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

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!

^^ 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

I've got a complete 7" ford Sierra ZF plated LSD for sale - any good ?
3.9.2 ratio - ideal with 15" wheels.

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.

quote:

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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.

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Oh ok, how much are you looking for?

£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.

Sorry it's way outside my budget, sound the business tho!

No worries...

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]

quote:

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Originally posted by andyhaase1
The link for the gearing table didn't work for me.

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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.

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

quote:

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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

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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.

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.

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

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]

Ok thanks.