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Compression Ratio Calculation Question

barttoday

barttoday

Active Member
Ok, I have been trying to figure a CR for my ride using the MVA cylinder heads. Here's what I have figured, correct me please if I am wrong.

Cylinder displacemnet - 1737cc - about 869cc / cyl.
Combustion chamber - 95cc

So, CR is approx.

(869 + 95) / 95 = approx 10.1 (granted, not taking into acct. head gasket)

Is my math sound?? Thanks!

Currently running stock heads so,

(869 + 85) / 85 = 11.2 (Still not accounting for head gasket. Running Cometic)????
 
There are two ways to look at compression ratio. There is the theoretical ratio and the dynamic ratio.

Theoretical is the ratio between the Total volume of the cylinder when the piston is at BDC vs what it is at TDC.

With Dynamic ratio you must factor in the cam shaft profile. Dynamic ratio will almost always be lower than theoretical.
 
I would think ccing the chamber out would figure in there somewhere because of piston's, stroke & head variations , been too long since I played with real performance I guess. I figure if S E parts call for 10.5 to 1, I drop a 1/2 point off for our 4500 ft elevation here and call it 10 to 1.
I bet prodrag could give you the formula and how to figure since he's into performance engine building.
 
Page 146 Big TwinHigh Performance Guide will give u the formulas for Compression ratios and how to cc the heads, not reall good at typing but will try to post it later, Chopper if u have this book u are the lead dog on this one, Jack
 
Jack, does the manual/formula let you enter the intake cam lobe closing degree? A radical cam can reduce a theoretical compression ratio by a bunch due to the intake valve closing after BDC. I assume Bart wants to know dynamic ratio but I may be wrong.
 
Hoople said:
Jack, does the manual/formula let you enter the intake cam lobe closing degree? A radical cam can reduce a theoretical compression ratio by a bunch due to the intake valve closing after BDC. I assume Bart wants to know dynamic ratio but I may be wrong.
Click to expand...

No Hoople, I will try to post the formulas and the one for cc ing heads on Sunday, Got to have my wife help me with this one cause I cant type very good and there are math equations involved, Jack
 
(cylinder displacement +net combustion )

This is the formula for calculating compression ratio, it is best to work in cc rather than cu in multiply cu in by 16.387 cylinder discplacement is the displacement of one cylinder specified in cc s discplacement=bore x bore x .7854 x stroke
 
we did a MVA with flat tops a while back,if i rememeber right it came out to 10.2-1,if anyone want their running compresion,let me know.we have a computer program we can run it thru.
 
prodrag1320 said:
we did a MVA with flat tops a while back,if i rememeber right it came out to 10.2-1,if anyone want their running compresion,let me know.we have a computer program we can run it thru.
Click to expand...

Just out of curiosity, what would you need to calculate my compression?? I am really curious as to what it is. Sometimes, even with the S&S easy start cams in, it seems like my starter is having an awfully hard time turning the crank. I have the S&S 106" kit, stock twin cam 96 heads (2008) and the S&S 551 easy start cams. Also, having worked with the MVA heads, whats your opinion on them? Would I be better off just getting a good port and polish job, or are they worth the cash? Thanks!
 
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