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| Deciding on a diff ratio https://www.ausmini.com/forums/viewtopic.php?f=2&t=89336 |
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| Author: | willy [ Sat May 02, 2015 10:14 pm ] |
| Post subject: | Deciding on a diff ratio |
Hi guys, I'm in the process of building a hot 1293 for a weekend car I am building, and so have the luxury of being able to select my diff ratio. It will be on 10" wheels (it is a Mini, afterall!) It will have a SCCR remote gearbox, S/C drop gears, 10.6:1 CR, and hopefully last a lot longer than my stroker did - through careful machining by a local trusted BMC specialist (Rhemac) and a build that is definitely not rushed and that has no corners cut. It will certainly be used as a weekend fun car - I have my 850 snail for church days! I am thinking a 3.44:1 might offer good highway revs as well as being decent off the line. But then again I was thinking how much actual sustained 100/110km/hr driving I would be doing, especially with what the cabin volume will be! Any advice / tips welcome. Cheers. |
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| Author: | drmini in aust [ Sat May 02, 2015 10:51 pm ] |
| Post subject: | Re: Deciding on a diff ratio |
IMO it will be a bit slow off the line with a 3.44 diff and SCCR gears. And even worse if you went for a 3.1. A 3.647 would be better but it's a bit busy on the freeway. I run one, at GPS 110kmh it's spinning at nearly 4,700 rpm. |
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| Author: | Irish Yobbo [ Sun May 03, 2015 12:22 pm ] |
| Post subject: | Re: Deciding on a diff ratio |
Have a look here: http://www.guess-works.com/Tech/ratio.htm It's a calculator - just enter the info for your car, and then change the diff ratio to see what speeds you will be doing at what revs. |
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| Author: | slide [ Sun May 03, 2015 6:39 pm ] |
| Post subject: | Re: Deciding on a diff ratio |
I have a sccr 3 synchro box with 3.44 diff in my Cooper S and it goes fine - just about reaches 60mph in first if you hang on for long enough! I tried a Morris 1100S ratio in my 1360 van and I didn't like it at all for normal street use, so swapped a 3.44 in there too. All on 10 inch wheels of course. |
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| Author: | aaron [ Sun May 03, 2015 6:42 pm ] |
| Post subject: | Re: Deciding on a diff ratio |
Hey willy. I run a 3.6 which is great for weekend runs, and motorkhanas. As you say, you generally don't run around at 100 let alone 110. and if you were of a mind to do a long trip, put 13s on. every mini is like a girl, it should always have more than one pair of shoes.  Cheers Aaron |
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| Author: | willy [ Sun May 03, 2015 7:53 pm ] |
| Post subject: | Re: Deciding on a diff ratio |
Aaron, you know long trips are no stranger to me!  Cheers for the tips. I think it'll be a 3.6:1. |
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| Author: | Morris 1100 [ Sun May 03, 2015 8:11 pm ] |
| Post subject: | Re: Deciding on a diff ratio |
I have a 3.65 in my Morris 1100 and it is great. But it is running tyres much taller than yours. I'm pulling 4000 revs at 100kph, on 10's it would be closer to 4600. So the secret is a pair of M1100 sized front tyres (155/80-12) for long trips. I use a handy program called Gearcalc to work out ratios and speed in gears, it is very handy. http://www.locost7.info/gearcalc.php 
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| Author: | 9YaTaH [ Mon May 04, 2015 10:00 am ] |
| Post subject: | Re: Deciding on a diff ratio |
Excellent little program Mike... I think the type of engine you have also needs to be taken into account...a torquey powerful road engine will pull just about any diff available (as long as you aren't in a hurry to get going!)...but a wilder cammed engine may be better suited to a run a lower diff... |
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| Author: | IndigoBlueCooperS [ Mon May 04, 2015 10:49 pm ] |
| Post subject: | Re: Deciding on a diff ratio |
Hi willy, What cam are you going to use? I love the 3.2 diff (or there abouts from the late A+ box) with the RE83 for cruising and around town. Good for top end - 6,000 RPM was about 170 KPH over the 500 metres. The cam runs out of steam after 6,500. I suspect something lumpy would need the higher ratio. I would put an ATB diff in there (instead of the straight cuts if cash is an issue). |
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