Axle ratio

[rsk289]

What rear axle ratios do folks run in their leafsprung Cobras?  I understand 'stock' was 3.54 for COB cars - has anyone changed this, and if so has the difference been noticeable?

[Jam2]

When I owned COX6011, Sometime arround 1980 I installed a much higher (numerically lower) ratio to facilitate long distance touring at speed. Sadly I cant remember what the ratio was, but it might have been 2.8.  The result was that getting off the line had to be done with care, but 70+ in first was amusing. Also put 130 miles into an hour on a deserted French motorway, cruising at under 5000rpm.  It made for a much more relaxed touring car, but definitely took the edge off at low speeds.

[A-Snake]

quote:Originally posted by rsk289
   
What rear axle ratios do folks run in their leafsprung Cobras?  I understand 'stock' was 3.54 for COB cars - has anyone changed this, and if so has the difference been noticeable?
   

   
   3.77:1 on CSX cars, I can't see why COB or COX cars would be any different unless requested by the customer.

[rsk289]

According to Rinsey Mills' Original AC Ace and Cobra, early Cobras were 3.54, but later CSX cars (can't remember the cut-off number) switched to 3.77.  COB and COX cars continued with 3.54 ratio, I believe 427s went to 3.31.

[A-Snake]

quote:Originally posted by rsk289
   
According to Rinsey Mills' Original AC Ace and Cobra, early Cobras were 3.54, but later CSX cars (can't remember the cut-off number) switched to 3.77.  COB and COX cars continued with 3.54 ratio, I believe 427s went to 3.31.
   

   You're correct on the very early cars. He states the change from 3.54:1 to 3.77:1 occurred after CSX2069

[rsk289]

quote:Originally posted by Jam2
   
When I owned COX6011, Sometime arround 1980 I installed a much higher (numerically lower) ratio to facilitate long distance touring at speed. Sadly I cant remember what the ratio was, but it might have been 2.8.  The result was that getting off the line had to be done with care, but 70+ in first was amusing. Also put 130 miles into an hour on a deserted French motorway, cruising at under 5000rpm.  It made for a much more relaxed touring car, but definitely took the edge off at low speeds.
   

   
   Yes, that's the tricky decision to make!  Do I 'lengthen its legs' with a lower number diff, or keep to original and leave it nice and 'squirty' on country roads?  I had thought of going 3.31 as an interim, but I do like the ratios of an 'L' gearbox set with a 3.54 rear.  I guess I just keep off motorways, or  use my old Mustang (with its modern 5-spd conversion) for touring.

[Jam2]

I suppose there is no room for an overdrive unit? Or are those available not up to the torque of a Cobra?  If there is space, a Mitchell 1000 overdrive might do the job!  It is effectively a synchro 2 speed box, which can be opperated in any gear.
   http://www.mitchelloverdrivemfg.com/our-products/

[tim isles]

If we start to consider overdrives then perhaps a simpler solution is to fit a 5 speed gearbox? My (4 speed) car runs a 3.77 diff. It's noisy and thirsty but it doesn't half go. And it still has a huge first gear. John, if you fitted Salisbury's highest ratio (2.88:1) then you really gave 6011 some motorway legs. You'll know this ratio was fitted to automatic 428s (max power @ 4,600 rpm) and gave around 30 mph per 1000 rpm on 15" 205/70 tyres. Manual 428s got the 3.07:1, which gave about 27.5 mph per 1000. I have a spare 3.31 diff which I may fit at some stage, but I think it's about as low as is possible to go without making that first gear just too long to use. Contemporary road tests of COB and COX cars record that these European cars got a 3.54 diff as standard. The Motor road test of COB 6106 car talks about the standard 3.54 ratio being replaced with a 3.31 diff on subsequent cars (but looking at the figures obtained in the road test I'd make a guess that the car had a 3.77:1 fitted anyway; others will know more.) Oh, and again reading road tests of the time, it seems that 427s were fitted with a 3.54 diff as standard too. But do I want to go touring in a Cobra? Er, not really. Give me an Ace, AC or Bristol powered, any day. 5 speed gearboxes for 289 Cobras? Seems a solution?

[Jam2]

Tim, you have jogged my memory, yes 2.88 was the ratio and steep hill starts were to be avoided. As for touring, I did numerous trips to the South of France and a memorable blast from New York to Monteray and back to Jacksonville. I think the car did close to 75k miles in my ownership. 125 - 130 was fine for cruising, anything more and the wind buffeting made it very uncomfortable.

[rsk289]

Prior to this car I built a Hawk 289, using a 289 6-bolt motor and a Tremec T5 5 speed gearbox with overdriven top (subject of the recent 2011 Euroclassic thread on this forum).  This had a 3.54 diff ratio which gave lovely driving around the twisty stuff, but with the 5th for motorway use.  That is always an option, but having gone to so much trouble to install the correct engine and gearbox in the current car I'd prefer not to muck about with 5 speeds.  That solution would also require some welding for the transmission mount, as the T10 (original) gearbox has very different mounts to a T5 5-spd, which I also don't want to do.
   As Tim has suggested, a Cobra is not the ideal car for touring in any case.  I took the Hawk to Rome, Madrid and a few other places, and whilst it's great while the sun is shining and the scenery's good, it gets very tiresome when it rains, is grey and damp, and in cities.  As noted I do have a Mustang which has a 3.25 rear and 5-spd with overdrive, so maybe I'll stick to that!
   Interestingly Sunbeam Tigers, which used the 260 motor, all came with a 2.88 rear (albeit a live axle).  My Jensen Interceptor 7.2 (now there was a car for touring) also had a US spec 2.88 rear axle ratio.