How is compression ratio measured?

[Fahrvergnuugen]

This may sound like a really dumb question, but I've never really thought about it before.

I know how you can theoretically calculate compression ration, but how is it actually/physically measured?

[Lorenfb]

It's the ratio of cylinder volumes, calculated with the piston fully down (BDC)
to it at TDC.

[cstreit]

THere is a comprehensive article on how to measure and calculate that in Bruce Andersons Porsche Performance Handbook. Worth buying if you don't have it!

[tsuter]

Is this a trick question???

[SteveStromberg]

http://www.aircooled.net/gnrlsite/resource/specgeninfo/calcs.htm

[ChrisBennet]

Originally posted by SteveStromberg
http://www.aircooled.net/gnrlsite/resource/specgeninfo/calcs.htm

Steve,
Have you looked at that page? It doesn't look like it would work for anything but flat topped pistons. Am I missing something?
-Chris

[ChrisBennet]

Originally posted by Fahrvergnuugen
This may sound like a really dumb question, but I've never really thought about it before.

I know how you can theoretically calculate compression ration, but how is it actually/physically measured?

You use burette to fill the head or whatever you're trying to measure with a precisely measured amount of liquid. With a burette and a way to accurately measure depth, you can calculate the volumes and arrive at the compression ratio using Loren's (excellent BTW) definition. For pistons with a flat area around the outside you can measure deck height much easier than using the Jo block method.
-Chris

[RoninLB]

domed pistons deck height volume requires a built up "plate" that covers the dome at TDC.. The head gasket CCs can be measured mathmetically. I never attempted to do any of this.

[ChrisBennet]

Originally posted by RoninLB
domed pistons deck height volume requires a built up "plate" that covers the dome at TDC.. The head gasket CCs can be measured mathmetically. I never attempted to do any of this.

To get the piston dome volume I put the top ring on a piston (with a little Vaseline to seal the ring gap) and push it to the top of the cylinder and then fill the cylinder to the top. I subtract the amount of fluid required from the calculated volume for the flat topped piston to get the volume of the dome.


-Chris

[ChrisBennet]

Here's a better calculator: Bruce Bowlings compression ratio calculator
-Chris

[126coupe]

Thats a cool burette have had no luck w medical supply stores any suggestions where i can buy a burette???

[thabaer]

Medical suppliers will have scissors, scalpels and slippers....

On a whim I went to yahoo under lab supplies:
http://dir.yahoo.com/Business_and_Economy/Business_to_Business/Scientific/Supplies_and_Equipment/Laboratory_Equipment/Glassware/

Thanks a lot, now I have an urge to spend more money

BTW: a flat sheet of tempered glass w/ a fill hole is normally used to help control the miniscus when cc'ing heads.

[ChrisBennet]

Originally posted by thabaer
Medical suppliers will have scissors, scalpels and slippers....

On a whim I went to yahoo under lab supplies:
http://dir.yahoo.com/Business_and_Economy/Business_to_Business/Scientific/Supplies_and_Equipment/Laboratory_Equipment/Glassware/

Thanks a lot, now I have an urge to spend more money

BTW: a flat sheet of tempered glass w/ a fill hole is normally used to help control the miniscus when cc'ing heads.

Use a piece of plexiglas. It's much easier to drill the hole and cut it to fit the head's (recessed) sealing surface.
-Chris

[Randy Webb]

Yes, plexi.

If anyone didn't pick up how it's done, here's an overview:

For simple shapes (perfect cylinder) you can just measure radius and length and calculate.

Fo more complex shapes, you fill the volume with a liquid (colored ATF is good) and measure the amount of liquid. A burette is a graduated cylinder with a stopcock that can accurately measure varying amounts of liquids over a range of volumes. You will need to approximate the volume first, then buy a burette that will cover that range. Also you need a burette holder and stand. See procedures above and various hot rod books for how to do this, such as How to Hotrod VW Engines.