950 engine tuning

[jaguar68]

Hi Chaps,

What is the best BL/BMC cylinder head to use on a 950cc engine? Also what camshaft and carburettors would be most suitable?

Regards

John

[liammonty]

Hi,

There's no such thing as 'best' really- it really depends on what you want from your engine. Low down torque and power, or more power at high revs at the expense of low-down torque etc. My 948 is bored to 1030, with flat top pistons to bring compression upa bit, and a 12G295 head, with an MG Metro cam, and a HIF 38 carb. It's a really sweet little engine, and I think it would give a standard 1098 a run for its money in a straight line, but it doesn't have quite the low down torque of a standard 1098. It flies at higher revs though.

Do bear in mind that if you use a much higher lift cam than normal, you will need to bear in mind that the 948 block doesn't have 'real' bearings for the camshaft in the centre or rear of the block. You can add them, but reckon on it costing around £150.

I'm sure others will be along to tell you what has worked for them, but this is my experience. A read of the Vizard 'guide to tuning the A-series' is well worth it too. Good luck!

[bmcecosse]

As above really - but use the 940 head with the 33/29 mm valves - it's SO much better. Fit a crankshaft damper - and be willing to rev the engine ! For a more gentle upgrade - the 202 head (skimmed 80 thou) and AEA300 camshaft from a 1098, and HIF 38 carb on GOOD inlet will easily give the engine 1098 power, at 10% more revs of course.

[IslipMinor]

Before we fitted the 1380 engine 15 years ago, the original 948 had been progressively treated to the 'at the time' classic 'goodies' - bored +0.060", flat top pistons, skimmed and ported 12G295 head, block cam bearings, 2A948 Cooper 997 cam, HS4 carb, LCB + twin box exhaust, duplex timing chain, balanced, crankshaft damper etc. This was long before such things as the 12G940 head, MG Metro cam and HIF SU's were ever fitted on to the A-Series!

It went very well, very sweet, good fun to drive and apart from a dropped (new) valve, was extremely reliable over 20+ years. After sitting in our garage for many years after fitting the 1380, we sold it to a Frogeye owner and got to see and hear it running about 9 months ago. He was very pleased with it, but had decided that at 80mph+ he really should do something to the suspension, although he did have disc brakes, so could stop when he got a bit nervous!

The standard 948 clutch is a bit marginal once you have more power, and a simple upgrade is to fit the 9-spring 6.25" cover plate used in the early Triumph Spitfire. The lack of cam bearings does cause real problems with higher lift cams, and was why we had the later bearings fitted in the block.

[bmcecosse]

The expense of fitting the cam bearings is not necessary /justified if using the 1098 cam -or the 997 (it's only slightly better) or even the MG cam - since they all have the same lift. The thing is NOT to go for stupidly strong valve springs - just enough to give you the revs you want. Best bet of course is to use a 1098 block with Mini 998 Pistons and rods on the 948 crank - giving a very nice 998 Minor engine - with cam bearings.

[IslipMinor]

The 948 had the same cam lift as the 803 and original 850 Mini from the BMC Minor Workshop Manual and Vizard? The 1098 cam had the same lift as the Mini 998, MG Metro, Cooper 997, and even the ubiquitous 731! Is this why they fitted cam bearings in all these engines?

The valve springs were probably more the cause of the block needing to have bearings fitted, and the use of higher than 'normal' Minor engine revs. Would stronger singles be enough?

[bmcecosse]

Yes indeed - the 948 cam is slightly less lift - although not much in it. Over strong springs cause the problem I think - I used stronger singles and they were ok when I ran 948 engines.

[jaguar68]

In have a couple of heads cluttering up the garage, 12A1456SM, 2A629, 12G295 and a small valve 12G940. Also I have an HS4 and a HIF 38 carb too.

Is the AEA 630 camshaft a modified profile from the standard 1098 one?

Regards

John

[bmcecosse]

Standard 1098 cam. Same timing and lift as the cam originally fitted to the Cooper S engines (but it had 1/2" wide lobes) - it's really quite a good cam provided you don't want stupidly high revs.

[jaguar68]

Would the 295 or 940 head need modifying to fit onto the 948 block?

[IslipMinor]

All will physically fit without modification, BUT there are other things to consider.

The first 2 in your list are standard 803/848/948 heads, so no advantage.

The 12G295 has a very large combustion chamber volume (28.3cc), so would need flat top pistons and around 0.030" machined off the head to get a CR of ~9.0:1. Very nice head, and ideal for the 948 engine.

The 12G940 has bigger valves (?maybe a bit too big for a standard 948 - has anyone first hand experience?), and a combustion chamber volume of 21.4cc. As you would also need to use a 1275 head gasket, the CR with standard 948 pistons would be ~8.6:1, which really could do with being a bit more, so 0.020" machined off the head, but would need to check the exhaust valve clearance to the block, as the valve head diameter extends outside the bore.

The bigger valves also put more load on to the camshaft running directly in the 948 block, so keep the revs down to no more than ~6,000rpm max.

Unless there is prior experience of poor results, I would be inclined to try the 12G940, with a 1275 head gasket, 1098 cam, HIF38 on a decent inlet manifold, better exhaust to let it all breath nicely, and see how it goes. The carb and exhaust are going to be needed whichever head you fit, so nothing really lost in trying it.

[bmcecosse]

As above really - but the exhaust valves will likely hit the block, especially if the head is skimmed - so the valves will need to be sunk into the head. By how much? Can't really say - you need to measure.... The valve lifts 320 thou - and ignore the gasket thicknes - that's your 'safety margin'...