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Author: Subject: making a custom diff
bi22le

posted on 14/4/16 at 10:16 PM Reply With Quote
making a custom diff

Im still in the hunt for higher ratio diff. I have found potential what is my best bet but. .

I have a 7" push in non LSD. The new one is 7.5" bolt on non lsd.

Now, is there potential to change the output type? I know my chassis needs to cater for the wider diff. I was also told that they are longer, can this be confirmed.

I don't want to change my drive shafts or prop. Simple is best for me right now.





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hkp57

posted on 15/4/16 at 06:13 AM Reply With Quote
What Ratio do you have now and what are you aiming for?





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bi22le

posted on 15/4/16 at 06:49 AM Reply With Quote
I have a 3.92 and i want higher. Im looking at a 4.27





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Please read my ring story:
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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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wylliezx9r

posted on 15/4/16 at 07:25 AM Reply With Quote
The nose on the 7.5" diff is 10mm longer (not a lot at all). When I changed up to the 7.5" in my car the 10mm difference was catered for by the prop slider.





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Doctor Derek Doctors

posted on 15/4/16 at 07:37 AM Reply With Quote
I think you mean a Lower ratio diff. You get lower number of output turns for the one turn of the input shaft. And so the gear ratio of the diff is Lower. In the same way that 1st gear is a Lower ratio that 5th in your gearbox.

1.6 MX5 differentials are 4.3:1 apparently, it would be a much more involved change but it will give you what you want. Also they do a 4.1:1 & 3.9:1 as well so you would have some choice to change back to Higher ratio if it wasn't what you wanted.





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hkp57

posted on 15/4/16 at 08:05 AM Reply With Quote
The easiest but not the cheapest would be to fit either a 4.1 or a 4.4 CWP to your 7" diff.

Quaife direct or Raceline supply





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adithorp

posted on 15/4/16 at 09:23 AM Reply With Quote
quote:
Originally posted by Doctor Derek Doctors
I think you mean a Lower ratio diff....


Beat me to it.





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bi22le

posted on 15/4/16 at 06:35 PM Reply With Quote
Still does not answer my question.

Can i take out the bolt flanges and put in my push in ones?





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Please read my ring story:
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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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hkp57

posted on 16/4/16 at 06:46 AM Reply With Quote
quote:
Originally posted by bi22le
Still does not answer my question.

Can i take out the bolt flanges and put in my push in ones?



In basic terms yes you can, take the back cover off and pull out the circlip and pull the drive flanges.

A Great Source of information is here

http://www.super7thheaven.co.uk/blog/differentials/sierra-rear-differential/






Part 16 is your Circlip.




I would guess your problem would be the 7.5" and 7" diffs have different shaft lengths and different splines.
I have not worked on a 7.5" so i am basing this on paper information.





[Edited on 16/4/16 by hkp57]





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bi22le

posted on 16/4/16 at 09:17 AM Reply With Quote
Hmm. So its seems to me that i cant change the bolt on for push in half shafts, the nose of the diff so to long and its not going to bolt easily in to my chassis.

Boo





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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Flat Pack

posted on 16/4/16 at 12:42 PM Reply With Quote
I had this same problem a few years ago and couldn't find a good solution for the diff I'm afraid.

I broke the 3rd gear synchro on my standard type-9 box and was looking at ways to sort out the gear ratios at the same time as replacing it.

One option was a bgh box and shorter diff to bring the overall gearing down. In theory custom 4.4 and 4.7 cwps exist for the 7" diff, but they're expensive and nobody actually seemed to have one or know when they might be able to get one.

In the end I left the 3.92 diff and fitted a Caterham 6 speed box instead. 6th is a 1:1 ratio, same as your 4th. It's not cheap and it's an engine out job, but I couldn't find a practical alternative at the time. It did completely transform the car though, the ratios are spot on now - it's definitely worth sorting out one way or another.

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bi22le

posted on 16/4/16 at 04:13 PM Reply With Quote
I would of thought it would of been cheaper to fit the 4.27 diff and change all three drive shafts!

You do get obvious other benefits from fitting the 6 speed if your pockets are deep enough.

What engine do you have in your striker?





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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Flat Pack

posted on 16/4/16 at 10:29 PM Reply With Quote
It's a 20v Silvertop 4AGE in my car. Rev limit is 8000 rpm and it develops most of its power at the top end of the rev range. It was geared for 170 odd mph in 5th before I changed it and about 140 in 6th now.

My gearbox was broken so I had to change it anyway and the cost of a bgh box and aftermarket diff ratio to make the ratios right was about the same as the Caterham box and bits and pieces to do the conversion. If you've got a close ratio box already then the diff will be much cheaper (if they're available) and should work well. I'm not sure a normal type-9 will work as well with the shorter diff though.

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bi22le

posted on 17/4/16 at 12:25 PM Reply With Quote
Nice work, cheers for the info.

I have a 20v blacktop in mine with a 8250 limit and it sings!!

As you know these, even with the VVT, have quite a narrow power band. I only use 5th on in and out laps so anything the wrong side of 1:1 is no good.

The scorpio 4.27 seems a lot of work to get in. I would rather go for a more modern lighter diff that is cheaper and gives the same ratio.

More to think about i guess.





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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Flat Pack

posted on 18/4/16 at 05:11 PM Reply With Quote
Yes you definitely need to keep them revving to make them work

I found the rev drop between 2nd and 3rd the most frustrating thing before I changed the box - I'd be trying to use every possible rev in 2nd (to the point I'd frequently hit the limiter) to try and get somewhere near the power band in 3rd.

Be interested to see what you come up with, something from a Subaru Impreza / Legacy might be an option too if you're going to be changing mounts, driveshafts etc.

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Rob Allison

posted on 19/4/16 at 07:47 AM Reply With Quote
quote:
Originally posted by bi22le
Im still in the hunt for higher ratio diff. I have found potential what is my best bet but. .

I have a 7" push in non LSD. The new one is 7.5" bolt on non lsd.

Now, is there potential to change the output type? I know my chassis needs to cater for the wider diff. I was also told that they are longer, can this be confirmed.

I don't want to change my drive shafts or prop. Simple is best for me right now.


I assume this is a frod diff your on about?

The 7" internal parts, including output drive flanges or push in shafts are not interchangable with the 7 1/2". but you can make up a hydrid drive shaft to use the inner lobo joint on a tripod type push in shaft. Its not easy to find the parts though. You need to measure the spline size at the shaft end that the cv joint mounts on to. Then find a matching cv joint in the bolt on type.
I'm not sure which parts though. Its been covered on the sierra owner forum before.






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