Beej

Paul, sorry to be thick but what does 7/21/13 mean? - I guess 13 is 13" wheels. Thanks everyone else and for the avoidance of confusion I was assuming a constant aspect ratio (hence rolling diameter would increase with tyre width). I am also aware that actual tyre width at a stated measure eg 175 seems to vary with manufacturer and tyre type (dunno why, it just does). My main concern was how a modest change in rolling diameter could put 20 mph on the top speed? I suppose I just need to do the maths.

Can anyone tell me what the correct (if there is one) final drive ratio is on an HPC racer? I am a tad confused at the moment. I have bought a very well known Vx engined racer which purported to achieve an average 85mph over 24 hours. However, the car currently manages around 70 mph (speedo doesn't actually work, so this was measured by sign language with my wife in the car in the next lane - v. scientific!) at 4000rpm in 5th (which on the type 9 box is apparently a 13% overdrive). It currently has a 3.9 diff and runs 13" wheels with (from memory - cos the car is back at C) 175 Avon ACB10s I have been advised that changing the tyre width will change the rolling diameter and hence restore a sensible ratio of around 19.5 mph per 1000 rpm. Does anyone know how realistic a simple tyre change is at improving the final drive ratio or should the car really have a 3.6 diff? I should add that the factory are trying to be helpful here but we are all struggling for an answer. I know that they are trying to get hold of Jez Coates to see if he can remember what wheels/tyres/final drive were orginally fitted. Cheers

can someone please tell me if the Vx racer type roll bar (i.e. driver's side only) is generally considered acceptable on track days FOR PASSENGERS? thanks.

no worries. If some kind soul would like to buy my bargain car then I can get a superlight and solve all these problems!

1987 standard factory spec x-flow supersprint, 5 spd box, dedion rear end - in other words it does NOT have the widetrack front suspension. Currently has spax all round with non adjustable spring platforms. I assume yours have adjustable spring platforms?

whats the history/spec on the x-flow? cheers Mark

is this for the wide track set up or will it fit my early car?

Oh yeah.. and its got high vis weather gear. Quite useful this time of year!

🤔 Colin, are you out there? I am not entirely certain how to contact you if your very complex e-mail address is to be believed. Yes, I do have one rather poor photo of the car which I can attach to an e-mail. Hope this helps.

for sale: excellent first time owners car.. 1987 (non Q plate) factory built supersprint, 5 speed, dedion. Subsequently uprated with S type seats, 5 point harnesses, fireeater, FIA roll bar, external battery cut off. Green with yellow nose & stripe, flared wings, 14" revolutions with Yoko AO21s, polished stainless 4 into 1 exhaust. Superb condition, 24,000ish miles. Make me an offer, I want a superlight. 😬

I am aware that (at least for track use) most people agree that the standard Girling front brake set-up (straight off a herald/spitfire) on my 1987 vintage 7 is a tad on the puny side. I am not clear if the same set up and uprights exist on the more modern versions but there seem to be a number of possible after market improvements eg Wilwood, AP, Caterham's own etc and I guess these might fit my early car. It occurs to me that a really easy and cheap solution is to swap the spitfire uprights for the beefier Vitesse/GT6/TR6 type, thus gaining bigger and stronger wheelbearings, larger diameter discs and larger caliper/pad size. Am I being daft? Is it really that simple? The brake force gains must be substantial even though they still run solid discs. The cost saving is massive (well it is for me cos I happen to have an ex Lotus Elan set lying around). I must be missing something.

Ah. Ahem, when you put it like that, I should really stop whingeing and be thankful for 110mph. I'll get my coat. Cheers Graham.

Thanks Eric, I don't mind bringing the car down to Marseille if it helps!!!!! (I wish) Your grandma is a very wise woman.. I will go back and check the ignition advance, check the throttle linkages, valve clearances etc. Bear in mind I am running 4 star LRP - not unleaded. Also although I will happily try warmer plugs - I can't quite figure how this will help with the engine running lean and I am running the coolest plugs known to man - it was quite hard to get them actually. I think ultimately the idle problem will be solved by richer idle jets and the full throttle performance will be solved by me just accepting the car won't go faster than 110mph!! I guess I just want someone somewhere to tell me that their 1700 supersprint does 130mph so I can justify spending more money.

I followed a 996 TT into work this morning.. awesome piece of kit. So where can I get a carbon fibre phone.. its obviously worth an extra 5mph? he he. I'm back down there at the end of the month in both cars. It will be interesting to see what the cerby will do now the cats are off (apart from spit foot long blue flames out the back). I am such a child. Of course, the truth is it really depends how good the driver is getting the power down through Lavant - in my case that probably explains why I am so off the pace - that AND the carbon phone.

