Calculating compression ratio and cylinder capacity

Engine capacity itself is simply bore x stroke. The radius of the bore times itself, times the stroke. This gives you cubic capacity, or swept volume. Or, radius squared times pi gives us surface area, surface area times height gives us volume. Compression ratio is the difference between the swept volume and the combustion chamber capacity remaining when the piston goes through top dead center. You need two find three other measurements. One is the cubic capacity of the cylinder head combustion chamber. The other is the dome volume of the piston, and the last one is the volume of the head gasket.

With the head upside down, you can place a glass, or heavy plexiglass panel over the combustion chamber sealed with vaseline. Valves in place, spark plug, and with a small hole in the glass, you instill a fluid to fill it to the opening, fully filling the chamber to the surface of the cylinder head against the glass. Then you need to remove the fluid and measure its volume, a syringe works very well. A good fluid to use is isopropyl alcohol. If you then put the removed contents of the fluid into a measuring beaker of some type, you can get your capacity.

For dome volume, I seal the sides of the piston with vaseline to the cylinder so it will hold fluid. Place a glass pane over the cylinder also sealed with vaseline, and press the piston lightly against it. Fill with fluid, and measure that volume. Next, carefully measure the distance from the top surface the glass was placed against, to the outside edge of the piston. The bore diameter, times the distance the piston is down the cylinder would measure a certain volume. That volume, minus the volume of fluid you removed with the glass against the piston, is your piston dome volume.

For the head gasket, you simply measure its thickness with the head torqued against it. Measure its physical bore diameter around the combustion chamber. Bore radius squared times pi, times height equals gasket volume.

Now your armed with your carefully (and duplicated repeatedly to insure accuracy) obtained measurments. Take your combustion chamber capacity, and add the gasket volume capacity and write it down. If your piston is domed, subtract the dome volume you measured from the other two. If the piston is dished, in which case your piston was likely pressed against the glass at the outer edges, then you would ADD whatever volume you obtained, to the first two figures.

This is the actual combustion chamber volume that remains when your piston has reached top dead center. Add this chamber capacity to your swept volume, and divide the result by the combustion chamber capacity. This is your ratio.

For example, the 308 has a swept volume of 365 CC. The head has some 40.5 CC volume. I cant recall off the top what the dome volume of the piston was, but lets add 9 CC, plus 1 CC for the head gasket. 50.5 CC. 365 + 50.5 gives 415.5 CC. 415.5 divided by 50.5 would equal 8.227 to 1.

When your doing this, carefully measure the actual point that the piston stops at TDC. Wiseco Pistons and a few others have documented the 308 piston stopping .055" before true TDC. Calculate that into your figures and you can see it would drop actual CR considerably. I have a set of Borgo 9.7 LM pistons that do the same thing, .055" below the edge of the liner. So actual CR with these 9.7 pistons is actually down around 8.4:1. This is why you hear figures of stock 308's having an actual CR of only around 7.5:1 If your going to measure all this stuff, it would be interesting to hear your results. I also got really anal and measured the capicity of the chamfer at the top edge of the liner.

Have fun~

 

another thing to keep in mind is that is calculating a STATIC compression ratio, a bit more math will get you the DYNAMIC ration which is what is really critical since that is what the engine is running on. it'll also be lower than static. here's a few sites for calcs and good reading, engineering types.

http://mb-soft.com/public2/engine.html
http://www.eng-tips.com/threadminder.cfm?pid=108&page=1
http://www.smokemup.com/auto_math/index.php
http://www.wallaceracing.com/Calculators.htm

 

It is the volume of the fluid that matters, not the density. (You are doing this at a reasonably constant room temperature.) I used mineral oil when I did mine. The point of it was more to equalize the combustion chamber volumes one to another rather than to arrive at or calculate some specail compression ratio down to the thousands. The engine is going to sort of make it's "own" true compression ratio depending on your cam overlap, RPM, intake and exhaust flow characteristics, etc. - so this volumetric thing is only a static approximation of a dynamic process.

BTW, about the easiest tool to use is called a burette - it is a verticle calibrated clear cylinder marked in CCs and has a precision valve to let the fluid out in perfect control. They use it in chemistry to do titrations - they make both glass & plastic, cheap or expensive.

BTW-2; doesn't this sort of discussion make you shudder in amazement at some of those hot-rod "engine build" shows where they just grab off-the-shelf parts and slap them together without ever measuring, weighing, or balancing a single thing?

 

This is a really good calculator for dynamic compression that I have used.

http://www.empirenet.com/pkelley2/DynamicCR.html

In regards to the original posters questions, I would hope that every single engine rebuilt has these calculations done. I know that a 18 year old kid with a beater 5.0 would never assemble a new motor without doing the math you describe, so I am quite alarmed how many motors, especially "exotic" are assembled to be just the way they were, just for the sake of originality.

And using a pistion that is .055 down the bore, on top of a .039 head gasket gives a quench of .094 which is perhaps the worst I have ever heard of, and will leave perhaps 20+ hp on the table. With the short stroke, open chamber and good rods of a Ferrari 3X8, I would be looking at cutom made pistons and about .028 deck clearance, which would be .017 out of the liner with a .045 Cometic head gasket. This type of machining and math is being done 100 times a day for the hot rod Fords and small block Chevys.

