What diff ratio?

[HK1837]

With those tyres the tailshaft should be doing 2270rpm at 100km/h, so in 4th with locked converter I get 1580rpm with a 0.696:1 4th. 3.08 will change that to 1750rpm. If ypu are running up around 1900 the converter musn't be locking. Is the engine standard? Exhaust? Do you have the right speed sensor and correct drive gear for the speedo cable?

A standard EFI 5.0L should easily cruise a HZ wagon at 100km/h at around 1600rpm but if the cam has changed it might simply need a new converter and the Memcal changed to suit (tune and torque converter lockup). If cam is too big it might need a lower diff.

[James]

VS SS engine with a small cam, extractors. VN g/box, speed sensor from V6 conversions which allows for cable speedo too. Not the right speedo drive it's reading probably about 5km/h lower at 100km/h. Was told Memcal had been changed to suit.

Reckon I need a lower diff mainly - what rev's should I be doing if it was 3.55?

Thanks for the info

[HK1837]

Just divide 1580 by 2.78 and multiply by 3.55, gives you 2017rpm. This is with a locked converter.

If your speedo is out then the signal to the ECU will be out. What you might have to do (regardless of the diff) is get the speedo right, and then get the memcal program looked at the see what ppk constant is used. ppk is pulse per kM, which is the pulses from the speed sensor, could be 2 pulse per rev or 10 pulse per rev. It might have to be adjusted to account for the tyre size and diff ratio so the ECU knows what speed the car is doing so it knows when to lock up the torque converter.
The cam might be too big for a standard converter too.

[James]

Yeah righto that makes sense. Thanks

[HK1837]

You can figure out the approx engine speed from the tyre size yourself if you need.

Just convert tyre size into diameter (eg 245/60/14, DIA = (254 x 0.60 x 2) + (14 x 25.4))
Then get the circumference, = pi x DIA.
Convert circumference into metres (divide by 1000).
Get the road speed in km/h into metres/min (multiply by 1000 and divide by 60).
Divide the road speed in m/min by the tyres circumference in m. This gives you the axle rpm.
Multiply the axle rpm by the diff ratio. Gives you the tailshaft rpm.
Multiply by gear ratio, gives you the engine revs on a manual or the rotor speed of a non locked torque converter on an auto (there will be a converter slip value if not locked).

For your case just assume it is a manual for a locked converter situation.

[matt159]

Not sure where you are but just found this
http://bit.ly/1c7NlQe
Sorry if it doesn't work, I'm on my mobile

[Blocker]

Has anyone seen a diff calculator online?

[Dr Terry]

The ideal rear axle would have been 3.08 with either 14" (70 series) or 15" (60 series) tyres, which is very close to how it was in the Commodore from the factory. However you mentioned that your engine has a 'mild' cam & extractors, this will push the available torque further up the rev range.

A stock 5.0 EFI will cruise happily at 110 km/h @ 1800 rpm, but your combination will struggle at those revs. You will need to go to 3.36 or even 3.55, depending on how 'mild' the cam profile is.

Dr Terry