14 replies to this topic

[madmax]

Wondering what diff ratio to run in my lh torana am doing a ls1 conversion with a 2002 t56 Camaro gearbox to a nine inch conversion using a true trac centre with Simmons v5 16 inch wheels with 55 series tyres setting up a diff looking to be doing a lot of highway driving and street anyone help with ratios
Thank you

[Bigfella237]

If you give me the exact rear tyre size I can crunch some numbers for you?

 

What are the overdrive ratios in that trans? I'd guess 0.74 & 0.50 but some were 0.80 & 0.63

[madmax]

It's from a 1993 to 2002 camaro lt1
Ratios are 2.66/1.78/1.30/1.00/0.74/0.50
Tyre size is 245/55/16 inch Simmons v5
Thank you

[myss427]

I have 3.9's with a .60 6th gear in mine and under 1800rpm @ 100kph so .50 would be 1650ish.

[Bigfella237]

Okay, so your 6th gear (0.50:1) is fairly tall anyway so cruising in 6th at 100km/h with a range of diff ratios gives you:

 

Tyre Size = 245/55R16

Tyre Diameter = 675.9mm (26.61")

Circumference = 2123.4mm (83.599")
Tyre Revolutions = 470 per km (757 per mile)

Gear Ratio = 0.50:1

Diff Ratio 2.60:1 = 1018 RPM @ 100km/h
Diff Ratio 2.78:1 = 1089 RPM @ 100km/h
Diff Ratio 3.08:1 = 1206 RPM @ 100km/h
Diff Ratio 3.36:1 = 1316 RPM @ 100km/h
Diff Ratio 3.55:1 = 1390 RPM @ 100km/h
Diff Ratio 3.70:1 = 1449 RPM @ 100km/h
Diff Ratio 3.90:1 = 1528 RPM @ 100km/h
Diff Ratio 4.11:1 = 1610 RPM @ 100km/h

 

I found a GM RPO code of "MM6" which seems to match your quoted gear ratios, so the graph on gear spacing is...

 

 

...and with a shift point of 5000 rpm I ran a few different diff ratios to give you a feel of where the gear changes will be...

 

 

 

 

 

 

 

What you're looking for above is not so much 1st gear, that will be next to useless with the lower ratios, but for example, you would be able to use 2nd up to 100km/h with a diff ratio taller than 3.55:1, but once you get into the lower ratios, 2nd will run out of legs down in the 80's. This is maybe not so important on the street, but if you're tuning the car for a specific track (or a certain piece of road ) it can make a big difference.

 

Now comes the problem, if using a standard 5.0L V8 Torana driveshaft (2.75" dia, 49.95" long, 1310 unis) you have a critical driveshaft speed of 7192rpm (that's the speed at which the harmonics meet the natural resonance and the shaft shakes itself to pieces).

 

Lets say a theoretical top speed of 240km/h (150mph), then we get driveshaft RPMs of:

 

Diff Ratio 2.60:1 = 4888 RPM @ 240km/h
Diff Ratio 2.78:1 = 5226 RPM @ 240km/h
Diff Ratio 3.08:1 = 5790 RPM @ 240km/h
Diff Ratio 3.36:1 = 6317 RPM @ 240km/h
Diff Ratio 3.55:1 = 6674 RPM @ 240km/h
Diff Ratio 3.70:1 = 6956 RPM @ 240km/h
Diff Ratio 3.90:1 = 7332 RPM @ 240km/h
Diff Ratio 4.11:1 = 7727 RPM @ 240km/h

 

So you can see that any diff ratio lower than about 3.70:1 is going to be exceeding the critical speed of the driveshaft with your size tyres at that speed. Limiting your top speed to say 200km/h brings all those diff ratios in under the critical speed, but I think you need to set a speed you will never reach and 200 seems a little low.

 

Then the ½ true critical speed would be 3589 RPM, so you need to avoid turning the driveshaft at this speed anywhere in your cruising range, let's say up to 120km/h:

 

Diff Ratio 2.60:1 = 2444 RPM @ 120km/h
Diff Ratio 2.78:1 = 2613 RPM @ 120km/h
Diff Ratio 3.08:1 = 2895 RPM @ 120km/h
Diff Ratio 3.36:1 = 3158 RPM @ 120km/h
Diff Ratio 3.55:1 = 3337 RPM @ 120km/h
Diff Ratio 3.70:1 = 3478 RPM @ 120km/h
Diff Ratio 3.90:1 = 3666 RPM @ 120km/h
Diff Ratio 4.11:1 = 3863 RPM @ 120km/h

 

So once again, with your tyre size, you need to stay above about 3.70:1 to avoid getting into harmonic vibration territory. If you plan on cruising at speeds any higher than that, you may need to use taller diff gears to move away from that ½ true critical speed figure.

 

It doesn't matter so much accelerating through this ½ true critical speed, you just don't want to be sitting around it for any length of time.

 

And of course a shorter length, or larger diameter, or lighter weight driveshaft will typically increase the critical speed, but I'm just working off basically a worst case scenario, well unless you were to use the 4.2L driveshaft (which is only 2.5" dia), then the critical speed drops to 6548rpm!

[madmax]

So a gear ratio of 3.6 would be a good cruising ratio with 16 inch wheels
Thank you big fella 327 I will order it tomorrow

[Ice]

3.55 would be good

[S pack]

3.08:1 would be better.

[Heath]

Bigfella237 mate you're a bloody champion. Above and beyond.

[Bigfella237]

Happy to help, and happy to run the numbers for anyone else's particular setup, spare time pending of course.

 

Note that the rear tyre size and diff ratio are the only things that actually affect driveshaft speed, so the above figures will of course be different for each setup. For example, the difference in rotation between a standard Torana SL/R tyre (185/70R13) and the tyres above is 70 revolutions per kilometre, that's roughly a 13% difference, which changes things a fair bit.

 

BTW that got me thinking about how the factory set these things up with such small tyres and how they avoided the critical driveshaft speeds, but then I realised that there weren't any overdrive gearboxes back then, so with 4th gear (1:1) being straight through the gearbox, engine RPM equals driveshaft RPM and there wouldn't be many engines straight off the production line that could pull over 6,500rpm in 4th gear!

 

And remember that any race cars that could rev like that typically had really tall diffs for high speed tracks like Bathurst. I think I remember hearing that Brocky used to leave the A9X in 2nd gear all the way up mountain straight on a qualifying lap. Those things must have been a bastard to get off the line on a standing start with such tall gears?

[madmax]

Thanks bigfella 327 looking up specs on a vx ss commodore they ran a ratio of 3.45 with a manual
gearbox and 3.08 on a auto running a 16 inch mag with 55 series tyres they have the same ratio gearbox
I am running from a camaro
First: 2.66
Second: 1.78
Third: 1.30
Fourth: 1.00
Fifth: 0.74
Sixth: 0.50
With a 3.45 ratio so I will be ordering that ratio for I will be doing a lot of highway kms not any drag strips
Ordered parts have turned up just need diff and rust repair to start hopefully my shed gets built soon
will post photos soon

[myss427]

Just noticed your tyre diametre of 676mm, thats a very tall tyre for a Torana.

[myss427]

635ish is legal for an A9X, might need some rego wheels?