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With my gearbox rebuild imminent (well, a few weeks away) and the comments made about the possibility of a new ring and pinion I have been doing some reading about what this entails. I have done loads of research but, as usual, cannot find the definitive answer to my primary question......

What will be the RPM difference be between an 8:31 and an 8:35 at 70mph?

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If you were doing say 3k @70 with a 8:31, then you'll see around 3400 with the 8:35 @70

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Yes but that's all people seem to care about, the mph and the quicker acceleration. It's actually about 7mph in 1st and second and 10 in fifth. With my car returning a fairly poor MPG as it is I would be loathe to make this worse with higher revs in top at 70mph. Also, what would concern me is more revs while cruising making the noise more uncomfortable.

1 hour ago, PeterK said:

If you were doing say 3k @70 with a 8:31, then you'll see around 3400 with the 8:35 @70

Thanks Peter. Where did you get this from or how did you work it out. I would like a much more stone bonker number.

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A quick browse suggested 3k Revs @ 70mph.  Changing the ratios from 31 to 35 will see 3000 become 3000 x 35 / 31 revs, or just under 3400

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34 minutes ago, PeterK said:

A quick browse suggested 3k Revs @ 70mph.  Changing the ratios from 31 to 35 will see 3000 become 3000 x 35 / 31 revs, or just under 3400

Thank you, you are such a geek 😉

It's weird, I spoke to Tandler here in the UK who stock the 8:35 and I was asking about the 8:31 and the guy said they had never been asked for an 8:31! I'm really hoping it is not the issue as they are hugely expensive.......

I do wonder what difference it will make to the acceleration though - worth considering.

Edited by Phill
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1 hour ago, Phill said:

Thank you, you are such a geek 😉

 

Sir is too kind 😂

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Thanks for that. I had seen the end results on Pelican but being able to enter the correct numbers is very useful. Although it doesn't give cruising  rpm for speed, per se, you can follow the graph lines to see what you will be doing. Having filled out the data it bears out Peter's calculations 👍

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47 minutes ago, Boydyrs said:

Phil why not just fit another 9.31? Last 9.35/I bought fir race car was £3k! Have they dropped in price? Presume this is 915 box?

8:31 seems to be few and far between and more expensive that 8:35. 8:35 is about £1800.

41 minutes ago, Leicestershire said:

Perhaps someone has removed the higher final drive from a 915 fitted on a later 3.2.

Could be worth a wanted on IB or a call to Tuthill??

Have you had your EFI system mapped?

Is the economy down on the old CIS setup?

 

 

  

Yes economy down on CIS. When cruising probably by about 4-5 mpg.

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All other things being unchanged you ought to be getting better fuel economy with ITBs and EFI than with CIS - the fuel and ignition mapping can be optimised for any combination of engine speed and load.

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Phill

From recent research and discussions with HT Racing, I think fully sequential injection is the only way to improve fuel economy.

A cheaper way reduce the gearing would be to use a 7:31 from a 4 speed, giving a final ratio of 4.428:1. If that's too low for cruising then you need to close ratio gears 1-4 and keep the 8:31 diff.

Pelican shows close ratio gearing options for the SCRS, but obviously the costs go up.

Maybe PM JG as he has a slightly higher ratio diff in the Orange!

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Jason

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