67 elan diff/top speed
Discussion
does anyone know what top speed ( in forth) you should get out of a standard s3 elan ( 1967)with a 1:3.77 diff?
i don't seem to acheive more than about 90mph at 6500rpm. the workshop manual seems to say you should get ~17mph per 1000rpm with 155x13 tyres, that gives a top speed of 110mph which seems more likely.
any ideas why i don't get this top speed? i have standard wheels and tyres
i need to replace the diff, do you know if there is much effect on acceleration if i change to a 3.55 diff to try to get a bit more top speed?
andrew
i don't seem to acheive more than about 90mph at 6500rpm. the workshop manual seems to say you should get ~17mph per 1000rpm with 155x13 tyres, that gives a top speed of 110mph which seems more likely.
any ideas why i don't get this top speed? i have standard wheels and tyres
i need to replace the diff, do you know if there is much effect on acceleration if i change to a 3.55 diff to try to get a bit more top speed?
andrew
With 3.77:1 diff ratio, Autocar recorded 124mph at 7,130rpm. That equates to 17.39mph/1000rpm, so at 6,500 revs you should be getting 113mph.
6,500 is a fairly conservative rev limit, though... 6,750 (= 117mph) is generally considered normal and 7,250 (which is what I apply on my slightly modified engine; = 126mph) is considered the safe limit for the standard bottom end.
Tyres have shrunk; the 'standard' 155 section tyres you now have fitted will probably be 70 profile, whereas the originals were equivalent to 80 profile, but it shouldn't make that much difference.
First thing I would check would be instrument accuracy; 40 year old Smiths instruments could be way out. There's the possibility that the speedo drive at the gearbox has been replaced at some stage with the wrong component, too. Check your tacho against the revs indicated on a digital engine analyser. Check you speedo against GPS (at a steady speed on a flat, straight section of road, since curves and gradients give spurious results on GPS). I'm betting that one or other of them is telling blatant lies!
6,500 is a fairly conservative rev limit, though... 6,750 (= 117mph) is generally considered normal and 7,250 (which is what I apply on my slightly modified engine; = 126mph) is considered the safe limit for the standard bottom end.
Tyres have shrunk; the 'standard' 155 section tyres you now have fitted will probably be 70 profile, whereas the originals were equivalent to 80 profile, but it shouldn't make that much difference.
First thing I would check would be instrument accuracy; 40 year old Smiths instruments could be way out. There's the possibility that the speedo drive at the gearbox has been replaced at some stage with the wrong component, too. Check your tacho against the revs indicated on a digital engine analyser. Check you speedo against GPS (at a steady speed on a flat, straight section of road, since curves and gradients give spurious results on GPS). I'm betting that one or other of them is telling blatant lies!
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