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MG Midget and Sprite Technical - 3.55 Diff

I have a noisy 3.9 diff and in anticipation of the CW&P being beyond resusitation I'm contemplating a 3.55 from Bastuck.de.
Has anyone had experience of their CW&P - good or bad?
Tony
Tony Wood

No issues with mine. Had it swapped over and set up by a transmissions guy in about 15 minutes. He reused the crushable spacer but it's worked out fine.
f pollock

I am interested in these too.

Doing some not so quick maths, with my engine and four speed box, it shouldn't actual have that big an effect on acceleration, around 0.2 sec to 60. But would have a huge benefit on cruising and top speed.
Malcolm Le Chevalier

Me too.

I swapped from 3.9 to 3.7 on my standard 1500 and I couldnt tell any descernable difference in acceleration. I wish i had gone with the 3.55 now.

Speaking to someone else who has a 3.55, and they love it, saying it makes first and second much more useable.

I think its a better alternative to overdrive and certainly easier and cheaper than a 5 speed conversion.

Chris Madge

Another alternative to reduce the engine speed for cruising is to increase tyre diameter (with others cons, obviously)

I am today on 145/80/13 from previous owner, and changing to 175/70/13 would reduce the engine speed by 100rpm at 70mph. It is going against the fact that I prefer narrower tyres for fun, but just did the maths for curiosity

Sorry, not the original question :P But looking in the archives, there is some positive feedback about them
CH Hamon

Yeah, I have 155/70s for "road" and 175/50s for "track". It's an easy way of altering the ride height for the two scenarios. I will go to 155/80s next time I need road tyres.

With the 3.9, for instance, it might feel like you are accelerating quicker as you are shifting sooner, but due to the fairly wide ratios on a four speed you end up in a higher gear sooner which results in less torque at the wheels (at that given road speed) than, for example, having a 3.55 diff and being in the lower gear.

It's on my list of upgrades, maybe next summer :-)
Malcolm Le Chevalier

I'm on 155 80s for that very reason.

I'm surprised the 3.55 isnt more common really on the 1500
Chris Madge

Cedric,
155/80/13 would give you (just about) the same (in fact a little more) as 175/70/13. If you look further in the Archives you will see that to an extent having a wider tyre does not increase its footprint on the road and its grip is about the construction and compound of the tyre.
Nigel Atkins

Chris,
I make yours 3,950 revs at 70 mph in 4th gear with 3.7 diff and 3,800 with a 3.55.

Peak torque as (book) 3,000rpm -
. with 3.7 gives - 16mph, 25mph, 37mph and 53mph
. with 3.55 gives - 16mph, 26mph, 39mph and 56mph.

Peak power as (book) 5,500rpm -
. with 3.7 gives - 29mph, 46mph, 68mph and 98mph
. with 3.55 gives - 30mph, 48mph, 71mph and 102mph.

These of course are paper figures and as you've already found small number difference can feel differ when driving the car.


Nigel Atkins

Thanks Nigel, useful information. The 3.7 at 70 is indeed a shade under 4000 rpm in my car according to the rev counter and GPS speedo.

Is the expense of a 3.55 worth it for a drop of 150 rpm at 70 for me....? hmmmmm
Chris Madge

Personally I cant see the point in changing for such a small difference, but then I'm used to buzzing along at 4750rpm at 70.

Trev
T Mason

I agree, Trev. I don't think that it makes sense changing from a 3.9 to a 3.55 just for the sake of it - but if you have a CW&P on the way out and need to buy a new replacement, then what's not to like about a Bastuck 3.55?
I think that this is driven by the high asking prices for second hand diffs of unknown quality. It would perhaps be a different market if s/h diffs were able to be graded for quality accurately.

Tony
Tony Wood

Just as a comparison, 1500 with 155/80/13 tyres, 3.9 against 3.55.

4,150 revs at 70 mph in 4th gear with 3.9 diff and 3,800 with a 3.55

Peak torque as (book) 3,000rpm -
. with 3.9 gives - 15mph, 24mph, 35mph and 51mph
. with 3.55 gives - 16mph, 26mph, 39mph and 56mph.

