Competition gear and diff setup 131 Abarth (replica)

[miro-1980]

Need guidance and advice :

I am expecting my new GC engine will be ready soon. With expected 180 + BHP. I am facing a totally new driving setup, which required adjustments to the gerabox/diff setup.

The car practically will not be used for other purpose than rallying and racing , but on rallies there are often 20 km of special (rally) stages and 300 km on regular roads between the special (rally) stage. It will not be driven just "for pleasure" ( except on very short distances )


Here is the key technical data.


CAR TYPE = 131 MODEL = Abarth (replica)
on 8 valve engine and two Weber 45's , original Abarth independent suspension and AND original Abarth differential.

ENGINE: PEAK POWER Kw = ca. 185 bhp rpm = 7200
PEAK TORQUE Nm = ca 200 Nm rpm = 5500


DIFFERENTIAL RATIO = 7/43 or 8/43

ROLLING RADIUS IN METERS =
a/ version 1 = 13 inch wheels ( Cromodora CD 66)
rolling circumference : 1.757 m
static ROLLING RADIUS: 0,261 m
dynamic ROLLING RADIUS 0, 280 m
b/ version 2 = 15 inch wheels (Cromodora CD 68)
rolling circumference : 1.757 m
static ROLLING RADIUS: 0,267 m
dynamic ROLLING RADIUS 0, 280 m

WEIGHT ON TRACTION WHEELS IN Kg. =1150


In this context I need expert advise:

Question one : should the gearbox be dog clutch or helical synchro type ?

Question two : I have a choice of going for Duo Block or Copia Frenata differential (from Colotti) or the Bacci diff type DAE03.

Anybody knows of any other choices for original Abarth differential ?

Anybody as experience with any of these ?.


Collotti offers the following :

Transmission kits:


T.247 – synchro gear 15/31 18/27 21/25 23/24 20/26 ,the 4th speed is direct
T.251 - dog clutch straight teeth with these ratios:
16/30 19/28 16/20 21/23 24/23 and the 5th speed is direct

LSD :

T.157 – Coppia Frenata differential for Fiat 131 Abarth
T. 196 – Duo Block differential for Fiat 131 Abarth


Bacci Transmissions option is reflected in the following data sheets (attached below) sheets:

131 1.757 7200.pdf

(19.08 KiB) Downloaded 725 times

Bacci 131 RCE12 7x43 13.pdf

(18.97 KiB) Downloaded 662 times

Also please look at the following thread:

viewtopic.php?f=6&t=1787

Miro

[mtbr]

Hello Miro,
Please take a look at the attached spreadsheet, it is an adaptation of a standard sheet I use to evaluate changes to my race car. I have used torque data for a similar engine to the one Guy is building for you and the gear ratios and tire data from your original post.
You can observe the effects of changing the final drive ratio and tire sizes on both road speed and drive force. You can also edit the torque data to accurately reflect the performance of your own engine, (when this data becomes available)

Of the gear sets and FD ratios you propose, I would suggest that the syncro box with the tall differential may well be the best option. (Change the T246 selected diff ratio to 5.38) You will note:
1. First gear is low enough, that you will be able to get the car off the line without killing the clutch
2. Top speed is 176kph at 7200rpm, you will probably be able to rev beyond this point (Set a rev limit with Guy and stick to it)
3. Each up shift puts you right on (or just past) peak torque. It looks like this box was made for a similar spec engine?

Remember the car will be traction limited in the first few gears, i.e. the car cannot use all the available torque.
Also consider the amount of time each shift takes, if you have to make too many gear changes, any time you gain through better acceleration will be used up making all those shifts. (and if you miss one....) The T246 ratios and 5.38 diff look like a good compromise.

Now having said all this, using a dogbox is going to be more efficient, shift quicker, allow clutch-less shifts and ultimately be more satisfying to drive. In an ideal world you would have the T246 ratios in a dogclutch box? As Guy has already noted in the previous thread, perhaps it's worth looking at a Ford case, Tran-X, Quaife, Elite. etc make a huge range of ratios in all configurations.

