Could some one explain to me how to chose the right head cc or compression ratio to match the right piston compression I dont understand how to match them up properly.
Thanks

 

most piston makes will list compression ratio with differant head cc volumns. if not you could contact the makers and ask them. there's formulas to figure compression ratio, but again you need gasket volumn, deck clearance volumns, the volumn of valve notchs, and the cc of the chambers in the head.

 

Here's your answer. Remember, Math is your friend.The basic equation for compression ratio (CR) is

Swept Volume + Clearance Volume
CR = ---------------------------------------------
Clearance Volume

The above equation can be rewritten as

Swept Volume
CR = ---------------------- +1
Clearance Volume

Swept volume is the size of 1 cylinder of the engine
This means the size of the engine divided by the number of cylinders
For example 350 divided by 8 equals 43.75

Clearance volume means all the air space above the top piston ring when the piston is at TDC (top dead center)
This includes the combustion chamber volume of the head, piston dish & valve relief volume, compressed head gasket volume, volume from space between piston top land & bore, etc.

Obviously to increase compression ratio the swept volume has to get larger, or the clearance volume needs to get smaller.

Swept volume bigger: Over bore the cylinder, +.030, etc

Clearance volume smaller: smaller chamber heads, milled heads, thinner head gasket, deck the
the block.

So, how do those equations help you in deciding how a volume change will affect a CR change? Just watch!

Use the 2nd equation and to a “what if” by assuming some CR.

Swept Volume
CR = ---------------------- +1
Clearance Volume

Assume a 350 engine with 9.0:1 CR

43.75
9.0 = ---------------------- +1
Clearance Volume

43.75
8.0 = ----------------------
Clearance Volume

43.75
Clearance Volume = -------
8

Clearance Volume = 5.4868 cubic inches

Now do the same for 10.0 CR, I’m not going to show the math, just do the above equation for 10 instead of 9.

Clearance volume = 4.861 cubic inches

Changing the compression ratio 1 number (from 9:1 to 10:1) requires the clearance volume to change( be reduced ) by 0.607 cubic inches. (5.468 minus 4.861 equals 0.607)

What is 0.607 cubic inches? That equals 9.95 cc’s.
How? 1 cubic inch = 16.39 cc
So, 0.607 times 16.39 equals 9.95 cc

We now know for a 350 engine to change the compression ratio 1.0 point, the clearance volume has to change 9.95 cc. Round off the 9.95 cc to 10 cc. Know you can say 1.0 CR = 10 cc

This can be taken a step farther, 1cc = 0.1 CR or said a different way, for a 350 engine, you change the compression ratio a tenth of a point for every 1 cc you change the clearance volume

I used copy/paste from a "WORD" document. Let's hope this works.
Regards, FJK
PS: I just saw how this looked after posting, not to good.
The dashed lines ----- represent divide by, but the text above & below the ----- is not centered. Mentally, shift the text to the right.


[This message has been edited by FJK (edited September 17, 2000).]

 

FJK,

Yup, most of us got it. Using a fixed-font style in Word will help with that next time, although the UBB variable font tends to trim the leading spaces, making all your spacing, which appears correct in your WP, look totally screwed on the UBB. Someday I may find a way around that, too.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Check your email- I jsut sent you a compression ratio spreadsheet so you can play around with different combos BEFORE you lay the bucks down on new parts.


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94 Firebird Formula M6- No options but Z rated tires. No mods over $10. 13.5@105.
79 Malibu "beater" w/junkyard 400 SBC, tweaked QJet 4bbl, finally a decent set of heads, a few other tricks. A maddening 13.000001 @ 108 on the motor with little traction. No nitrous runs with the new heads yet.

"One of the last remaining QJet tuners on Planet Earth!"