Theoretic Technical question about gearbox...

[Nappe1]

Hi,
As I am now finalizing one version of that GT Legends simulation version of GT-R, I am trying to make it as competitive as possible. For this I have been making gearbox and right now it looks like this:

input / drop gear ratio:
(20/27)

1st: (19/35) --> 2.487
2nd: (21/26) --> 1.671
3rd: (25/23) --> 1.242
4th: (27/20) --> 1.0

and for the final gear driver / player has full range to choose from:
(8/37) --> 4.625
(10/43) --> 4.30
(9/37) --> 4.11
(10/41) --> 4.00
(9/34) --> 3.778
(11/41) --> 3.727

...and this is from PR91W homologation as alternative gear set. However Bellett 1600GTs in thier different forms (PR90, PR91) also have homologated two more sets, which have input ratio of 19/28.

So, can you change the input gears without changing the layshaft and inputshaft in bellett? My knowledge is rather limited here, I do know that in Mini you can change the "drop gears" which change the ratio of input and layshaft, but Mini differs a lot from Front Engine, Real Wheel Drive cars.

and if you can, is the car stil valid for Group 2 historics as the homologation does not mention this combination?

[dave]

Hey surely someone can help Nappe1 out with this?

I've not pulled any of my cars apart enough to know the answer!

Anyone?

Cheers,

Dave

[Glenn]

my PR60 Sports lists as follows:

1 3.207
2 1.989
3 1.356
4 1.00
Rev 3.5962

Diff 3.727

I have no idea though what can be done in changing gearsets.

Can anybody tell me how my ratios compare to a 1600 GT ?

[JT191]

Specs for GTs:
http://www.isuzuperformance.com/isupage ... belgt.html

September 1969 - September 1973 Bellett 1600 GT PR91W

Gear Ratios
First 3.467
Second 1.989
Third 1.356
Fourth 1.000

Axle Ratio
Final Drive Ratio 3.727

[Glenn]

Specs for GTs:
http://www.isuzuperformance.com/isupage ... belgt.html

September 1969 - September 1973 Bellett 1600 GT PR91W

Gear Ratios
First 3.467
Second 1.989
Third 1.356
Fourth 1.000

Axle Ratio
Final Drive Ratio 3.727

interesting, only a 1st gear ratio change on the so-called 'close ratio' GT box.

Some interesting info in the link too.

I was under the impression that the later cars like my 72, used unleaded fuel. Anybody know the fuel situation in Japan at that time ?

[JT191]

I was under the impression that the later cars like my 72, used unleaded fuel. Anybody know the fuel situation in Japan at that time ?

I ran a couple searches...
One page said unleaded was first introduced in Japan in April of 1972.
Another said they began refining unleaded in Japan in 1975. So maybe the "introduced" gas was imported?
The last said that unleaded became widely available in Japan in 1976.

Designed-for and capable-of-running-on might be two different things, if they switched over the rings and seals a few years early, anticipating the change.

The 1969 161SS engine appears to be leaded only. Or someone upstream seemed sure, since it included jugs of lead additive.

[Glenn]

I was under the impression that the later cars like my 72, used unleaded fuel. Anybody know the fuel situation in Japan at that time ?

I ran a couple searches...
One page said unleaded was first introduced in Japan in April of 1972.
Another said they began refining unleaded in Japan in 1975. So maybe the "introduced" gas was imported?
The last said that unleaded became widely available in Japan in 1976.

Designed-for and capable-of-running-on might be two different things, if they switched over the rings and seals a few years early, anticipating the change.

The 1969 161SS engine appears to be leaded only. Or someone upstream seemed sure, since it included jugs of lead additive.

I can't figure anythiong from the japanese owners manual I have for the PR60.

so could well be leaded then. Most of us here run 98 octane unleaded (no leaded freely available here) Some use a lead replacment additive i think. If an engine was to be re-built most would have hardened valve seats fiited. Or, like most of us, with very low milage per year, just accept unleaded.

glenn

[Nappe1]

Specs for GTs:
http://www.isuzuperformance.com/isupage ... belgt.html

September 1969 - September 1973 Bellett 1600 GT PR91W

Gear Ratios
First 3.467
Second 1.989
Third 1.356
Fourth 1.000

Axle Ratio
Final Drive Ratio 3.727

Actually, this is the standard street box for GTR. homologation document (for PR91W) knows another, much closer ratio alternative box, which was most likely used for factory racing cars in the late 60's - early 70's:
1st: 2.487
2nd: 1.671
3rd: 1.242
4th: 1.0

for PR90, there's huge selection of gearsets in homologation documents:
in 1964 homologated gear sets:

Code: Select all

***   set1   set2   set3   set4
1st   3.444   3.444   3.17   3.05
2nd   2.133   1.82   1.96   1.89
3rd   1.387   1.28   1.276   1.33
4th   1   1   1   1

and in 1966 they homologated PR90 again, this time with just two gear sets:

Code: Select all

***    set1   set2
1st   3,444   3,444
2nd   2,133   1,82
3rd   1,387   1,28
4th   1   1

so, if your PR90 is 1965 or earlier, and you are in to historic racing, you have 2 valid gear sets more than if the car is from 1966. (not to mention that most likely it will end up in class where cars are from 1966 to 1970, making the opposition much more faster...)

...from PR91 I have only homologation documents from 1969, in which it declares two gearsets:

Code: Select all

***   set1   set2
1st   3,207   2,487
2nd   1,989   1,671
3rd   1,356   1,242
4th   1   1

...and from same year, PR91W homologation, it's same two as the PR91:

Code: Select all

***   set1   set2
1st   3,207   2,487
2nd   1,989   1,671
3rd   1,356   1,242
4th   1   1

I do know that 1800GT was also homologated, but I don't have the documents. It would be interesting to see, if the higher low end torque has made any changes to gear ratios... If you look at the exact tooth counts (if you have the homologation docs or the gearbox spec sheets, I am a bit lazy to write it all here... you see the toothcounts of ultra short box in my first post as example.), the differences between the sets aren't really big and technically, all these (excluding the ultra short one starting with 1st being 2.487) use rather small different set of physical gears inside the box.

I am working hard to get some version of the GT Legends simulation car done before next season in our online championship racing series starts in Melbourne mid march and hopefully Isuzu natches few drivers to drive the car.

What comes to my original question, I got that figured out; the input ratios most likely are not interchangeable, but you need to change all gearbox shafts (input shaft, lay shaft and output shaft) at once.