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Old 07-20-2009, 11:03 AM
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Question about compression.

I know this might have been brought up a few times before but can't find anything.

I understand that in a gasoline V8 engine a compression of about 10.5:1 is pushing the limits of pump gas.

But what puzzles me is why does my motorcycle (2008 Kawasaki Ninja ZX-6R) came from the factory with a 13.3:1 compression and ran on premium gas without any problems what so ever, also my new motorcycle (2009 Kawasaki Ninja ZX-14) runs a 12:1 compression on premium.

I also know that diesel motors (the ones I work with) run almost 18:1 then are turbocharged to insane amounts of charge PSI. I understand that this is because of the differences in firing methods.

What makes these motorcycle engines allow a higher compression then a V8 or other factory car will allow?

Hopefully someone is able to answer this, been reading alot and now I'm puzzled.
Eric

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Old 07-20-2009, 01:14 PM
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Combustionchamber design, aluminum heads and block dissapate heat much faster.And the engine is far more lightly loaded.
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Old 07-20-2009, 03:42 PM
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As already stated, aluminum, chamber design, and vehicle weight is a huge one on a bike as they are so light.
Also, cam design plays a role, as most bikes have redlines in the 9000-14000 RPM range.
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Old 07-20-2009, 04:59 PM
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Static compression isn't the only thing in play here. How much PSI you make at low rpm is an important factor too. Quench also is important. The cam plays a huge role in high compression engines using pump gas. The combination of the engine is key. If you lived during the 70's, you saw plenty of low compression, no quench, sorry camed engines from the factory ping themselves half to death and get sorry fuel mileage too......btw, how much cranking psi do you have?

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Old 07-20-2009, 06:20 PM
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Aluminum motorcycle enigines expand quite a bit when they are running.
This cylinder barrel, head,crank case expansion actually lowers the physical compression ratio a significant bit.
(A small change in piston compression height makes a big difference in a small cubic inch motor)
They also have highly advanced high swirl fast burn combustion chambers that don't need as much spark advance, much better control over cylinder to cylinder AFR, Mixture swirl, quench, through out the useable power band and newer ones have advanced electronic knock control and individual cylinder spark timing control.
A automotive V8 is limited to the worst knock limit cylinder of the 8 cylinders ant any one time.
The power rpm band of a high performacne bike motor is quite a bit higher than a typical V8 so low rpm/ high load lugging is not common. below 3000-4000rpm. The in cylinder running pressure is low(er) at low rpm {BMEP}
untill the engine gets up "on the cam" and the induction wave and exhaust system scaveging are "in phase" creating high cylinder charge filling at the higher rpm band.
The intake valve timing closing point (begining of actual compression) can be quite a bit later toward TDC to optimise the high(er) rpm resonace wave tuning of the induction and exhaust system.

If the typical high performance bike motor gets too far off tune for some reason: lean AFR, carbon,oil in combustion, excessive induction path or combustion chamber heat, over heated valves and detonation and or priignition sets in, they $$self destruct in a hurry$$$ just the same.

Even a bad tank of gas can kill it at WOT. Thats what the knock detection/electronic timing control is for.

Small(er) bore size engines are naturaly more knock resistant.

Last edited by F-BIRD'88; 07-20-2009 at 06:30 PM.
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