How to measure compression ratio

[capriceowns]

With my basket case engine I got it came with new rods/pistons with the pistons pressed onto the rods aleready, I figured out there Speed pros by a number on the side.

But it either comes up as a cast aluminum one, or hyperuerutic. both with very dif comp rations.

Is there a way to measure the compression Ill get with these pistons without installing them/etc?

[silicon212]

Originally Posted by capriceowns

With my basket case engine I got it came with new rods/pistons with the pistons pressed onto the rods aleready, I figured out there Speed pros by a number on the side.

But it either comes up as a cast aluminum one, or hyperuerutic. both with very dif comp rations.

Is there a way to measure the compression Ill get with these pistons without installing them/etc?

You can measure the volume of the dish, if present, with fluid measured with a graduated cylinder. You can also add this number to your combustion chamber size, as well as compressed head gasket* volume and deck height, divided into total cylinder volume, to come up with a compression ratio.

For example, suppose you have a 350 engine, about 43.73 cubic inches displacement in a cylinder. In metric terms, which is the general measurement in an engine, this comes out to ~717 cc. To determine total cylinder volume, you add the displacement to the volume of the combined unit of the chamber/gasket/piston.

Now, using MY engine as an example, you have 62cc chamber heads, .005 deck height, .041 compressed height* of Fel-Pro 1003 head gasket, and a 22cc dish (reverse dome on piston). Your .005 deck height equals a little more than 1cc, we'll keep it there for simplicity. The head gasket itself has a volume of about 8cc (again, for simplicity sake). Add all of this up and you get 62+22+1+8 = 93cc.

Now, add the 93cc to the 717cc for total cylinder volume, in this case, 810cc. Divide 810 by 93 to determine compression ratio. This engine runs at about 8.7:1 compression doing the math above, which is about right.

* Compressed gasket height is the thickness of the gasket after the heads have been torqued to spec. The gasket is thicker and displaces more out of the box, until it's installed.

[capriceowns]

Ill have to try that out.

But can magnets stick to hyperuerutic pistons? its either a cast alum one or hyper.

[capriceowns]

wow ill get 8.34:1 compression, thats not good.

[silicon212]

A hypereutectic piston is a cast aluminum piston produced from an alloy with a high silicon content. The silicon adds strength to the aluminum, making the hyper piston stronger than a standard cast piston. Being an aluminum alloy, it is non-magnetic.

Are you sure of your numbers? Remember that a 1003 Fel-Pro head gasket is for a 350 and has a larger opening, and about 2cc over the 305 version. A shim steel gasket will have the lowest amount of volume, so you can use one of these to obtain a higher ratio. Keep in mind that with steel shim gaskets, you will need to retorque the head bolts about 500 miles or so from first startup.

[Blue Bowtie]

Or, if you're lazy (and have MS Excel) you can plug the numbers into this:

Engine Calculator

[capriceowns]

Originally Posted by Blue Bowtie

Or, if you're lazy (and have MS Excel) you can plug the numbers into this:

Engine Calculator

thats a cool thing BB. I did it with a 5cc flat top valve relief and .020 headgasket and came out with 8.8:1 but with a .040 gasket i get much lower, it makes a big dif.

I guess I know now that I can get almost 9:1 with these like I was planning.

Is there wway to get a compression ratio while the engine is together?

[flemmingSundra]

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