How to choose a diff ratio (question)

[Piero franchi]

Question: As topic says, how do you work out (on papper) what diff ratio to choose???

To give a example, I have put a fiat twin cam engine into my spit, it will also run the fiat Gear box.

The fiat 131 2liter gave about 115 bhp, (will have to fill in were the power came in)
it ran a 3.9 diff (again will have to conferm)

The 130 strada arbath (same engine, more power) gave 128 bhp at 5,900. it ran a 3.4 diff (again will have to comferm)

Now looking at that, one might asume that with the extra power, and lighter car, you needed a longer gear.

I hope to tune the 130 to about 175 bhp, and as it is now in my spit, the car is now, even light.

Do I go for an even longer diff ????????

The power will NOT be lost at bottem end, if it is tuned right, and with the weight of the spit, there should be no worrys about it pulling away from the lights.

Any help will go a long way guys

[the_nutter]

I guess the obvious question has to be how fast do you want to go?  I used a simple spreadsheet to look at tyre sizes, diff ratios, and speeds, over a range of engine speeds.  I'll send the spreadsheet if it would be of use, but a picture of it is below.  Also consider what size tyres were used on the original car as that will effect the gearing if you then use a different size.

Hope this is of some use.

[Piero franchi]

I didnt think it was that simple!!!!!!!!!!!!!!

Surley it depends on were and how much power you have for a given weight.

what would be the point of lets say, saying I want it to go X fast in top.
If the car can not get there due to it not being able to pull away from the lights, or if it goes way past or never reaches the power band in top gear!!!!!!!

Dont get me wrong, The replys are just what I want, I am just trashing the question around so I can understand it.

[the_nutter]

I agree with you completely, top speed is only one part of the equation, but sometimes when you start digging into the detail you can loose sight of the obvious.  Top speed and acceleration are a balance, too much of one (cos you're unlikely to get both without serious wonga, or a stripped out kit car with a bike engine) and you'll compromise the other.

I suppose basic physics applies, F=ma, ie, given a constant mass acceleration is directly proportional to the force (power) applied.  Fitting a lower ratio diff will give you higher speed, but at reduced acceleration.  Leave the ratio as original and you'll get the same top speed as original (everything else being equal), but get there faster.

Personally I'd not reduce the diff ratio further because the engine has more power (if it gives an acceptable speed) but use the power gain to accelerate quicker, which is a probably the objective.

Hope this is a better explanation.

[gilesdive]

One other consideration, not just the power you think you will get out of your engine, but at what revs.
A big lazy lump that produces a load of power and torque at 3,000 rpm will want different ratios to a little twin-cam that produces the same power at 7,000+ rpm.

Don't overgear it, to the point that you lose acceleration and it won't pull up to a decent top speed either if you overdo it. Also depends what you are planning on doing...fairly tall gear if you will be strolling up the motorways a lot, but as short a gear as you like if you will be hill-climbing/sprinting it.

[Piero franchi]

Thanks guys, sort of getting my head round it.

If I keep the same gearbox ratios, and the same diff ratios as the strada, lets say.

But just tune the engine for more power, are you saying I should have the same top speed at the strada, (if tyre sizes are the same)???????

I will be going from the standard strada bhp of 130 at 5,900 rpm to about 175 at 6,500 rpm

Surly wind resistence is big factor at top speed as well????

[CharlieB]

Have you seen this calculator: http://www.mintylamb.co.uk/gearspeed/
Brilliant!

[Richard B]

Top speed would depend on max power and the areo-dynamics of the car. However the peak RPM of the engine with the ratio of top gear - the diff ratio & the diff ratio will give your theroretical maximum. Although 4th was always the 1-1 gear that the car was optimised at rather than an overdriven gear.

I think the art is to get the theroretical maximum gearing as close as possible to the areo-dynamic maximum to give you the best all rounder.

Otherwise you end up with a quick car that tops out too early because the engine cannot go any faster.

or

A slow car that cannot pull it's top speed at max bhp (or is it torque?)


Richard waiting to be shot down in flames........ with thats B0110ck5 etc