entropy
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posted December 10, 2003 01:21 PM
who has actually determined zx12 Comp Ratio???
Hey guys,
JohnnyCheese has been tasked with measuring/calculating my 1427 CR as part of putting it back together (head gasket failure).
I want to know (not guess from CC pressure) what the CR is so I can use the right fuel & ign timing. I'll vary # of base gaskets to get the CR to about 13:1, while aiming at .050"-ish distance between piston & head in "squish" area.
Has any actually done this process??? any tips???
I am sure the Cheeseman knows what to do, but I'd like to hears other's experiences.
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Y2KZX12R
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posted December 10, 2003 01:53 PM
 Edited By: Y2KZX12R on 10 Dec 2003 13:55
Entropy, theres many varriables. Its not cut and dry. I've done my bike twice. before and after chamber work, also before and after removing the block shim.
Is the stock shim under the block?
is the block decked?
is the head milled?
Are you using the factory head gasket?
is there chamber work done?
is the head milled?
are you using factory valves?
are the seats sunk?
How many cc are the spark plugs?
Whats the assembled deck height?
What height pistons (if aftermarket)
How many cc valve reliefs?
For dynamic compression you should measure IVC and use a swept cylinder volume chart for the zx-12r's stroke,rod length,and bore to determine the bench dynamic compression.
Actual dynamic compression can only be determined the old fashond way by using a compression guage on a hot just run engine with the throttle wide open while cranking.
Theres software all over the net for playing "what if" if you want to see what different combos do. Just be 100% accurate in the info you input if you want accurate results.
I'm running a very tight head to piston clearance. Alot less than .050"
And also a very tight intake valve to piston clearance because the cam is at 103 deg. and the block shim is removed. This required the stock pistons to be flycut .015"
My compression is 12.7 to 1 static.
I wish i could answer the question but i'll need more info.
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MadMike
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posted December 10, 2003 02:54 PM
Very good Info, I am going to do the same with my motor and see what the exact everything is for mine! Great Post
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entropy
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posted December 11, 2003 12:34 PM
Edited By: entropy on 11 Dec 2003 12:38
y2kzx12r
damn near everything has been modified incl base haskets, spacer, 4.6mm crank, 87mm pistons, milled block and head (to make em flat), ss valves...
All the mods is why I want to get a static CR determination.
I can get to my desired state of about 220psi cold cranking pressure by playing with the number of base gaskets (3 or 4), but I wanna know the static CR.
I guess the process involves
1. cc-ing the head,
2. taking a bunch of measurements,
3. getting a factor from the piston manufacturer (MTC),
4. plugging all that info into a formula.
It would be VERY USEFUL to know how much CR impact adding/subtracting each .010" base gasket makes. Any idea????
I'd like to go to min safe squish height, but don't want to jack my compression pressure up to where it was (255-260psi).
thanks for yr info.
i owe you MANY beers!!!!
PS
"For dynamic compression you should measure IVC and use a swept cylinder volume chart for the zx-12r's stroke,rod length,and bore to determine the bench dynamic compression."
what's IVC?
where do i find a sept cyl vol chart.
goddamn i am ignorant, out of my depth... 
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Y2KZX12R
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posted December 11, 2003 02:30 PM
Yea you got it.
Measure the deck height of the lower assembly you are going to run.
With the head off, use a degree wheel find tdc with a dial indicator on the piston top by marking on the degree wheel, the degrees when the top of the piston is .050"to each side of tdc. Set the piston to TDC.
Use 4 head bolts to put some clamping on the block and base shim. use sockets etc to take up the space of the head. Snug them down a little.
Use a depth guage (or fealer guages and a steel straight edge) to get the deck height. (distance from the piston top to the block deck)
CC the head with the valvejob, valves, plugs you are using. Lower the surface tension of the water first.
CC the valve reliefs on the piston.
Measure a used head gasket of the same type that you want to use.
Now you have the info you need to do the calculations.
Removing the block shim will raise the compression (with factory valves and pistons and stock cc chambers) about 1/2 a point. From 12.2 to 12.7-12.8
By IVC i mean Intake Valve Closed. Or seat timing. Seat timings are as important as duration and is often confused with it. It all depends on the cam ramps and how agressive the cams are on and off the seats.
