Tuning for a big bottom - Stroke

[Nourish]

I would like to understand and select the right bits to build a lazy torque-y engine and was hoping we could discuss each aspect of the individual contributions here - if that makes sense?
If that's OK with you all - could we start with the 'Stroke' of an engine. Why when the piston diameter is smaller than the stroke an engine will produce bigger bottom end power?

[Bsareg]

I'm no techy but I would have thought a lot of it was down to inertia of longer and heavier conrods, flywheels to maintain momentum due to the longer throw,. Maybe shorter flame front could help. Would be good to hear a real techy's explanation though. I used to love the way a Gardner or AEC would pull over
 Shap before the Motorways, slow on the flat but nothing could touch them on hills (going up). 

[groily]

There's a ton of reading out there on the relationship between, and effects of, variations in bore and stroke ratios.
The words below I pinched from carbuzz.com

Unless you are thinking of doing very radical surgery there's not a whole lot you can do to alter the basics of an existing design (if that's what you were thinking?). But you can choose pistons, camshafts and carburettor options, also gearing maybe, that offer optimal low speed performance, just as you can go the other way for a revvy and more powerful one. The bhp differential between a soft- and highly-tuned 650 twin from the same stable and with the same external engine appearance would be maybe 35%, more even -  which makes one a nice plodder and t'other a missile (well, relatively speaking!)

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Undersquare (Long Stroke, narrow bore)

This means the piston has to travel further in the cylinder for every revolution of the crankshaft. The longer travel distance increases the piston's speed within the cylinder, but it also increases the leverage on the crankshaft. This leverage advantage of the undersquare engine design results in more torque at a lower rpm. It gives an engine great torque and pulling power even at lower engine speeds. This does limit an undersquare engine's ability to achieve high rpm, also stifling its ability to create as much horsepower as an oversquare engine. The undersquare engine design is typically preferred for towing applications and heavy loads where high torque is paramount over high horsepower.

Advantages:
Higher torque at low RPM
Better low speed efficiency
   
Disadvantages:
RPM is limited
Higher friction due to longer piston travel
More wear on internal engine components


Oversquare (Bore wider than stroke is long)

Because of its shorter piston travel, such an engine can achieve higher RPM with lower piston speed, ultimately producing more combustion events, thus creating more power at high RPM. Because of an oversquare engine's larger cylinder bore, bigger intake and exhaust valves can be accommodated. This characteristic allows for more air flow through the combustion chamber, allowing the engine to breathe better, bolstering horsepower production. Generally, oversquare engines are preferred for performance and racing applications due to their ability to spin at higher RPM and create more horsepower.

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Although I don't think our post-war machinery was unduly affected by it (just a lingering design hangover probably), the notorious RAC Horse Power formula was responsible for the design of a lot of pretty average engines back in the day, with long strokes to keep down the horse power rating upon which vehicle taxation was based. The old formula went 'Diameter of bore squared x no. of cylinders, divided by 2.5'. So short stroke big bore engines were penalised quite severely in the UK compared to other places and manufacturers had a disincentive to make better faster or more technically-interesting things for Mr Average. Nowadays we're not so one-eyed about things, or should I rather say 'we just tax every little thing there is in existence until the pips squeak'.

[Nourish]

Thank you for that Bill. I'd not heard of the RAC Horse Power Formula and so looked it up on Wikipedia - and a quote from there explains a lot - "The emphasis on long strokes, combined with the nature of British roads in the pre-motorway era, meant that British engines tended to deliver strong low- and mid-range torque for their size, but low maximum speeds. The long stroke also meant that piston speeds and the load on the big end bearings became potentially damaging at high power outputs. Many smaller British cars did not cope well with sustained cruising at 60 mph (97 km/h) or more, which led to reliability problems when the vehicles were exported to other markets, especially the United States. Cars such as the Austin A40, the Morris Minor and the Hillman Minx all achieved notable initial sales success in the US in the late 1940s, until the short service life of the engines when asked to routinely drive long distances at freeway speeds became clear."

[RDfella]

Second Groily's explanation. Makes a change from the usual hogwash found in chatrooms etc where the authors talk absolute rubbish. I remember Enzo Ferrari once saying torque wins races, bhp sells cars. Dunno if it's quite that simple...

Of course, most things in engineering are a compromise, and stroke / bore is no exception.
An under-square engine (long stroke, small bore) means more leverage at crank, therefore more torque, but - the push on the crank is less because the piston is smaller.
Over-square means less crank leverage but higher pressure from piston. There are also other considerations - a larger bore means more room for bigger valves etc. Then there's cam and induction design and much else. Hello compromise.

