Diff options

[callyspoy]

Hi folks,
Wondering if anybody could give me their diff experiences please.
I have a '58, with standard 4.55 diff.

My engine is:
998cc (flat tops, high compression)
12G940 head
HIF38 with K&N
Currently 1098 cam, but will be 731 at some point as i have one spare.

With that configuration, i found on the motorway, on gps, it is screaming at 60mph. If you look at my other thread, it did appear to have a head gasket issue, which could explain why it didn't sound so refined.
So, what diff is best? I did pick up a 3.7 diff at a good price, i think it will kill the fun of the engine, but it was cheap and will sit on the shelf in case i fit a bigger CC engine at some point.

Would i be best getting a 4.22 diff? I notice that it should drop the revs by 300rpm at 60mph, which is not insignificant.
Or sell the 3.7 and buy a 3.9?

I am aware that the 948/998 is not a torquey engine, so a tall diff isn't ideal, but similarly, i will be using the car as a daily driver, including 6 miles either way on the motorway (well, A27 if you're from Hampshire). At the moment, i avoid the motorway as it was just a bit too noisy for me.

Any thoughts appreciated as ever.

[ianmack]

My ‘61 had a 1098 engine and with the original diff it was a bit undergeared. Fitting a 4.2 axle improved things a lot.

I don’t know how the power of your engine compares to a standard 1098 lump so it’s hard to advise and it will also depend on your personal preference balancing fun against relaxed cruising. With the modest power of most Mogs it is easy to overgear them.

If the choice is down to 4.22 or 3.9 the 4.22s are plentiful and relatively cheap so could be a good start.

Bear in mind that the 4.55 diff has the oil level and filler plug in the nose whereas the 4.22 has it in the axle casing. The later diff fits the early case but then your only oil access is the breather hole. I fitted a complete late axle.

For good acceleration and cruising you could consider a five speed gearbox.

[panky]

I've got a 3.7 diff on my traveller with a mildly tuned 1098. It went great with the original 4.22 diff but did scream a bit at higher speeds. The 3.7 has dulled the accelleration to where it was with the 4.22 but more relaxed cruising.

[IslipMinor]

I would be very cautious about fitting a 731 into 'road' 998 engine. It has the same lift as the 948 and 1098 cams, and 268° duration for both inlet and exhaust valves. For a road 998 it has too little lift and too much duration, which makes it a VERY peaky cam, and to my mind would not be nice to drive at all.

Have you fitted cam bearings into the 948 block? Before fitting any sort of performance cam, the block must be fitted with cam bearings for the centre and rear bearings - normally the cam runs directly in the block.

If you stay with a BMC cam grind, then the Cooper 997 (2A948) is good, as is the MG Metro cam, but more for higher displacement engines. For the MG Metro cam, you will need to change the oil pump drive type to suit it. They all have the same lift, which is the limitation for torque on a small A-Series engine.

For more torque you need more lift, before more duration, and for that you will need to look at the aftermarket suppliers. I have no direct experience of them in a 948 engine (ours was +0.060" = 994cc, and had a 2A948 cam, which is still running well in a Frogeye Sprite!). The popular wisdom for high lift 'moderate' road cams is the Kent 'High Torque' MD246, Swiftune SW5 or AC Dodd ACD-RT. These all have virtually identical lift that is 8% more than the BMC grinds, and in the case of the MD246 it has the same duration as the 2A948, so almost can be seen as a 'high lift' Cooper 997 cam.

As you say a 4.22 diff ratio gives almost 300rpm drop at 60mph, a 3.9 will give 570rpm drop and a 3.7 720rpm drop. With a 998 engine, I would not go further than a 3.9, and would expect to notice an acceleration drop-off, even with a high lift cam. Maybe fit a 4.22 and spend the money saved from buying a 3.9 on a sound proofing kit to provide a bit more audible comfort?

[philthehill]

For a road going 998cc 'A' Series the best BMC stock cam is the 2A948 or near equivalent.

White metal bearings must be fitted to all camshaft bearings or there is a high probability of the camshaft picking up in the block and seizing.

The 998cc engine I had in my Ser2 would easily sit at 70 on the motorway have plenty of low speed traction when required with only a 1 1/4" HS2 to breath through and it usually left everything behind.
The rear axle was a Marina unit 4.2 and fitted with 155 x 13" wheels and it was not uncomfortable or noisy to drive.
At one time it was doing 800plus miles a week mostly on the motorway.
I would suggest that you fit a Minor 4.2 unit (nice and cheap) and see how it works out.
Phil

[les]

Going back In time, I fitted a 731 cam in my cooper, I think special tuning called it a half race cam, and again I think it was designed for the larger cc A series. I'm using a car now with one fitted.

[callyspoy]

Hi folks,
Thanks for the wisdom thus far.

I hadn't really driven it so much since the build, then the head gasket went awry, so i was basing it on the runs i did before the gasket let go.
Having now driven it since gasket replacement, i can confirm that it sounds much, much better. This may just give me a bit more confidence when driving it, with regards to noise.
Regarding bearings for the camshaft, no, it is running standard. I built the top end of tbe engine on a budget and it will remain as such until a time that suits financially. This means machining of the block to fit bearings was a no go.
The bottom end was gifted to me by my dad.

I will take your advice regarding the 731, especially now i've driven it without HG issues and it runs so nicely with the 1098 cam.

And also, the diff, it would appear we are in agreement about that. As i said, it was an absolute bargain, so it was silly to not get it.
A 4.2 may just work nicely and as stated, are generally pretty cheap.
I have a later axle casing in my car as it is, so fitting a different diff is no issue.

[oliver90owner]

It’s not only top speed that gives problems with high ratio diffs. The gear ratios can have a bearing, too. One needs some bottom end grunt with wide ratios in the box.

Can’t say much about a ‘cooking’ Moggie, but a 3.78 diff was better suited to a Ford Cortina only if it was over 80 HP, or so.

My Mk1 Escort thrived on the 3.78 but it was (almost) Mexico spec in the engine dept. and pulled to over 6000rpm in top with a four speed box.

[liammonty]

A 4.22 should suit well - my ‘62 with original 948 bored out to 1030 cc goes well with a 4.22:1 diff. It gives a useful reduction in revs at speed without costing too much in acceleration or hill climbing ability. With your engine, I wouldn’t go for a 3.9 or 3.7:1 as there won’t be sufficient torque to pull it. My 1098 Traveller pulled a 3.9:1 diff well, but that engine has more torque than the shorter stroke 948.