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Discussion Starter · #1 ·
What is the corrolation, given two bikes that are exactly the same in every way except for the engine size?

Looking for an equation if possible (even a loosely modeled one).
 

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What aspect of performance are you wanting to look at?

Acceleration, top speed, lap time, fuel mileage.... or ?
 

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Discussion Starter · #3 ·
whatever youve got. I know there's not a good way to generalize performance, i guess I should have specified which aspect. Actually those four you listed are the main ones I can think of.
 

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If you swap 2 valve for two valve, the larger engine will be faster with lower gas mileage.

More cubic inches, more hp = more performance.
 

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There is also more to the equation than just displacement. Bore and stroke directly affect the power and torque outputs of an engine at a given RPM. If two engines are identical, but one has a longer stroke, it will generally have more torque and over a broader range, though its ability to reach high RPM's will be limited at some point due to larger forces on the piston, rod, crankshaft assembly.

This is just one example of the many cause-effect relationships that small changes in engine design may have.

-Danimal
 

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If two engines are identical otherwise (valve size, intake dia. & length, cams, stroke etc.) and only the bore is increased, the torque curve will be flatter, occur at a lower rpm, and produce higher torque. Mother Nature abhors a vacuum and will fill the cylinder of the bigger motor in the same time period as the smaller one, so the intake velocity must go up. I'm thinking it's an inverse relationship so a little increase in velocity of intake charge makes a relatively big difference in torque. That
 

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Discussion Starter · #7 ·
Danimal said:
There is also more to the equation than just displacement. Bore and stroke directly affect the power and torque outputs of an engine at a given RPM. If two engines are identical, but one has a longer stroke, it will generally have more torque and over a broader range, though its ability to reach high RPM's will be limited at some point due to larger forces on the piston, rod, crankshaft assembly.

This is just one example of the many cause-effect relationships that small changes in engine design may have.

-Danimal
Interesting!

LA said:
If two engines are identical otherwise (valve size, intake dia. & length, cams, stroke etc.) and only the bore is increased, the torque curve will be flatter, occur at a lower rpm, and produce higher torque. Mother Nature abhors a vacuum and will fill the cylinder of the bigger motor in the same time period as the smaller one, so the intake velocity must go up. I'm thinking it's an inverse relationship so a little increase in velocity of intake charge makes a relatively big difference in torque. That
Do go on!


So would you say for instance that a 20% increase in CCs would give a 20% increase in acceleration (i.e. a linear relationship)?
 

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Aguacate said:
So would you say for instance that a 20% increase in CCs would give a 20% increase in acceleration (i.e. a linear relationship)?
If you keep *everything* else the same, yes.
 

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Aguacate said:
Interesting!

Do go on!


So would you say for instance that a 20% increase in CCs would give a 20% increase in acceleration (i.e. a linear relationship)?
Not really. Maybe if the bike was run in a vaccum and with super lubrication with no friction.

As the CC's go up, torque will go up, almost linear. But acceleration has other factors, including wind drag and friction. They are not linear, and will rob more power as you get going faster.

I don't know the exact relation, but the HP difference to go from 90 to 100 mph is X, the HP difference to go from 190 to 200 is like 10X.

mitt
 

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I don't know the exact relation, but the HP difference to go from 90 to 100 mph is X, the HP difference to go from 190 to 200 is like 10X.
I don't know the exact relationship either but I think it would be something more like X squared (how do ya do an exponent). At any rate I know it is a log relationship between speed and hp.
 

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mitt said:
Not really. Maybe if the bike was run in a vaccum and with super lubrication with no friction.

As the CC's go up, torque will go up, almost linear. But acceleration has other factors, including wind drag and friction. They are not linear, and will rob more power as you get going faster.

I don't know the exact relation, but the HP difference to go from 90 to 100 mph is X, the HP difference to go from 190 to 200 is like 10X.

mitt
That's the difficulty with this type of question.
By the time you adequately define the boundary conditions, the specific answer is only valid in a fairly ridiculous situation.

Engine torque is basically linear in proportion to displacement.
A geometrically similar design that is twice the displacement will have twice the torque.

I've thought about his fairly narrowly defined question a bit more:
Aguacate said:
--------------------snip--------------------
So would you say for instance that a 20% increase in CCs would give a 20% increase in acceleration (i.e. a linear relationship)?
If you can magically increase the displacement of the engine in a particular bike by 20%, without changing *anything* else (weight, aero drag, rpm capability, etc.), your acceleration will be *at least* 20% better.
At higher speeds, where the aerodynamic drag begins to dominate, the acceleration will be more than 20% better.
Beyond the top speed of the small displacement bike , the acceleration of the large displacement bike will become infinitely better, as it can still continue to accelerate due to making more power.

The horsepower required to overcome aerodynamic drag increases as the cube of speed.
So, to go 2x as fast with the same aero package requires 8x the power.

That's why an M620 can comfortably cruise at 60mph, dishing out 8 HP to overcome the aero drag, but can't get to 120 mph, because it can't produce 64 HP.
 

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Interesting post, Speeddog. [thumbsup]

So, can we put this into real-life perspective? What kind of expectations should I have for a big bore kit in my M800? I been thinking about one of these kits from Chris with the Mahle pistons. Once installed, this would create about a 7% increase in engine displacement (from 803 to 859cc).

In its current state of tune, my 803cc motor makes 85hp @ 8500 rpm and 56 lb-ft @ 6800 rpm (measured at the rear wheel). All things equal, should I expect about a 7% increase in hp and torque to correlate with the 7% increase in displacement?
 

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Ddan said:
Aren't those also high comp. pistons?
Yep -- a "true" CR of 11.2:1
 

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bluemoco said:
Interesting post, Speeddog. [thumbsup]

So, can we put this into real-life perspective? What kind of expectations should I have for a big bore kit in my M800? I been thinking about one of these kits from Chris with the Mahle pistons. Once installed, this would create about a 7% increase in engine displacement (from 803 to 859cc).

In its current state of tune, my 803cc motor makes 85hp @ 8500 rpm and 56 lb-ft @ 6800 rpm (measured at the rear wheel). All things equal, should I expect about a 7% increase in hp and torque to correlate with the 7% increase in displacement?
You’ll likely get more than a 7% boost, because those pistons increase the compression ratio as well.

If the pistons only increased the displacement 7% without increasing the compression ratio, then you would get approximately 7% more torque at and below the torque peak, but a diminishing advantage past the torque peak, as the engine is airflow-limited after the torque peak.

The rpm at which peak torque is acheived will go lower, as the engine will become airflow limited earlier.

Basically as shown below:
 

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Re: Engine size correlated to performance?

Speeddog said:
You’ll likely get more than a 7% boost, because those pistons increase the compression ratio as well.
<snip>
Not that I didn't trust you, Speeddog, but I independently confirmed this with my local engine builder. ;D Our consensus was that this kit from Chris would yield about a 10% bump in HP and torque across the powerband. This would essentially lift the entire power curve upward.

Chris' kit is a good value, and very tempting.... >:D
 

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I've no problem with you getting a second opinion, it's your money. [laugh]
 
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