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Old 07-05-2008, 05:49 PM   #1
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Default Bore Stroke Ratio

Aren't sqaure and slightly oversquare the ideal for street automotive applications? Why did Honda stick so almost exclusively with an undersquare geometry for it's 4cyl engines until the K20?

Why/how exactly is square-oversquare better, how is comubstion affected by this?
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Old 07-07-2008, 02:08 PM   #2
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Default Re: Bore Stroke Ratio (vinuneuro)

Quote:
Originally Posted by vinuneuro
Why/how exactly is square-oversquare better, how is comubstion affected by this?
Oversquare is typically suitable for high rpm applications where you want a relatively small stroke. The relatively large bore preserves some of the displacement. Square is an all-around compromise from a packaging standpoint, and also gives enough torque to get 2 metric tons past a red light.
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Old 07-07-2008, 02:32 PM   #3
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Default Re: Bore Stroke Ratio (vinuneuro)

Quote:
Originally Posted by vinuneuro
Aren't sqaure and slightly oversquare the ideal for street automotive applications?
Not for producing power at low rpms.

Quote:
Originally Posted by vinuneuro
Why did Honda stick so almost exclusively with an undersquare geometry for it's 4cyl engines until the K20?
Torque mostly

Quote:
Originally Posted by vinuneuro
Why/how exactly is square-oversquare better, how is comubstion affected by this?
If you are referring to rod/stroke angles, then a higher r/s angle would put less pressure against the sidewall of the cylinder. In theory this would make a longer-lasting motor that would be able to rev higher, safer. A longer stroke will produce more torque than a shorter stroke though. I'm not entirely sure of the effects it has on combustion if any.

This information has been covered. A lot.
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Old 07-07-2008, 02:38 PM   #4
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Default Re: Bore Stroke Ratio (flyrod)

Quote:
Originally Posted by flyrod

Oversquare is typically suitable for high rpm applications where you want a relatively small stroke. The relatively large bore preserves some of the displacement. Square is an all-around compromise from a packaging standpoint, and also gives enough torque to get 2 metric tons past a red light.
Here's a table of basically all modern engines that includes bore stroke ratio. The overwhelming trend is oversquare. Why would Honda choose undersqaure in it's high-rpm applications, to preserve some torque?

An abstract from an SAE Paper (holding volume constant):
http://www.sae.org/technical/papers/1999-01-3342

" ...

The most important conclusions were:

Engine power was greatest with the compromise bore/stroke ratio of 1.0 or 1.2

Combustion efficiency tended to decrease with increasing bore/stroke ratio.

Mechanical efficiency tended to increase with increasing bore/stroke ratio.

The specific fuel consumption tended to rise with increasing bore/stroke ratio.

The experimental investigation concluded that a square or slightly over square bore/stroke ratio produces an engine with the best brake performance. "



If engine power was greatest with a bore/stroke of 1.0 or 1.2, why did a larger decrease combustion efficiency and increase mechanical efficiency? How are the two related? How/why does bore/stroke affect these three parameters?
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Old 07-07-2008, 02:52 PM   #5
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Default Re: Bore Stroke Ratio (vinuneuro)

Fairly simple, STREET cars like stroke, better cyl. fill(higher piston speed) at lower RPM's and RACE cars like larger bores (same displacement) for more BHP. TQ at a higher RPM. 96.4% of the people that buy a STREET car, your Mom,Dad, Sister, other brother, want it to perform @ 2000-3000 Rpm. The long stroke works there.
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Old 07-07-2008, 02:53 PM   #6
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Default Re: Bore Stroke Ratio (DonF)

I get that longer stroke for street is better, that's obvious- but:

How does bore/stroke ratio affect combustion + mechanical efficiency and bsfc?
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Old 07-07-2008, 04:18 PM   #7
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Default Re: Bore Stroke Ratio (vinuneuro)

That is easy, read ALL the automotive SAE papers, not just one on 2 strokes. Also read ALL the articals on profblairandassociates.com. when you get it, re-post. The Smokey, Yates, and David vizard books would help also. Get a Superflow manual with all ther formulas for head work, and engine flow, while you are at it.
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Old 07-07-2008, 04:20 PM   #8
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Default Re: Bore Stroke Ratio (vinuneuro)

Larger bore loses energy due to having a higher surface area and longer burn time due to the cylinder walls being further out than a small bore large stroke motor. BSFC is usually higher in smaller bore longer stroke engines because of less heat loss and faster burning. Highly simplified =/.

Also helps to remember that honda makes motors for fuel efficient consumers first and foremost. Yes, they can make really good power but they are still main stream market motors designed with efficiency in mind.

A longer stroke will produce higher piston speeds at a lower RPM, which is what every "normal" American consumer wants - low end power.


Modified by ZeroTraction at 5:28 PM 7/7/2008
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Old 07-07-2008, 04:41 PM   #9
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Default Re: Bore Stroke Ratio (ZeroTraction)

Quote:
Originally Posted by ZeroTraction
Larger bore loses energy due to having a higher surface area and longer burn time due to the cylinder walls being further out than a small bore large stroke motor. BSFC is usually higher in smaller bore longer stroke engines because of less heat loss and faster burning. Highly simplified =/.

