How Oversquare Can An Engine Get And Remain Streetable?

General engine tech -- Drag Racing to Circle Track

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af2
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by af2 »

englertracing wrote: Thu Jun 03, 2021 2:36 pm
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.
I don't understand what you just said. I ran a 420 Husqvarna with a short stroke big bore in my outlaw cart some 30 years ago and found major power when widening the exhaust and transfer and intake ports to where the ring gaps had very little bore surface. Ran right with the big boys CR 500 and KTM 550,s
Again I don't understand?
GURU is only a name.
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by Momus »

nicholastanguma wrote: Thu Jun 03, 2021 1:09 pm
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.
I don't think you understand engine fundamentals.

Do you know the relationship between torque, horsepower and rpm?
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Re: How Oversquare Can An Engine Get And Remain Streetable?

Post by adam728 »

"Nothing but torque" is just being descriptive of the powerband. Sorry you are taking it literally and judging my knowledge based on it.

The DR650 is known for being a big, lazy single that makes relatively low hp. In stock form its about 36 whp at a bit over 6000 rpm, and 32 ftlbs at a bit under 5000 rpm. For oversquare and 4 valve head it is not a screamer, nor does it make much hp, even though internet generalization says an engine with specs like it has should be a high revver with crap for low rpm power.

Big bore it and torque swells to 52 ftlbs, whp to about 50-54 at under 6k. As a comparison a 105/80 bore/stroke KTM 690 makes 50ish ftlbs but >70 hp, and doesn't have the off idle torque off the DR.

Point is, bore/stroke doesn't tell much, and you need to know the engine as a whole to predict what has good torque and drivability.
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