How Oversquare Can An Engine Get And Remain Streetable?

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nicholastanguma
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How Oversquare Can An Engine Get And Remain Streetable?

Post by nicholastanguma »

I'll use a thumper as my example, since this is the engine I like and know best in all of the motorcycling world.

Way back in the day undersquare configurations were standard, torque was the name of the game, and transmissions had only three or maybe four gears. Even racing bikes were like this.

Metallurgy improved, machining tolerances improved, carburetion improved, so pistons started getting bigger in diameter while strokes started getting smaller, and transmissions started using five gears as standard. Horsepower ratings started rising above pounds feet of torque.

Now, since the 80s, ninety nine percent of thumpers are using big bore/short stroke configurations, and transmissions always have a minimum of five gears, but six is entirely common, too. Even dual sport thumpers, which need lumpy torquey bottom and mid ranges, are oversquare. And some of them have positively enormous bores in comparison to their strokes, the kind of bore/stroke numbers that decades ago were only being used by high rpm racing singles.

Just how oversquare can an engine get while still remaining streetable?

I'll use the middle-of-the-road 500cc class for this example: 97mm bore/68mm stroke, with a six speed transmission. Can this thing be tuned for a nice fat idle-to-redline power curve instead of just high rpm horsepower?
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by n2omike »

Ford 289 has a 4" bore and a 2-7/8" stroke... and in factory form was a docile as anything out there.
I'm sure those boundaries can be pushed pretty far. You'd probably be getting into F1 territory before you hit a true limit. Of course, it all depends on your definition of 'streetable'. "Fine for a hotrod" and perfectly, theoretically ideal for an off idle grocery getter are two different criteria.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by nicholastanguma »

n2omike wrote: Tue Jun 01, 2021 7:11 am Ford 289 has a 4" bore and a 2-7/8" stroke... and in factory form was a docile as anything out there.
I'm sure those boundaries can be pushed pretty far. You'd probably be getting into F1 territory before you hit a true limit. Of course, it all depends on your definition of 'streetable'. "Fine for a hotrod" and perfectly, theoretically ideal for an off idle grocery getter are two different criteria.

Great example, thanks.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by panic »

And the answer is: crankshaft inertia solves all problems.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by PackardV8 »

n2omike wrote: Tue Jun 01, 2021 7:11 am Ford 289 has a 4" bore and a 2-7/8" stroke... and in factory form was a docile as anything out there.
For true. Ford did it with small ports to keep velocity high at low RPM.

FWIW, Studebaker made their '55-'64 V8 in 224", 259" and 289", all with the same ports and cam timing. The 224" used a 2-13/16" stroke (2.8175") and thus 1/16" shorter than the Ford. To make more torque with the 224", the keyway on the crankshaft was changed to advance the cam 4 degrees, IIRC.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by frnkeore »

I suppose what is really being talked about, is bore/stoke ratio? W/o a 4V head, it's hard to get the ideal combination of squish, compression and valve flow, the higher you go.

The 289 had a bore/stroke ratio of 1.394, Jacks Stud, 1.267 but, the king of the hill, as far as common, domestic engines go, is Fords little 144-6 @ 1.4/1 (3.5 x 2.5). Fords ultimate bore/stoke ratio was the 997cc, Kent engine @ 1.67 (3.19 x 1.91) and in Formula racing, it did quite well!
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by englertracing »

Manufacturer Ducati
Engine Liquid cooled 90° 1,285 cc (78.4 cu in) L-twin,
4-valve/cyl desmodromic
Bore / stroke 116 mm × 60.8 mm (4.57 in × 2.39 in)
Compression ratio 12.6:1
Power 205 hp (150.8 kW) @ 10,500 rpm (claimed)

The bore and stroke have less to do with how Streetable an engine is than the port velocity and cam characteristics.

Also
Back to 2 strokes, the way cylinder ports work, the port area is smaller on a big bore short stroke than a small bore long stroke.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by Momus »

nicholastanguma wrote: Tue Jun 01, 2021 5:19 am I'll use a thumper as my example, since this is the engine I like and know best in all of the motorcycling world.

Way back in the day undersquare configurations were standard, torque was the name of the game, and transmissions had only three or maybe four gears. Even racing bikes were like this.

Metallurgy improved, machining tolerances improved, carburetion improved, so pistons started getting bigger in diameter while strokes started getting smaller, and transmissions started using five gears as standard. Horsepower ratings started rising above pounds feet of torque.

Now, since the 80s, ninety nine percent of thumpers are using big bore/short stroke configurations, and transmissions always have a minimum of five gears, but six is entirely common, too. Even dual sport thumpers, which need lumpy torquey bottom and mid ranges, are oversquare. And some of them have positively enormous bores in comparison to their strokes, the kind of bore/stroke numbers that decades ago were only being used by high rpm racing singles.

Just how oversquare can an engine get while still remaining streetable?

I'll use the middle-of-the-road 500cc class for this example: 97mm bore/68mm stroke, with a six speed transmission. Can this thing be tuned for a nice fat idle-to-redline power curve instead of just high rpm horsepower?
Big bore or a long stroke; the engine can be equally tractable. The premise of the question is based on a fundamental misunderstanding.

A long arm gives leverage, a large bore increases the area for the expansion forces to work: same effect.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by lc-gtr-1969 »

Momus wrote: Wed Jun 02, 2021 10:00 am
nicholastanguma wrote: Tue Jun 01, 2021 5:19 am I'll use a thumper as my example, since this is the engine I like and know best in all of the motorcycling world.

