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Wondering if Spit has the right diff fitted?


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yes thats the correct one for your car so if a gearbox designed for a 3.89 diff ratio has been installed you will have a speedo which under reads by about 4 mph at 3000rpm (assuming perfect speedo and tacho calibration).

Unfortunately the gearbox number cant be seen wthout removing the interior cover...

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to get the turns per mile  ( TPM) 

mark out 52.8ft down the road 

undo speedo cable from the back of the speedo

stick a card needle on the drive cable 

push/drive the car accurately the 52,8ft 

count the turns on the needle 

multiply turns by 100 =  the TPM 

eg a 1000 tpm speedo should give you 10 turns of the needle (revolutions of the cable) 

 

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I may be confusing myself here but surely if the gearbox was from a car with a 3.89 diff and my car has & 3.65 diff then the Speedo would read too fast, not slow. 

For a given road speed with the 3.89 diff the  gearbox output would make a certain number of revolutions. If the diff was changed to 3.65 then the gearbox & Speedo drive would turn faster for the same road speed.

 

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With a 3.98 diff for every 3000revs in top gear, the wheels will revolve 771 times.  Assuming a rolling circumference of 6 feet, the car will travel 1542 yards or 0.876 miles.  So at 3000 rpm the car will travel at 52.6 mph giving a overall gearing of 17.5 mph / 1000rpm.

With a 3.65 diff the wheels will revolve 3000/3.65 = 822 times and the car will travel 1644 yards or 0.934 miles. So at 3000rpm the car will travel at 56 mph giving an overall gearing of 18.7 mph/1000rpm.

Therefore changing from the intended Dolomite diff to a Spitfire will cause the speedo to under-read .  To correct the reading, assuming it was correct in the first place, you need to multiply the mph by a factor of 1.066

I knew that the A level in maths and mechanics would come in useful one day !

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6 hours ago, GrahamB said:

With a 3.98 diff for every 3000revs in top gear, the wheels will revolve 771 times.  Assuming a rolling circumference of 6 feet, the car will travel 1542 yards or 0.876 miles.  So at 3000 rpm the car will travel at 52.6 mph giving a overall gearing of 17.5 mph / 1000rpm.

With a 3.65 diff the wheels will revolve 3000/3.65 = 822 times and the car will travel 1644 yards or 0.934 miles. So at 3000rpm the car will travel at 56 mph giving an overall gearing of 18.7 mph/1000rpm.

Therefore changing from the intended Dolomite diff to a Spitfire will cause the speedo to under-read .  To correct the reading, assuming it was correct in the first place, you need to multiply the mph by a factor of 1.066

I knew that the A level in maths and mechanics would come in useful one day !

Yes, you are absolutely correct and what I wrote was the opposite of correct. In my defence I had drunk more than several beers when I wrote it 🥴

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Yes, I agree, Graham has got it. Please see my response to him to understand why I said otherwise lol.

Also agree with your simplified explanation although as my Speedo is reading 15% slow, I am thinking it would need to be like putting a 3.63 diff in a 4.11 car?

Also, I have noticed that if I shift into 1st gear just before stopping at a junction the box crunches a little, leading me to think it might not have synchro on 1st & from an earlier car.

I will have to get out of the habit.

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Your right that the error is pretty big but theres some reasons why it would be surprising if an early gearbox (and it would have to be very early to be only 3 synchro) had been installed:

Yours should be a single rail which only came in 3.89 and 3.63 flavours and all of them 4 synchro. This type can be confirmed by the position of reverse on the gear lever which should be lift, right and forward?

The earlier 3 rail has reverse on the left and is an inch shorter so a making a swop a bit more complicated.

Our theory is that the earlier boxes all had the same speedo drive gears and the correction was done in the speedo itself.

A crunch on selecting a gear can be due to a worn synchro, dragging clutch and/or high tickover and is not unusual in first.

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48 minutes ago, johny said:

Your right that the error is pretty big but theres some reasons why it would be surprising if an early gearbox (and it would have to be very early to be only 3 synchro) had been installed:

Yours should be a single rail which only came in 3.89 and 3.63 flavours and all of them 4 synchro. This type can be confirmed by the position of reverse on the gear lever which should be lift, right and forward?

