Diff Question

[AlfaCorseChris]

Was looking for a 1.3 ltr gearbox to swap on my 16v 33 so I could get better gear ratios for the track.

However, as its not something you can find easily, and I already have a rebuilt gearbox with no cranking gears, I thought I can only change the diff


So, please correct me if I'm wrong here...

The diff crown of the 16v 33 is 35 teeth, yes ?
Then the crown of the 1.5 Ti is 37 teeth which is shorter alltogether
and finally the diff crown of the 1.3 is 39 teeth

Would that be accurate ? So that I'll know what I'm looking for.

At the moment, with my current gearbox and lifted rev limiter, I have a 2nd gear which goes up to 120khm, my third gear is around 170 Khm, 4th gear is at 200khm and 5th at 240 give or take.

Problem is, with this setup the car is pretty much undrivable on the track, where taking the corners in 2nd gear is too high and I need to brake earlier and slow down, and 3rd gear is too high, so i will either take the corner with lots of load on the engine, or speed up in 3rd gear before the corner and fly to the moon once I hit the apex.


Any suggestions ? Solutions ?

[Johnboy]

Hi Chris
Will this help you . I've got a 146 16v I will be trying in my 33 .no jokes
Here's the list

[Veesix75]

Not sure if this helps as I'm not sure of your gear ratios - however the hallowed 4:11 diff from a 1200 ti gearbox can also be found (I think) in a series 3 gearbox, I think the 105 Green Cloverleaf.

In my opinion, the next best thing to a 1200 ti gearbox for lower ratios and track use out of the factory is the series 2 1500 ti gearbox.

Tim

Edit : ha ha - the diff answer is in JB's table......

[902a]

You can´t swap only the diff..!

Crownwheel and pinion always belong together! You can´t change only the crownwheel without the pinion!!

Ali

[Spacenut]

I seem to remember there is also a difference in shaft bearings between early a late model gearboxes. When the casing on my 1.5 Ti (Series 2, with the 3.889 final drive) was broken in an attempt to remove the speedo pinion, I got a replacement Gold Cloverleaf gearbox, which has the same 4.111 final drive as the 1.2 Ti. Alex Jupe sourced a new bearing in order to install all of my old 1.5 Ti ratios into the GC box but retain the factory 4.111 for the hot set-up, but like a fool I said I would take the GC box as it was - with the result that the overall gearing was even taller with the 4.111 final drive than my old 1.5 Ti with the 3.889!

Lauren

[Veesix75]

Yep, the gears are taller inthe GC box, it's the final drive that's the same.

I think shrew posted a link to calculate speeds at given rpm based on gears/final drive and tyre diameter.

So with a lot of time and loads of gearbox spares you could make ideal box.

I think the best box I had was 16v 1st and 2nd, 1200ti 3rd, 4th, 5th and diff.

[PETROLHEAD]

I have 1.3 gearbox if you need it Chris?

[AlfaCorseChris]

ok... hang on a second...

Ali, I did swap just the diff on my racing Sprint, and from 35teeth diff I put the 37teeth one in. So far no problems, but it was the complete diff that I changed, not just the crown wheel.

Lauren, so you are saying that the ratios of the 1.2 were higher than the 1.5's ? :/

Shrew, is it in good condition mate ? I'll consider it but I need to make sure its gonna work the way I need it to before paying a fortune on shipping

[PETROLHEAD]

I've not driven it or used it myself of course Chris, but visually it looks very good.

at some point i could pop it over to BLS for their beady eye if you like?

[AlfaCorseChris]

Trying to figure out this table John posted...

So lets go through some of these together, coz it can get a bit confusing...

1st of all if we put the diffs in order from highest to lowest ratios, we have the hightst 9/35 (which is whats on my 16v now as well), then 9/37, then 10/37, then 11/39 and the shortest is 11/35. Is that right ?

So if we establish that, then there's the question Ali brought up,
IF lets say I find a shorter diff, can I just change that alone and keep the rest of my gears intact ?


And now, lets say that it cant be done for whatever reason.
What strikes me as odd, is the gear ratios of the 16v in compared to the 1.5 and 1.7 Ti S2's.

I have an S2 1.5 Ti gearbox with the 9/35 diff, an S2 1.7 QV gearbox with the 10/37 diff and my 16v gearbox with the 9/35 diff (assuming that nobody swapped gearboxes on those cars before I took them apart, with the exception of my 16v which I rebuilt myself and I know its the 9/35 diff in it).

So that been said, from the table John sent me, on all these 3 gearboxes, ALL OF THEM have THE SAME GEAR RATIOS ! :/
That cant be right... Can it ???


HEEEEEEEEELP

[AlfaCorseChris]

HEYYYYYYYYYYYYYYYYYYYYY.... I just noticed... the table says that the Series 2 1.5 Ti has the EXACT SAME GEARBOX as the 1.7 16v S3 !!!

