![]() ![]() Most performance street and street/track motors have DCR in the range of 8-8.5:1. Because DCR is dependent upon IVC, cam specs have as much effect on DCR as does the mechanical specifications of the motor.ĭCR is much lower than static CR. The DCR is what the air fuel mixture actually “sees” and is what “counts”, not the static CR. The ratio of the cylinder volume at IVC over the volume above the piston at TDC represents the dynamic compression ratio. Once IVC is reached, the air fuel mixture starts to compress. Compression does not begin until the intake valve closes (IVC). Consequently, even though the piston is rising up the bore, there is no compression actually occurring because of the open intake valve. This is the critical point as far as understanding DCR. The piston reaches bottom dead enter (BDC) and starts back up. Fresh fuel and air are drawn into the cylinder. The exhaust valve closes and the intake valve opens. The piston reaches TDC and starts back down. As the piston rises it is helping to push the spent combustion gasses out the exhaust port. ![]() The intake valve is closed and the exhaust valve is open. The power stroke has been completed and the piston is heading up in the bore. How is this so? Well, think about the Otto cycle and how a four stroke engine works. ![]() Why? Because without reference to the camshaft specs, talking about (static) CR is next to meaningless! The above brings up the question that is often on the mind of performance enthusiasts and engine builders: how high should my CR be? Even if you know all about your engine and have decided what fuel you are going to use, the question cannot be answered as phrased. Higher octane = more resistance to detonation and the ability to tolerate more compression. Once these mechanical aspects of the engine have been fixed, the main variable is fuel octane. These include combustion chamber design, head material, use of combustion chamber coatings, etc. The amount of compression a given engine can handle is determined by many factors. Detonation kills power and it kills engine. So why not bump up the CR even further? Once CR exceeds a certain point, detonation will occur. Higher CR also improves fuel efficiency and throttle response. Simply put, higher compression makes more hp. Conversely, if we hogged the chamber out to 60cc, the CR would now be 510/60, or 8.5:1.Įveryone knows that high performance engines typically have higher compression ratios. If we were to mill the head so that the volume above the piston crown was decreased to 40cc, the CR would now be 490/40, or 12.25:1. For example, if a hypothetical cylinder had a displacement of 450cc and a 50cc combustion chamber (plus volume over the piston crown to the head) the CR would be 500/50, or 10:1. This is a simple concept and represents the ratio of the swept volume of the cylinder (displacement) to the volume above the piston at top dead center (TDC). The first thing to understand is that “compression ratio” (CR) as it is usually talked about is best termed “static compression ratio”. While seemingly esoteric, this is an essential concept in designing an engine for performance use. In our attempt to help our customers understand performance and what makes an engine produce power we are going to explain the concept of dynamic compression ratio (DCR). Electronic Fuel Injection – Aftermarket ECU’s. ![]()
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