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with stock pistons and .50mm head gasket, what would my compression ratio be? what is the formula to calculate this?
 

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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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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. :hello:
 

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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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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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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.

JJ
 

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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.

JJ
 

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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 :dontknow:

BD
 

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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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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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I think that Gofaster´s formula is correct.
No matter the piston dome shape, the cylinder holes of the gasket are about circular. That holes form a cylinder itself, the height (thickness) of the gasket.
As the .45 gasket is .2 thinner than the stock gasket (Kawi numbers..) you can calculate a .2mm height cylinder, and subtract it from the theorical combustion chamber volume.
Then you make this calculation independent from the piston dome shape.

If you want to know the exact (real) comb. chamber volume, then you may find the exact TDC in any cylinder, and fill the chamber up with oil, using a syringe until the oil begins to reach the plugs thread. Be carefull to record the amount of oil exactly. Cylinder bank has to be as 90° to the floor as possible to minimize the possibility of keeping an air bubble in the chamber.
You can take the oil out using the same syringe and a hose.
 

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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.

I wouldn't be surprised one bit if the compression ratio is not what they claim it is.
The formula your suggesting will only give the difference the change in the gasket thickness will make. If I am understanding you correct, then you are correct.

However it is not the true compression ratio is the statement I was making. I think we are on the same page.

JJ
 

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I think that Gofaster´s formula is correct.
No matter the piston dome shape, the cylinder holes of the gasket are about circular. That holes form a cylinder itself, the height (thickness) of the gasket.
As the .45 gasket is .2 thinner than the stock gasket (Kawi numbers..) you can calculate a .2mm height cylinder, and subtract it from the theorical combustion chamber volume.
Then you make this calculation independent from the piston dome shape.

If you want to know the exact (real) comb. chamber volume, then you may find the exact TDC in any cylinder, and fill the chamber up with oil, using a syringe until the oil begins to reach the plugs thread. Be carefull to record the amount of oil exactly. Cylinder bank has to be as 90° to the floor as possible to minimize the possibility of keeping an air bubble in the chamber.
You can take the oil out using the same syringe and a hose.
I tried that B4, far to much monkey motion involved. So I graduated up to how real engine builders do it using a graduated burret and a piece of plexiglass with a hole in it. The piece of plexi glass is placed over the Combustion Chamber and sealed with grease. Then it is filled thru the hole.

The way you describe I could never get accurate and most importantly, repeatable readings even with multiple attempts. Then there is of course the trouble of trying to tilt an engine so the pistons point straight up and then you never know if an air bubble existed. Good Idea in theory, but it doesn't really work.

I dont know anyone who actually does it this way, and I think its more for demonstration purposes to get the point across than to be actually attempted.

JJ
 

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If I'm understanding that method correctly, the flat piece of plexiglass gets laid against the cylinder head with the engine apart. It will give the volume inside the head itself, but will not account for the shape of the top of the piston, the distance from the top of the piston to the top of the block, or the thickness accounted for by the compressed head gasket. And obviously the engine needs to be apart. (Or is that piece of plexiglass going somewhere else?)

The other method thru the sparkplug hole works with the engine assembled and accounts for all of the above. Yeah, it's a pain one way or the other, but something like the burette/syringe method is the only way of measuring the ACTUAL volumes of these chambers.
 

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No Garth, it won´t pass any oil at all; unless the rings are really worn out.
The compressed air (or gases) has a difficult way to go through the rings, so there is almost no possibility that oil would go past them.
The method is a PITA, but is better done with the engine apart from the bike
 

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The plex glass and burret are good for CCing heads to get a measuerement, but like you say there is the piston crown area to take away.

There are several ways of measuring the compression ratio by taking the volume at TDC away from the Volume at BDC using displacement tools or you can take an impression of the piston crown using modelling clay and use the same perspex sheet and burret to measure the volume of the impression and deduct that from your combustion chamber result.
 

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If I'm understanding that method correctly, the flat piece of plexiglass gets laid against the cylinder head with the engine apart. It will give the volume inside the head itself, but will not account for the shape of the top of the piston, the distance from the top of the piston to the top of the block, or the thickness accounted for by the compressed head gasket. And obviously the engine needs to be apart. (Or is that piece of plexiglass going somewhere else?)

The other method thru the sparkplug hole works with the engine assembled and accounts for all of the above. Yeah, it's a pain one way or the other, but something like the burette/syringe method is the only way of measuring the ACTUAL volumes of these chambers.
but will not account for the shape of the top of the piston, (All piston manufacters such as Wiseco / JE have this figure and can give it to you, or you can measure it your self using a berret, a piston, the block and piece of plexi glass like I have shown.)


the distance from the top of the piston to the top of the block,(This is called deck height and it is either a positive or negative # in the math equation depending on whether the piston quench area falls below the top of the block at TDC or protudes above the top of the block at TDC)


or the thickness accounted for by the compressed head gasket. (Take an old head gasket and measure it)

Listen, not trying to be an ass, or insluting, but I have tried what you state numerous times, and it doesnt work. Its not repeatable, and there is really no way to account for when the oil reaches the top of the CC chamber and about to enter the sparkplug threaded hole. This is when trying to look down in a deep sparkplug well, as well. It only takes a very tiny bit to make an accurate measurement way off.

I even account for the gap above the top compression ring when I measure it. I can assure you no serious engine builder measures compression the way you suggest, because it isnt accurate and doesnt work.

FWIW, the burret and plexi glass have numerous uses. For example you can measure and adjust all 4 Combustion chambers to have the same volume, and when porting heads, you measure all 4 intake ports and port match them to all have the same volume.

This is how engine builders really build power, and is just one of the tricks. Port matching intake runners and Combustion chambers builds engines that make huge power, ESPECIALLY when the engine is built with EQUAL compression ratios on all 4 cyclinders. There is a lot of variance from the factory. I know, I have measured a lot of engines.

JJ
 
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