Now I know theres something wrong here

[PETROLHEAD]

What i earned from the table that JB put up is as follows.



He has fully charged his phone on works electricity, its not yet 3'oclock and he's skiving off surfing the internet like its its Sunday afternoon,

and typical I-bone, he's got no sodding signal!

[902a]

ok... hang on a second...

Ali, I did swap just the diff on my racing Sprint, and from 35teeth diff I put the 37teeth one in. So far no problems, but it was the complete diff that I changed, not just the crown wheel.

It´s your diff and your box, but don´t say, I didn´t tell you..

[Johnboy]

I'm working very hard as always. I have know

As for the gearbox God knows , I'll leave all this to the pros . Chris what do you think of the 146 box if true? Plus they come with the thick alloy sump too

[AlfaCorseChris]

I think its worth a shot.

Looks like 1st and 2nd gears are shorter anyway and the rest of the gears would get sorter with the shorter diff ratio, yes ?

[AlfaCorseChris]

ok... hang on a second...

Ali, I did swap just the diff on my racing Sprint, and from 35teeth diff I put the 37teeth one in. So far no problems, but it was the complete diff that I changed, not just the crown wheel.

It´s your diff and your box, but don´t say, I didn´t tell you..

Ali, I'm not doubting you mate. I'm sure what you say is true, its just that I havent had an issue yet.
At least when it breaks I'll know why

[Johnboy]

I think its worth a shot.

Looks like 1st and 2nd gears are shorter anyway and the rest of the gears would get sorter with the shorter diff ratio, yes ?

Well I'm guessing as the 146 was 200 kg's heavier or around that. Alfa needed to help the little get off the line . So should be good in a 33 hopefully.

[AlfaCorseChris]

Thats a good point actually.

I called up a friend with a 146 1.6 boxer engine.

His rev limiter is at 6300 rpm.

He verified that he has roughly about 40 khm per gear. 2nd gear to the limiter is at 80 khm, 3rd gear to the limiter is 120/125 khm and so on.

... on this note, and given that I rev up to 7500 rpm, I believe this is the box for me !

I'll get the flywhel off the engine as well if its 58 teeth as that lighter already and can host a 215mm clutch plate in it

[Johnboy]

These are the specs for a 1.7 16v 146 95-96 year
Transmission / Gear ratio
1th gear ratio: 3,55:1
2th gear ratio: 2,05:1
3th gear ratio: 1,32:1
4th gear ratio: 1,03:1
5th gear ratio: 0,85:1
6th gear ratio: -
7th gear ratio: -
8th gear ratio: -
9th gear ratio: -
Reverse gear ratio: 3,09:1
Final drive: 4,11:1
RPM at 120 km/h (theoretical): 3.850 rpm

[AlfaCorseChris]

Sounds about right mate.

You've convinced me. Just bought a 1.6 Boxer 146 Gearbox :p

Picking it up next week.

I'm excited to see the results with that. I'm expecting a miracle !!!

[Johnboy]

no surprise there then I hope it works out !

[SteffenJ3316V]

HEYYYYYYYYYYYYYYYYYYYYY.... I just noticed... the table says that the Series 2 1.5 Ti has the EXACT SAME GEARBOX as the 1.7 16v S3 !!!

Now I know theres something wrong here

Nope as far as I remember that is correct

[KevJTD]

ok chris.
there really is no need to guess on what the ratios would give for speed. you have all the details to calculate what a given engine speed would give in mph. assuming some small acuracies.
we simply need;
engine rpm (revolution per minute)
overall gear ratio ( multiply the gear ratio by the diff ratio) (diff ratio is calculated by dividing the crownwheel teeth by the pinion teeth, so 9 into 37 gives 4.111 in the 1.2 ti box)
tyre rolling circumferance.

important things to note.

we are aiming to find out a given speed which is in miles per hour (mph) and the engine speed is measured in revolutions per minute (rpm) so that needs factoring in.
a gear ratio is a fixed, mechanical connection that gives a certain rotational distance which can be calculated given a few details, assuming no clutch slip of course.
tyres need to be measured in rolling circumferance not overall diameter as the true diameter (distance per rotation) is less than the static outer diameter due to the squash of the weight of the car reducing the diameter.

so armed with this data i made a simple, repeatable calculation that just needs the relevant data adding to get an mph reading

so if we say that,
A is crankshaft speed
B is overall gear ratio
C is wheel speed in rpm
D is tyre rolling circumferance in Metres

this gives the formula ;

A ~ B = C x D x 60 ~ 1609.3 = MPH

now please note that the ~ symbol is the nearest thing i can find to a divide symbol on the keyboard so please allow for that!



