Classic cars forum & vehicle restoration.

[SpiggyTopes]

Hi All,

It seems to me that the design of British engines in the 1930s was dictated by the fiscal horsepower rules?

One of the best engines I've had the pleasure of was a Saab 2.0 litre which, I believe, is square; it revs beautifully and lasts forever.

Anyway, I wonder to what extent the long stroke works against engine design, longevity, efficiency and similar?

Does a long stroke engine have any merits at all?

[Rick]

[Ashley]

The long stroke isn't much of a problem because it limits revs (and produces less power) but unlike modern cars, most oldies had very low top gears. A Morris Minor is doing 4000 at 60mph for example, so the engine must be pretty good to stand that! I expect a Fiesta does a little more than 2000 at the same speed.

[mikeC]

Give me a long-stroke engine over a short-stroke every time
Buckets-full of low speed torque and tractability win over outright power at screaming high revs

[Ray White]

I would argue that there is a downside to long stroke/small bore engines. The fact is that due to much higher relative piston speeds, they are prone to premature wear and burning oil. There is also a greater stress placed on crankshafts causing big ends to run and crank breakages more likely.

Any engine design students in the house want to respond?

[peter scott]

I agree with Ray.

Peter
_________________
https://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[goneps]

I'd suggest that the disadvantages Ray mentions are more a matter of overall design and technology of the respective period, rather than characteristics necessarily associated with a long-stroke configuration.

Anyone who's seen inside a Bentley 4½-litre engine of the late 'twenties will surely have marvelled at the con-rods, great long things with undersized big- and small-ends and negligible webbing, that appear quite incapable of handling the forces of damned great pistons thrashing about. No wonder they gave up with monotonous regularity when raced or driven hard in countries where high speeds could be maintained for more than the relatively short bursts between towns on the Great North Road. By comparison, the con-rods of a Morris Eight motor (only a few years later) are much better designed, with significantly larger ends relative to the piston size.

Another factor is that of traditional white-metal bearings. In my experience these seemed to be fine on a new engine, but once the crankshaft is ground the white metal layer becomes thicker to compensate, and that's when it all starts to go wrong. Babbit metal is by its nature soft and unstable, and the thicker the layer the more unstable it becomes, leading to stress fractures and subsequent break-up. Every Morris Eight engine I've stripped has revealed bearing metal breaking up. Not surprising, then, that many such engines have been upgraded to the later Series E or MM Minor crankshaft and rods, with all shell bearings. No matter what the crankpin undersize, shells have only a very thin layer of bearing metal.

We should also remind ourselves that many cars of the 'twenties and 'thirties were built down to a price, so inevitably corners were cut. Even those that were not so constrained were, in many respects, poorly engineered by modern standards, and metallurgy was not the finely developed science it is today; cast iron blocks and heads were rough and grainy had to be left to 'mature' for a couple of months before machining.

The logical conclusion to all this is that the less desirable characteristics of long-stroke motors are as much due to the technology of their era as to the imperatives of a ridiculous taxation system that hobbled British engine design for decades. Obviously long-stroke engines are not suited to the sustained high speeds that are the norm today, neither are they 'revvers', and in most cases, particularly side-valves, their poor breathing is not conducive to either. Thus it makes sense to drive them in the more relaxed style of their age, and enjoy the scenery. There's an almost infinite variety of modern cars with nigh-indestructible engines to choose from, but surely a prime reason for owning an oldie is for the satisfaction of applying the skill required to meet its demands.

Richard

[colwyn500]

Richard summarised it so well.
You could apply the principles of his argument to explan all of the attractions of old car engineering and design.
I can't think of any practical reason for me choosing to drive somewhere in my long-stroke engined Austin Ten over my modern
The reason is that it makes me feel good.