Originally Posted by SloRice (Post 9195812)

dynamic compression (ie. combustion) is significantly higher than static compression (ie. 9.5:1, 10:1, etc).

With that being said, do you really think 1 point of compression ratio is really going to affect the max amount of boost you can run??

ps - up to 34psi on 93 octane with my car without any adverse affects. ;)

Originally Posted by chetrickerman (Post 9196183)

there is no such thing as dynamic compression. dynamic compression means that the compression ratio can change, and obviously compression ratio of an engine doesnt change. if you build it as 9.5:1, it is not going to change to 11.5:1 then go back to 9.5:1.

the only thing that is dynamic, is the amount of Air/fuel going into the engine and rpms.

Originally Posted by EVOhead (Post 9199218)

Depending on cam design cylinder PRESSURE does change. There are other factors in this also like exhaust back pressure, but cam design plays a big role. Please go educate yourself before posting about something you don't understand.

Originally Posted by widge (Post 9199443)

Im running

manley turbo tuff I-Beam Rods
Weisco 9.0:1 Pistons

Originally Posted by chetrickerman (Post 9199288)

compression ratio is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

cams do not change that. nothing changes that except taking out the pistons, rods and crankshaft and placing in a larger or smaller size of each.

your right, cylinder pressure does change. it changes with more air/fuel sucked or forced into the cylinder, or rpms, but the COMPRESSION ratio does not change.

not trying to start a huge debate, but before you go and tell someone to go educate themselves, you better make sure your facts are straight.

Originally Posted by kozmic27 (Post 9201125)

Headwork can also change the compression ratio, particularily if the head is milled for any reason. This is not all that uncommon, particularily if an aluminum head engine is subject to overheating. Changing the thickness of the head gasket also will change the compression ratio.

Originally Posted by kozmic27 (Post 9201125)

While the mathematical compression ratio does not change, IE the relationship between cylinder volume at BDC and TDC, cams and cam profiles do change the EFFECTIVE compression ratio, ie the amount of air at by MASS at BDC vs TDC. This is because cam overlap and duration effect the amount of air that will leave the cylinder during compression stroke prior to the valves both fully closing. It is a well established fact that longer duration high overlap cams will allow a higher MATHEMATICAL compression ratio than milder duration low overlap cams. This is true on any engine, wether it is a twin cylinder engine in a Harley Davidson, a big block v8 in an old muscle car, or a small forced induction 4 cylinder like in the Evo. Variable cam timing engines, like in the evo, in effect do have a dynamic compression ratio because the effective cam profile changes during operation.

Originally Posted by EVOhead (Post 9202207)

Well you said this:

Dynamic compression is Effective compression. Slorice's post was not wrong.
There is enough info after Slorice's post for anyone reading this to figure it out. Oh and Dynamic compression is not VE. It contributes of course.

Just do a google search for dynamic compression, for anyone who wants to understand this more.

Here is a calc and some info:
http://www.rbracing-rsr.com/comprAdvHD.htm

Originally Posted by Wikipedia (Post 9196183)

Dynamic compression ratio

The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke, and that the volume compressed is the actual volume.
However: intake valve closure (sealing the cylinder) always takes place after BDC, which may cause some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. Intake port tuning and scavenging may allow a greater mass of charge (at a higher than atmospheric pressure) to be trapped in the cylinder than the static volume would suggest ( This "corrected" compression ratio is commonly called the "dynamic compression ratio".
This ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or "nominal" compression ratio.
The actual position of the piston can be determined by trigonometry, using the stroke length and the connecting rod length (measured between centers). The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)
The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine with lower compression but earlier intake valve closure.

Originally Posted by SloRice (Post 9195812)

dynamic compression (ie. combustion) is significantly higher than static compression (ie. 9.5:1, 10:1, etc).