Avons standard diff. So 80MPH is just over 4000 revs. Still fairly low geared as my other car is 34MPH approx at 1000 revs Or 2000ish at 70!! Good luck Hugh Robinson Link to comment Share on other sites More sharing options...
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