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Ok, another basic question (engine related)

Compression...relating to piston types - this is something I've never understood...so bear with me... If you have piston/head combination that produces, say 9:1 compression - and you swap the pistons

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Old 08-10-2009, 09:23 PM   #1 (permalink)
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Default Ok, another basic question (engine related)

Compression...relating to piston types - this is something I've never understood...so bear with me...

If you have piston/head combination that produces, say 9:1 compression - and you swap the pistons with some that claim 11:1 - all things being equal, shouldn't the compression really be the same? By that, I mean, if you calculate the volume of the cylinder + combustion chamber at BDC and TDC for the 9:1 compression pistons, and, calculate the cylinder + combustion chamber volume at BDC and TDC for the 11:1 pistons - the ratio should be the same...

The only way I can figure on the compression ratio increasing is if you increase the stroke of the piston, so that it has a higher BDC volume compared to the TDC volume...

Or am I missing something?

Thanks...
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Old 08-10-2009, 09:34 PM   #2 (permalink)
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Piston shape is different......Taller actually which usually results in Valve pockets being machiined into the Piston tops for valve clearance whle the outside circumference or the center area is taller. They are generally shaped to come up into the combustion chamber therefore compressing the air more..
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Old 08-11-2009, 08:44 AM   #3 (permalink)
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Originally Posted by Modshack View Post
Piston shape is different......Taller actually which usually results in Valve pockets being machiined into the Piston tops for valve clearance whle the outside circumference or the center area is taller. They are generally shaped to come up into the combustion chamber therefore compressing the air more..
Right, but by the same notion, wouldn't there also be a reduction in the internal volume of the cylinder by the same amount due the piston shape difference?
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Old 08-11-2009, 09:39 AM   #4 (permalink)
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Right, but by the same notion, wouldn't there also be a reduction in the internal volume of the cylinder by the same amount due the piston shape difference?

Well, yeah. An engine is spec'd to it's original design size (3.7, 3.5 liters etc). If YOU change things than displacement will change (but only very slightly).
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Old 08-11-2009, 10:57 AM   #5 (permalink)
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The higher compression piston lowers the volume of the combustion chamber.


It's a matter of taking the 0.616 liters (3.7/6)of air/fuel mixure you sucked in on the intake stroke and compressing it into a space that now has less volume. Compressing 0.616 liters into the volume of a golf ball will have higher pressure than compressing into into the volume of a baseball.

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Old 07-11-2011, 06:53 PM   #6 (permalink)
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The higher compression piston lowers the volume of the combustion chamber.


It's a matter of taking the 0.616 liters (3.7/6)of air/fuel mixure you sucked in on the intake stroke and compressing it into a space that now has less volume. Compressing 0.616 liters into the volume of a golf ball will have higher pressure than compressing into into the volume of a baseball.
The way this thread was going I was worried nobody was going to point that out.

It's like saying that a piece of dynamite loosely packed versus tightly packed has a different stick size...
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Old 07-11-2011, 07:37 PM   #7 (permalink)
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Ahhh, I get it now. I was writing a post agreeing with kannibul when the answer dawned on me. See if this example helps (or tell me if I'm all screwed up).

Lets make a theoretical cylinder that has 0.6L capacity on its down stroke and 0.1L capacity on its up stroke. I'll call that a 6:1 ratio (I don't know if that is how they measure a real cylinder but it will work for this example). So when the cylinder travels down it sucks in 0.5L of air since 0.1L was already there at the top of the stroke. Now we change out the cylinder head with a taller cylinder and leave everything else the same. You now have 0.05L on the up stroke and 0.55L on the down stroke. This gives you a new compression ratio of 11:1.

Your stroke is still the same. It sucks in 0.5L of air just like the original cylinder but it squeezes it into a smaller volume.
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Old 07-12-2011, 05:56 PM   #8 (permalink)
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Originally Posted by axeman71 View Post
Ahhh, I get it now. I was writing a post agreeing with kannibul when the answer dawned on me. See if this example helps (or tell me if I'm all screwed up).

Lets make a theoretical cylinder that has 0.6L capacity on its down stroke and 0.1L capacity on its up stroke. I'll call that a 6:1 ratio (I don't know if that is how they measure a real cylinder but it will work for this example). So when the cylinder travels down it sucks in 0.5L of air since 0.1L was already there at the top of the stroke. Now we change out the cylinder head with a taller cylinder and leave everything else the same. You now have 0.05L on the up stroke and 0.55L on the down stroke. This gives you a new compression ratio of 11:1.

Your stroke is still the same. It sucks in 0.5L of air just like the original cylinder but it squeezes it into a smaller volume.
Looks good to me! We have our winner!!!!
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