11 replies to this topic

[Stu1961]

Hi All,

 

Just started another engine build. Present spec 1275, Metro HLE gearbox with 3.44:1 diff. Rebuild spec 1293 if it needs a re bore  Sw05-7 camshaft, moderately ported head, HIFF44 and Aldon yellow dissy, not looking for absolute  max top end speed or acceleration just something that works well. So with that in mind what would the optimum diff ratio be.

 

Any help much appreciated.

 

Cheers

Stuart  

[Cooperman]

That spec will have good mid-range torque with a wide power band so a 3.1:1 would work well.

[nicklouse]

I would not change it.

[Stu1961]

Thanks for the reply's Cooperman and Nick those were the answers I was hoping would be suggested. I guess a lot of it would be down to personal preference within reason, how noticeable would the difference in acceleration and max speed be between 3.1:1 and 3.44:1. Though I maybe wrong (Please put me right if I am)  the 3.44:1 would give slightly better acceleration for the same revs, though how noticeable this would be I don't know and the 3.1:1 would be slightly better throughout the mid range. Top end not sure what the difference would be though I suspect the 3.1:1 would be better on 'A' roads and a little less noisy due to less revs for the same speed. Hope that makes sense nearly confused myself.

[Cooperman]

When I built a 1990 Cooper and fitted an MG Metro cam, cleaned up the head a bit, put the CR up a bit and generally built everything very carefully and accurately I used a 3.44:1 and it was wonderful. It would pull to almost 7000 rpm in the gears and cruise at around 4500 to 5000 rpm (5000 is just over 80 mph. Acceleration was excellent. Remember, that is the gearing for the original 1275 'S' and they 'din arf goo'.
However, for a lot of main road work a 3.1:1 will pull ok and give more relaxed cruising.
Your choice really.

[Guess-Works.com]

don't forget the Metro HLE had the wide ratio gearbox ( double groove gears, with over drive drops ) you still have a ridiculously short 1st gear being 4:1

[Cooperman]

I've used a 4.1:1 twice. Once was in a full-on 1071 'S' and the other was a 970 'S' which revved to absurd numbers. In my 1275 'S' I have a 3.9:1 and that works well. Those are with a SC CR gear set.

[carbon]

As Guessworks says, with those HLE wide ratio gears you won't need to use first gear much...

 

With that wide ratio gearset you could still easily use the 2.7 diff, it would allow more relaxed cruising.

[Stu1961]

Been having a looking at and reading a few things this afternoon, and have come up with this. My understanding is my original intended set up using the box already fitted to the engine (HLE gear set including drops) along with a 3.1:1 diff will as Guessworks has stated give me a very short first gear (32mph at 6500rpm), so fitting the 2.76:1 will give a longer first (37mph at 6500rpm) so conversely fitting the original 3.44:1 will make first even shorter (29mph). Had a look at Guessworks Ratio Calculator and engine speed at 85mph will be around 4328rpm for the 3.1:1, whereas the 2.76:1 at the same speed will be around 3854rpm.

 

My understanding of a short gear is, it will get there quickly but will use a lot of revs doing it. Is my understanding correct?

 

My biggest issue then as I see it is the short first gear so fitting a 2.76:1 diff will overcome this to some extent don’t know what the perfect figure for first would be in my case given the engine spec.

 

At the end of the day all I want is something that is useable like I said not to worried about absolute max speeds and absolute max acceleration. Any more input is most welcome

 

Stuart.

[nicklouse]

It should not have a 3.44 diff if it is the original for the block. So check the ratios.

The 1.3 HLE and 1.0 HLE had different FDs. Just check.

[Stu1961]

Fair point Nick, have just put number into Guessworks decoder (12HE25 107714) and it comes up with
Under Engine Features - Wide Ratio Rod Change gearbox with Pot Joints
Then under Gearbox features comes up with - Std Ratio Rod Change gearbox with 3.4:1diff
Engine originally came out of a 1990 Metro HLE I know this because the reclamation company I had it of informed me of this
Box is definitely double groove and diff is 3.44:1

Edited by Stu1961, 18 February 2018 - 08:11 PM.

[Stu1961]

Drop gears are 30 tooth primary and 28 tooth input