What the highest ratio Sierra diff they made? (could be lowest, I never get that right). I need a 4.1 or maybe even higher preferably.

3.92 seems to be most common

I thought the higher the number the lower the ratio. 3:1 high 4:1 low, to do with division. Wheel size could be a factor to influence your calculations.

yours, Pete.

You could be rigth Pete, like I said I always get that wrong! From maths point of view, a 4.1:1 ratio is higher because the difference is greater, but in this situation we're using it to slow the wheels down more, so its a lower ratio!

I just know I want something like a 4.1 diff!! A 4.4 would still give me a theoretical max of 170mph, can't help but feel I'll still be wasting some accelleration.

So, did Fraud ever make an IRS diff with a lower ration of 3.9?

Just remember that a seven-style car has the aerodynamic qualities of a house brick past 125mph!



David

David,

Thats exactly my point!! No point being geared up to 200mph if the aerodynamics are gonna limit you to 130, might as well use the power for accelleration Only benefit I can see is a "quieter" 5th gear cruising speed!

I thought a 4.4:1 would have a lower top speed than say a 4:1, cos doesn't the number represent what the number of turns of the prop are divided by at the wheel?

May be wrong. I thought a 1:1 would have a theoretical top speed of IRO 5000m per min, assuming rev limiter at 5000rpm and a rolling radius of 1m.

The lower the ratio, the lower the engine speed for a given road speed. For a car using 185x65x14 tyres and assuming the tyre stays perfectly round at 70mph a 3.89:1 diff gives and engine speed of 3237 rpm, a 3.62 gives 3012 rpm and a 3.36 gives 2976 rpm. So the higher the first number of the ratio, the higher the engine speed when compared to road speed.

rallynuts do crown wheel & pinion sets for sierra difs ranging from 3.6-4.7, so they are available (though cost £310+vat new!)

Ned.

what engine?
how many revs?
40" rims?

I'm using a Mazda 13b rotary.

Max revs 8000.

15" rims with 205/50 tyres.

The problem is the box has high gearing, with a 4.1 diff on the RX7 it was ok, but at the moment I have a 3.14 Sierra diff which gives me a crazy theoretical max speed of 241mph! A 3.92 brings that down to a more sensible 193mph! Obviously I'll never get there because of the naff aerodynamics of a seven.

Won't stop me trying though!!

Your using the mazda box then? If so is the 5th gear an overdrive? 4th should be 1:1 and you'd then be about the same as most people with high performance engines. eg. my engine will rev to 7000 (sustained)with a 3.62 diff and I dont give a toss about theoretical top speeds.

gjn, you're right, 4th is 1:1, 5th is 1:0.711

with a 3.92 diff I'll be "cruising" at 120 with almost half my rev band left. Knowing that the cars not going to go much faster than that I'd rather have lower gearing to make better use of the spare revs to give me more accelleration, especially beyond 60mph which is where a 7 will start feeling the effects of its brick shape.

[Edited on 18/9/2003 by GO]

I understand your point, but too engineer it out will cost££££££££££. Also if you where to reduce your theoretical top speed by 1/2 you'll be forever changing gears. I dont think any road car is designed to be able to redline in top gear. If so my Safrane should do 180mph!!

true, but the 3.14 diff I have at the mo is a bit silly!!!

True get a lsd, there 3.62

....and sell the 3.1 to one of the BEC boys....

ill swap ya a 3.6 lsd for ya 3.1