Also helps to remember that honda makes motors for fuel efficient consumers first and foremost. Yes, they can make really good power but they are still main stream market motors designed with efficiency in mind.

A longer stroke will produce higher piston speeds at a lower RPM, which is what every "normal" American consumer wants - low end power.


Modified by ZeroTraction at 5:28 PM 7/7/2008
You are 1/2 right. Think about it.
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Old 07-07-2008, 04:51 PM   #10
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Default Re: Bore Stroke Ratio (DonF)

Oh no, tech.
Thinking. Which point was wrong?
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Old 07-07-2008, 05:34 PM   #11
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Default Re: Bore Stroke Ratio (ZeroTraction)

Keep thinking about it. 2000 psi on a 2 in circle or 4 in circle. figure out the pressure. Click the image to open in full size.
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Old 07-07-2008, 05:40 PM   #12
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Default Re: Bore Stroke Ratio (DonF)

Higher net pressure on the smaller circle, so shouldn't that increase mechanical efficiency of a smaller bore/stroke? Hmm, that's the opposite of what's said in the abstract of that paper.
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Old 07-07-2008, 05:48 PM   #13
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Default Re: Bore Stroke Ratio (vinuneuro)

um... ∏R^2

2" piston = 3.1415 square inches times 2000psi = 6280 pounds force

4" piston = 12.566 square inches times 2000psi = 25100 pounds force...

Which piston has increased mechanical efficiency again? Click the image to open in full size.
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Old 07-07-2008, 05:49 PM   #14
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Default Re: Bore Stroke Ratio (ZeroTraction)

I don't know how much you can rely on data conducted using a two-stroke expirement.

With that said, it would be my guess that Honda used undersquare (<1.0 B/S Ratio) engine construction to help with idle and cold-start emissions. As is the reason behind the offset primary and secondary lobes on VTEC camshafts. These characteristics help enduce better low engine speed cylinder filling by causing 'swirl'. Basically, the swept volume is ingested at a faster rate at low engine speeds with a undersquare engine when compared to an oversquare engine.

As anyone in the automotive industry knows, idle and cold-start emissions are critical to the sucess of the company. As a side note, ~90% of all emissions expelled from an ULEV rated vehicle occurs during the first 20 second of cold-start.

You will never see a purpose built race engine designed with an undersquare geometry. Most race engines will have a 1.4-1.6 B/S ratio. This is to allow them to achieve their peak BMEP at a higher RPM, which equates to more net work or "power".

BEMP * Swept Volume = Net work
# of cylinders * Net work * rps/rpc = power

As far as mechanical efficiency goes, mechanical efficiency is BMEP/IMEP.
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Old 07-07-2008, 05:54 PM   #15
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Default Re: Bore Stroke Ratio (DonF)

Ok I think Im prolly gonna way off with this one, and if I am I blame the weed.

Find the pressure of 2 circles with 2000 psi. P = force / area, so I know the area of 2 inch circle (1 inch radius) is 3.14 sq in and 4 inch circle (2 inch radius) is 12.56 sq in, so calculating for f you get 6280 lbs of force on a 2 inch circle and 25120 lbs on a circle double its size. Not linear?

If Im totally wrong and off it wouldn't be the first time.
Ok so I wasn't. Anyways some one tie this up all neat and nice in a concise statement now lol.
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Old 07-07-2008, 06:15 PM   #16
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Default Re: Bore Stroke Ratio (PirateMcFred)

Quote:
Originally Posted by PirateMcFred
um... ∏R^2

2" piston = 3.1415 square inches times 2000psi = 6280 pounds force

4" piston = 12.566 square inches times 2000psi = 25100 pounds force...

Which piston has increased mechanical efficiency again? Click the image to open in full size.
Click the image to open in full size. I read it as 2000 pounds.

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Old 07-08-2008, 06:03 AM   #17
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Default Re: Bore Stroke Ratio (DonF)

Quote:
Originally Posted by DonF
Keep thinking about it. 2000 psi on a 2 in circle or 4 in circle. figure out the pressure. Click the image to open in full size.
Well the pressure would be 2000psi in either case, no? I have not read the paper, but I'd guess that mechanical efficiency describes the ability of the engine to convert cylinder pressure into torque/power while combustion efficiency describes the ability of the engine to convert air/fuel into cylinder pressure. That said, the only reason I can think of for why bore/stroke would affect mechanical efficiency (given equal displacement and speeds) is the increased friction and pumping losses from a longer stroke. Otherwise, pushing a big piston with more force for a shorter distance should be pretty similar to pushing a smaller piston with less force but for a longer distance (again, given equal displacement and just talking mechanical efficiency). But like I said, I didn't read their paper and maybe they are not holding displacement and speed as constants when they say "Mechanical efficiency tended to increase with increasing bore/stroke ratio."


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