andyhaase1
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| posted on 9/5/16 at 11:40 AM |
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What's the right Sierra diff ratio
I'm starting to gather all the running gear to build a MNR VORTX, I've got a 2 litre duratec, now I'm told I need a mark3 mx5 box
and Sierra lsd diff as this will give me wider track than the mx5 diff. Can anyone confirm that I need a 3.62 ratio diff or is it down to driver
preference. It will be a 90% track day car if that helps.
Andy
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cliftyhanger
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| posted on 9/5/16 at 12:05 PM |
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It will also heavily depend on tyre size.
Butthe best bet is to start with a rpm/speed table for your chosen gearbox ratios, see where the gearchanges will be and so on. All very well having a
car that is relaxed on the motorway, but you never need 4th on track. So camshaft choice and where you engine develops power etc are all important.
Took me ages to get my head round what I wanted, and then looking at charts so I could see where a gearchange would leave me helped hugely. ie no
point in having a change form 2nd redline to third out of the powerband.
Hope that sort of makes sense!
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mcerd1
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| posted on 9/5/16 at 12:09 PM |
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^^ you don't 'need' any special diff ratio - its your preference as is the choice between an LSD and an open diff...
you can however work out what each of the available ratio's would mean for you 
use a gear calc program (I like this one: http://locost7.info/gearcalc.php) - you'll need to tell it the correct gearbox ratio's and tyre
size aswell as the diff ratio.
These programs will tell you a theoretical top speed in each gear for a given rev limit, but remember they don't account for the power your
engine produces or aerodynamics etc, so the figures for the high gears are not going to matter much
More importantly the program will tell you your engine rev's for any given road speed and gear - so you'll know if its going to be
screaming its head off at 70mph in top gear or if its going to be chugging along at 900rpm, it'll also give you an idea of how many gear
changes you'll have to make
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Barkalarr
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| posted on 9/5/16 at 07:28 PM |
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I've got a complete 7" ford Sierra ZF plated LSD for sale - any good ?
3.9.2 ratio - ideal with 15" wheels.
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andyhaase1
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| posted on 9/5/16 at 08:53 PM |
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Thanks for the offer, but I think I'm going with 13 inch wheels.
The link for the gearing table didn't work for me.
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Barkalarr
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| posted on 9/5/16 at 09:02 PM |
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quote:
Originally posted by andyhaase1
Thanks for the offer, but I think I'm going with 13 inch wheels.
The link for the gearing table didn't work for me.
It came from a 5 speed (type 9) Caterham with 13" wheels if that's any good ?
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andyhaase1
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| posted on 9/5/16 at 09:23 PM |
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Oh ok, how much are you looking for?
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Barkalarr
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| posted on 9/5/16 at 09:28 PM |
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£900 - it's a complete diff. It's push in shafts. 6000 miles. No whine. Ramps are Caterham specific. I've a receipt from
Caterham for it.... it was purchased direct from them.
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andyhaase1
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| posted on 9/5/16 at 09:54 PM |
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Sorry it's way outside my budget, sound the business tho!
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Barkalarr
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| posted on 9/5/16 at 10:01 PM |
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No worries...
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scoop
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| posted on 13/5/16 at 02:20 PM |
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For 90% track use I'd go 3.9 LSD.
There are tables out there that you can work out the speeds in each gear with whatever diff, if you know them.
Oops already been said. I'm forum rusty 😉
[Edited on 13/5/16 by scoop]
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mcerd1
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| posted on 13/5/16 at 03:00 PM |
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quote:
Originally posted by andyhaase1
The link for the gearing table didn't work for me.
try this:
http://locost7.info/gearcalc.php
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bi22le
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| posted on 13/5/16 at 04:04 PM |
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For your info. My I feel my 3.92 is not enough. I don't touch 5th on track.
I'm looking at going to a 4.4 to give me more zip in 3rd and 4th. I still think I'll only just hit limiter in 4th at 125mph.
I only use 5 th for cool down and warm up laps.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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andyhaase1
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| posted on 13/5/16 at 07:06 PM |
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thanks guys,
so ive tried 2 different tables, both give same results, using the mx5 mk3 5speed ratios, with 205/60/13 tyres, 7000rpm rev limit, and 2 theoretical
sierra diffs.
First one is with a 3.62
gives in gear max speeds of 42, 69, 98, 131, 161
second one is with a 3.90
gives in gear max speeds of 39 ,64, 91, 121, 151
So it looks like bizzle might be right, both the above could be too long for me. Maybe a 4.4 is the way to go!
Andy
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bi22le
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| posted on 13/5/16 at 07:22 PM |
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quote:
Originally posted by andyhaase1
thanks guys,
so ive tried 2 different tables, both give same results, using the mx5 mk3 5speed ratios, with 205/60/13 tyres, 7000rpm rev limit, and 2 theoretical
sierra diffs.
First one is with a 3.62
gives in gear max speeds of 42, 69, 98, 131, 161
second one is with a 3.90
gives in gear max speeds of 39 ,64, 91, 121, 151
So it looks like bizzle might be right, both the above could be too long for me. Maybe a 4.4 is the way to go!
Andy
Dont.get.me.wrong, you need a cruising gear for track, its just not the same requirment as on road. 80mph lowest revs possible.
Unless your running north of 200 or a very slipery car you wont see above 130 on track. So 120 should be just above max power in my eyes.
Looking at your ratio I think a 3.62 would be good. You can run into.straights grabbing 4th knowing that you wont need to change up again. 3.9 may
see you snatching 5th at the end of a straight which would be aqward.
I think my gearing is about the same as your 3.62 ( I have a 3.92 with stock type9 and 14" wheels). The main problem i have is only 150bhp so
although i can get 120+ on the strainghts i have no grunt for 3rd. Or mod range 4th.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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andyhaase1
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| posted on 13/5/16 at 07:35 PM |
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Ok, I see what you mean. I should have roughly 190 bhp and all the seven copies have the aerodynamics of a brick don't they? So I'm back
to a 3.62 now!!!! Whatever I get its bound to be wrong. I suppose it's not a massive job to change it if I find its too long. And I suppose
it's track dependant anyway.
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loggyboy
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| posted on 13/5/16 at 10:08 PM |
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Dont get to hung up on wheel size - the circumference of 195 50 15 (most common 15 inch tyre for kits) and a 205 60 13 (fairly common 13 inch tyre) is
nearly identical.
Mistral Motorsport
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andyhaase1
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| posted on 27/5/16 at 03:59 PM |
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Ok, I decided to stick with the Sierra 3.62 diff, bought from Dan on here. Does anyone know what size/ thread,bolts I'll need? I have a 3/8unf
tap that feels close, but I don't want to force it without being sure.
Andy
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mcerd1
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| posted on 27/5/16 at 04:52 PM |
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The bolts are all metric
M12 for the mounts
M10x1.0 For the propshaft (very fine pitch) - you can get them from burton power
Driveshaft bolts are M8x50 if it the 100mm CV's
[Edited on 27/5/2016 by mcerd1]
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andyhaase1
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| posted on 27/5/16 at 06:46 PM |
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Ok thanks.
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