One time we tested 3 camshafts of the same lift and duration (advertised) and degreed them in the same in the same 355 short track chevy engine and saw several hp difference. below .050" lift the cams were very different. the seat timings were very different. Agressive cam ramps are hard on parts and in most cases require premium valves and related parts.
Swept cylinder volume can be calculated easily with an engine program.
There are many out there that sell from free (shareware) to thousands. But there are several in the $20-$150 range that are good.
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MadMike
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posted December 11, 2003 03:43 PM
what if you have domed pistons, like I do? how do you get to volume you need? and Thanks for all of this information!!
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entropy
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posted December 11, 2003 10:48 PM
Y2K
"Removing the block shim will raise the compression (with factory valves and pistons and stock cc chambers) about 1/2 a point. From 12.2 to 12.7-12.8"
Do you mean the .080" shim????
Thanks again for the info.
MUCH appreciated (i am saving this material...  )
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Y2KZX12R
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posted December 12, 2003 03:06 AM
Edited By: Y2KZX12R on 12 Dec 2003 04:26
Mike, either call the manufacturer and get the dome volume info, or if you have milled the domes and changed them, then cc them. you can make a negative impression in a mould and then cc the mould.
Or to just check to see how much you machined off the dome use a displacement tank. Its a water tank with a graduated scale for the water level in 1 cc increments. do a before and after.
ENT, no i mean the .015" shim below the cylinder block.
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dougmeyer
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posted December 12, 2003 09:49 AM
Here's an easy way to cc a piston dome:
Just put the piston a known distance down in the cylinder, say 10 mm.
When you put your flat cc'ing plate on top you have now created a space that is bore size x 10mm. On Entropy's 1427 that would be 87mm x 10 mm or 59.5 cc. If the piston was a flat top with no relief or dome, there would be no change to this number when you fill the area. If it is dished, this number would increase by the amount of dish and the resulting difference would need to be added to the chamber volume. If the piston is domed, the number would decrease and the resulting difference would need to be subtracted from the chamber volume.
Y2, You're right about the base gasket giving about a 1/2 point change, but I think you'll find that a compressed base gasket is about .008" thick, not .015".
Doug
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posted December 12, 2003 10:18 AM
Edited By: Y2KZX12R on 12 Dec 2003 10:20
I must be confusing what you guys are calling the "base gasket" with somthing else?
The "gasket" under my cylinder (that was under it) is solid stainless steel not a composite and it is .014"+ very close to .015".
In fact i didnt have any of my engine data handy to respond to this post so i actually went out to the shop and measured it today.
It will not compress any measureable amount because it is a solid stainless shim. You guys are making me wonder if mine is somehow different than other production bikes.
Are they selective fit on the assembly line?
This is scarry because i've previously given out info to people believing that the .015" shim (gasket) is the same on all 2000 zx12r's. This is not good.
Doug, so yours is a compressable composite? And you are positive its .008"?
And you are talking about the "gasket" between the cylinder block and the crankcase?
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MadMike
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posted December 12, 2003 01:55 PM
Doug ? do you know the volume of my pistons, that are the non-machined pistons in my 1361 kit? thanks
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Ra12r
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posted December 12, 2003 02:01 PM
Y2k, if I remember U have a very low production number bike. I have not seen a base gasket that did not compress. You sound like you have a spacer plate?! Stroker eh.........oops told on yourself,,,hehehe
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MadMike
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posted December 12, 2003 02:59 PM
my stroker kit also has the base gasket??? ut oh did I just loose out on some compression????
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Y2KZX12R
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posted December 12, 2003 03:38 PM
Ra12r, yea 003407 low for one sold in the usa. From what I've seen anyway. There was a zx12r regestry i remember reading thru a few years ago.
Do you guys still have the factory "gasket" that was below your block? If so, do me a favor, measure its thickness and tell me how many layers it is.
I'm wondering of the different spec engines that are listed in the service manual use different block "gaskets".
This is one reason I've always refered to it as a shim. I've noticed how most people call it a gasket. This has me wondering. somthing is up.
LOL if i had a stroker, it would make a hell of a lot more than 181 hp.
Mike i wouldnt worry about it. I'm sure Muzzys has optimized the combo in the kit.
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entropy
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posted December 12, 2003 03:58 PM
this is a GREAT thread, somone ought to archive it...
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Johnnycheese
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posted December 13, 2003 07:10 AM
Thanks guys I am reading this.
Karl, your bike is in good hands.
