14 replies to this topic

[bungle0408]

hello just want to know if i put a 3.1 in my car will it make a big difference to my top speed as its a 3.6 thats in . i have a 2.9 but only the crown wheel and no pinion if i had one o them then i think i would go for that , its a 1.3i cooper 1993 so i dont think power is a problem as i want more top end less revs , cheers if anyone can help

[Fast Ivan]

http://www.guess-wor.../Tech/ratio.htm

 

type your info in to this

[bungle0408]

will give it a try cheers

[Artstu]

Didn't they have a 3.1 diff as standard anyway? or has it been changed to the 3.6? what revs can you pull in top gear?

[bungle0408]

am at around 4000 for 70 , i think its been messed around with at one point . like i say i have picked up a 3.1 and part of a 2.9 . i know the 2.9 would be the better but i am struggling to find the pinion 20 tooth

[Dr s]

4k at 70 isn't a 3.6? Mine is a 3.6 and its 4k at 60...

[Artstu]

am at around 4000 for 70 , i think its been messed around with at one point . like i say i have picked up a 3.1 and part of a 2.9 . i know the 2.9 would be the better but i am struggling to find the pinion 20 tooth

 

A 2.9 diff isn't going to make the car faster, apart from the very odd occasion where you find a very long downhill stretch with a tailwind thrown in for good measure.

[1968andyf]

The 3.76 gives 15.6mph per 1000 rpm in top the 3.44 gives 16.1 mph in top as you have more than this I would say you already have a 3.105 which is 59/19 a 2.95 is 59/20 and a 2.76 is 58/21 not sure of mph for these but am sure some body on here can help.

[carbon]

I'm running 1275 with 3.1 diff on 10 inch wheels, for fast road use I find this a good combination with the 'GT' helical ratio gears. You can cruise at 70mph and engine is not too noisy...

 

And in top gear your 1275 with 3.1 diff will be quicker than one with a 2.9 diff, even going down a steep hill.

 

What's inside the gearbox - is it A+ standard helical gearset?

[bungle0408]

thanks for all your help the one that came out was 17/62 tooth the one i am putting in is 19/59. i not sure whats inside the box i suspect standard 

[1968andyf]

Yes looks like it was a 3.6 but if you got a speedo reading of 70 at 4000 either your tàcho is out or your speedo check the speedo pinion for your 3.1 it should have 16 teeth

[bungle0408]

will check it out

[Ethel]

It won't matter is four final drive is A or A+ as long as they match.

 

In case it hasn't been made clear, a taller FD , like 3.1 or 2.9 is likely to make your top speed slower. Turning less rpm means less power for shoving air out the way, though you do get less noise 'n better economy as compensation.

 

If you're really looking for maximum speed find out your maximum power, and what rpm it occurs at, and select the ratio that matches that to the speed/drag/power characteristics for your car.

 

http://www.wallacera...-Calculator.php

[69k1100]

No one asked if the tyre/rim combination is standard.

[Cooperman]

No one asked if the tyre/rim combination is standard.

 

It doesn't really matter as the rolling radius of the 10", 12" & 13" tyres is virtually the same due to the different aspect ratios of the tyres used.

The actual difference with the wider tyres is the increased aerodynamic deag caused by the extra tyre width which will affect the maximum speed very slightly. Cars with narrow (145 width) tyres and no wheel arch extensions will be faster than those with 175 section tyres and huge arch extensions.

The huge wing mirrors on later cars also knock a couple of mph off the top speed.

 

The problem with really high FDR's is that the drag can limit the speed by preventing the car accelerating to the point at which maximum power can be achieved. There are cases in which a car can be faster when flat-out  in 3rd gear than in top. In the 50's & '60's many cars had overdrive on 3rd & top gears and it was often the case that when the car was flat out in 4th, the engaging of overdrive actually caused the car to slow down as the rev drop reduced the power developed and that was insufficient to overcome the drag at max in 4th. It is quite a complicated area of calculation, but remember, if the speed id doubled, the aerodynamic drag increases by a factor of 4 times.