Jay

PS You will note that cam timing has a much bigger effect on dynamic compression than actual cc of the head. So now you are going to teach yourself all about cam timing too!

 

amazing isn't it, 'Blueprinting' a common thing when building chevy and ford motors seems to get passed on. I would assume that those here who are building the 'monster' motors are doing that though, I hope.

http://www.superchevy.com/tech/0110sc_blueprint_your_motor/

 

I would argue that its much more than only a static approximation, you need to know specifically what you are dealing with in order to discuss the issue with a piston manufacture and to order a new custom piston. And as Scott eluded, you also need that information to formulate dynamic compression. While burettes and beakers are very good tools and all, you can get just as good of accuracy with insulin hypodermic needles. Lots cheaper and much easier to find out here in BFE, lol.

I have been shaking my head for years at the things the "hot rodders" do without barely a clue whats up. Thats the kind of crap I did at 11 years old with a mini bike, swapping parts over from other engines without a care in the world. But when you see grown men doing it you really wonder. I knew a guy who bought what he thought were 11:1 pistons for a 350 Chev. He slapped it together without measuring anything, and the engine could barely turn over it had so much compression. It always knocked, it always dieseled, and it didnt last very long. But it did go like a bullet for a week or so. Not nearly as funny as a kid I knew who put a 671 blower on a old worn out 396 Chev in a 68 Firebird though. Stepped on the gas, the tires smoked, the front tires lifted off the street, and it litterally blew itself to bits in less than 200 feet. Everytime I see that blown Challeger in "Cars" it reminds me of that Firebird, and I just fall over laughing.

Tojo, as James pointed out, the fluid you use makes no real difference, though for me alcohol seems to be easier to fully remove and measure. Your simply measuring static volume at room temperture, and unless your going to be taking all day, evaporation shouldnt be a factor. I took all my measurements three times to be absolutely sure of accuracy. If any one came up different, I did it again. I want to see three equal measurements. You dont need rings on the pistons. Just smear vaseline in around the piston, and carefully wipe away any excess. And when you do the CCing on the head, alcohol is wonderful at testing sealing of the valve seats. Keeping a watchful eye, if you see even the slightest amount of leakage while looking in through the ports, your valve sealing is in question. But again, a wipe with vaseline around the valve seat will seal enough to make your measures.

 

I perfer syringes too. However I perfer to measure the fluid going in rather than coming out...I don't know how you would ever get every drop out.

Anything with a low viscosity and more important, low surface tension so it doesn't trap air bubbles will work. I like alcohol be cause it dries fast with a little air sprayed on it so you are ready to repeat the measurement faster than with mineral oil. paint thinner is good too, but is eats thought the Vaseline/grease faster. Rubbing alcohol (30% water or something like that) seems to barely touch white lithium grease, which is my favorite

oh, you left pi out of your displacement formula in your first post.

 

Measuring the fluid going in rather than going out was the way I did it, too. I happened to have the burettes around because I was in the chemical business back then. I did the mineral oil because my Guru engine builder master told me to use it, as it would not evaporate fast. He also made me assemble the whole engine onto plastigage strips, then take it back apart and check all the clearances. Also to assemble the valve gear, and carefully measure each and every pushrod & rocker travel at the valve stem. And weigh and balance each and every moving part. It took weeks to get all this done.

I think he was trying to teach me decent work habits as much as trying to help me build the best possible engine...

 

One source is the Summit burette kit: SUM-911581 $109.95 SummitRacing.com

 

I think your getting hung up on weight, not volume. This makes perfect sense because as the temperture of fuel changes, and as it expands and contracts, its mass would change. Aircraft that haul cargo and/or passengers have very critical weight and balance criteria, and specific density of fuel vs temperture becomes very critical when your dealing with thousands of pounds and thousands of gallons of fuel. They generally only want enough fuel onboard to stay within FAA guidelines for 45 minute reserve beyond thier time enroute. Carrying extra fuel creates higher costs by burning more fuel.

With the small amounts were dealing with, and the fact that everything should be room temperture, specific gravity and density are really not factors. And while alcohol may be a more rapidly evaporating fluid, within the time it takes to fill and evacuate the the part being measured, evaporation is not significant. I measured it going both ways to double check myself. If 40.5 cc's went in, I wanted to see 40.5 cc's come back out, and I did.

 

When I was younger, I built a couple of motors (using JE pistons by the way and had to blue print everything. Calculating the compression ratio is pretty straight forward (as you seem to have figured out). It's just a simple formula. The important part is getting the correct measurements off the cylinder head chamber. If JE made your pistons, they should be able to give you specs on any volume on them +/- due to cuts, etc.

Another important factor, along with figuring out your compression ratio, is making sure all the chambers in the cylinder head are equalized. You want to make sure all the cylinder head volumes are as close as possible. Sometimes very slight variations in the height of the valves, seats, etc. can affect the CC. When I built one motor, I had to adjust the cylinder head volumes a tiny bit in order to get everything exactly right.

Don't forget to take into account the volume around the top ring on the piston by the way

Ray