14 replies to this topic

[Dribble159]

Hey all, Merry Christmas.

 

I've been having some issues figuring out what ratio gearbox and diff etc is in my 1978 Austin Mini 1000. Ever since I acquired it i knew that it was revving quite high while cruising, after getting a tacho, I found i was right. 

 

When revving to 6500 rpm

1st- 20kph

2nd- 40kph

3rd- 70kph

4th- 100kph

 

I have been constantly revving the engine to 8000rpm and sometimes more . When on the highway I usually sit at 120kph which pegs the tacho to 8000rpm. I've cruised like this for hours without issue. I am worried now that I know how high it is revving   .

 

My thoughts were that someone built a gearbox for a small tight circuit and an aggressive cam, and that it has somehow made its way into my car. As far as I have found no gearboxes were this way from the factory (I'm still new to Minis and my knowledge is very limited)

 

Any ideas or thoughts are appreciated. 

 

Thanks everyone

[Quinlan minor]

I'd respectfully suggest that the tacho may be over-reading.

Perhaps it might be wise to mark the tires and the crankshaft pulley, take the sparkplugs out, select 4th gear and pull the car forward one wheel revolution while counting how many revolutions the crankshaft makes.

Feel free to check the other gears,too.

Edited by Quinlan minor, 27 December 2022 - 09:33 AM.

[Cooperman]

Either the tacho is over-reading by a large amount or the transmission has an extremely low final drive ratio.
It does seem unlikely that a 998 engine would pull a sustained 8k rpm in top gear. In fact, unless a modified unit it certainly could not
First thing to do is to measure the actual final drive ratio by jacking up one front wheel and with 4th gear selected and the plugs removed, measure the exact angle turned by the wheel for EXACTLY one revolution of the crankshaft. Divide the wheel angle into 360 and that gives the ratio. Then let us know what the ratio is. You must do this very accurately.

[nicklouse]

Can’t think of a set up that would give you the numbers you are giving. So as suggested above you are way out on any or all your readings.

[Dribble159]

Thanks for the suggestions. 

 

I have found my diff ratio using cooperman's method as accurately as I could with my available tools. I test 3 times and got - 200°, 199° and 201°. I believe its 360/200=1.8 x 2= 3.6. Assuming my testing was out by 5°ish this would mean i have a 3.647 ratio diff? I have been playing around with guessworks calculator and nothing seems right.

 

I agree that my tacho may be over-reading, I am going to see if i can borrow a known working one from somewhere to test. But even if its out by 2k rpm its still odd.

 

I have attached a link to a quick video with sound I took today https://youtu.be/PLAyi_yCH1I

 

Thanks again

[nicklouse]

Sounds like my old pick up.

 

next thing check your speed using your phone. Loads of apps. Eg Waze.

[smurfomatic]

What make/model of tacho is it? Sounds like it may be reading double what it should be

[Quinlan minor]

Sounds like my old pick up.

 

next thing check your speed using your phone. Loads of apps. Eg Waze.

I, also, think the speedometer may also be somewhat optimistic.

[viz139]

According to Guessworks calculator there is no diff low  enough to give those figures. Need to check your Tacho and speedo  and as said above jack up one wheel and turn by hand to see what diff you have.

Edited by viz139, 27 December 2022 - 12:28 PM.

[Homersimpson]

Check that your tacho is set for 4 cylinder operation, some are adjustable for 4/6/8 and it might be set wrong.

[pete l]

According to Guessworks calculator there is no diff low  enough to give those figures. Need to check your Tacho and speedo  and as said above jack up one wheel and turn by hand to see what diff you have.

 

Do you not have to jack up both wheels and spin them both at the same time to do this check ?

[Lplus]

 

According to Guessworks calculator there is no diff low  enough to give those figures. Need to check your Tacho and speedo  and as said above jack up one wheel and turn by hand to see what diff you have.

 

Do you not have to jack up both wheels and spin them both at the same time to do this check ?

 

No, just halve the amount the free roadwheel turns for the calculation.

[Dribble159]

 

 

I have found my diff ratio using cooperman's method as accurately as I could with my available tools. I test 3 times and got - 200°, 199° and 201°. I believe its 360/200=1.8 x 2= 3.6. Assuming my testing was out by 5°ish this would mean i have a 3.647 ratio diff? 

I think I have test it right, but if someone could please confirm. I had one wheel on the ground and did one rotation of the crank. Because only one wheel is off the ground I think it spins double. It spun 200° so if i halve it thats 100°   360°/100°=3.6.

 

 

This is the tacho i have, its a cheap autogauge one.  It is definitely set to 4 cyl, although ill try other settings just incase it is that crap.  I will see if i can find a known good one to test with today. 

 

My speedo is reading roughly 10kph high at 50kph. i will try with the waze app and see what that says.

 

Thanks for the help

[Cooperman]

A 3.67 FDR will give about 15 mph per 1000 rpm in top gear or thereabouts.
The 3.76:1 is much more common as it was the standard ratio in a lot of different models.

[Dribble159]

Thanks for the help Cooperman.

It turns out everyone was correct, the tacho is miles out and absolutely buggered. I test with another gauge and my revs and speed sit with what guessworks says it should with a 3.765:1. Much happier now.

 

Thanks for the help everyone.