I have a '68 valiant with 225 the short block is good, but I would like to raise the compression by shaving the head. How much can I shave it to raise the compression to say 9:1 ?

Thanks

(1) pull the head
(2) measure the volume of the combustion chambers in the head
(3) measure the volume of the area above the piston when the piston is at TDC
(4) calculate the volume of the combustion chamber area of the head gasket
(5) calculate the swept volume of the piston chamber
(6) determine current compression ratio
(7) calculate how much material needs to be removed from the head to achieve desired compression ratio.

Far more useful than static compression is your dynamic compression. It is more complicated to calculate, but is worth doing. Search the site for instructions.

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Casually looking for a Clifford hyperpak intake for cheap.

Reed

is there a simple way?

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1970 Dodge Dart /225

That is the simple way.

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/6 '67 Barracuda convertible, electronic ignition, 4-OD, street cam, SBP KH discs, 3.55 SG 7.25" 1" t-bars. Bilstein.
340 '67 fastback, Doug Nash 5-speed.
1988 Toyota pickup work truck

Simple way to do what? You must know the volume of your combustion chamber and the clearance between the top of the piston and the top of the block to calculate the compression ratio. Compressed thickness for a FelPro gasket is about .040" and the gasket's bore is 3.5".

http://www.kb-silvolite.com/calc.php

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Joshua

ok
how to measure?

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1970 Dodge Dart /225

I use a depth micrometer to measure & get an accurate measurement.
That measurement can also be done with a caliper with a depth probe.



Note that small changes move the compression a lot so work to get accurate measurements.

Example: Using the KB calculator and a 56cc head chamber plus all the other details, I used a -.180 deck height and got 7.891 static compression.
Use a -.140 Deck H. and you get 8.384 (.040 difference)
Plug-in .100 DH and you get 8.953
At .095 you have 9.03 to 1 CR

If the head's combustion chamber is not 56 cc, all the numbers above are "bunk".
DD

The easiest way to measure the volume of a combustion chamber is to:

(1) get a flat piece of plastic that is larger than the diameter of the combustion chamber.
(2) Drill two small holes in the piece of plastic.
(3) put a thin ring of grease (such as wheel bearing grease) around the perimeter of the combustion chamber you want to measure.
(4) Press the piece of plastic over the combustion chamber to seal the chamber and center the two holes over the middle of the combustion chamber.
(5) Get a needle-less syringe that is marked in one CC increments. i had trouble finding one even though I live in a big city. Other folks who live in rural areas can go down to the feed store and buy these all day. I eventually found mine at an online shop. however, CeeJ was kind enough to give me one as well.
(6) Get a bowl of water and add your favorite color of food coloring.
(7) Fill the syringe with 60 CCs of the colored water
(8) put the syringe tip into one of the holes in the piece of plastic and SLOWLY fill the combustion chamber. The head will need to be firmly mounted flat. One hole will let the water in, the other will let the air out.
(9) stop filling when the water reaches the bottom of the air-hole in the plastic
(10) look at how much water is left in the syringe. Subtract that amount from 60 to get the volume of the combustion chamber.

You must measure the volume of each chamber since manufacturing tolerances allow for differences in volume.

Measuring the volume of the block is easier. bore X depth of piston crown below block deck height.


EDIT: listen to Doc and Josh. They know way more about this than I do.

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Casually looking for a Clifford hyperpak intake for cheap.