10 replies to this topic

[Curley]

Just trying to get some 2nd opinions. I have a stripped out track/summer day car that's currently producing around 92bhp at the wheels. The problem is the car is so light I don;t need 1st gear - it happily pulls away in 2nd - and you've quickly run out of gears by about 70 mph.

 

Given it's a track car, the cam comes onto power around the 3K mark, would you go for the GT A+ gears with it's longer 1st gear or bite the bullet and go for a straight cut gear set?

 

I should point out I'm considering adding 50 shot of N2O and expect my current box to give out at around 100-120bhp.

[nicklouse]

What final drive?

And you run out of revs at 70mph?

[Curley]

Not exactly sure to be honest, I did not build the gearbox. I'm not convinced my speedo is reading correctly but 3K is around 60mph, 4k is about 70mph and 5.5K is 90mph. I've considered moving to a 3.2 or 3.11...

[nicklouse]

Not exactly sure to be honest, I did not build the gearbox. I'm not convinced my speedo is reading correctly but 3K is around 60mph, 4k is about 70mph and 5.5K is 90mph. I've considered moving to a 3.2 or 3.11...

Sorry you estimates can not be correct. IF those are in fourth and 3k is 60 then 4k should be 80 and 5.5 110mph

If 5.5 gives you 90 then 3 will give 49 and 4 will give 65

Or if we take 4 giving 70 then 3 will give 52.5 and 5.5 will give 96.

The ratio between RPM and MPH is fixed for any given gear.

As you can see you have a big difference just on your guesses. 90 to 110!

Final drive can be worked out with the plugs out and the car in 4th. And measuring the ratio between wheel rotation and engine rotatio.

I would have thought there would be a how to on here.

There is no point suggesting anything until you know what you gave got.

Personally I like the old remote housing and run a 4.3 (or is it a 4.1) was a few years ago I fitted it.

Oh and wheel size makes a difference.

[jaydee]

Another thing to consider is with a 3.2 or 3.1 your engine might fell off cam

[carbon]

And if you don't come up onto the cam until 3000rpm then this won't be so user friendly with GT ratios.

 

With 4-syncro boxes (pre A+) the only difference between std helical and GT ratios is the closer spacing between 3rd and top in the GT box.

[Curley]

 

Not exactly sure to be honest, I did not build the gearbox. I'm not convinced my speedo is reading correctly but 3K is around 60mph, 4k is about 70mph and 5.5K is 90mph. I've considered moving to a 3.2 or 3.11...

Sorry you estimates can not be correct. IF those are in fourth and 3k is 60 then 4k should be 80 and 5.5 110mph

If 5.5 gives you 90 then 3 will give 49 and 4 will give 65

Or if we take 4 giving 70 then 3 will give 52.5 and 5.5 will give 96.

The ratio between RPM and MPH is fixed for any given gear.

As you can see you have a big difference just on your guesses. 90 to 110!

Final drive can be worked out with the plugs out and the car in 4th. And measuring the ratio between wheel rotation and engine rotatio.

I would have thought there would be a how to on here.

There is no point suggesting anything until you know what you gave got.

Personally I like the old remote housing and run a 4.3 (or is it a 4.1) was a few years ago I fitted it.

Oh and wheel size makes a difference.

 

Wow - that's pretty cool. Math rocks!

 

You're right about the estimates - I should have said the speed gear is almost definitely not the right one for my gauge (some old Smiths, probably jaguar) - another reason why I want to take the gearbox off.

 

Looking at them all written down like that i'd say it's closer to your numbers than mine. The car's running on 165 on a 12" rim. I'll have a look for that guide and let you know what I've got. Either way the box is coming off this winter :)

[Curley]

And if you don't come up onto the cam until 3000rpm then this won't be so user friendly with GT ratios.

 

With 4-syncro boxes (pre A+) the only difference between std helical and GT ratios is the closer spacing between 3rd and top in the GT box.

 I understood the GT gears to have a more usable 1st gear. In that for a given RPM you'll be doing a fast speed than a std box. Is this not correct?

[Gulfclubby]

I can only recommend doing the math here: http://guess-works.com/Tech/ratio.htm

Works out much simpler than doing it by hand.

[Guess-Works.com]

And if you don't come up onto the cam until 3000rpm then this won't be so user friendly with GT ratios.

 

With 4-syncro boxes (pre A+) the only difference between std helical and GT ratios is the closer spacing between 3rd and top in the GT box.

 

Not true,

 

The 1st motion shaft has one more tooth, which means that the laygear turns a little faster than the equivalent standard A series gearbox... The means that 1st 2nd and 3rd are brought closer together, and consequently closer to 4th..

 

The same principle applies to the 4 piece Straight cut close ratio clubman kit... This uses the standard 31 tooth 1st gear but spins the laygear at a faster rate to achieve the near same ratios to the original ST Straight cut gearset without having to change the 1st and reverse idler.. Hence making it cheaper.

 

on the flip side,

 

The wide ratio A+ gearset has one less tooth than the standard A+ set on the 1st motion shaft, which means the laygear turns slower and widens the gear ratios

[carbon]

I will rephrase my earlier comment...With 4-syncro boxes (pre A+) the difference between std helical and GT ratios is the closer spacing between 3rd and top in the GT box. The spacings between 1st and 2nd gears, and 2nd and 3rd gears are the same (ie change in engine rpm is the same)

 

4-syncro A-series std : 3.525, 2.218, 1.433, and 1:1

4-syncro A-series 'GT' : 3.330, 2.094, 1.353, and 1:1

 

Relative differences between ratios (ie how close are the gears spaced)

4-syncro A-series std : 1.59, 1.54, 1.43

4-syncro A-series 'GT' : 1.59, 1.54, 1.35

 

As you say, it's all down to that one tooth difference on the 1040 layshaft.