More sporting gear ratios

[kfitzgerald]

I used a BWCR 12 when I raced an Arnolt Bristol. First was gear to start the car in motion only. I used the ring and pinion from a BMW 328 to help acceleration. Final ratio was 4.2 and good for 110 to 120 mph. If you are racing you must use ATF in the transmission, the transmission overheats and you will lose your synchros if you don't.

[Julian Caples]

Lewis

Bristol made a number of close ratio boxes (all with higher first gear) for racing. Early Frazer Nash boxes (to go with FNS engnes, contemporary with the 85/85A/85B/85C engines) were CR3 type.
Later (Borg Warner synchro type) were BWCR6, BWCR9 and the BWCR12 mentioned below. they all have closer ratios (which will sort out your large gap between 1st and second and third) and a higher first gear (with no synchro)

the other implied problem in your question is your overall highest ratio. this could be dealt with by either and overdrive (use the adaptor kit and more modern overdrive to fit inside the footprint of the normal Bristol gearbox plus extension) or a higher final drive (a 3.66 ratio was used on some cars - normal 400/401/403 ratio is 3.9, 405 final drive is 4.2 with overdrive giving an effective 3.3ish ratio)

none of these options would be cheap, but they would be historically 'correct'. Rob McDermott can probably help you if you want to go the CR gearbox route

the ford option could be cheaper

cheers

Julian

[Julian Caples]

Found the actual ratios for you

400 gearbox (for 85A engine)
1st 4.3:1
2nd 2.172:1
3rd 1.295:1 (box for 85C has lower third gear 1.41:1)
4th 1.0:1

CR3 (also BWCR 6, 9 & 12)
1st 2.92:1
2nd 1.825:1
3rd 1.292:1
4th 1.0:1

BWCR 5 (403)
1st 3.16:1
2nd 1.825:1
3rd 1.292:1
4th 1.0:1

my 403 runs a CR5 box with a higher 1st (2.92:1) no freewheel (so essentially a CR3/6/9/12 set of ratios. i think they are nicely spaced for my car (3.9 diff) your lighter car could probably easily pull a higher diff without too much loss of acceleration in the intermediates

if you go to the CR3 ratios, you will essentially have a first gear just below your current second ratio, a better spaced second gear and third and top essentially the same (well third is fractionally higher in the CR3 ratios)

hope that helps

Julian

[Julian Caples]

a correction

for the BWCR5 box the service manual (and handbook) gives the ratio as 3.16:1 (which i put in the table below), if you actually calculate the ratio using gear teeth ratios it is actually 3.61:1 (which gives the correctly quoted 14:1 overall ratio with a 3.9 diff) - a simple typo that seems to have been perpetuated in a number of sources

so a straight CR5 box will still have a first gear that is too low for you - you will need the 2.92:1 ratio of the sports boxes

Julian