Diff ratio/tyre size/rpm

Gents

I've been having a look at what i'm going to do to my drive train.....

I've been looking for a subaru r180 plated LSD at a sensible price, I think I may have found one. Its a 4.44 ratio.

Going for 55 profile/15 inch wheels, and going off a B type gearbox ratios

1st 2.906
2nd 1.902
3rd 1.308
4th 1 :1
5th 0.864

rev drop speed
1st to 2nd 2418 39
2nd to 3rd 2186 59
3rd to 4th 1648 86
4th to 5th 952 113
5th top speed 131

Final: 4.4 :1

Using a L28, rebuilt, dynamically balanced crank/clutch/flywheel etc, 40's, 10.5:1, decent exhaust and for example a 'stage 3' type cam, I'm guessing a max of 7000rpm, would this be a save max/sensible target?(not decided on forged or cast pistons yet).

The 4.44 diff should give a nice boost to in gear acceleration, and a 75mph of 3500rpm. Along with the a 10 year old diff and LSD. Should also enable the car to use all of its potential rev range across the whole gearbox. Also it works well if I ever went to the s14's box.

Please feel free to give your thoughts.....
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
What section size of tyre?

I use 195/55/15 with a 3.9 and S13. That's just about right and I use 7k.

If you are only doing trackwork it may be ok. On the road it would be too low geared (and thirsty).
 
The above is based on a 205/55/15, I may use a 60 section or 55 section 15's.

If you used the same box then yours would be;

1 2418 43
2 2186 66
3 1648 96
4 952 125
5 max 145

so you're 75mph cruise is about 3250rpm?
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
My road tyres are 205/60/15 and a 6th gear would be good ;)

So in your road car what revs are you showing in top at 70mph?
 
Just to add, I see the 5th gear of a s14 box is 0.759, effectively an overdrive, so with the 4.44 diff ratio would give me all the plus points of a lower ratio and move 70mph to about 3100rpm.....

So the best of both worlds? Its a lower cruising rpm than a standard 5speed box and 3.9 diff.

For those with an s14 box, and a 3.9 diff, do you find the ratio's 'leggy'?
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
Franky, you are trying to convince yourself that the Subaru diff will be perfect. Be careful, I reckon it will be too low geared unless you fit silly tyres.
 
I'd agree with a 240z 5speed, however with a s14 200sx box it'll be a lower rpm cruise than standard in 5th, with better acceleration 1st-4th,

I might be convincing myself slightly but the numbers/rpm to mph look perfect with....

205/55/15 tyres
4.44 subaru diff
s14 gearbox conversion.....
 
Working it out to exact rpm with the standard 240z 5sp+4.44 diff, 205/55/15

70mph = 3746rpm in 5th, max of 132mph at 7000rpm

Using s14 box with its lower 5th ratio(exactly the same 1-4)

70mph = 3252rpm in 5th, max of 152 at 7000rpm.

Assuming that the info i've found out so far that the s14's 5th ratio is 0.759 :)

On the same tyres, a UK spec car with 5speed box and 3.9 diff gives;

70mph 3305rpm 148mph max at 7000rpm.

So the maths stacks up for the change, better in gear with 5th as a motorway gear.

Can someone else check the maths, it wouldn't be the first time i've cocked up my math!!!!
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
Just to add that a genuine 70mph (GPS measured) is 3,200 in top in my track car. 205/60/15 tyres yesterday. On the track I usually use 195/55/15, but switching to some 205/50/15s I've just aquired.

I noticed the other day that 70 in top in my 370Z is 2,200! (7 speed Semi-Auto).
 
Rob, cheers for the info, at 70 i'd be just under 3600rpm. If I can get a 0.75ish 5th i'll be at 3100rpm which would be the ideal for me.

I'm going ahead with a 4.4, I found a diff that was too good to miss, its so so smooth compared to every other diff I've had in hand before, and its a plated type.

Plated LSD diff/S14 gearbox/shortshift/Rebuild kit/output flanges and bits needed to fit for under 1k :)


DSC_0185.jpg

DSC_0187.jpg
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
Ok Franky I knew you'd buy it ;)

If you get a 0.75 5th gear you'll be dropping 25% of your revs from 4th to 5th :eek:

Anyway lets see how it works out.
 
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