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Discussion Starter #121
shaggy said:
bowtie0069 said:
You can use them to estimate what you're looking for, but until you have the actual numbers, you're guessing.
Everything you said makes perfect sense just wished there was a way to calculate rather than a guess.

you will need to actually CC your chambers, and piston dome volumes. You will also have to know your deck height and calculate the CC's for that too. Your gasket CC's you should be able to get that from your gasket manu.

Then you will punch those numbers into the formula and that will give you your CR not a guess...but your actual CR.

BUT the only way to get that is to measure and get the actual CC's of the items listed...
 

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Discussion Starter #122
Are you looking for your Compression ratio or your Piston pin height?
 

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Reason I ask is because I read that some pistons will require milling while others dont. And compression height for any specific piston will dictate that. In other words advertised compression could be for a zero deck height while others are at a standard deck height for specific pistons. I mean if you want to shop for pistons the manufacture seem to never tell you this.
 

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Discussion Starter #124
hmmm....well normally wouldnt you select a specific pin height that will work the the stroke and rod length that you are going to run...I have heard of milling pistons but in the instances I have heard of it they did it to lower the compression of a combo down to a more streetable level...such as milling the domes off of dome pistons...or cutting the dome in half or something like this....I would think if you selected the correct pin height to match the combo you are building you shouldnt have to mill them to get the pin height right...BUT hey...what do I know...If you ask my wife she'll say "Sweet FA thats what he knows".....lol:D
 

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I got this info off a different engine board that not all pistons are created equal because some "rebuilder pistons" will actually have the impression that you will deck the block anyhow. The reasoning from what I understand is that they feel if your rebuilding an engine that the block be decked in all cases. But not all pistons are made this way.
 

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Discussion Starter #126 (Edited)
shaggy said:
Reason I ask is because I read that some pistons will require milling while others dont. And compression height for any specific piston will dictate that. In other words advertised compression could be for a zero deck height while others are at a standard deck height for specific pistons. I mean if you want to shop for pistons the manufacture seem to never tell you this.
Well the books I have read said you cannot take the dome volumes that the manufactures state as fact...often they do not take the valve reliefs into account and normally when CC'd many pistons domes will CC smaller then what the manufacture stated them to be......they say the only way to truely figure out your actual compression is to actually CC everything....PLUS the CC of combustion chambers are rarely as stated...my heads are supposedly 64CC but again I have read where Visard says this head routinely CC's out to 68 CC's NOT 64 CC's as GM says they should be...all of theses little differences add up and will give you a false compresion ratio if you just use the numbers supplied by the various manufactures...to get an accurate CR for your motor you will have to measure your motor...:)
 

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See thats the problem is if the manufacture doesnt divulge that wether the compression piston height is for a zero or standard block then it makes it difficult to buy the pistons either requiring you to deck the block or not. And of course you cant cc anything without buying the pistons first. Seems like catch22 if you ask me.
 

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Discussion Starter #128
shaggy said:
See thats the problem is if the manufacture doesnt divulge that wether the compression piston height is for a zero or standard block then it makes it difficult to buy the pistons either requiring you to deck the block or not. And of course you cant cc anything without buying the pistons first. Seems like catch22 if you ask me.
I do see what your saying but if your going to try to zero your block you will need the pistons before so you can mock it up (after the bores have been machined of course) and measure how much the block will need to be cut to get to zero with this combo...SO you should be able to buy the pistons that you want to run and then if decking is required do that afterwards as you will need the pistons to figure out deck height anyway...:)
 

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Compression height is listed by piston manufacturers but if what you are looking for is how to determine what CH will work with your combo it's this:

(You need to know the following)
Block height (this is crank center line to deck).
Stroke
Rod length
Deck clearance you want (for figuring quench it's simply DC + GASKET)

CH= BH - ((stroke/2) + RL + DC)

Stick 350 example: 9.025" -( 1.74" + 5.7" + .025") = 1.56 CH

Once you've figured what CH you need then you can plug it into the CR calculation.
 

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69NovaSS said:
I do see what your saying but if your going to try to zero your block you will need the pistons before so you can mock it up (after the bores have been machined of course) and measure how much the block will need to be cut to get to zero with this combo...SO you should be able to buy the pistons that you want to run and then if decking is required do that afterwards as you will need the pistons to figure out deck height anyway...:)
I guess my beef is with the manufacturers for not advertising wether the pistons are for 0 or standard deck height as im sure they have this type of information. It could also save time and money for shadetree mechanics who would rather just use a ridge reamer and be done with it. Sometimes you buy pistons and they are setup for a standard deck and then the next time you get pistons for a zero deck you will look at them and say wow those pistons are sunk down more than I thought. Or how about you buy standard deck height pistons and your deck height is already decked. I found out its because piston manufacturers dont reveal this info.
 

