Determining compression ratio

So I have been asked on several occasions what my compression ratio is. As a matter of fact, I am damn curious myself. Since the previous owner had no information to offer me, how can I determine this? I was told by one member to get a casting number off a piston. If I can't find one, what is an alternative method? I need specifics here....
Thanks
MC J

The compression ratio is determined by cylinder combustion chamber size (they vary with manufacturing tolerances between heads even with the same casting numbers) head gasket thickness and diameter, piston dome/valve relief size and the height of the cylinder block (again this varies with manufacturing tolerances, some are taller than others even with the same casting numbers). The best way to figure ratio is to measure all of the different dimensions of the parts involved.
More importantly is the question "what is my cranking compression with a compression tester?"
A low compression ratio engine with a small duration camshaft will have the same compression as a high compression ratio engine will have with a long duration camshaft. If the camshaft closes the intake valve early (small duration cam) the compression cycle will start sooner and build more compression. If the intake valve closes later (long duration cam) the compression cycle starts later and will not build as much compression. Matching the camshaft and compression ratio so you have around 180 PSI of cranking compression will give you an efficient engine for use with premium pump fuel. Keep it down to 160 PSI for regular pump fuel.
Hope this helps someone!
Sincerely
Dennis Moore

Dennis Moore:
The compression ratio is determined by cylinder combustion chamber size (they vary with manufacturing tolerances between heads even with the same casting numbers) head gasket thickness and diameter, piston dome/valve relief size and the height of the cylinder block (again this varies with manufacturing tolerances, some are taller than others even with the same casting numbers). The best way to figure ratio is to measure all of the different dimensions of the parts involved.
More importantly is the question "what is my cranking compression with a compression tester?"
A low compression ratio engine with a small duration camshaft will have the same compression as a high compression ratio engine will have with a long duration camshaft. If the camshaft closes the intake valve early (small duration cam) the compression cycle will start sooner and build more compression. If the intake valve closes later (long duration cam) the compression cycle starts later and will not build as much compression. Matching the camshaft and compression ratio so you have around 180 PSI of cranking compression will give you an efficient engine for use with premium pump fuel. Keep it down to 160 PSI for regular pump fuel.
Hope this helps someone!
Sincerely
Dennis MooreMy cans are all at 205-10 except one that is like 179...

[QUOTE]Originally posted by Dennis Moore:
Matching the camshaft and compression ratio so you have around 180 PSI of cranking compression will give you an efficient engine for use with premium pump fuel. Keep it down to 160 PSI for regular pump fuel.
Hope this helps someone!
Sincerely
Dennis Moore
OK so how does 180 at cranking with a blower compare to 160 cranking & a blower??? Then throw in the % percent of the drive or additional boost that could excced the cranking compression, what are your thoughts Dennis?

To answer your direct question, the proper way to find out your compression ratio is to measure it with water.
You must remove the head and measure the amount of mililiters of water the combustion chamber of the head will hold with the valves installed of course. That will give you the amount of cc's. 1cc holds 1ml of water.
Do the same with the cylinder with piston at TDC and the head gasket should have it stamped on the package. Add them up and look it up. I do have a formula laying around here somewhere, that I (of course) cant remember right now. When I find it I'll let ya know.
Anyway thats how to determine the comp ratio of your engine.
DAVE

Heres a link that both tells you exactly what it is (simple statement) and how to calculate it...
http://www.turbofast.com.au/TFcomp.html

Dear Gnarly,
Tough question, that borders on a question that only an engineer could answer. I hope that we can get someone to respond to it!
Dennis moore

Dennis Moore:
Dear Gnarly,
Tough question, that borders on a question that only an engineer could answer. I hope that we can get someone to respond to it!
Dennis mooreI'm not an engineer but I think I can answer this, Static C/R + Boost [lb/sq/in]= Dynamic C/R. wink

058:
Dennis Moore:
Dear Gnarly,
Tough question, that borders on a question that only an engineer could answer. I hope that we can get someone to respond to it!
Dennis mooreI'm not an engineer but I think I can answer this, Static C/R + Boost [lb/sq/in]= Dynamic C/R. wink Nice!.... :D

OK yes you can do it with water but the way you can do it without removing the heads.
Titl engine on side so spark plug hole it directley up.
remove that cyl. lifter. goto TDC adn measure the amount of oil it takes to fill the cavity.
write this number down. Then move engine to BDC and continue filling with oil. ANd this amount to the first amount and write that number down.
take the 2nd number ( Bigger ( and divide it bvy the 1st ( Smaller ) the answer will then be the first number in the compression @@ to 1.

[QUOTE]Originally posted by 058:
Static C/R + Boost [lb/sq/in]= Dynamic C/R.
Something just does sound right about this. If your static & say 150 at cranking compression & you added the amount of boost which may be say 10 PSI then 160 would be the dynamic compression. This does not sound right as cranking comp will be lower than running comp at a given RPM not to mention the difference in volumetric efficiency of the assembly. ???

"Rexone" Awesome turbo site thanks!!!!! :D :D :D

Charley, No, It isn't as simple as that. There is a mathamatical formula for calculating dynamic C/R and I don't have it handy right now. But if my feeble old memory serves a 8 to 1 engine boosted to 1 atmosphere [app. 14.5 lbs] will make about 15 to 1 dynamic C/R. or almost double the static C/R because you are stuffing approximently twice the voluume of a/f into the cylinder. There is more info regarding this in books for turbochargers and superchargers.

