Overdrive - revs difference when on and off

[Mr Elsie]

Hi

I'm not quite sure what diff I have on the gt6 but when I engage the overdrive switch there isn't a massive difference between the revs. Does the type of diff fitted make a difference?

[JohnD]

In fourth, at about 60mph, you should see the revs drop by about 1000 (ball park)
If there is no discernable difference, your O/d isn't working.
Try engaging O/d when parked, ignition on, engine off, in third/fourth gear.  There should be loud click as the O/d solenoid actuates, but Canley's have a troubleshooting guide on their website.  Work out which you have here:
http://www.canleyclassics.com/?xhtml=xhtml/infodatabase/overdrives.html&xsl=infodatabase.xsl

The diff ratio is irrelevant.
John

[Mr Elsie]

Cheers john. Od is definitely working as there is a noticeable drop in revs but doesn't appear to be much. Tbh not got much to compare it against so what I'm experiencing May be ok.  One related question, if I do have a diff fitted that is suited for acceleration presume that if I then go into overdrive the perceived revs will be higher.  

[thebrookster]

Yes and No.

The diff does not effect the range through which the O/D works. If you engage the O/D at 4500rpm, the drop will be exactly the same, regardless of the diff fitted.
However, if you are basing your observation on speed, and are simply putting the O/D in at say, 60mph, then what you see on the rev counter will differ, but this will happen regardless of the diff fitted.

In basic sequence, your engine has an rpm output. This is transmitted to the gearbox, which applies a ratio to this rpm, thereby changing it. Normally this ratio makes the gearbox output shaft spin slower than the input. This gives less speed, but far better torque (hence why setting off in 1st is OK, but if your try setting off in 4th you stall!!)
The O/D takes its rpm from the gearbox, but alters the speed the opposite way from the gearbox, it increases your output rpm. This give closer step changes than the gearbox by itself, but has most use in 4th gear, which is normally a straight through ratio (i.e. what goes in comes out the same). The diff is the final piece that alters what rpm your wheels see. It simply takes speed from the propshaft, and applies a set ratio to it which doesn't change.

So yes, your perceived revs will be higher/lower FOR A GIVEN SPEED. This is the problem with perception, we all see it differently :)

I do hope that this makes some sense, and that I have not rabbited on too much!!

Cheers,

Phil

[Richard B]

If its a D'Type then the ratio is 22%


Sort of a 4th and a 1/2 gear