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Discussion Starter #131 (Edited)
Now this was on the Keith Black website as to how they calculate their advertised compresion ratio:

KB Website said:
All weights and compression ratios are based on a .030" oversize piston for that listing. The compression ratios in each listing are calculated using a combined deck clearance and gasket thickness of .040" and bore diameter of .030" oversize, unless otherwise noted in the listing under the notes.
So depending on the gasket you picked the piston would either be about .020"-.025" in the hole or nearly at zero deck...it would all depend on the thickness of your gasket...Possibly if you looked around other pistons manufactures website you might find simular information that would be helpful....

http://kb-silvolite.com/article.php?action=read&A_id=15
 

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That still leaves you guessing wether the compression height they advertise is for 9.025 or a zero deck.
 

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Discussion Starter #133 (Edited)
KB Website said:
combined deck clearance and gasket thickness of .040"

shaggy said:
That still leaves you guessing wether the compression height they advertise is for 9.025 or a zero deck.

NO...it doesnt leave you guessing....now it might requre that you figure a few things out but you shouldnt have to guess....If you use a thick composite gasket which are in the .039" compressed range you will have to deck the block so the piston is only .001" in the hole (so you end up with the .040" total)...basicly zeroing the block..if you use a steel shim gasket...they are about .015" thick when compressed so then the piston would need to be .025" in the hole (again so you would end up with the .040" total)

You can buy a set of pistons...mock it up to see what your actual deck clearance is and then decide on the gasket you want to use...then you will know if you have to deck the block or not to end up with the combined thickness of .040" that KB uses to calculate their Compression ratio.....fairly straight forward it would seem:)

BTW the gasket manufactures will be able to tell you the COMPRESSED thickness of their various gaskets so you wont have to guess on this part either......
 

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check Speed Pro's site for example... they show the figure for deck at a measured spec for their posted compression rations...
 

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shaggy said:
That still leaves you guessing wether the compression height they advertise is for 9.025 or a zero deck.
It doesn't matter what your deck is. The CH is the CH. It's the distance from the center line of the piston pin to the top of the piston.
The deck clearance depends on the block height. Don't assume any block is 9.025" That's a nominal dimension. It's the Actual block height that you may mess you up. It will vary and it will also vary front to rear. Assume you'll need to square your block so allow some milling wiggle room. Don't try to buy your way to a zero deck.

shaggy said:
I guess my beef is with the manufacturers for not advertising wether the pistons are for 0 or standard deck height as im sure they have this type of information. It could also save time and money for shadetree mechanics who would rather just use a ridge reamer and be done with it. Sometimes you buy pistons and they are setup for a standard deck and then the next time you get pistons for a zero deck you will look at them and say wow those pistons are sunk down more than I thought. Or how about you buy standard deck height pistons and your deck height is already decked. I found out its because piston manufacturers dont reveal this info.
If you need a ridge reamer to get the pistons out, your bore is worn. Zero decking is worthless with excessive blowby and slapping pistons. The purpose of getting the piston flush with the top of the block is with a std. .039" gasket gives a tight quench distance.
Minimum quench is a NEAR collision of the piston to the head. .039" is not a lot of room! A rocking piston in a loose bore will smack the head if you don't maintain precision dimensions and tolerances.
Zero decking for minimum quench is something you have to sneak up on by test fitting and carefully removing material from the deck. You are committed to that gasket size. You certainly can't install a .015" gasket if it's set up for .039".
It has to be done at a competant machine shop. If you think otherwise you are kidding yourself.
 

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I never decked my block so im not sure of the deck height. It was bored and honed out to .030 with a 3.75 stroke and 5.7 rods. I was scared I would run to close like you stated and ran a 0.041 compressed gasket. I have this with 76cc heads and flat top pistons with 7cc valve reliefs. In the calculator it says im about 9:1 compression just right for pump gas i guess. I just wasnt sure if maybe my compression was even lower. The pistons I got were 9.5 with 76 cc heads but Im not sure if they were for a 0 deck or not and the manufacture of the pistons no longer exist.
 

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shaggy said:
The pistons I got were 9.5 with 76 cc heads but Im not sure if they were for a 0 deck or not and the manufacture of the pistons no longer exist.
OK let me repeat one more time. Pistons aren't FOR any paticular deck clearance. That's why the manufacturers don't list deck height. It's for you to calculate from the stack up dimensions you have.

They do have a compression height which is:

the distance from the wrist pin centerline to the top flat surface of the piston.

Got it? it's not the distance to the deck. They don't know or care what deck clearance you may have or want. It's the responsibility of the engine builder to measure everything and figure it out.

For example:
If you have .025" deck clearance with 1.55 CH and want "zero deck" (pistons flush with deck), then you'd need either remove .025" from the deck or order pistons with a CH of 1.575" (1.55 + .025")

To calculate estimates from nominal dimension figures still all depends on the true block height dimension (centerline of crank to deck). You don't know if your block was ever machined. Since you don't know your actual block height you'll have to measure it.

Three ways.
One is to buy a special tool for doing so.
Or you can use a 12" caliper and measure the saddle to deck dimension and add half the main bore.
Or you can mock up known components and measure the clearance and calculate the BH using the formula I posted above.

9:1 is very low compression. Probably not worth the trouble and risk to try for minimum quench if it can't be done properly.
 
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