compression ??? - Kawasaki
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post #1 of 18 Old 10-05-2007, 07:46 PM Thread Starter
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compression ???

with stock pistons and .50mm head gasket, what would my compression ratio be? what is the formula to calculate this?
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post #2 of 18 Old 10-06-2007, 02:04 AM
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You'd need the volume of the combustion chamber to figure it precisely, but the stock head gasket is .65mm, so it's not a big jump, maybe a 1/4 point, so ~13:1.
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post #3 of 18 Old 12-24-2007, 08:13 AM
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Ok , I'm using the kaw .45mm head gasket in my 05 , anyone know what my compression ratio will be ??
Also , will premium gas be ok ?
Thanks , later.
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post #4 of 18 Old 12-24-2007, 12:48 PM
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I thought with the .45 gasket it was about 13:1 but Ive heard from engine builders that compression ratios say ours is 12.7:1 is really about 12.5:1 but around 13:1 I'm sure.... pump gas is still fine with this... heck I have 13.5:1 pistons and use pump gas with 10 degrees advance timing lol..... 0 detonation....
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post #5 of 18 Old 12-24-2007, 04:01 PM
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Calculate the existing combustion chamber volume from the existing known displacement per cylinder and the known existing compression ratio:

Chamber volume = 249.5 / (CR - 1) = 249.5 / (12.7 - 1) = 21.32 cm3

Calculate the difference in volume from the known bore size (7.6 cm) and the known difference in head gasket thickness - all dimensions converted to cm:

delta V = ((7.6 ^2) x 3.14159 / 4) x (0.065 - 0.050) = 0.68 cm3

Calculate new combustion chamber volume 21.32 - 0.68 = 20.63 cm3

Calculate new compression ratio

CR = 249.5 / 20.63 + 1 = 13.09

there ya go, now you know how to do it and can re-figure it for different situations if you want!

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post #6 of 18 Old 12-24-2007, 05:33 PM
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There is also piston deck height, and piston dome height that has to be considered in the CR calculations. What the last poster stated would be accurate only for a completely flat top piston that had 0 deck height and no valve pockets, none which applies to the 10R, or most engines.

Years back I bought a graduated burret, can been seen in one of my threads. I used it to measure the combustion chamber volume of my ZX7 Superbike heads, and valve pockets on what I thought was 13 to 1 Wiseco pistons.

After I finished up the math, even with the 13 to 1 pistons I was only getting 11.5 to 1. So I called and talked to a tech at Wiseco. Here is what he told me.

Compression Ratios as advertised by the manufacters is as about inflated as the horsepower figures they advertise and the ZX7 really only had about a 10 to 1 Comp ratio stock from the factory, so I was actually getting a 1.5 bump with the Wiseco pistons. Which is significant, but no where near 13 ro 1 as advertised.

He says they (Wiseco) obviously can't advertise the pistons as 11.5 to 1 because that would be less than what Kawasaki claims for the stock engine. They would never sell any pistons and are therfore forced to play the same game of inflating #s.

So not only are HP #s inflated, so are claimed compression ratio figures.

Unless you have a graduated burret, a piston deck height measuring tool, know for certain or can measure the dome volume of the piston and the valve pockets, you will never get an accurate reading.

My guess stock is less than 12 to 1. and the .45 gasket don't really bump it up much more than a 1/4, if that much.

I know this is hard to beleive, but I have measured many engines and have found it to be so.

Also consider 13.00 to 1 is the max compression ratio that NASCAR allows and they have the most effiecient heads and piston domes ever designed, and detonation is the limiting factor for HP because they are only allowed 100 Octane gas.

So NO, we do not have 13 to 1 CR in our engines, even with the .45 gasket. It would NEVER run even on 93 pump gas and stay together at 13 to 1.


Last edited by JimmyJam; 12-24-2007 at 05:37 PM.
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post #7 of 18 Old 12-24-2007, 05:41 PM
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Here is another interesting tidibit. Notice I stated NASCAR only allows a 13.00 to 1 CR.

That is because a few years back when Rusty Wallace was still racing the rule book said 13.0 to 1. So they figured 13.09 was allowed and would be legal and almost a tenth bump and built an engine as such, was found legal, but the rule book was changed the next day to add a second 0. 13.00, because 13.009 would be insignificant. 13.09 hwoever is about the same as 13.1. I thought that was very creative on their part. Didn't last long though.

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post #8 of 18 Old 12-24-2007, 06:30 PM
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In the interest of getting a desired CR one would been wise to deck the head and or the block to acheive this yes?? perhaps even biased towards the exhaust side


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post #9 of 18 Old 12-24-2007, 06:36 PM
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GoFaster has provided all the information you need , the combustion chamber volume is in the first equation derived from the percentage of compression.

By reducing the combustion chamber volume by the difference in volume between headgasket thicknesses you arrive at a new percentage, this is then applied to the already given compression ratio by the factory and gives you your new number.

Now if you didnt know the compression ratio already , you couldnt work it out.
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post #10 of 18 Old 12-24-2007, 06:50 PM
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JimmyJam, the equation that I gave is accurate as long as you are changing the compression by changing the head gasket thickness only. Piston domes and valve pockets have absolutely no bearing, because you are physically moving the entire cylinder head (valves and chamber and all) closer to the top of the piston by the difference in thickness of the head gasket.

Big Daddy - 'Course, you're right that one must watch out for piston-to-head and piston-to-valve clearances when doing this sort of thing, and if machining becomes necessary to get adequate clearance then that changes everything. And, if you machine the head, you're right because the area that you are machining off includes some of the squish bands around the outside. Normally if you are changing ONE thing that's meant to be a direct-replacement you should not have trouble (but not always!), but if you are changing multiple parts from different sources that might not necessarily have been designed to work with each other as a package, the only solution is to check the clearances ...

I wouldn't be surprised one bit if the compression ratio is not what they claim it is.

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