Hi all,

Reflecting on a late night conversation whilst camping last weekend.

I think my acceleration compared to some of the other cars was very slow. I have a MT 75 gearbox with the following ratio's;

1st 3.61

2nd 2.08

3rd 1.36

4th 1.00

5th 0.83

My diff I believe it's a 3.62 (although not checked yet) but purchased from ex-club member..First job on the weekend after sorting the miss-fire.

So not fussed about top end speed, but I recall someone saying they do about 2700 @ 70mph. Mines is a lot higher.

Could I fit a 3.92 diff to improve acceleration?

 

Surely if you fit a diff for better acceleration you would increase your rpm at 60mph. I’m sure I have a 3.92 fitted to a type 9 and do around 3000 at 60

If a change of diff cuts out a gear change this might help getting to 60. My setup requires 2 changes to get to 60, I would be quicker if I only needed a single change.

As 1st is a sierra stump puller gear probably your best bet is a modified box with a higher first.

11 hours ago, magh said:

Surely if you fit a diff for better acceleration you would increase your rpm at 60mph. I’m sure I have a 3.92 fitted to a type 9 and do around 3000 at 60

Yes it will, but I do try and stick to A and B roads although going to Le Mans next year it could get annoying on the motorway.

10 hours ago, IanS said:

If a change of diff cuts out a gear change this might help getting to 60. My setup requires 2 changes to get to 60, I would be quicker if I only needed a single change.

As 1st is a sierra stump puller gear probably your best bet is a modified box with a higher first.

Thanks I will look into this but gearboxes tend to be very expensive compared to diffs, even if I decide to go LSD. Not sure what mods are available for MT75 but I will investigate.

Mark

Assuming you have 15" wheels with your gearbox and 3.62 diff then 70mph in 5th will be approx 3000rpm. 3.92 diff, 3300 and 3.38 diff 2800.   In 3rd 5000 revs would be 70.

This is according to a gear calc. I wrote many years ago.

Edited July 23, 20205 yr by Peter Bell

51 minutes ago, Peter Bell said:

Mark

Assuming you have 15" wheels with your gearbox and 3.62 diff then 70mph in 5th will be approx 3000rpm. 3.92 diff, 3300 and 3.38 diff 2800.   In 3rd 5000 revs would be 70.

This is according to a gear calc. I wrote many years ago.

Thanks Peter, very useful I will check this weekend.

So cruising on motorways it would be better to have a 3.38 diff and to put power on the ground a higher ratio of 3.92 but be prepared for more engine noise.

Yes higher ratio is best for more relaxed cruising but if you are particularly looking for a time to X may also save a gearchange, otherwise a lower ratio should give marginally quicker accelleration.

With the right ratio diff you could probably forget first and just use 2nd

Mark, come visit and I'll show you what a proper set of ratios can do for you

In short (ha - see what I did there) a shorter diff will improve acceleration across the board but obviously at the expense of revs; your engine will be revving higher for any given road speed, compared to before. It's a trade off really. For me, I don't mind, I don't have this car to do long motorway cruises.

Edited July 23, 20205 yr by brumster

Not forgetting Brumster's excellent gear ratio spreadsheet (first post on the second page).

 

 

My Superspec has the MT75 gearbox and a 3.92 diff. Seems to be the luck of the draw when the kit was bought as some other Specs came with the 3.62 diff.

The 3.92 ratio does give better acceleration through the gears, although first gear is almost redundant. I often start off in 2nd. The downside is cruising on dual carriageways and motorways when it all gets a bit frenetic and buzzy with 70mph in 5th coming coming in around 3500rpm (speedo isn’t that accurate). We’ll certainly frenetic and noisy compared to cruising in my diesel car!