Here at Treadstone one of our main goals is to educate our customers to provide them with the knowledge to safely boost any vehicle they desire. This is why we have decided to introduce “the basics,” which will be a series of blogs that will present information in a simple to understand format with liberal use of lamens terms. Today we will cover the topic of equivalent compression ratio and how it related to static compression and boost pressure. First some quick definitions and examples, as some of you may or may-not know, the compression ratio of an engine is basically the largest volume within the cylinder over the smallest volume within the cylinder (volume at bottom dead center (BDC)/ Top Dead Center (TDC)) this is what is referred to as static compression. i.e. An engine with a cylinder with 10 units of volume and a chamber with a volume of 1 has a 10:1 compression ratio. Equivalent Compression is the resultant compression when boost is introduced. If an engine with a fixed compression ratio is boosted the equivalent compression ratio is no longer equal to static compression as it would be in a Naturally Aspired engine (NA) because the added boost introduces another level of compression. i.e. An engine with a cylinder with 10 units of volume and a chamber with a volume of 1, with a boost pressure of 10psi of boost has an equivalent compression ratio of about 16.8:1 instead of the 10:1 calculated previously. Compression ratios is the result of a number of engine factors, primarily the compression ratio and boost pressure. If the resultant equivalent compression ratio is too high a number of dangers can occur, such as pre-ignition which can result in server engine damage. To ensure this does not occur the first step one should take is reduce the boost or static compression ratio to within the threshold limit of the internals of the engine. Another step to counter act these effects is for the end user to upgrade higher octane fuels, forged components, and tuning among others.
This chart will help give a visual of how static compression coupled with different boost levels equate to a relative equivalent compression level, note how simple it is to add pounds and pounds of boost to the lower compression without going into the danger zone with is in sharp contrast to the higher compression ratios were just a little boost will prove hazardous without the correct supporting mods. We achieved the chart above using the formula described below: Equivalent Compression Ratio (ECR) = ((Maximum Boost into the engine (PSI) / Altitude in PSI) + 1) multiple by the static compression ratio = (Boost (PSI) / 14.7(PSI)) + 1) x SCR Boost = Maximum Boost 14.7 = Psi. at Sea Level (for those not at sea level use this link here!) SCR = Engine Static Compression Ratio For example if you were to live in Miami with an engine with a 9.5 compression ratio with 20 psi of boost you're calculations for equivalent compression ratio would look like this (20 / 14.7) + 1) x 9.5 = 22.4:1 or to look at the inverse of the equation above, you have a desired equivalent compression ratio that you must not exceed due to the limitations of an internal component, which is set at 25.0:1 maximum compression and a desired boost pressure of 25 psi what is the compression ratio you need to achiever your goal? 25.0/ ((25/14.7) +1) = 9.26:1 Static Compression Ratio *note colors are just for dramatization purposes, plenty of our customers have but 15 pounds of boost with a 10.0:1 compression ratio with the correct supporting mods without a problem. Almost anything can be with the correct supporting mods and fuels!