sould someone explain to me the what dynamic compression ratio is. i know the compression in my car is about 165psi, but what do those 10.5:1 ratios mean?

For a very simple explaination, the 10.5 means the air mixture gets compressed 10.5 times as the piston travels from the bottom of the cylinder to the top of the cylinder.
This definition ignores how well your intake manifold and headers perform as well as what kind of camshaft (valve timing) you are running.

Well to start, you need to know that there is a difference in static and dynamic compression ratios.
The ratio 10.5 : 1 is a static compression ratio
Both compression ratios are simply a ratio of two different volumes.
It's set up like a quotient: (quotient = numerator / denominator)
or one volume divided by the other.

STATIC COMPRESSION RATIO:
Is having your numerator being the total cylinder volume when the piston is at Bottom Dead Center plus the volume of cylinder head chamber, and the denominator being just the cylinder head chamber volume.

(cylinder volume + chamber volume) / chamber volume


DYNAMIC COMPRESSION RATIO:
To understand this you need to remember that the piston doesn't begin to compress the fuel mixture until the INTAKE valve is fully closed, and that depends on the unique INTAKE valve timing of the camshaft. INTAKE valve timing is in reference with degrees of crankshaft rotation. For instance, the INTAKE valve on my camshaft closes at 61 degrees of crankshaft rotation after Bottom Dead Center. That means the cylinder is not sealed shut until the piston has traveled 61 degrees of its 180 degree rotation from Bottom Dead Center to Top Dead Center (this being the compression stroke). So at this point the piston is already up in the cylinder a little bit, no longer at Bottom Dead Center. Well, if you're still with me after all that, you know that the volume the cylinder is now less than the volume it had at Bottom Dead Center.

The only difference with this calculation from the static one is that the numerator is the volume that the cylinder has when the INTAKE valve finally closes. The denominator stays the same, being the cylinder head chamber volume.

((cylinder volume @ IVC) + chamber volume) / chamber volume

pictures usually help:
http://www.eden.rutgers.edu/~vath/327/crank.gif
where the horizontal lines are the piston locations in the cylinder.

To get really precise with these calculations you have to take into consideration all the little volumes such as the quench volume and also if your piston have valve reliefs or a positive or negative volume dome, ect…
I hope that wasn’t to confusing…


------------------
1968 Coupe (http://www.eden.rutgers.edu/~vath/Camaro/), 327/210hp project in process. PS/PB, Factory AC, adding camelhumps and a CompCam XE262
1986 Chevy Stepside (http://www.eden.rutgers.edu/~vath/Stepside/), dying 4.3L, TH-400, no A/C, 2wd, 1/4 million miles.


[This message has been edited by JIM68 (edited 11-22-2002).]

<BLOCKQUOTE>quote:</font><HR>Originally posted by JIM68:
DYNAMIC COMPRESSION RATIO:
To understand this you need to remember that the piston doesn't begin to compress the fuel mixture until the exhaust valve is fully closed,…

<HR></BLOCKQUOTE>

I think you meant "intake valve"



------------------
Chris Avery

"If you can't afford to do it right, you can't afford to do it over"

'69 Nova, 355, 9.3:1, Keith Black 12 cc D-cup hypereutectic pistons, 180 cc Dart Iron Eagles w/64 cc chambers, Comp XE-262, Crane Energizer 1.6 ratio roller rockers, Performer RPM, Holley 650 DP, TH350 w/B&M shift kit, B&M 2400 RPM Torkmaster converter, 4.10 10-bolt posi. Best 1/4: 13.098@103.41, 1.949 60'

Static compression ratio: total volume of the cylinder (piston all the way down) plus the volume of the combustion chamber divided by the combustion chamber volume. 10.5:1 means that the stroke of the engine plus combustion chamber is 10.5 times the size of the combustion chamber alone.

Dynamic compression ratio: how much you are actually compressing air in the cylinder. If your dynamic compression ratio is 8:1 you are compressing the air in the cylinder to 1/8 of its original size.

Like the others said, compression doesn't even start to happen until the intake valve closes. With high performance cams the intake is left open after the piston passes through bottom dead center because air/fuel mix continues to fill the cylinder because of velocity through the port. Once the intake valve closes, compression begins.

The 165 psi you have represents the dynamic compression ratio of the engine. There is no simple way to do it, but in theory you could actually calculate your damanic compression ratio by converting back from psi. I'm going to figure out the math one day http://www.camaros.net/forum/biggrin.gif

Eric,
The theoretical formula is related to P1*V1*T1=P2*V2*T2 using absolute units. The P2 is the peak cranking pressure, P1 is ambient air pressure. V1 is trapped volume when the intake valve closes while V2 is the combustion chamber volume at TDC. T1 is ambient temperature while T2 is the temperature of the air in the chamber after it has been heated by the heat of compression. Delta T (T2-T1)could be ignored for simplicity by assuming constant temperature if you wish. We also want to ignore losses past the rings and losses due to restrictions on the intake/exhaust as this is difficult to quantify with any accuracy. You would need to know the bore and stroke as well.

Cranking pressure is a function of cam timing (big effect) and mechanical compression ratio(small effect within the practical range) for any given displacement but...

From a practical application standpoint, the speed of the stater motor has a big effect along with ring/valve sealing losses etc so empiricaly, you can not use the cranking pressure and cam timing to determine the mechanical or dynamic compression ratio.

Yes you could do the math. No you can not practicaly determine the static/dynamic compression ratio with any accuracy using cranking pressure. Maybe that is why nobody ever does.

-Mark.

thankyou guys for the info. i understand it now better than before. still have a few years until i get that ASE master certification, but why not know these things now? anyways, thanks again!

wow thanks fuji.... i'll edit my post... i can't believe i did that...

jim

------------------
1968 Coupe (http://www.eden.rutgers.edu/~vath/Camaro/), 327/210hp project in process. PS/PB, Factory AC, adding camelhumps and a CompCam XE262
1986 Chevy Stepside (http://www.eden.rutgers.edu/~vath/Stepside/), dying 4.3L, TH-400, no A/C, 2wd, 1/4 million miles.

Mark,

Thanks for the info. I've been inetersted in learning how to "work it backwards" because we always seem to get those questions about a combo where someone has no idea what cam and compression ratio they already have. You know the line, "bought a car from a guy who put in a 3/4 race cam and domed pistons". I think it would be neat to be able to "ball park" IVC and static compression ratio by taking info you DO know and working it backwards algebraically.

You know I've thought about losses due to rings leaking and different cranking speeds, but I've had some repeatability with my car. 1st combo was 10.3:1 static, medium cam, DCR of 8.2 and cranking pressure of 215#. Second combo was 11.3: static, bigger cam, 8.2:1 DCR, and 210# cranking pressure. Interesting huh - I've thought about using my data and coming up with constants for leak by and cranking RPM. Just for grins http://www.camaros.net/forum/biggrin.gif