Peak power as (book) 5,500rpm -
. with 3.9 gives - 27mph, 44mph, 65mph and 93mph
. with 3.55 gives - 30mph, 48mph, 71mph and 102mph.
Nigel Atkins

how much are these 3.55s? they don't appear to be listed on their website...
David Smith

Here : https://shop.bastuck2.de/index.php?&cat=70000&hg=71000&ug=71009&detail=1&lang=en

;)
CH Hamon

thanks :-)
David Smith

This is going to sound bonkers i know - but - my 1275 with 175/70 tyres and a 4.2 diff does 55mph by GPS (60 on the dash) at 5000 rpm.
The revs cant be that far out can they - it definitely feels like 5000 rpm.
S G Macfarlane

that would mean my rev counter is over reading by nearly 1500rpm, meaning i should be able to go to 7000rpm on the dial quite comfortably and still have some to go which is impossible
S G Macfarlane

Yes, that does sound bonkers.

Something’s definitely not right there.
Dave O'Neill 2

Stuart,
are you, your Spridget, rev counter, speedo or GPS device cider drinkers by any chance(?). :)

If your GPS is a free "App" on a phone it's refresh rate could make it inaccurate. One of the best examples of an oxymoron I can think of is a "smart-phone". I'm sure these and Alphabet/Google, Microrsh*t and Amozonion will lead us to the end of the world sooner than later.

I think 5,000 revs for you should be about 77mph and 55mph would be about 3,550rpm.

Perhaps you're running with your rear tyres severely under-inflated, or one of more of your measuring instruments is in error.

Failing that a visit to SpecSavers. :)


Nigel Atkins

The tallest diff is a 5.3, but that would still give you 12.4 MPH/1000,or thereabouts. Still you might want to check the stamping on the diff case. I used to run one of those in London with a straight cut box (+ 649) and it was a screaming riot.
f pollock

"I'm sure these and Alphabet/Google, Microrsh*t and Amozonion will lead us to the end of the world sooner than later."

The spirit of Prop is still alive and well on the BBS!

Malcolm Le Chevalier

Early electronic tachos had serious calibration drift issues, as much as 30% due to thermal instability. I understand it was the use of germanium in the units which were originally positive earth that were most susceptible.
At least I think that's the cause. Although originally a physicst who should know these things, it was so long ago I can't remember. I prefer to think it's being the memory chips have had this data overwritten with more useful stuff like nuts and bolts :)
What ever the cause, the early tachs can drift out enough to explain Stuart's discrepancy.
Paul Walbran

So even though it feels like the tacho is right, get it checked. When I have experienced over-read my eyes mislead the rest of my senses, making me think the engine really was humming along that fast.
Paul Walbran

I was visited and probed by Prop only the other night, well I think it was Prop.
Nigel Atkins

I've checked the diff - its a 4.2 based on 2 rotations of a single rear wheel turning the propshaft 4 turns and a bit ending up between 2 and 3 0'clock if you know what I mean
The dash speedo over reads against the GPS by about 5mph as above.

I checked the GPS against the speedo in my modern, the GPS reads about 1 to 2 mph less - so the GPS - I think - is bang on.

I'm going to check the rpm with a dwell / tach meter this weekend - but I suspect its fairly accurate

There is no way at a recorded 7000rpm on the clock that I can still rev it by another 1500!

I'm totally stumped - at 5000 revs I should be doing 22mph faster than I actually am (77 not 55)!!




S G Macfarlane

I think it's quite common for modern cars to overread by 2mph. My midget previously was spot on and a lot more accurate than my wife's previous modern, when I had it recalibrated I had it set to slightly overread to save having to remember when switching between cars.

I thing you and your standard engine would know if you sustained 7,000rpm.

I'm not sure I could tell the revs in my Midget other than by the exhaust and general noise perhaps and it takes a bit of work and time in a standard Midget to get to 5,000 revs in 4th.
Nigel Atkins

Nigel
I saw this the other day and if you have been probed in your dreams by olemate maybe just maybe
lol
willy


William Revit

Most modern cars that I've had my satnav in, have been between 3 and 4mph over-reading.

My 'B' reads spot-on, but maybe that's down to the 175/70 tyres.

Interestingly, if you change your diff ratio, the relationship between the speedo and tacho readings doesn't change, although the speedo reading could be either more, or less accurate than previously.
Dave O'Neill 2

A 4.22 diff suggests an early 1275. That's the years of the problem tacho, so the results of your calibration check will be interesting.
Paul Walbran

Willy,
I wasn't dreaming, but I might have been abducted by an alien American spirit or being, and they have certainly been messing with my memory (same as the Goggle lot).

Nigel Atkins

This thread was discussed between 06/09/2019 and 20/09/2019

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