058,Yeah I think it's time to do a lot of reading! I'll get the book out again, I haven't read it in years but it would be good to read it again. I don't think it should be so hard to understand but adding an air compressor to the motor changes EVRYTHING. I don't need the biggest or baddest, but what I have should be as dialed in & efficient as humanly possible to maximize the available power & when not wide open getting the best GPH I can tune in, it’s a fine balancing act.

Charley, What are you building and what supercharger [or turbo] are you using?

058, I sent you a private message.

Charley, Got it. Anything I can help with?

Remove the cylinder head and tell us what you see. We'll walk you through it and come up very close. Or do it this way: Remove the spark plug of one cylinder. Put the piston at Bottom dead center. Turn the motor upside down (easier if it is removed from the boat). Fill the cylinder to the bottom of the spark plug threads with a liquid. Solvent works well, maybe with a little atf for color. Record how much fluid you could get into the cylinder. Now drain the cylinder by rolling the motor over (still easier if motor removed from boat). Now put the piston at Top dead center with both valves closed. Fill the cylinder and record the how much fluid it takes. Give me the numbers and I will tell you: A: your compression ratio. or B: you spilled some.
Or just do like most everyone else that has a "blueprinted" motor. 13.5 sounds good so tell 'em that. Also do not rely on compression gauges as many are not accurate and camshaft duration, where it's a degreed, and atmospheric pressure all have to do with compression readings. Remember the engine is just an air pump. More air available, it pumps more air. Air changes more than you think on each day. We used to tune our Pro Gas Flat by a compression gauge. We took compression readings at each track. Best air density gauge we ever had. I better quit. My age is showing!

Well I think it'll be hard to rotate the boat :) to check the compression. I know of several ways to ck compression ratios but I wanted to see what this engine is really supposed to be at static. After all I wouldn't really want to blow 8 psi on to a stock 9.5:1 motor with cast slugs & std marine gaskets. But an 8.0:1 motor with cast would be a whole different story. The bottom line is knowing what your compression is to supply it with the right boost, timing & octane ect..... I think that I found out what it is but need to verify with secondary sources.

Charley, For a secondary source try a race track such as Petaluma, they usually have one of those devices that verify comp. ratio. They are quite accurate.

058, thats a great idea actually! But I don't thinkI will need to do that. By the way the device I think if memory serves me correct is a whistler, I should know that sine I used to live in the tech lanes :cool:

I'm not sure if anyone really answered this one yet about the blower, but one piece of info you need to remember is that the blower is not producing 10 lbs of boost statically. In fact, most only come to full pressure at full RPM. For example, my blower motor in the Daytona was actually drawing vacuum at idle, and all the way through 1100 when it started making boost. At 6500 RPMs, I had it set for 10 lbs even. So my "static" compression was only 7.25:1 with the pistons I was running.
If you were asking regarding total developed compression at RPM, I don't know how to figure that out. You would have to do the math for PSI over the area in question for total pressure.

Or checkout Kuhlsuperchargers.com they have tech section that has info about static and dynamic comp ratios at different levels of boost. :cool:

I wonder if you can determine the compression ratio of you and your sweetheart.....

Blown 472:
Or checkout Kuhlsuperchargers.com they have tech section that has info about static and dynamic comp ratios at different levels of boost. :cool: Way cool, That is what I was hoping for! You got 5 stars from ME :D I knew that some one else had done all the math long before I had the question so this info had to be somewhere! :D thanks again.
http://www.kuhlsuperchargers.com/tech%20p05.htm

Thank you, your are welcome glad I could help. 78, you funny I tell you what when I get ontop of her it gets into a dynamic thang :D :D
Then there is a whole new meaning to blown. eek!
[ September 12, 2002, 01:36 PM: Message edited by: Blown 472 ]

[QUOTE]Originally posted by Blown 472:
I tell you what when I get on top of her it gets into a dynamic thang
Kind of like being Blown & Injected???
[ September 12, 2002, 01:44 PM: Message edited by: gnarley ]

Blown 472:
Thank you, your are welcome glad I could help. 78, you funny I tell you what when I get ontop of her it gets into a dynamic thang :D :D
Then there is a whole new meaning to blown. eek! It is dynamic because stroke and duration change with condition. Position also becomes a factor and if she has gained a few pounds that might get the ratio even lower. If she is a little loose like Anna Nicole Smith and it's like throwing a vienna sausage down a hallway, you are going to have some serious problems with both discharge and lubrication and cylinder exhaust.
J

No problem with gettin loose, she stays hooked up just fine, in fact it is almost like sprayed vht. And dont forget the cylinder lube, hell she could set up shop and sell the stuff. :D

Blown 472:
No problem with gettin loose, she stays hooked up just fine, in fact it is almost like sprayed vht. And dont forget the cylinder lube, hell she could set up shop and sell the stuff. :D Well, now. She should put it on e-bay and see how much she gets...

Somehow I think we are not talking about superchargers anymore.....funny how these topics get turned around :D :D :D