ok I had DPR do my head over a year ago, i had it done at stage 5 all to specifications that we discussed... after he was done he said he wanted to mill the deck to clean it up and so we can raise compression... i said ok go for it, well when i got it back on the paperwork with all the specs he didnt include how much he milled it..
so here is my dillema, i want to determine my compression ratio before i go and build the block and so forth but i dont know my deck hight.. so do i have to take the head off and measure if so at what point do i measure to compair to the stock figure
or is there another way by running a compression test maybe that i can figure it out

 

ill include some reasons why this is important, he said he didnt raise compression a great deal and i have an oportunity to go turbo so i want to know how low i have to lower the compression and also if i continue all motor what pistons to get and what octane to run.. if i go all motor i still plan on claying the motor anyway but i need to know about gas and also what parts to order before hand

 

I'm thinking the best way is to measure the CC of the combustion chamber....then you would know what your true CR is

 

thanks, i knew it wouldn't be easy

 

well does the helms have the head measurements? It may be in there...

 

but i dont know how much he milled off though either

 

just take it to a shop to figure it out.... they will beable to tell you the compression of the motor. i used to have an ecuation to figure out the full motor compression when you give the comression of each cylinder, but i think i left that in highschool.

 

You can estimate it with a compression test, but to do that it would be best to have both engines side by side, so atmospheric conditions and the tester itself can be eliminated as variables. If you have friends with stock USDM or JDM engines, or engines with known compression numbers and cams similar to yours, you can try comparing compression test numbers with them. You can also estimate the compression if you can figure out how much things were milled. Milling the head, decking the block, welding the chamber, or oversize valves generally increase CR, while valve jobs and chamber porting lower it. Here is another thread on CR:

https://honda-tech.com/zerothread?id=438456

Like the other guys were saying though, the best way is to have everything CC'ed. If you have aftermarket pistons, the manufacturer may be able to tell you how they compare to stock pistons in terms of CC's. As I said in the other thread though, valve lash, cams, etc. can affect your dynamic compression, which is what really counts. hth....

 

Oh one more thing. If the head is on the block and you don't want to take it off, you can get the CR by filling the clyinder with oil through the spark plug hole and carefully measuring how much went in, minus the displacement (BDC-TDC), etc. A local shop should be able to do it, just to let you know it can be done with the head in place...

 

when filling it with oil do you do it at TDC? or when the piston is all the way down

 

you measure how much you put in at BDC and how much comes out when at TDC witht he spark plug out, its not a very accurate method because the sparkplug is not in place and it takes up a substancial volume in the combustion chamber and the amount of human error possible is very high. aslo this method is EXTREMELY messy. its kinda the podunk shadetree way to do it. i know cause i grew up in wilson (wide awake baby!)

 

I have not done this before, but here is what I would do if I wanted to try it: Go to a pharmacy or vet supply and get a large syringe. Like 20cc or bigger. Stick a length of thin tubing on the end of it that will reach down the spark plug hole. Place the cylinder to test at TDC on compression stroke (i.e. all valves are closed). Fill syringe to the 20cc line or some even number, and drop the thin hose down the spark plug hole. Then squeeze the syringe and look down the spark plug hole with a bright light. Keep filling until you see the oil level at the bottom of the threads of the spark plug hole. Presumably you know the bore and stroke, and now you know the combined combustion chamber volume (the amout of oil you put in). CR is (combustion CC + cylinder displacement)/(combustion CC) It shouldn't be all that messy. You can suck most of the oil back out with the syringe. If it's on an engine stand, just turn it upside down over a bucket and go get a beer.