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ninja12
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posted December 13, 2003 07:41 AM
y2k yes all the 12's i've seen has a base gasket. I don't think the actual gasket material will compress either, but there is raised ridge running all around in the center . I assume the ridge is to help seal.
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keith
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posted December 13, 2003 12:32 PM
My base gasket measures .011". However, the pistons , (Muzzy 1270) still clipped the head with .75mm clearance static. i could have machined the pistons but put in another base gasket instead. So I lost a bit of compression - I'll have to put nitrous on the bike then! Keith
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Y2KZX12R
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posted December 13, 2003 02:58 PM
Ok, checked the sheet i have on my Motor.
It has .010" written down.
So i went out again and measured. cleaned it off and noticed the embosment. My mistake. I hastily measured the shim the other day.
I double checked to make sure i even had the right one.
All is cool.
So anywway, on a stock bore stroke engine every .010" is about 1/2 a point.
The combo of advancing the intake cam to 103 and the extra 1/2 a point of compression the engine really has nice midrange (5000-9000)
I lost 3 hp from 10,600 and up but gained several hp everywere below 10,500.
Its the combo i like best so far.
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dougmeyer
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posted December 13, 2003 07:13 PM
I guess you've re-checked. But if you measure the gasket on the "flat" portion (you must assume that the embossing squishes flat) you'll find it about .007-8". No, they are not selective fit at the factory- all the same stainless shim.
Rule of thumb- The larger the bore in relation to the stroke, the greater the effect of changing the "height" of the combustion chamber, that is by increasing the squish, deck height or cylinder height. The larger the bore, the greater the change in volume per change in height.
Mike, no, I don't. But hey, you're a big boy- steal a burette somewhere and measure them yourself (or buy one from Goodson- www.goodson.com- lot's of cool engine building tools)
ANYWAY- Once you have the burette here is the quick, dirty and entirely adequate way to figure your compression. -
"Dry" assemble your engine with the one piston and one (used) ring. Grease (use thick grease) the piston and ring and park it at TDC. Bolt the head on with the valves for that cylinder in place, a used head gasket and snug. Position the entire aseembly with the spark plug hole pointing opposite from the center of the earth. Take your burette, fill it with some solvent and fill the cylinder and chamber up to the top of the spark plug hole. Note the CCs.
Subtract 1.2 (this is the approximate volume of the spark plug that is not there). This is the TDC volume.
Figure the cylinder volume - Area of cylinder bore x stroke
add the the TDC volume - this gives you the "Total Volume" (at BDC)
Divide this total volume by the TDC volume alone
Voila- The resulting number is your compression ratio within a half a point or so.
To do this really accurately you can make a hollow tool that screws into the spark plug hole, measure it's internal volume (with your burette when you're done) and fill that tool to the "top" when you measure the TDC volume and subtract this amount out rather than the spark plug estimate cited above. It's much easier to see when you're "full" this way.
Doug
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dougmeyer
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posted December 13, 2003 07:26 PM
Oh what the hell-
Here's an article I wrote for "City Bike" about ten years ago- You guys might find it interesting-
Compression ratios are one of the established engine parameters
that a lot of people ask about. Many of you know that compression ratios in most modern motorcycle engines range around the 10 or 11:1 range and there is no question that increasing the compression ratio in an engine is a clear and direct way to increase power, but too much can definitely be a bad deal. It is that ever present engine killer, detonation, that limits how much compression an engine will tolerate. However, certain engine designs, being much more detonation resistant, will allow a higher compression ratio to be used for a given fuel. Hemispherical combustion chambers (like the old Z-1) are the least resistant to detonation due to the long distance that the flame front must travel to achieve complete combustion. Wedge shaped chambers (like a small-block chevy) are only a little better and by far the best design is the four valve "pent-roof" style chamber found in most modern motorcycle heads.
This style chamber is very good because it offers a relatively small volume, short flame travel, a centrally located spark plug, and lots of turbulence to ensure smooth and complete combustion.
The compression ratio is simply a comparison between the volume of the cylinder when the piston is at the bottom of it's stroke and the volume when the piston is at the top. A compression ratio of 10:1 simply means that the volume at TDC is 1/10 of the volume at BDC.
