Bore x Stroke.

[JOWETTJAVELIN]

Aware of the fact that the 1098 engine is under-square, presumably a hangover from the old RAC tax rating days, has me thinking about the principles of engine design, specifically cylinder swept volume and stroke, and the corresponding engine life. US cars of the period - 1950s and beyond (and US marques in Britain) generally had square or over-square engines.

So can we expect the 1275 to last longer than the 1098? Why did the Austin Maxi have such a long stroke having been designed in the mid 1960s?

[liammonty]

I think generally speaking, a shorter stroke will give less wear, both on bores and the crank, mains, big ends etc, due to the decreased piston speed compared to a longer stroke engine. Funnily enough, I don't think the 1098 was such a long stroke purely due to echoes of RAC taxation, but it was also a result of creating 2 new A-series configurations from the 948- initially the 998 (larger bore but same stroke as 948), and then the 1098, which is more or less a stroked 998. The 1275 a couple of years later I think, and initially only an 'S' engine- itself a stroked version of the earlier 1071 and 970 'S' engines. It was only after BMC found it was a good configuration that it became more widespread and made in non- 'S' form.

I believe the E-series (Maxi engine) was made long stroke and small bore to enable the E6 version (2.2 six cylinder- a 1500 with 2 extra pots on it) to be able to fit transversely in the Landcrab. Due to the design of the block, it wasn't possible to overbore the block, which meant that when it was found to be gutless in the 1500 Maxi, the only way to increase the capacity was to stroke it to make the 1750- even worse bore : stroke ratio! I suspect that most of these things happened due to politics and a lack of planning, unlike other British manufactures (eg Ford, Rootes) who were heading towards oversquare engines by this stage...

[bmcecosse]

Exactly - the 'wear' is not significantly different in either configuration. The ability to 'rev' certainly is - Ford 997 engines were revved to over 10,000 in 'screamer' race trim (as was the 998 Imp (ex Coventry Climax) engine- the 970 Cooper S engine could be revved up to 10,000 although not above. Advantage of big bore = ability to use large valves and high revs! But low rev torque was pathetic - the long stroke engine gives nice low revs torque - but not the high revs. So as with most things in life - a compromise lies in between the two extremes.......

[JOWETTJAVELIN]

Thankyou both for the information.

[moggiethouable]

Regarding the hang over from the old RAC Bore or horsepower Tax, it was dropped just as the Minor went in to production.
A 3 cylinder 2 stroke, double crank six piston trial engine was dropped by Morris Motors in favour of another trial engine the flat 4, in order partly to meet said tax.
But in the interim, Miles Thomas (later Sir Miles Thomas) hired in 1924 by no less than RWM himself,conviced the then Chancellor of the Exchequer Hugh Dalton to drop it (the tax) in favour of a flat rate purchase tax.
This meant all Morris had to do was increase the bore of the existing Morris 8 engine to get more horses, without the penalty of more tax on said ponies.
Hence its design and the demise of other options.
As for the mechanics, all of the above is beyond the ken of a common mortal like me and is no doubt correct.

[JOWETTJAVELIN]

Many thanks for the info.

[ErnstBlofeld]

Used to get really hung-up about this after reading the noted journalist LJK Setright in "Classic Cars" many years ago harping on about how unfortunate many British engines were. He said something like it can be mathematically proven that the best engine has the greatest number of cylinders,the largest possible bore, and the shortest possible stroke.

Hey, though, you know the odd thing about this?

It wasn't only the British or BMC who did the whole long-stroke thing.

If the Maxi 1750cc unit is a more than a bit undersquare (76.2mm bore x 95.75mm stroke), and so too was the 1098cc A-series unit (64.58mm x 83.72mm) what are we to make of the Honda D-series?

After all, Mr Setright was very fond of Honda and was ever more fond of calling them "the world's best engine builder."

But the D-series 1600cc unit (75mm x 90mm) is an undersquare lump, if not quite so undersquare as the 1748cc E-series or the 1098cc A-series.

The 1.7 litre D-series Honda unit (75x94.44mm) was just about as undersquare as the E-series unit, though.

But there was a Honda unit even more ridiculously undersquare: the 66mm x 90mm bore ER unit sold from 1981 to 1986.

So much for Honda.

Got into a bit of spat with someone on another forum who said that the Rover T-series engine (84.5m x 88.9mm) was hopelessly outdated owing to its undersquare dimensions and that was one reason the unit was punted out of production.

I did point out that it was barely undersquare, so much so that it was not worth mentioning, and that the unit was actually less undersquare than the 2-litre VW unit (82.5mm x 92.8mm) that people seem so ready to admire - and is still on sale today.

Moreover, the T-series bore/stroke ratio does seem to have seeped into BMW's consciousness because their latest N20 four-cylinder is very O-series/M-series/T series in its bore/stroke dimensions: 84mm x 90.1mm.

And then there is the 1600cc Peugeot/BMW Prince unit in the current MINI, that's undersquare too: 77mm x 85.8.

The Tritec 1600cc unit used in the R59 MINI was exactly as undersquare as the Prince unit, which is to say slightly less undersquare than the 76.2 x 87.3mm BL S-series Montego engine of 1984 - a heavily redesigned E-series unit that shared only the valves, valve springs, and con-rods from the Maxi engine- which takes us back to where we began.

And what of Fiat, once the exemplar of the oversquare engine? Why, it's 1272cc FIRE engine (70.8mm x 78.9mm) is almost as undersquare as the 1275cc A-series (70.61 x 81.28mm.)

Not sure what all of this proves.Yes, I am: the undersquareness of British engines was - curiously - technically backward and at the same time incredibly far-sighted. Packaging constraints and emissions compliancy have made the hugely oversquare - the 1969 Fiat 128 featured an 1100cc unit with an 80mm bore and 55.55mm stroke - passenger car engine a thing of the past.

You do have to wonder if Austin-Rover was all that wise forcing the hand of Norman Tebbitt to stump up the cash to develop the K-series. For £150 million of taxpayer's money what miracles could have been wrought upon the S-series and the O series?