Thanks guys for the help,
Doing things backwards seems to be my specialty lately.
The tranny is back in the car so I can?t redo the revolutions directly on the
tranny. I do have the rear end on jack stands putting on some new brake lines
so can I pull the spark plugs and just rotate the rear wheel while counting the
crank pulley revolutions to get the ratios for each gear?
I did count the teeth on the gears, but I didn?t measure the diameters because
I didn?t think to do that.
1st = 30 teeth
2nd = 26 teeth
3rd = 22 teeth
4th = 19 teeth
Reverse = 33 teeth
Pics below.
The ratios I pulled from the mintylamb gearspeed calculator showed for a GT6
gearbox (for KC/KF/KE gearboxes) are:
1st 2.646
2nd 1.779
3rd 1.254
4th 1.0
These are real close to the ratios shown for a TR7 4 speed gearbox. I have a
refurbished (by TRF) TR7 4 speed awaiting an adapter to my Spitfire MK III bell
housing. So if/when I need to replace the GT6 tran my plan is to use the TR7 4
speed as it is supposed to be stronger. But I?d like to know what ratios I have
now before that decision point is reached.
1st 2.66
2nd 1.78
3rd 1.26
4th 1.0
I used a 3.89 welded diff initially with this gearbox but I switched to a
welded 4.11 which is now in the car.
Thanks again.
Bud
Extreme Parts Racing (more than just a haircut)
71TR6 CC57365 (Good 6)
71 Spitfire MK IV Race Car #3
69 Spitfire MK III (back up FE engine/dinghy car)
93 Minnie Winnie Race Support Vehicle
77 Z-50A Hardly Davidson Honda Mini-Trail Bike (Triumph Pit Bike)
On Jan 11, 2018, at 7:31 AM, Ron KRANTZ <rkrantz77 at comcast.net> wrote:
> Bud & Bob,
>
> Your going at the ratios backwards. Mark the output flange and tail housing
> and the input shaft. Turn the OUTPUT flange 1 revolution while counting the
> input shaft revs. The number of input revs is the ratio. example, 1 output
> rev has input turning 4 revs = 4.00:1 ratio.
>
> Not ALL transmissions have a 1:1 4th gear ratio. I build a racing trans that
> 4th gear can be overdrive, underdrive or 1:1. Spitfires do not have a good
> selection of differential ratios which is why I designed and built my trans.
> I can also give you any 1st, 2nd, 3rd gear ratio you want. I use
> Hewland/Webster gears and best of all it is a bolt in trans for any Spit.
>
> Ron Krantz
>
> ReK Racing
>
>> On January 11, 2018 at 8:19 AM Robert Lang via Fot <fot at autox.team.net>
>> wrote:
>>
>> Hi Bud,
>>
>> Those numbers look fishy. 4th gear should be 1:1, e.g. turning the input
>> shaft one turn should yield one turn of motion at the output flange. The
>> lower gears would turn the output flange less than one turn. A 1.33:1
>> overall third gear ratio would see the output flange turn 3/4 of a turn per
>> one turn on the input shaft. Similarly, a 1.75:1 2nd gear would result in
>> slightly more than half a turn (.57 to be exact) of the output flange per
>> turn of the input shaft. Note that 1.33 and 1.75 are random numbers shown to
>> illustrate and not actual gear ratios. I think the GT6 used 1.78 and 1.25
>> for 2nd and 3rd. At least the Mk3 GT had those ratios.
>>
>> What you reported seems to be the opposite of my example.
>>
>> It is possible that you are seeing some sort of "sticktion" where gooey oil
>> in the gearbox is coming into play. But the key is that 4th should always be
>> 1:1 unless there's an overdrive and that O/D is engaged.
>>
>> But the only way to know for sure of what the ratios are is to count teeth
>> and "do the math".
>>
>> Can you remove the top cover and count the teeth on any of the gears on the
>> main shaft? That should make it easier to identify what gear clusters are in
>> there. You probably only need to count the teeth on the main shaft but I
>> know squat about GT6 gearbox internals other than they are more
>> Spitfire-like than TR-like so the lay shaft gear ratios may be fixed as
>> they are on the Spitfire. The point being that there are only so many ratios
>> available that would fit in the gearbox, so it should be fairly easy to
>> figure out what you have there.
>>
>> Regards,
>> Bob Lang
>>
>>
>>
>>
>>
>> On Wednesday, January 10, 2018 9:09 PM, Bud R via Fot <fot at
>> autox.team.net> wrote:
>>
>>
>> Listerati,
>>
>> I?ve been trying to figure out what gear ratios I have in the gearbox that
>> came with my race car but don?t know how to do the proper math. The PO told
>> me it was a close ratio GT6 gearbox but one never knows until you actual
>> check.
>>
>> I at least took some ?data? while it was out so I?m hoping someone can tell
>> me how to figure out the gear ratios and if it is indeed close ratio GT6
>> gears or whatever it might be.
>>
>> When the input shaft was turned one full rotation the output shaft turned
>> the following amount.
>>
>> 1st gear it turned 1/4 turn
>> 2nd gear it turned 1 1/8 turns
>> 3rd gear it turned 1 5/8 turns
>> 4th gear it turned 1 3/4 turns
>>
>> Any help on how I should do the math is appreciated.
>>
>> Also does anyone know what happened to the minty lamb gearspeed website? It
>> does not appear to be up any longer and I used to play with it a lot. Very
>> useful site that now appears gone. Waaahhhh.
>>
>> Thanks
>> Bud Rolofson
>>
>> Extreme Parts Racing (more than just a haircut)
>>
>> 71TR6 CC57365 (Good 6)
>> 71 Spitfire MK IV Race Car #3
>> 69 Spitfire MK III (back up FE engine/dinghy car)
>> 93 Minnie Winnie Race Support Vehicle
>> 77 Z-50A Hardly Davidson Honda Mini-Trail Bike (Triumph Pit Bike)
>>
>> _______________________________________________
>> fot at autox.team.net
>>
>> http://www.fot-racing.com
>>
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>> at yahoo.com
>>
>>
>>
>
>
>
>> _______________________________________________
>> fot at autox.team.net
>>
>> http://www.fot-racing.com
>>
>> Donate: http://www.team.net/donate.html
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