Final Drives, are they interchangeable?

[KoogiA17]

Does any one know if you can fit the final drive from any 900 box into a later 99 box or vice versa? Are the final drives a universal part?

[DrewP]

They do interchange, but depending on what years you are interchanging you may have to move the C900 differential basket and tripod joints over to the 99 for it to work - the offset of the ring gear changed.

Also depending on the year the pinion bearings and brackets are different, but *should* plop in. Which means you may also have to swap in the C900 gear stacks.

I put an early style '73 diff and drivers into a '78 box to get my yellow car driveable right when I got it, and it mostly swapped right in, the early C900 stuff should be the same situation.

[Luke]

What part are you refering to as final drive?
Usually the final drive is a numerical value (ratio) that combines the ratios of the primaries, gears, and ring and pinion ratios into one... basically the overall ratio between the engine and the road...

[KoogiA17]

Sorry I meant the ring and pinion wheels in the rear of the box. I used it as that is what Saab call the ring & pinion in the list of gearbox ratios.

Thanks!

My box is from a 1980 99, G34601 and was hoping that I could slot in a 3.67 final drive from an 81-88 900 gearbox. The less I have to change the better.

[DrewP]

The diff carriers in both those might be the same - you'll still have to take the entire box apart to swap the output/pinion shafts though.

So you might just be able to put the internals of the newer box in your 99 case?

You want to keep it a 4-speed? No interest in just putting in a 5-speed?

[KoogiA17]

The diff carriers in both those might be the same - you'll still have to take the entire box apart to swap the output/pinion shafts though.

So you might just be able to put the internals of the newer box in your 99 case?

You want to keep it a 4-speed? No interest in just putting in a 5-speed?

Yeah, I know about the pinion shafts. The ring and pinion are supposed to be a matched pair.

Nope, no interest in a 5 speed, want to keep it a 4 speed but give it the longest possible ratios.

[nutcase]

5 speed pinion shaft is longer?

[KoogiA17]

5 speed pinion shaft is longer?

The plot thickens!

Have found a diagram of the 5 speed box internals and compared it to a 4 speed and the pinion shaft does appear to be longer. I think you are correct.


Therefore, would need to find a 4 speed box with a 3.67 final drive. Almost makes swapping over the ring and pinion pointless apart from the fact that my 99 box is a chillcast.

[simo96]

What chain drives have you got?

[KoogiA17]

What chain drives have you got?

The plan is to have type 8s (if and when I find any), otherwise it is type 7s

[Luke]

The 4 and 5 speed pinion shafts and definately not interchangeable... like you noticed, the 5 speed one is much longer... on the 4 spd (chain drive primary) the input shaft is integrated with the drivegear for the intermediate gears, on the 5 speed that gear is a separate piece and slides over the longer pinion shaft.
I think a 4spd with type 8 primaries would likely be very slow and not fun to drive with anything but a huge HP motor with a very flat torque curve.

[Crazyswede]

The 4 and 5 speed pinion shafts and definately not interchangeable... like you noticed, the 5 speed one is much longer... on the 4 spd (chain drive primary) the input shaft is integrated with the drivegear for the intermediate gears, on the 5 speed that gear is a separate piece and slides over the longer pinion shaft.
I think a 4spd with type 8 primaries would likely be very slow and not fun to drive with anything but a huge HP motor with a very flat torque curve.

I put a Type 6 primary in a 4 speed that was under a low power NA 16v a number of years ago. First gear was very tall and the car could cruise on the highway at 75 mph in 4th gear at about 3000 rpm. It was not a very good setup with low hp.

[KoogiA17]

The 4 and 5 speed pinion shafts and definately not interchangeable... like you noticed, the 5 speed one is much longer... on the 4 spd (chain drive primary) the input shaft is integrated with the drivegear for the intermediate gears, on the 5 speed that gear is a separate piece and slides over the longer pinion shaft.

Thanks. Problem comes with trying to find a 4 speed box with 3.67 drive.

I think a 4spd with type 8 primaries would likely be very slow and not fun to drive with anything but a huge HP motor with a very flat torque curve.

Give that man a cookie (or whatever your prefered sweet delicacy is)

Have done the math and you would be amazed how rapid such a box can be.

[turbo stephan]

What part are you refering to as final drive?
Usually the final drive is a numerical value (ratio) that combines the ratios of the primaries, gears, and ring and pinion ratios into one... basically the overall ratio between the engine and the road...

HA, that's a good point. I recently created an Excel sheet which allows to select gearbox, gear # , RPM (in 250 steps) and tire size, and then give you the resulting speed in kph and mph. I noticed that I called the pinion and gear ratio final drive ratio as well, probably because somebody here called it that at some point. Anyway, good that you mentioned it because certain phrases or wording can often be very confusing.

...
I think a 4spd with type 8 primaries would likely be very slow and not fun to drive with anything but a huge HP motor with a very flat torque curve.

I agree and it confirms what I can see in the above mentioned Excel sheet. For example, taking a 900 with the 3.67 pinion and gear ratio and combining it with a primary 7 leads to almost the same speed as selecting a 3.89 pinion and gear ratio and matching it with a primary 8. Theoretically, combining a 3.67 pinion and gear with a primary 8 would give the longest ratio and highest speed per rpm. However, as Luke mentioned, this would only make sense if you have quite a bit of power and would be especially bad if you're in city traffic.
On another note, IIRC, the very first 900 gearboxes up to 1980 came at least in Europe with the 3.89 pinion and gear set.

[nutcase]

Wonder if there's enough material to machine a 5 speed pinion shaft down to suit a 4 speed box...

I understand early boxes were 3.89 too.