GO
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| posted on 15/9/03 at 10:10 AM |
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Sierra Diffs...
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.
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Alistair Mc
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| posted on 15/9/03 at 12:13 PM |
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diff
3.92 seems to be most common
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Peteff
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| posted on 15/9/03 at 02:43 PM |
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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.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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GO
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 posted on 15/9/03 at 02:53 PM |
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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?
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David Jenkins
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| posted on 15/9/03 at 03:00 PM |
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Just remember that a seven-style car has the aerodynamic qualities of a house brick past 125mph!
David
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GO
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| posted on 15/9/03 at 03:10 PM |
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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!
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JoelP
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| posted on 15/9/03 at 06:39 PM |
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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.
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Browser
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| posted on 16/9/03 at 12:25 AM |
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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.
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ned
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| posted on 16/9/03 at 12:39 PM |
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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.
beware, I've got yellow skin
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accident
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| posted on 16/9/03 at 09:02 PM |
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what engine?
how many revs?
40" rims?
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GO
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| posted on 17/9/03 at 09:07 AM |
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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!!
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gjn200
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| posted on 18/9/03 at 05:05 PM |
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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.
<- Me!
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GO
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| posted on 18/9/03 at 05:15 PM |
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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]
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gjn200
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| posted on 18/9/03 at 05:25 PM |
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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!!
<- Me!
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GO
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| posted on 18/9/03 at 05:28 PM |
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true, but the 3.14 diff I have at the mo is a bit silly!!!
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gjn200
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| posted on 18/9/03 at 05:46 PM |
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True get a lsd, there 3.62
<- Me!
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Stu16v
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| posted on 18/9/03 at 10:10 PM |
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....and sell the 3.1 to one of the BEC boys.... 
Dont just build it.....make it!
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accident
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| posted on 18/9/03 at 10:50 PM |
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ill swap ya a 3.6 lsd for ya 3.1
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