diff & gearbox ratios

[KevJTD]

handy list courtesy of alfa pages

http://www.alfa-pages.co.uk/gearbox.php

[Johnboy]

Morning KEV
What do you think of the ratios of the 146 16v box ? . Looks quite close to the 1.2 box . But it's all Russian to me

[KevJTD]

Hi JB.

the diff ratio is the same, used in quite a few 'boxes really the 4.11 diff.

the 1.2 ti & 145 'box have similar enough 1st and 2nd gears but 3rd starts to change things before 4th takes it a stage further but it's really 5th gear in the 1.2 ti that is the lowest geared of all. ok on paper they look close enough, but what does that all equate to? fair enough question...


for the comparison we'll use a 195/50R15 tyre dimension, common enough size for a sud/33/sprint/145 that could easily be found on any


so 3rd gear.

on the 145 at 5,000rpm you'd be doing 62.15mph.
in the sud you'd be pulling 5419revs to achieve the same speed, or look at it another way,
at 5000rpm in the sud you'd be doing 57.34mph

not a huge difference but we're starting to see where the sud 1.2 ti 'box has what can be considered almost "close ratio" top 3 gears..

so, 4th gear

the 145 at 5000rpm would be doing 80.07mph
the sud would need 5429rpm to achieve that speed in 4th,
or at 5,000rpm it'd be doing just 73.75mph


5th gear though is where you can feel the difference with the 1.2 ti ratios...

the 145 at 5,000rpm would be doing 96.29mph
the sud would need 5451 rpm to reach that speed,
or at 5,000 rpm you'd be just reaching 88.32mph

so although the numbers of the gear ratios themselves are pretty similar they make quite a difference in real times.




however, lets look at things another way...and a real bugbear of mine....

I know loads of you guys like bigger and bigger wheels, I don't. I like a car to accelerate as best as it can to a given speed, having it geared to do 150mph is pretty pointless if you haven't got 300bhp to get you there, or a legal amount of space to achieve it

we can chase lower gear ratios etc when the easiest thing is to lower the tyres rolling circumference

so assuming the usual upgrade tyre size of 205/40R17

the sud 5,000rpm in each gear goes from,

3rd 57.34mph to 65.84mph
4th gear goes from 73.75mph to 76.29mph
5th jumps from 88.32mph to 91.37mph

just in changing the tyre size. let alone what the trend for 18 and above does, let alone the changes in gyroscopic values which affect handling more with larger diameters..
but I admit they are nicer to look at

so the easiest/cheapest way to get better acceleration is to fit smaller rolling circumference tyres

[Johnboy]

Wow great write up KEV . Looks like it's the next best box to a 1.2 . Or close to. Agree smaller wheels is the easiest way to go. But we all want looks too

[PETROLHEAD]

Aren't you running 15's or 16's anyway jb?

[Johnboy]

Hi chap
15's but I've got those 16's off you . All gear box stuff is all Russian to me tho the 33 should be coming home soon. So I'll be able to start on it

[PETROLHEAD]

Well don't forget JB that a 195 50 15 has an almost identical circumference to a more historic 185 60 14 as used on something like a sud ti/33 wheel. Some used 185 65 14 too iirc so the 195 50 15 tyre would then be slightly lower gearing than an 80's Sud at that point.

If you jump to the 16" wheels, then a change to 195 45 16 tyres will keep a very similar comparison again, slightly larger circumference but we are talking mm's, less than 1% diffeence!

But i feel that 195 maybe a little skinny for those 16's, so a 205 45 16 would suit better, and even that means an increase in circumference of just over 2%


At these differences compared to the old commonly fitted 14" tyres, its not worth dojung the maths, the difference is virtually meaningless.

Just chose the gearbox you want that suits the road speed you want and know that with a 16v fitted it'll go just fine!



I agree with Kev that its when folk start to look at 17's and 18's that it starts geting awkward really, but having said that my 16v 33 is sat lovely on 18's

[Johnboy]

Hi chap
Great write-up and taking the time to do it. Those 16's I got off you will need wide rubber on them.im guessing 225 to be level with the rim.they are wide

[PETROLHEAD]

Yes, 9j as i recall.

Well, a 215 40 16 is identical circumference and gearing to a 195 50 15, and a 225 40 16 just a tad larger, so you're still ok.

Depends if you'd like a little stretch on the tyre. Which i suspect youll need getting a 9j in a 33 arch,but it is possible, ive had 17x9j's on mine before

[Johnboy]

Lol
Morning chap
Good info again .I'll let the vw boys have the stretch look still looking into making an integrale style kit for it .so we shall see

[PETROLHEAD]

Grale' arches?

There's your answer right there! Yummy!

[Johnboy]

Yeah been wanting to do it for years .as it's the closest I come to owning one