4.11 Differential? Likely? - NOW ANSWERED!

[revilla]

Comparing my road speeds against RPMs in various gears assuming a standard 3.92 differential, the measured values are definitely out of line, however if I plug in 4.11 for the differential ratio I get good agreement (4000 RPM in 4th, measured 65, calculated 65.2, 400 RPM in 5th, measured 79, calculated 79.5, rev limit in 3rd, measured 96, calculated 98.4). Road speeds are checked on GPS, tacho calibration seems right based on rev limiter and tyre measurements are correct at 195/50R15 (I'm on R1Rs not 45-profile CR500s). Calculations are here.

 

How common are 4.11 diffs? Is it likely that I would have one on a 2003 SV RoadSport VVC 160? Were they fitted as standard or is it likely to have been an upgrade for acceleration?

 

When I get a chance I'll see if I can see the tags on the casing, but until then just wondering if I'm likely to be right as I would have expected to have a 3.92 diff ratio.

 

Edited to add that it's a standard 7" Sierra open differential.

 

Edited by - revilla on 15 Apr 2014 09:58:01

 

Edited by - revilla on 18 Apr 2014 08:30:11

[Jonathan Kay]

Was it ever used for Academy or other racing classes?

 

Jonathan

[revilla]

I don't think so - I don't know the full history but it looks like it has spent it's whole life configured as a road car, there's nothing about it that looks like it was configured for racing.

[Mark Durrant]

A 4.11 ratio is rare so much so that they have recently started making them again. I would therefore be surprised if you have this ratio fitted.

[Richard Price]

4.1 diffs are not at all common.

Your spreadsheet shows max revs of 7600.

Isn't the normal VVC rev limit 7200?

Have you have you had your rev limiter lifted to that value by Z&F?

My tacho often shows 8000rpm at the limiter, but my datalogger shows a more plausible 75xxrpm limit.

 

 

[revilla]

Yes my RPM limit was increased to 7600 RPM by Z&F and the tacho reads about 7600 when the limited kicks in.

[Mankee]

Quaife/Tran-X make CWPs for 7" diffs lower than the "normal" 3.92. Was just looking at them yesterday actually. £400-odd I think. Not something that I would splurge my money on though!

[ECR]

I ran a 4.1 diff for many years when sprinting. Recently refitted my 3.9 as I'm on circuits more often now

[Englishmaninwales]

I ran a 4.1 ratio with standard type 9 ratios with a 7600 limit a few years back for hillclimbing. Fitted with 15'' 195/45 Stunners (I dont have a record of the rolling circumference now) and I got 35/66/95/119/and a theoretical 144 mph which are close to your figures in the link.

Edited to add - just found it 1737mm for the Stunners

 

Edited by - Englishmaninwales on 15 Apr 2014 14:07:36

[Jonathan Kay]

Quoting revilla: 

When I get a chance I'll see if I can see the tags on the casing...

Or turn the propshaft and count?

 

:-)

 

Jonathan

[revilla]

And the answer is ... NO! (Unsurprisingly)

 

Jonathan I did what you suggested, except instead of turning the propshaft I just jacked one back wheel off the ground and turned it whilst watching a chalk mark on the propshaft flange at the diff (it didn't involve getting so dirty!).

 

20 revolutions of the propshaft equated to 10.2 revolutions of the wheel (would be 5.1 revolutions of both wheels on the road but I only jacked one). So differential ratio is 20/5.1=3.92. So bog standard.

 

I'm now intrigued as to why my speeds don't work out. Something must be reading or measuring wrongly; the tyres are new and a known size, the speedo checks out against GPS and Pi hasn't changed very much since the time of ancient Greece ( 😬) - so I guess that leaves the tachometer.

 

I'll cross-check it against the ECU's reading of RPM via OBDII when I get a chance but I suppose the difference between 4.11 and 3.92 is less than 5% so wouldn't need to be far out. I am correct in assuming that the tachometer reads the RPM independently and directly from ignition-timed pulses from the ECU, and not from some kind of digital output of the ECU's calculated figure, aren't I?

[Jonathan Kay]

Quoting revilla: 

I'm now intrigued as to why my speeds don't work out. Something must be reading or measuring wrongly; the tyres are new and a known size, the speedo checks out against GPS and Pi hasn't changed very much since the time of ancient Greece

Was the car ever registered in Indiana?

 

I'm trying to think of other possible explanations... what is your maximum speed?

 

Jonathan

 

Edited by - Jonathan Kay on 18 Apr 2014 08:48:44

[Wrightpayne]

Speedo accuracy has never been a strong point also affected by changing size of wheels and tyres.

 

On the older cable speedos you can get different drives for the gearbox with more or less teeth. The electronic ones can be programmed differently using a number of toggle switches on the back of the speedo IIRC from what I've read on blatchat.

 

I have a holux gps sport for accurate speed.

 

Ian

[revilla]

@ Jonathan - Mmmm Lorentz contraction and time dilation, hadn't really thought of those as possibilities! Not sure what my maximum speed is but for some reason I was asked to shift a load of neutrinos from CERN to Gran Sasso a while back and we weren't half shifting!

[mav]

my tacho reads between 250 and 500 rpm too much when i compared its reading against the emerald i had fitted for a short while...

[MADMALC]

YHM

[revilla]

BM received. Thanks Malcolm, I'll double check all my measurements and calculations.

[revilla]

My original spreadsheet made no allowance for static & dynamic rolling distortion of the tyre. Reading around this on the web, most sites (such as here) seem to suggest that this is difficult to model precisely but an industry standard factor of 0.96, applied to the static unloaded circumference calculated as I was doing previously, will be pretty close. Plugging a 0.96 factor into the spreadsheet leads to pretty good agreement with my measured figures and a 3.92 diff (it's the missing "less than 5%" factor I mentioned in my earlier post and incorrectly assumed was the difference between a 4.11 and 3.92 diff).

 

Update Spreadsheet here