Let's see what that means in a "theoretical" engine through it's four cycles. Starting with the piston at TDC we'll open the intake valve (theoretical, remember) and lower the piston in the bore. This creates a vacuum allowing the outside ambient air pressure to flow into the cylinder. Atmospheric pressure on a "standard" day at sea level is 14.7 PSI. So assuming you have a fully filled cylinder at BDC, that is the pressure therein. This condition would be called 100% volumetric efficiency). If we could then close the intake valve and move the piston to TDC we would increase the pressure in the cylinder times 10, which would yield 147 psi.
Except it doesn't. If you measure the pressure with a compression gauge (or check the specs in the book) you may find 200 psi or more. Have all of you hard cores out there ever wondered why that is? Tell you in a minute. So, back to TDC, we fire the plug and create the big bang. Due to the basic laws of thermodynamics, where the burning mixture rapidly expands, the pressure in the cylinder will rapidly rise to about 550 psi. Remember, that's per square inch, so given a 76mm bore like on a ZX-11 that offers 7.26 square inches of piston surface per cylinder, you get about 4000 pounds worth of push at peak pressure.
Cool, huh? Now, given our theoretical engine, tests have shown that the actual power increase bears an almost linear relationship to the increase in compression.
So, if we raise the compression ratio one point, to 11:1, the resulting peak pressure would go up 10% to 4400 pounds. That's a lot more push. Once the piston is back at BDC, having done it's work, we open the exhaust valve and all this burned stuff rushes out and makes that sweet music we love so much. Of course, the 4000 lb. figures quoted above are PEAK pressures that occur for only a very short time during the combustion process. Engine designers use a convenient measure of the mean pressure called Brake Mean Effective Pressure or BMEP for short. BMEP is the mean pressure in the combustion chamber throughout all of the four strokes (including the negative values during inlet). Normal BMEP values range from around 130 to 200 psi, with racing engines usually between 175 and 200 psi. (Remeember this is a calculated reference number, not an actual measurement)
One might ask, if this works so well, why not just raise the compression ratio until we reach the power level we want? Because at a certain point (different for each engine design and fuel grade) the pressure and temperature in the chamber trigger detonation during the burning process and the resulting pressures destroy the engine. How high can we go?
An engine that has to work over a fairly wide power band (streetable), and must use "pump gas" is limited to about 13:1.
Special purpose engines like pro - stock drag racing can run in the 17:1 range with high octane fuel. (Diesels, which you'll remember actually run on auto - ignition or "self detonation", run ratios as high as 22:1 ). Whenever you restrict or increase the quantity or quality of the air entering the engine the compression ratio can be altered to suit. For example, when you turbocharger or supercharge an engine, compression ratios in the 8 or 10:1 range are common. Conversely, if you restrict the amount of air the engine can get, or feed it air at a lower ambient pressure (high altitude), a higher ratio is appropriate. I build my Bonneville engines to at least a 15:1 ratio because of the less dense air at the 4000 foot altitude of the salt flats. The highest generally known compression ratios currently found in racing are NASCAR Winston Cup engines for the Superspeedways where a a restrictor plate under the carburetor is employed. Because this plate limits the amount of air the engine can get ( to limit power and speed) they can run as high as 16:1! The really trick guys actually optimize the compression ratio for each cylinder because one cylinder may have a higher detonation threshold than another due to temperature and airflow variations within a given engine. Another factor effects the relationship between the "mechanical" ratio and the actual pressure in the cylinder.
In theory, a volumetric efficiency of 100% ( that's 100% cylinder filling on the inlet stroke) would be pretty hard to beat, but in practice it happens pretty regularly.
When the engine is at high rpm the column of air in the intake tract has a velocity and therefore, has inertia. This inertia continues to pack the cylinder as long the inlet valve is open, even after the piston has begun it's upward movement. This is why the valve closes some distance after Bottom Dead Center.
Now about that pressure thing.
When you reduce the volume a gas occupies, it's pressure rises. When the pressure rises, the gas gets hotter.
When a gas is heated , it wants to expand. It is this expansion that adds to the already increasing pressure caused by the reduction in volume. In most engines this addition in pressure is a factor of about 1.2 to 1.4 times. Check it out.
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MadMike
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posted December 13, 2003 10:14 PM
Thanks Doug! great information, thanks for sharing!
MM
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entropy
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posted December 14, 2003 03:43 PM
quote:
...
Karl, your bike is in good hands.
damn right my bike is in good hands!!!
BTW Dr. Cheese, its about time you showed up on this forum :thumbsup:
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