Way back in the day undersquare configurations were standard, torque was the name of the game, and transmissions had only three or maybe four gears. Even racing bikes were like this.

Metallurgy improved, machining tolerances improved, carburetion improved, so pistons started getting bigger in diameter while strokes started getting smaller, and transmissions started using five gears as standard. Horsepower ratings started rising above pounds feet of torque.

Now, since the 80s, ninety nine percent of thumpers are using big bore/short stroke configurations, and transmissions always have a minimum of five gears, but six is entirely common, too. Even dual sport thumpers, which need lumpy torquey bottom and mid ranges, are oversquare. And some of them have positively enormous bores in comparison to their strokes, the kind of bore/stroke numbers that decades ago were only being used by high rpm racing singles.

Just how oversquare can an engine get while still remaining streetable?

I'll use the middle-of-the-road 500cc class for this example: 97mm bore/68mm stroke, with a six speed transmission. Can this thing be tuned for a nice fat idle-to-redline power curve instead of just high rpm horsepower?
Big bore or a long stroke; the engine can be equally tractable. The premise of the question is based on a fundamental misunderstanding.

A long arm gives leverage, a large bore increases the area for the expansion forces to work: same effect.
The changes to max piston speed allows for more potential revs (safely) which allows the advantage of heads/cam packages that can take advantage of the higher rpm potential, however this engine would have a soggy bottom end- hence the OP was referring to the need for 5-6 speed gearboxes to help gear the engine into the torque curve.

Cam it sensibly and the oversquare engine should be fine on the street imho.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by BLSTIC »

If you're using enough of the head area for flow you can likely keep shorter duration cams for a given top end [citation needed]. So in this manner larger bore engines could give a wider spread of torque as there's less reversion at lower rpm.

I'm sure many of the engines with 'too large' ports are based around this principal. Lower velocity with high flow gives 'ok everywhere' rather than 'great somewhere'.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by af2 »

englertracing wrote: Wed Jun 02, 2021 2:00 am Manufacturer Ducati
Engine Liquid cooled 90° 1,285 cc (78.4 cu in) L-twin,
4-valve/cyl desmodromic
Bore / stroke 116 mm × 60.8 mm (4.57 in × 2.39 in)
Compression ratio 12.6:1
Power 205 hp (150.8 kW) @ 10,500 rpm (claimed)

The bore and stroke have less to do with how Streetable an engine is than the port velocity and cam characteristics.

Also
Back to 2 strokes, the way cylinder ports work, the port area is smaller on a big bore short stroke than a small bore long stroke.
Until you widen it..
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by adam728 »

Using thumpers as a reference, a DR650 is 100mm bore, 82mm stroke. Or opt for Pro Cycle's 790 kit, takes the bore out to 111mm. The bike makes nothing but torque. 4 valve heads, small valves (33mm IN, 28mm EX), small cam, and small ports.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by nicholastanguma »

adam728 wrote: Thu Jun 03, 2021 12:11 pm Using thumpers as a reference, a DR650 is 100mm bore, 82mm stroke. Or opt for Pro Cycle's 790 kit, takes the bore out to 111mm. The bike makes nothing but torque. 4 valve heads, small valves (33mm IN, 28mm EX), small cam, and small ports.

Terrific example, thanks.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by englertracing »

af2 wrote: Wed Jun 02, 2021 9:14 pm
englertracing wrote: Wed Jun 02, 2021 2:00 am Manufacturer Ducati
Engine Liquid cooled 90° 1,285 cc (78.4 cu in) L-twin,
4-valve/cyl desmodromic
Bore / stroke 116 mm × 60.8 mm (4.57 in × 2.39 in)
Compression ratio 12.6:1
Power 205 hp (150.8 kW) @ 10,500 rpm (claimed)

The bore and stroke have less to do with how Streetable an engine is than the port velocity and cam characteristics.

Also
Back to 2 strokes, the way cylinder ports work, the port area is smaller on a big bore short stroke than a small bore long stroke.
Until you widen it..
You can't widen it enough to make up the difference.
That's why you'll see 20,000rpm ica kart engines are undersquare and 45,000 rpm rc engines also undersquare.
There are some square and oversquare 2 strokes but non of them are outstanding.

Port width is limited to a percent bore size, and does not take advantage of any 3.14 trickery.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by digger »

Momus wrote: Wed Jun 02, 2021 10:00 am
nicholastanguma wrote: Tue Jun 01, 2021 5:19 am I'll use a thumper as my example, since this is the engine I like and know best in all of the motorcycling world.

Way back in the day undersquare configurations were standard, torque was the name of the game, and transmissions had only three or maybe four gears. Even racing bikes were like this.

Metallurgy improved, machining tolerances improved, carburetion improved, so pistons started getting bigger in diameter while strokes started getting smaller, and transmissions started using five gears as standard. Horsepower ratings started rising above pounds feet of torque.

Now, since the 80s, ninety nine percent of thumpers are using big bore/short stroke configurations, and transmissions always have a minimum of five gears, but six is entirely common, too. Even dual sport thumpers, which need lumpy torquey bottom and mid ranges, are oversquare. And some of them have positively enormous bores in comparison to their strokes, the kind of bore/stroke numbers that decades ago were only being used by high rpm racing singles.

Just how oversquare can an engine get while still remaining streetable?

I'll use the middle-of-the-road 500cc class for this example: 97mm bore/68mm stroke, with a six speed transmission. Can this thing be tuned for a nice fat idle-to-redline power curve instead of just high rpm horsepower?
Big bore or a long stroke; the engine can be equally tractable. The premise of the question is based on a fundamental misunderstanding.
this, it has nothing to do with streetability
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