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What is Dynamic Compression Computing your dynamic compression is very critical when designing an engines combination if you want it to perform properly, especially if you are to run pump gas. This is an essential concept in designing an engine for performance use so we will attempt to help you better understand what it is.
We first need to understand what “compression ratio” (CR) is, also known as "static compression ratio". This term represents the ratio of the swept volume of the cylinder (displacement) to the volume above the piston at top dead center (TDC). An example of a hypothetical cylinder having a displacement of 660cc and a 60cc combustion chamber (plus volume over the piston crown to the head) the CR would be 660/60, or 11:1. If we were to mill the head so that the volume above the piston crown was decreased to 50cc, the CR would now be 660/50, or 13.2:1. Now, if we hogged the chamber out to 65cc, the CR would now be 660/65, or 10.15:1.
It is understood that high performance engines typically have higher compression ratios than the average street cars. Higher CR improves fuel efficiency, throttle response & increases horse power. So why not bump up the CR even more, once the compression ratio exceeds a certain point, detonation will occur. Detonation kills power and can destroy the engine. The amount of compression a given engine can handle is determined by many factors. Some of these include combustion chamber design, head material, use of thermal coatings, cam profile, octane of fuel to be used, etc. Higher octane fuel has a higher resistance to detonation therefore it can accommodate higher compression.
A common question among many of my customers is how much compression can I run? Even if you know all about your engine and have decided what fuel you are going to use, the question can not be answered without determining your dynamic compression. How do we determine what the dynamic compression ratio is, by referencing the camshaft specs?
Think about how a four stroke engine works. The power stroke has been completed and the piston is heading up in the bore. The intake valve is closed and the exhaust valve is open. As the piston rises it is helping to push the spent combustion gasses out the exhaust port. The piston reaches TDC and starts back down. The exhaust valve closes and the intake valve opens. Fresh fuel and air are drawn into the cylinder. The piston reaches bottom dead center (BDC) and starts back up. This is the critical point as far as understanding DCR. At BDC. the intake valve is still open, consequently even though the piston is rising up the bore, there is no compression actually occurring because of the intake valve being open. Compression does not occur until the intake valve closes. Once intake vale is closed then the air fuel mixture starts to compress. The ratio of the cylinder volume at IVC over the volume above the piston at TDC represents the dynamic compression ratio. The DCR is what the air fuel mixture actually "sees" and this is what matters, not the static CR. Dynamic compression is dependent upon the intake valve closing. Cam specs have as much effect on DCR as does the mechanical specifications of the motor. Also remember that when under boost or spraying nitrous that your DCR will be dramatically higher then when not under boost or spraying nitrous.
Your dynamic compression will always be lower than static CR. Most performance street and street/track motors have DCR in the range of 8-8.5:1. With typical cams, this translates into static CR in the 9.0-11.0:1 range. When running compression any higher than this there could be detonation problems with pump gas. Engines with "small" cams will need a lower static CR to avoid detonation. Engines with "big" cams having a later IVC point can tolerate a higher static CR. When race fuel is used, much higher dynamic & static compression ratios can be used. This is because of the higher resistance that race fuels have against detonation.