ChrisLeary
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 posted on 5/8/11 at 12:03 PM |
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Whats the difference?
Afternoon everyone,
I hope you're all well?
I'm just wondering, what difference will changing the diff in the seven make?
I've got a 2.0l zetec with throttle bodies and megasquirt, with an MT75 box. After some tuning problems it seems to be running reasonably well,
but acceleration really isn't up to much. (Apparently the guy that built it had touring in mind).
I think the running gear is from a Granada, so my questions are:
1. How do I tell the difference between a 7in and 7.5in diff? I'm guessing there's a measurement to be take from somewhere.
2. How can I tell what the ratio of the diff is at the moment?
3. Which diff ratio would give me an average performance, more towards acceleration than top end?
4. Will it make that much difference changing it or is it more down to tuning?
Thanks in advance,
Chris
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mookaloid
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| posted on 5/8/11 at 12:23 PM |
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These are the diffs side by side
 
diffs
The higher the diff number the better the acceleration so a 3:1 diff will be slower on acceleration than a 4:1 ratio.
You can tell what you have either by reading the tag on the back of the diff if it is still present, or by taking the back off the diff and reading
the ratio off the crown wheel, or most easily by jacking the car off the ground and counting how many turns of the prop it takes to make one turn of
the wheels.
It does make a difference.
ETA: In my opinion a 3.9:1 or thereabouts is good for a zetec engine.
[Edited on 5/8/11 by mookaloid]
"That thing you're thinking - it wont be that."
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ChrisLeary
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| posted on 5/8/11 at 12:30 PM |
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So if I've got a 7.5in diff fitted at the moment, could I change it for a 7in one or will I have to change other components to accomodate it?
Thanks,
Chris
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MikeRJ
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| posted on 5/8/11 at 12:35 PM |
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The chassis will need modifying to accommodate a 7.5 inch diff. They are quite a bit heaver as well.
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blakep82
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| posted on 5/8/11 at 01:56 PM |
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quote:
Originally posted by MikeRJ
The chassis will need modifying to accommodate a 7.5 inch diff. They are quite a bit heaver as well.
by all accounts, he's already got the 7.5, and considering changing to 7", so would any chassis mods be needed to go smaller?
i would imagine acceleration should still be very good, even on a longer diff. i'd be looking to make sure you engine's tuned up properly
first
[Edited on 5/8/11 by blakep82]
________________________
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Dusty
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| posted on 5/8/11 at 03:00 PM |
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You can use spacers to take up the mounting clearances if you fit a 7" diff. Don't know if half shafts need changing.
Wheel/tyre size also has a big effect. 13" wheels or 17". Looks v acceleration.
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Liam
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| posted on 5/8/11 at 03:41 PM |
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If you think the ratio is bad there's not neccessarily any need to change to a 7" diff as I'm sure there is an equally wide range of
standard ratios available in 7.5", isn't there? You just need to decide what ratio you want and find one in 7.5". And as Blake says
make sure your engine's working properly too!
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ChrisLeary
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| posted on 5/8/11 at 04:42 PM |
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Thanks for your replies so far.
I've just got back from the garage, I jacked up one rear wheel and marked a reference point, I also marked a reference point on the prop shaft.
I turned the prop shaft and it took just under 2 whole turns of the prop to turn the wheel one revolution.
Surely that doesn't mean I've got a 1.something:1 ratio diff? I'm confused?
I'm still working on the engine, it's standard bar the gsxr throttle bodies and megasquirt, I'm just wondering where to go next.
Thanks again,
Chris
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blakep82
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| posted on 5/8/11 at 04:45 PM |
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no, because if you only turn one wheel, half of the revolution is lost to the diff. so if its just under 2 turns, you double it, so its a 3.
something.
a more accurate way is to push the whole car backwards and forwards, with all wheels on the ground, and count the turns of the prop
if you take the back cover off the diff, the actual number is usually written on the crown wheel.
[Edited on 5/8/11 by blakep82]
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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ChrisLeary
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| posted on 5/8/11 at 04:51 PM |
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quote:
Originally posted by blakep82
no, because if you only turn one wheel, half of the revolution is lost to the diff. so if its just under 2 turns, you double it, so its a 3.
something.
a more accurate way is to push the whole car backwards and forwards, with all wheels on the ground, and count the turns of the prop
if you take the back cover off the diff, the actual number is usually written on the crown wheel.
[Edited on 5/8/11 by blakep82]
Ahhhh right ok, I thought I was missing something, 1.something:1 ratio really didn't make sense.
I'll get the back cover off and have a look.
I think a rolling road might be the next option, I dont have too much money to start spending on engine mods.
Cheers,
Chris
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Liam
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| posted on 5/8/11 at 05:18 PM |
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Sounds like something in the region of 3.6 or 3.9 then which are probably the most common ratios, but if you already have a 3.9 you can't get
lower with a standard ratio I dont think (or are those later 4.somethings 7.5" diffs??). Your acceleration ought to be OK with a 3.6 or 3.9 I
would have thought - what are your speeds in each gear? Maybe your engine really isn't running right?
[Edited on 5/8/11 by Liam]
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ChrisLeary
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| posted on 5/8/11 at 06:10 PM |
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quote:
Originally posted by Liam
Sounds like something in the region of 3.6 or 3.9 then which are probably the most common ratios, but if you already have a 3.9 you can't get
lower with a standard ratio I dont think (or are those later 4.somethings 7.5" diffs??). Your acceleration ought to be OK with a 3.6 or 3.9 I
would have thought - what are your speeds in each gear? Maybe your engine really isn't running right?
[Edited on 5/8/11 by Liam]
I'll have a drive out tomorrow Liam and I'll post some speeds up so you can have a gander.
Thanks for your time,
Chris
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Gary 7
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| posted on 5/8/11 at 07:00 PM |
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You can get a 4.27 7.5 diff from a scorpio, if changing from a 7 inch three of the mount holes will line up ,one top mount will need moving
(passenger side viewed from back of diff).If you are going from 7.5 to 7 you just need a spacer on the one side of the top mount . The bolt on
driveshaft flanges are the same distance apart on both diffs. The 7.5 is also around 10mm longer than the 7 on the propshaft end.
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