But the rarely discussed issue is piston speed. Clearly the piston of a long-stroke engine is moving further / faster than that of a similar capacity short-stroke. Max piston speed is generally accepted to be 4,000 ft / min. Above that the piston is risking stress failure and the rings are reaching the point where they will end up against the land top, denying gasses from getting behind the ring to seal the bore and assist heat transfer from piston to bore. Meaning that piston will fail in seconds. High-revving engines employ thin or dykes rings to minimise the problem.
For example, an engine with a 3" stroke moves its piston 6" / rev. meaning at 8,000rpm the piston is doing 4,000 ft/min. If you want to rev higher, you have to shorten the stroke - as Nortons did with their TT racers.

[Nourish]

Because I don't know  - I'll make a few comments RD if I may
  "the push on the crank is less because the piston is smaller" - but it's pushing for longer?
  "3" stroke moves its piston 6" / rev. meaning at 8,000rpm the piston is doing 4,000 ft/min" but only twice - at the 1 1/2" point where it's travelling the fastest?

[BagONails]

Interesting discussion.

All fair points and as stated the generalisations 'longer stroke, smaller bore generates more torque but less power / shorter stroke larger bore generates lower torque but more power - these are of course for any given capacity.

Power = torque x revs  so you can generate more power by increasing torque or by increasing revs or increasing both! This is where forced induction comes in!

BMEP (Brake mean effective pressure) is the all important thing. The pressure generated in the cylinder by combustion is what "pushes" on the piston. Oversquare engines can rev higher generating more combustion events per minute of running therefor generating more power (capacity to do work).  They also have greater area above the piston so if the force pushing on the piston is = to the pressure x the piston area then more force will be generated. So in summary oversquare enables more force acting on the conn rod plus more power strokes per minute.

Undersquare engines tend the other way, can't rev as high so fewer power strokes per minute and less area for the combustion pressure to act on so less force acting on the conn rod at the small end. The longer stroke however redresses this to some extent as the big end throw is greater which for a specific force on top of the piston generates higher turning force on the crankshaft or higher torque.

[Radlan2]

I look toward the A65 which was the next generation of our engines and very similar despite (can I say this) the odd look of the unit bikes. BSA used the short stroke of the A7, with a 75mm bore, to give 650cc. In theory the A65 can have up to a 20% power increase 50HP@7,000RPM  compared to 40HP@6,000 for the A10.
       Chris

[BagONails]

Funny you should mention the A65 Chris, here's one I did earlier, well am still doing currently...

The early models were certainly a bit frumpy looking, I am led to understand BSA never actually had a stylist on their books which explains a lot. I've always had a thing for the Spitfire though, an older lad I knew had one in the early '70's and I thought it was the dog's dooh dahs. UK/Aus spec of course cafe racer style with the lovely curvy 4 gallon tank and low bars and now at last I might get a chance to ride one. This is a 67 MkIII slightly more civilised than the original fire breathing 55HP MkII (there never was a MKI strangely, due I think to the A10 Spitfire models sold in the States)

When they introduced the Spitfire in 1966 they weren't mucking around. Aimed squarely at America where they just couldn't get enough power or torque for that matter. This model delivered with 10.5:1 compression, twin GP carbs, optional close ratio gearing, Borrani alloy rims, 190mm Rocket Gold Star front brake, 55HP at 7000 rpm all this on a bronze bush timing side main ala A7/A10 

Cycle World tested one at 123mph and a 13.6 standing quarter mile, it was the fastest production road bike they'd ever tested at that point. They also commented that while you could wring its neck and rev it to the red line there really wasn't any need and they achieved their figures while not exceeding 5,500 rpm.  This would suggest that further trade off of torque for power could possibly result in even higher power figures. They certainly put Meriden's nose out of joint with consecutive production racing wins in 65 and 66 (Hailwood and Cooper)

Anyway I shan't be trying to improve on those numbers but here's the engine all buttoned up, not such a bad looking unit really  Note the head steady securely bolted to the cylinder head casting as per the original long stroke A7 in '49!

Nice looking front wheel with the 190mm brake. Bigger hopefully also being better???

Rear wheel has the older style half width drum which also performs well I'm told (relatively well that is).  BSA going backwards to improve on the cycle parts is something of an indictment maybe? Looks much more stylistic than the full width hubs tho don't it?

[sean]

I would also recommend having your head flowed to match your cam and mods