Just thinking Pt 1 Final drive ratios

[Graham]

Ok those of you that are regular readers of this forum will probably by now have realised that i spend far to much time surfing the web, probably a symptom of having a 'challenging' desk based job that I have to escape from time to time !

 

Anyway I've just been looking at an article which suggests changing the standard 3.45:1 CWP out for a 3.7 and then chaining the OD ratio from the standard 'A' type 22.5% to 28% gives lower ratio non OD gears for hooning about whilst providing a slightly higher ratio in OD top for cruising, has any body carried out this mod and was it worth the effort.

 

BUO's box and diff. are out for rebuild and the initial reports are that the CWP is marginal anyway.

 

If my appalling maths is correct the final drive ratio in top would be 2.32:1 with the standard set up and 2.29:1 with the modified top.

 

I have another 'what if' I'll post latter today but for now

 

ATB Graham

[simonjrwinter]

I have 3.45 diff and a Stag gearbox with 28% overdrive.

Not the question you're asking I know, but the extra long legged overdrive makes top gear overdrive cruising much more relaxed (and enables me to get 30+ mpg out of the V8 on a run)

Simon

[TriumphV8]

You should not approach that problem from the ratio side but from the engine:

 

The stock TR6 as CR or carb version refuses to rev and is pretty harsh.

If you go for CP-PI or a tuned version you get a different response.

 

My engine revs to 7000 and is limited to 6500 and it happily fires into the limiter

in the lower gears. Also I balanced crank, rods an piston and from that get a

sweet humming engine in higher revs.

To be honest it does not need the OD under all circumstances. You simply do

not get the feeling that the engine runs out of gears till you reach the red line.

 

So this plays more a role than the little difference between the ratios.

The 3.7 instead of the 3.45 lets the car act more crispy in the lower gears

but on highway the difference is not that big to make a real difference.

 

If you are hunting for highest speed you should be aware that in one gear the

highest speed is achieved just over the maximum BHP rpm.

 

With the OD engaged you normally will not reach that speed and with lower ratio

in 4th gear engine might rev too high to perform that. But for me thats academic,

I do not go that fast with the 6-cylinder.

 

My recommendation is to keep the OD as it is because the results are marginal

and choose the axle ratio from 3.45 over 3.7 to 4.1 that it suits your engine and

makes you feel comfortable.