mk1-forum.net

Hello chaps

bit of maths here for you - im rebuilding the 8 port and want to change the Final drive ratio, the current 4.32 is to short , a 4.1 is to near to what i have already and a 3.9 is to high when i calculate speeds out

so can i use drop gear ratios to get to a 4.0 ( say a 3.9 cwp and the appropriate drop gear)

im running a salisbury lsd, 10 inch wheels , with the current ratio i have to be really carefull and clock watch all the time as it easily gets up to 8k rpm

thanks chaps

pad

3.9 with 1.04 primary gear gives 4.06.

Thanks fella - thats confirmed my maths - looks like swiftune supply the bits i need - ah well best remortgage the wife then

pad

Double Check! It's not as simple as "3.9 x 1.04 = 4.06"...

The ratios are so close that without a tooth count it is not
really possible to work things out accurately. You could end
up changing things and getting exactly what you had to start
off with, or thereabouts! (And a very angry wife!).

The "4.1" is 15/62 which is 4.1333

The "3.9" is 16/63 which is 3.9375
A 24-30-25 drop gear combination gives 1.0417 = 4.101
A 23-30-24 drop gear combination gives 1.0437 = 4.108
A 22-30-23 drop gear combination gives 1.0455 = 4.116

There are plenty of options...
... but they all calculate at "4.1".

Ian

PS
Pad, have you seen this?

"Drop Gear - Gearbox - Diff - Ratio - Speed" calculation Tool:
http://mk1-performance-conversions.co.u ... anical.htm

Ian

There is a Trannex drop gear set .9853 (24/23)

With the 15/62 you'll get 3.961.

Hardly worth the cost over a 3.939.

Get out the calculator and look up some more final drive ratios. Very close to 4.0 can be achieved with the newer Salisbury replacements.

Possible typo there, olddad!
24-30-23 = 0.9583 OVERDRIVE
23-30-24 = 1.0434 "UNDERDRIVE"! As olddad says, there may also be "specials" that are available,
but be very wary of spending serious money on stuff that will
end up the same as the current setup.

Always use the tooth-count and don't round up until the
final figure is found.

Ian

IMO, driving it, it 4.0 or 4.1 FD, you won't notice it.

I have a couple of special 16 / 64 FDs, made and given to me many years ago by JKD (when it was a decent company), they were prototypes that as far as I know didn't go in to production. I know these is a tease, but I don't want to part with them.

ianh1968 wrote:Double Check! It's not as simple as "3.9 x 1.04 = 4.06"...

The ratios are so close that without a tooth count it is not
really possible to work things out accurately. You could end
up changing things and getting exactly what you had to start
off with, or thereabouts! (And a very angry wife!).

The "4.1" is 15/62 which is 4.1333

The "3.9" is 16/63 which is 3.9375
A 24-30-25 drop gear combination gives 1.0417 = 4.101
A 23-30-24 drop gear combination gives 1.0437 = 4.108
A 22-30-23 drop gear combination gives 1.0455 = 4.116

There are plenty of options...
... but they all calculate at "4.1".

Ian

Well I couldn't find the calculator, so did it with a pencil

I totally accept your point that it's just playing with numbers that would only matter on a particular race track.

Reminds me of the story of John Reed/ Uncle Bunt, who used to build specialist motor cycles. He built a drag race bike for this chap. He came round one Saturday and spent the afternoon playing with the bits and measuring everything.
" John, do you know the back wheel is 0.0015" off centre on the bike?" "So!"
"Well it will be out of balance going up the strip!" "So hang your testicles on the other side of the frame and balance it up!"

i can see your point about its only playing with numbers to suit a particular race track but i think the points been lost a bit.

Currently the 8 port rally car is a pig to drive, first gear is pointless and the other 3 are used up so fast im having to really watch the rev counter

The car has now been retired from forest rallies and im going to use it the odd trackday and some historic (longer distance) rallies - it will never be a circuit car but running out of revs at a small track like blyton is a real killer , Aintree was done mostly in 4th gear on the rev limit

70 mph on the motorway is about 6300rpm

I might try a 3.9 final drive , see what its like as its not to much of a job to change the drop gears at a later date

Pad

ianh1968 wrote: (And a very angry wife!).

ian


you have met pads missues then...........................

pad4 wrote:I might try a 3.9 final drive , see what its like
as its not to much of a job to change the drop gears at a later date

If you use the 23-30-23 drop gears, you have the option of
gearing it up or down by just changing the primary gear...
... no need to even take the transmission case off!

22-30-23 Would gear it down by 1.045 = 4.116
23-30-23 Would be 1:1 = 3.937
24-30-23 Would gear it up by 0.958 = 3.773

Ian

pad4 wrote:i can see your point about its only playing with numbers to suit a particular race track but i think the points been lost a bit.

Currently the 8 port rally car is a pig to drive, first gear is pointless and the other 3 are used up so fast im having to really watch the rev counter

The car has now been retired from forest rallies and im going to use it the odd trackday and some historic (longer distance) rallies - it will never be a circuit car but running out of revs at a small track like blyton is a real killer , Aintree was done mostly in 4th gear on the rev limit

70 mph on the motorway is about 6300rpm

I might try a 3.9 final drive , see what its like as its not to much of a job to change the drop gears at a later date

Pad

have you calibrated your rev counter? 7000 rpm on a 4.1 is 91mph iirc

Surely the 8 ports got enough torque to pull a much higher ratio?.ours have If you actually intend to use it for more normal stuff why not consider a 3.7?
we do a fair bit of motorway miles to get to shows so both 8 ports have 3.4's but brother has the Minispares sc 5 speed.
The disadvantage is on club runs we're running off cam most of the time to stop leaving the others behind.

pad4 wrote:

ivor badger 2 wrote:

pad4 wrote:i can see your point about its only playing with numbers to suit a particular race track but i think the points been lost a bit.

Currently the 8 port rally car is a pig to drive, first gear is pointless and the other 3 are used up so fast im having to really watch the rev counter

The car has now been retired from forest rallies and im going to use it the odd trackday and some historic (longer distance) rallies - it will never be a circuit car but running out of revs at a small track like blyton is a real killer , Aintree was done mostly in 4th gear on the rev limit

70 mph on the motorway is about 6300rpm

I might try a 3.9 final drive , see what its like as its not to much of a job to change the drop gears at a later date

Pad

have you calibrated your rev counter? 7000 rpm on a 4.1 is 91mph iirc

Sorry meant to type 5300rpm is around 70mpg (using 4.32 ratio as currently fitted) , still its painfull

pad

pad4 wrote:
Sorry meant to type 5300rpm is around 70mpg (using 4.32 ratio as currently fitted) , still its painfull

pad

70 mpg, so it's very economical on fuel then.

it is - ive only filled it up once since blyton