Sorry, I didn't mean to imply it was a 7 that would do 130 on the back straight - although there was a burgundy R500 that was definitely in that ball park last week. My road car is a Cerbera 4.5 - which, with 420bhp, is just plain cheating of course. But I hit 130mph in a bog standard 993 Carrera 4 last week so I just assumed that this was within the realms of possibility for a 7. I just have to reset my expectations and find some twistier circuits!! Problem with that approach is it presupposes I can actually drive well, and not just rely on power to weight ratio. O well, I guess I am just going to have to learn to driver well.

thanks eric, sounds like maybe my original idea (that the idle jets were too weak) was correct. I was a bit surprised to discover that the "correct" jets were fitted but because I assumed that Caterham did their homework when setting the carb specification I figured the problem was with something else. It may be that my engine is not exactly to Caterham 1700 supersprint spec and it does seem worth checking out a richer idle jet configuration. That being said, I am not too concerned about how the car behaves at and off idle because I tend to drive it flat out 😬. So my primary concern was to understand why the engine would not pull more than 5000 - 5500 rpm in 4th gear (and less in 5th). I posted a comment on this on another thread and various people have commented that this seems normal for a 1700 SS - equates to an indicated 110mph max speed. If so, I may need more horsepower because my nearest tracks are Goodwood and Combe and I am used to hitting 135 - 140mph on the straights in my everyday car (although its obviously slower thru the corners). Before I catch upgraditis big time I just wanted some reassurance that I was getting the best out of the current spec.

This is all quite heartening. I thought I had a major problem with my 1700 x-flow because it was struggling to pull more than 5,000 rpm in 4th (5 speed box) - although thanks to new chokes I can now get it to 5500 rpm. It seems I am not alone -its the old bhp versus drag thing. O well, I guess I need more bhp and less drag then. Does an indicated 110mph achievable on a sensible straight such as the one after Lavant at Goodwood seem average?

Graham, smee again! Need some more help. OK - I ran it with the 130 mains and it ran ok - certainly not too rich but definitely still not right. It was still popping and f@rting at both ends.. which has totally confused me. I replaced the O rings with sandwich plates and reset the rubber bobbins at the same time. Everything else checks out ok.. BUT no matter how hard I tried it would not pull more than 5,000 rpm in 4th and dropping to 5th actually lost me speed. I was at least 20mph slower than most other cars down the back straight (still stuffed them thru the corners though!!!). Since then I have changed the chokes to 32s and I can now get 5500 rpm in 4th (indicated 110mph but probabaly less) - with possibly more to come but it was really struggling - and getting very hot. It is still spitting back at and off idle and clearly is not happy on the main circuit. I assume you can spin your 1700 up to 7,000 in all gears? Any ideas? Cheers

*thumbup*Thanks guys - very much appreciated. I guess 28 was pushing my luck a bit!

help? what pressure should I be running for track use in Yoko A021s - the intermediate tyre. I know they are not ideal but its what I've got so I need to make the best of them. I rang yokohama yesterday and they quoted 28 cold.. then they phoned me back and said 19 cold!! So thats clear then. I felt subjectively quicker round Goodwood at the lower pressure but the car handled better at the higher pressure (thats the bog standard suspension's fault - not the tyres). Also, should the presures be different front and rear?

Cheers, I bought the car specifically for track days so, although tractability is important, I don't want to lose out on outright power. My everyday car does tractability quite well (420bhp) but its too darned expensive to run on the track. I had planned on changing the mains to 130s, and if that improves the balance I would change the chokes to 32s. I am not sure about the origins of the engine, it looks like a totally genuine, straight out of the factory 1700 with all the right bits but I wouldn't rule out the 1600 theory. Either way I had planned on transplanting a twink head this winter so I'll probably have to rejet it again!!

Graham - it turns out that the idle jets are correct, BUT the chokes are 30s, the mains are 110 and the air is a max at 180. So I am not really surprised at it running lean but it doesn't explain the off idle problem. Any further thoughts?

Thanks very much - thats really helpful. I can cross check this against the current settings. I strongly suspect they will be different - it just runs too lean all the time and spits like camel having a bad day at the office. Cheers

can anyone please tell me what the correct jet sizes are for a bog standard 1700 X-flow super 135 (supposedly) bhp. Its the idle jets that I am especially interested in. I can't get it to pull clean on a part throttle and I think a previous owner may have been mucking around with the jet sizes. Any clues?