blue2cv
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| posted on 21/1/07 at 03:25 PM |
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Ideal final drive ratio
Anyone got some thoughts on a good start point for final drive ratio on a BEC running 195/50x15 tyres.
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smart51
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| posted on 21/1/07 at 03:34 PM |
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It depends what engine you're using. Each has a different gearbox, with different ratios plus a different red line.
I have a 1998 R1 engine, a 3.62 diff and 195/50/15 tyres. My car will do 53MPH in1st, 73 in 2nd 90 or so in 3rd, 100 ish in 4th, 110 in 5th and 120
in 6th. Acceleration is good. Top speed is more than I had enough room to get to on a track day (short straights) but it is a bit revvy on motoways
and dual carriagways. 2nd is a good overtaking gear.
A 3.14 diff would make all the top speeds all 15% higher - 61, 83, 105, 115, 126 and 138, although I'm not sure there is enough power for 138
MPH. RPM at 70 is 6000 RPM, down from 6800. Acceleration in each gear will be 15% lower than with the 3.62 diff
A 3.38 diff would be between the two.
A 2004 R1 has a taller 1st gear and shorter 2nd - 6th. You need a 3.38 or 3.14 just to keep the top speed up. You end up with a 1st gear that is
really tall, keeping the in gear acceleration down. You do have another 2000 RPM though so you can keep going for longer in 1st. You might never
need to shift into 2nd at legal road speeds.
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blue2cv
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| posted on 21/1/07 at 04:03 PM |
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Having a dead brain day, forgot the engine bit, Blackbird, with little mods
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smart51
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| posted on 21/1/07 at 04:33 PM |
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What are the gear ratios and primary reduction ratio of your engine? What will it rev to?
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DIY Si
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| posted on 21/1/07 at 04:38 PM |
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PR = 1.571,
1st = 2.769
2nd = 2.000
3rd = 1.579
4th = 1.333
5th = 1.167
6th = 1.042
Redline = 10,800 rpm
Assuming standard box and no rev raising mods.
[Edited on 21/1/07 by DIY Si]
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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smart51
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| posted on 21/1/07 at 05:40 PM |
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With the figures supplied, here's what top speed you get in each gear.
3.92 diff with 195/50/15 tyres
1st = 41 MPH
2nd = 57
3rd = 72
4th = 86
5th = 98
6th = 110
70 MPH = 6850 RPM in 6th
3.92 diffs are cheap but unloved. You get the best acceleration, which is good for sprints or very short circuits.
3.62 diff with 195/50/15 tyres
1st = 45 MPH
2nd = 62
3rd = 78
4th = 93
5th = 106
6th = 119
70 MPH = 6300 RPM in 6th
This will give you the best 0-62 time possible. and reasonable gearing for road use. 3.62 diffs are common, cheap and available with LSD if you
want.
3.38 diff with 195/50/15 tyres
1st = 48 MPH
2nd = 66
3rd = 84
4th = 100
5th = 114
6th = 128
70 MPH = 5900 RPM in 6th
3.14 diff with 195/50/15 tyres
1st = 51 MPH
2nd = 71
3rd = 91
4th = 107
5th = 123
6th = 137
70 MPH = 5500 RPM in 6th
This will give the best top speed and reasonable cruising. It will be the slowest accelerating and most expensive though.
Which is best? Its a balance of outright acceleration, top speed, revs at cruising speed, cost and whether you want an LSD.
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blue2cv
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| posted on 21/1/07 at 06:37 PM |
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Gents, thanks for the input, as usual a font of useful information. All i have to do now is stop myself cutting it up and modifying it before its
finished, its a mk ??? already
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Wadders
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| posted on 21/1/07 at 07:40 PM |
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I know the figures will say otherwise, but both my 0-60 and 1/4 mile times improved after switching diffs from 3.62 to 3.14, maybe because of less
wheelspin (with the 3.62 it would light up in the first 3 gears) and less gearchanges. The car feels to accelerate just the same, only it holds in the
gears longer. weirdly it feels no different when setting off slowly, no fuss or clutch slipping needed.
Al.
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