for an example using my old 16V sud with the 1.2 ti gearbox using 195/50 R 15 tyres we have the following calculation to find the theoretical mph speed at the rev limit of 7,200 rpm in 3rd gear

7200 ~ 5.895 = 1221.374 x 1.76 x 60 ~ 1609.3 = 80.144MPH

to explain more we have the chosen engine speed, 7200 RPM, which we divide by what we know to be the gear ratio of 3rd gear to find out the RPM of the crown wheel which of course is the same as one revolution of the wheel. so we can find the wheel speed/revolutions per minute - 1221.374
we then times that number by the rolling circumferance of the tyre (how far it moves along the road for one revolution)(1.76M) to find out how many metres the car travels in one minute - 2149.618.
as we want to know how far it will go in one hour (MPH) then we need to multiply that by 60, which gives us a distance in metres that the car will travel in an hour - 128977.09.
to convert the metres per hour figure into miles per hour we need to divide it by the number of metres per mile, 1609.3
this gives us our desired figure, a mph number at a given engine rpm in a given gear.

it is also useful to try different tyre sizes out using that formula without having to go to the trouble and expense of buying them to know what differnece they'd make



hope that makes sense.....

[KevJTD]

also if you wanted to use KM's/H instead of MPH you just divide it by 1000 instead of 1609.3 in the last part to give a KPH reading

[AlfaCorseChris]

Who is the rocket scientist here ? You ? Or Lauren ?


This is VERY HELPFUL mate ! Thanks !!!

So assuming that I rev up to 7500 rpms, and my tyres are 205/45R16 (donno the exact diameter), and overall gear ratio is already given to us by the chart John posted earlier, ...

I need to figure out what diameter my wheels are first.

[KevJTD]

all a matter of following logic mate.
crank turns at one revolution so that turns the unput shaft one revolution, once you know the gear ratio & the diff ratio it's easy to work out how many revolutions it takes of the crank to turn the crown wheel once, which is the same as the wheel rotating once.
when you know how far the wheel moves along the ground once it's a matter of entreing the data to get to what you need to know.
simple algebra, you know.....the stuff we swore we'd never use outside of school



205/45r16's have a rolling circumferance of 1800mm according to my michelin book

the rolling circumferance is always less than the overall diameter due to tyre squah at the contact point making the effective diameter smaller than a tyre just sitting on the shelf. something other calculations don't take into account

[Spacenut]

Who is the rocket scientist here ? You ? Or Lauren ?

I'm the rocket scientist. Kev is helpful

[AlfaCorseChris]

Ok, can you calculate mine then please ?

My maths are worst than my geography !

Lets say I rev up to 7200 as well, coz I dont hit the limiter if I can avoid it, and I'm using 205/45/16 tyres.

Do 2 calculations please mate.

One with my current 16v gearbox (so we can verify the numbers) and one with the gearbox of the 146 boxer please


Stephan, I'm sorry but there is no way the 1.5 Ti gearbox is the same as the 1.7 16v in terms of ratios. I know what they say on paper, but really, No Way ! I'd bet on it !

[KevJTD]

lazy!!!

anyway, here you go mate.
based on 7,200 rpm and 205/45R16 michelin data

..................2nd gear............................3rd gear......................4th gear

33 ---- 66.67mph/107.29kph ------ 93.95mph/151.19kph ------121.03mph/194.78kph

145 ----57.33mph/92.27kph------- 88.85mph/142.99kph-------114.47mph/184.22kph

[Spacenut]

Hmm, don't have the specs for 33 or 145 gearboxes to hand, but this is the output for the 1.2 Ti and post-1982 1.5... without the squashy tyre correction I might add...

MAXIMUM SPEED IN GEARS CALCULATOR
Early 1200/1300
1st 42.9310885078 km/h 26.6768710046 mph
2nd 94.6274477535 km/h 58.8003776509 mph
3rd 136.0681989315 km/h 84.5511706528 mph
4th 174.9971275944 km/h 108.7411468306 mph
5th 209.5830260664 km/h 130.2324153771 mph
REVERSE 63.1257836518 km/h 39.22561589 mph
Post 1982
1st 52.0324792714 km/h 32.3323676576 mph
2nd 95.1813645209 km/h 59.1445749835 mph
3rd 140.6790174966 km/h 87.4162788148 mph
4th 189.992012919 km/h 118.0587913497 mph
5th 236.5112694155 km/h 146.9653075347 mph
REVERSE 63.1257836518 km/h 39.22561589 mph

Kev, I guess the squashy-tyre correction is dependent on tyre pressure and loading isn't it? So it won't be a constant for different tread widths, for example? In spite of all the decimal places (it's my freeware spreadsheet you know) I only developed this to allow me to compare different gearboxes, rather than calculate the absolute speed in the gears... that's my excuse anyway.

Still isn't very helpful I'm afraid

Lauren