Differential Ratios

[gjkrv8]

Originally Posted by Merv and Sharon

Exactly how do you calculate a diff/final drive ratio (without counting gear teeth)? I put a mark on the drive shaft near the diff housing and some tape on a rear wheel relative to the edge of the body/sill. So the final drive is the number of rotations of the drive shaft produced by one wheel rotation? I multiplied this by 10 to get some better accuracy and 10 wheel rotations produced 22 drive shaft rotations (in neutral). FD = 2.2 therefore???!!!@@## It has been a long day and the brain is obviously missing something very simple ....


Merv

Theory sounds right to me Merv but not the result.

i.e. I calculated my Wheel/Tyres 17" 315/35/17 are spinning at 812 RPM at 100 KPH. At this speed with a diff ratio of 3.45 my Tailshaft is going around 2800 RPM. In 6th Gear (ratio on T56 is 0.50 in 6th) this equates to around 1400 RPM which is about what I am seeing.


All this talk about diff ratios got me thinking about my own Cobra - I actually made a handy little Excel Spreadsheet that allows you to input your Gear Ratios, Wheel/Tyre sizes and Max Engine RPM and gives you what KPH per gear at what RPM - even a little graph.

The websites I checked only had Mile per hour not KPH.

PM me if you want a copy.

Rgds

Gregg (I need a beer now my head is done in)

[Merv and Sharon]

Thanks Gregg. Actually Geof/Cob 07 did the same thing today!

Great minds...

Still not sure where I went wrong on those calculations unless it should be Input (drive shaft)/output (wheel turns)?

Merv

[gjkrv8]

Originally Posted by Merv and Sharon

Thanks Gregg. Actually Geof/Cob 07 did the same thing today!

Great minds...

Still not sure where I went wrong on those calculations unless it should be Input (drive shaft)/output (wheel turns)?

Merv

I think you had everyone with their calculators out trying to work out our own ratios etc. (I know you got me wondering as my diff is a VE diff and I wasnt sure of the ratio).

If your diff ratio is 4:1 then I would have thought you'd get 4 Tailshaft revolutions to a single Wheel Revolution.

Rgds

Gregg

[stumpeater]

Hello Merv, did you have both wheels off the ground, and were they turning in sync with each other, The diff will let the wheels turn at different speeds. Maybe the other wheel did the extra turns?? If one wheel was stationary, you would get twice the tail shaft Revs.
Peter

[Merv and Sharon]

No one wheel was on the ground but I seem to be getting half the rotations...

Merv

[Merv and Sharon]

All is revealed! Stumpeater gets the prize. It was the LSD. I raised the other rear wheel and counted my rotations and exactly (over 10 turns) 4.1! After driving the car at a consistent 100 km/hr today it seems very comfortable and running at about 2250-2350 rpm (not entirely calibrated that tach yet, and suspect some over read) and great sound from the engine and Gilmer. Fewer rpms than the WRX.

So I should get that S15 diff and put it aside - just in case I get tired of the noise of this acceleration.

Merv

[07cob]

Originally Posted by Merv and Sharon

... get that S15 diff and put it aside - just in case I get tired of the noise of this acceleration.
Merv

That's not going to happen!!

But a good idea for when you add some extra hp. Then the 4.1 will only be useful for painting black stripes on the track.

Geof

[sambo]

Merv, the diff swap was straightforward as expected. A few pics below and see my gallery for larger photos and a few comments.







Sambo

[Merv and Sharon]

Excellent Sambo! Will be interested in your driving impressions. Also was your drive shaft length an issue? I have mine already installed to the R33. I has some 'play' but not a great deal.

Merv

[sambo]

No idea Merv, I haven't bought my tailshaft yet. I won't be ready to drive the car for at least another 6 months unfortunately, possibly more. Keep us informed if curiosity gets the better of you!

[Merv and Sharon]

OK then the suspense increases! I will order mine as soon as I get back from Norway. No ICV's there I am told but I have seen a huge array of 50s and 60s American cars! They just love them there. At +AUD3.00/litre, they are an expensive hobby!

Merv

[Pete Dore]

Warren, I have the same dilemma. I have an R32 but with the 5.4 engine first seems very low - not that I have driven it much - and first is not something I want to use too often. That's how I made my last (and hopefully only) minor mess. What is the end result we should be looking for? Aussie Mike provides some great figures about cruising and gear change speeds, but is there a 'standard' that is best? I have the T45, 5 speed gearbox behind the 5.4 SOHC.
BTW, am planning on coming home for good in 3 months which means I will get to learn all this quickly.... Cheers. Pete

[WKB]

Hi Peter,
Saudi summer, Australian winter, you will feel the difference when taking the Cobra for an early morning drive when you return.
Steve and Peter replaced there diffs (R32)with a 3.7:1 (3.692 if you want to be exact) from a S15 200SX. They both use first gear now, and the cars are notably quiter at highway speeds. Steve's car uses the the T45 so the gearing will be the same as yours. I think this conversion would be worth the effort. If you could get diff and axles like Sambo and Merv it would save getting adaptor plates made between the axle and diff.

Warren

[Pete Dore]

Thanks Warren, that's well worth knowing. I'll look into the bits and maybe Scott can get this done as well while he finishes the car. Other wise I'll make it my mini-project when I get back.

It's well over 40 every day and heading for 50 or more so I will need cooling off by the time we get home.

Cheers
Pete

[Merv and Sharon]

Well thanks to Scott at Venomcobras I have finally started this delayed project. I have an R33 DE 3.58 ratio diff to replace the existing R33 (4.11).



This should all go in with relative ease, although I will replace the rubber bushes on the nose of the diff with solid ally ones. I believe that the relative size of the old and new diffs are the same (both R33)



The predicted hassle is the half shafts (well described by Paul and Warren above). They have a different bolt pattern (6) from the existing ones (5). In addition they can have a different number of splines on the shafts themselves.



I can get some different shafts I guess but some have used an adapter of some kind. I am wondering how that works? Surely it would have the effect of lengthening the half shaft?

Replacing the half shafts would seem to be the best option - I would, of course, then need to match the flanges (above) as well as getting the length and spline pattern right.

Any opinions welcomed!

[sambo]

Hi Merv, that six bolt flange pattern looks exactly the same as the 3.7 ratio helical that I sourced from a manual S15 200SX. As I think I mentioned earlier the auto version used a viscous centre and may be different.

I got the S15 half shafts with the centre and it went straight into my R33 cradle. They seem to be interchangeable in that regard in the R32, R33 & R34 Skylines. The S15 diff is borrowed from the R34, so it also matches - not sure about earlier 200SX variants though. As for splines, exact match for me.

Try to get the 12 bolts and nuts with the shafts. They have a unique shoulder on the bolt that is hard to match at the bolt shop. Genuine Nissan items weren't cheap!

Please let us know how the road manners change with those solid bushes. I'm also looking to replace the old rubber bushes in mine.

[Merv and Sharon]

Thanks Paul. Will update when it's done.

[Merv and Sharon]

Thanks also Warren. Will try those half shafts of mine on the new diff.

[Merv and Sharon]

Helical diff it seems Paul - as you said. I must start reading up on what that means in operation!

[sambo]

Merv, look up torsen (TORque SENsing) diffs. Here's a couple of links:

How the TORSEN Works

HowStuffWorks "Locking and Torsen"

Torsen - Wikipedia, the free encyclopedia

And I found some photos of a stripped down S15 centre below. You can see the inner half shafts but not the outer.

FS: S15 Torsen/Helical standard diff [SYD] - Hardtuned.net