The earlier 3 rail has reverse on the left and is an inch shorter so a making a swop a bit more complicated.

Our theory is that the earlier boxes all had the same speedo drive gears and the correction was done in the speedo itself.

A crunch on selecting a gear can be due to a worn synchro, dragging clutch and/or high tickover and is not unusual in first.

Ok, reverse is right and forward so that confirms it's not an early box I guess. Maybe I'm just trying to engage 1st at too high a speed or not fully depressing the clutch. Thanks for the clarification 👍

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reverse to the right is a single rail box  correct for a 1500 spit

Im just rebuilding a 3 rail from a GT6 been off the road for 30 years

all baulk rings worn out and the constant clashing of teeth when it was on the road has wrecked most of the dog tooth chamfers,  a classic case of clutch not clearing

most have no idea what goes on with synchromesh , its driven from the mainshaft and in the blink of an eye

the rings have to have 1/2 tooth misalinment   ( rock over)to baulk the change till its spun the clutch disc up /down to match then the ring allows the sleeve to progress to lock the dog teeth

any clashing/crashing chimbles the dog tooth chamfer angles to end up as a scrap gear or hub

here is a Rootes take on explaining it all

so lesson is   do not clash /crash selections 

 

 

syncho and howi1.jpg

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5 hours ago, Pete Lewis said:

 

reverse to the right is a single rail box  correct for a 1500 spit

Im just rebuilding a 3 rail from a GT6 been off the road for 30 years

all baulk rings worn out and the constant clashing of teeth when it was on the road has wrecked most of the dog tooth chamfers,  a classic case of clutch not clearing

most have no idea what goes on with synchromesh , its driven from the mainshaft and in the blink of an eye

the rings have to have 1/2 tooth misalinment   ( rock over)to baulk the change till its spun the clutch disc up /down to match then the ring allows the sleeve to progress to lock the dog teeth

any clashing/crashing chimbles the dog tooth chamfer angles to end up as a scrap gear or hub

here is a Rootes take on explaining it all

so lesson is   do not clash /crash selections 

 

 

syncho and howi1.jpg

Indeed. I'm trying to get back into the habit of being nice to the gearbox by blipping the throttle during a down change to match engine to road speed. Something I haven't done for a long time.

 

 

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I think maybe your box is a bit tired because yours will have the later bigger synchro rings which should be more effective than the original type in mine. With the torque of the six plus a weaker bearing design you can imagine how gentle I am with it although its true gear changes are less frequent as I get it into 4th asap and pretty much leave it there...

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28 minutes ago, johny said:

I think maybe your box is a bit tired because yours will have the later bigger synchro rings which should be more effective than the original type in mine. With the torque of the six plus a weaker bearing design you can imagine how gentle I am with it although its true gear changes are less frequent as I get it into 4th asap and pretty much leave it there...

I will try and treat my box with respect, hopefully that will come naturally as I drive it more. But if and when it's time does come I would probably take the opportunity to fit overdrive. Hopefully that won't be too soon as I have enough things to do already 🙄

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of course the other thing to take into account is that materials and machining tolerances have moved on a lot since our boxes were manufactured. Current synchro rings and gears are made of metals that better resist wear and are lighter so spin up much more easily during the synchronisation process giving faster and possibly smoother changes. You have to alter your style when driving our cars and work at it a bit more but I think thats part of the enjoyment👍

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58 minutes ago, johny said:

of course the other thing to take into account is that materials and machining tolerances have moved on a lot since our boxes were manufactured. Current synchro rings and gears are made of metals that better resist wear and are lighter so spin up much more easily during the synchronisation process giving faster and possibly smoother changes. You have to alter your style when driving our cars and work at it a bit more but I think thats part of the enjoyment👍

Yes, I agree. It used to be second nature driving these cars and it will be again. I think there is a need for a degree of "mechanical sympathy" that modern vehicles don't need, although they also benefit from it if shown.

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