Regards, Mark

[Walezy]

From my experience you will have few problems with the gears. First is that both of your final drive ratos are too short for that engine and if you fit any other close ratio gearbox with direct 5th gear then you will have no speed at all. That engine does ot rev too much and you will just change the gears all the time and get no speed.
The other problem in your car is weight, 1150kg will not not be a rocket and 180HP will not help it too much, you need to go down with the weight to get reasonably fast car.
I use in my car 8/43 final drive but the engine revs to 9000rpm so on 15inch wheels with overdrive 6th gear i get about 195-200kmh which is reasonable max speed for a rally car.
I think that you should look for 4,55 ratio or something close to that.

I am using duo block diff for some time and it is fine, no problems at all but the locking ratio is not as big as in ramp style diffs(with no preload on clutches). It makes the car to handle nicely and predictive on twisty roads.

[Guy Croft]

Helpful post Tomasz, thanks!

In case you frighten Miro into changing his engine to a 16v at this stage: Whilst your observation about 180bhp is perfectly reasonable and based on a lot of rally exp let's remember that the bhp governs the straight line flat-out speed only and it is the torque and distribution of it that governs the acceleration - which - let's be honest - is a big factor in winning rallies.

GC

[Walezy]

Of course it will have plenty of torque and this will help with acceleration but the problem is that with such short final drive ratio there will be a problem on any longer straights, if you want to get any advantage of such final drive you will have to use the car on very slow special stages(not in Poland as i was gettin on rallies here top speed at about 180-190kmh even with FSO Polonez 2.0 8v car that I previously had) or you need to have the ability to rev the engine to about 8500rpm even if just on the 5th gear for a short period of time to get a good top speed.

I would suggest using Ford Type 9 gearbox with tranX gearkit, it has good ratios and can still be synchromesh which makes it reliable. Such gearbox is relatively cheap and can be fitted to Fiat TC easily(if you need help with it then just PM me and i will give you all details).
Here is a movie with my 131 and 180HP engine on such gearbox and 4,55 final drive http://www.youtube.com/watch?v=pXcpXxKqn-I and here is how the car performs on a testing ground http://www.youtube.com/watch?v=6-jm0oMt5Hc - please note long first gear - you need a good clutch to use such long gear as standard may slip after couple of hard starts.

As for the 5,38 final drive ratio you may see how such car performs with engine that revs to 9000rpm here http://www.youtube.com/watch?v=XRUpxx3rePo The car is using standard Honda s2000 gearbox with FSO Polonez diff and ratio 5,38(duo block diff). If you take almost 2000 revs from it then you will not get too much speed on each gear. On that stage i got average speed of 80kmh and was braking too early.

[Guy Croft]

some great advice Tomasz, thanks very much for engaging in this thread. I am doubly grateful for your exp on this as Miro had sought my advice by email and unfortunately I had to say I just don't have the time to advise in depth in gearboxes and diffs at the moment.

I would certainly second the TranX option and - Miro - if you're reading - I still have a couple of Fiat 131 bellhousing adapters left. The clutch for the Type 9 would need to carry a 1" x 23 spline, not the Fiat 7/8 x 20 one.

GC

[miro-1980]

Guy, Tomek, Mark !

Thank you gents for very interesting comments .

I need to chew on it to see if I translate my understanding of the concepts into understanding practicalities.

One general comment on my part, though. As strange as it may seem I will not modify my 131 Abarth replica by introducing non-Fiat specific gearbox or diff. In this sense I am a total purist , even at the expense of performance , reliability. etc.

Re: Speed. I understand what you are saying Tomek, but ... I do not have as opponents cars which are capable of reaching such speeds as 200 km/h , and even if some of them do - the stages we rally are very short. All events events like Krokusy, Zubry , or Debica have special stages of 1 km in length all steeply up hill with a lot of very, very sharp curves. None of my opponents is eally much faster than I am. I just calculated average speed of the class 5.1 winner in this year's Zubry Rally. It was 62 km/h !!! Mine was 51 km/h. I was 17% slower . with power ratio to the winner of 63 %. Just to out it in perspective: The winner on Subaru Impreza WRX (no corrections) had average speed of 72 km/h and Sobieslaw Zasada (former Europe Rally Champion and one of the best drivers in Europe of his time , who knows all the Zubry stages by heart) had average speed of 64 km/h).

This shows that my racing is very specific and does not require that much speed as requires "drivability" : traction , acceleration, torque. GC engine will give me all I need in terms of power and toque. I now need to make sure that I can use it best. This is what I am after with my questions.

Miro