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Final Drive Ratio's

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Joe_flanagan
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Final Drive Ratio's

Post by Joe_flanagan » Tue May 09, 2017 3:08 pm

Hey lads,

Just looking for some advice on final drive ratios and what people typically run. I recently swapped out my 998cc engine for a 1275 high compression MG Metro engine. The engine number is 12HA83AA 101408. Its running great but the only thing I have noticed is that the final drive ratio is different to standard. I bought the engine second-hand and the guy had said that he thought the engine was used for racing in Mondello previously so the rpm to speed ratios are different to the standard spec which makes me think it has a different final drive ratio. Its really torquey at low speed and really nippy which is great fun but once I get onto a motorway she is screaming once I hit 60mph. At 60mph it would be around 4000rpm or more as far as I can remember.

Just wondering what ye lads think and what ratio you would recommend for that engine. I do mainly city driving in Dublin so it being nice and torquey at low speed is good fun but I sometimes drive down to Wexford or if I am going on a Mini spin its pretty impossible with the current setup as its just at such a high rpm for so long. Any recommendations be great or even if ye think it could be a different gearbox that has been put in it as I'm sure ye will have come across this before.

Cheers,
Joe



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dayoungfella
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Re: Final Drive Ratio's

Post by dayoungfella » Tue May 09, 2017 5:24 pm

3.4 is usually the best all round ratio for daily driving. Typically a race or rally car will have a ratio between 3.9 and 4.6 depending on what discipline it's being used in.

Swapping out the diff ratio isn't a straight forward job, though. Engine out, split from gearbox, fit new crown wheel and pinion, and reverse the process!
Neil

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The Marcos Graveyard
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Re: Final Drive Ratio's

Post by The Marcos Graveyard » Tue May 09, 2017 8:33 pm

The 3.44 diff does roughly 17mph per 1000 revs on 12" wheels.
Some Metro City and L models came with a 3.6 diff which would be closer to 16mph/1000rpm.
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macker
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Re: Final Drive Ratio's

Post by macker » Wed May 10, 2017 1:42 am

Generally the higher the numbers of the diff (3.9 - 4.6) the lower top speed you will have, but your acceleration will be better,
The lower the diff (3.7 - 3.1ish) the higher the top speed, but your acceleration will suffer.

By the sounds of it you may have a 3.7 or 3.6 fitted to your own

BTW, from memory, the 3.6 was much sought after by the mini racing guys and was at one time like gold dust to get :)
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woody
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Re: Final Drive Ratio's

Post by woody » Wed May 10, 2017 11:42 am

Depending on how accurate your numbers are (and tyre size) 60mph @4000rpm gives you a diff ratio of about 3.9 to 4.1.
3.44 is considered the best compromise of top speed and acceleration.
If you feel the engine has a bit more torque and your not into standing starts off the lights I'd consider 3.2 or 3.1 to give more comfortable cruising.
Follow this link and input you tyre size and other details and it will calculate your speed per rev. It will also give you the worm and pinion gear sizes you need to keep your speedo right.
http://www.guess-works.com/Tech/ratio.htm
Although you have to remove the engine from the car it is possable to change the final drive without splitting the engine and box. If you go to the lower ratios you will also need to change the main bearing retainer or machine the one you have.
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zakgt
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Re: Final Drive Ratio's

Post by zakgt » Wed May 10, 2017 12:28 pm

Joe,

John Guess the author of that website will be at the IMM as a trader 'Guessworks'. He is very approachable, if you want to discuss things with him.

Adrian

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macker
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Re: Final Drive Ratio's

Post by macker » Wed May 10, 2017 8:40 pm

woody wrote:Depending on how accurate your numbers are (and tyre size) 60mph @4000rpm gives you a diff ratio of about 3.9 to 4.1.
3.44 is considered the best compromise of top speed and acceleration.
If you feel the engine has a bit more torque and your not into standing starts off the lights I'd consider 3.2 or 3.1 to give more comfortable cruising.
Follow this link and input you tyre size and other details and it will calculate your speed per rev. It will also give you the worm and pinion gear sizes you need to keep your speedo right.
http://www.guess-works.com/Tech/ratio.htm
Although you have to remove the engine from the car it is possable to change the final drive without splitting the engine and box. If you go to the lower ratios you will also need to change the main bearing retainer or machine the one you have.
That's the link I was looking for :D
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Joe_flanagan
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Re: Final Drive Ratio's

Post by Joe_flanagan » Fri May 12, 2017 5:30 pm

Thanks for the quick replies lads I really appreciate the advice. I am running 13" wheels as well which I should have mentioned. I didn't know it was a complete engine out job. Might have to leave it for the time being seen as I just put the engine in and had to use my mates garage to do it. Unfortunately I am away for the IMM as well. Would have been great to chat with him about it. Think I'll have to make do with it for the moment though and get the exact rpm and speed figures. Its good fun having the extra acceleration anyway driving around the city.

Cheers lads.

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