Dynamic Compression Ratio

 

Dynamic Compression Ratio

 

 

Will an engine run on pump gas without detonation (pinging)? That's the question many people ask themselves before building or modifying an engine. Most people think the compression ratio is the biggest determining factor. Well that's partly true but there are two types of compression ratio (CR), STATIC and DYNAMIC. Static compression ratio (SCR) is the most commonly known and is most often referred to in engine specs. It's the ratio of cylinder volume when the piston is at the bottom of it's stroke vs at the top of it's stroke. There is a simple formula that plugs in the cylinder specs to figure it. Cubic inches, and combustion chamber, piston dome, and head gasket volumes most affect SCR.  Dynamic compression ratio (DCR) is a more important factor in determining how well an engine runs and whether it will run OK on pump gas (93 octane or lower). DCR, quite simply, is the static compression ratio after some of it is bled off by the intake valve closing after the piston passes bottom dead center. It is most affected by the effective camshaft intake valve duration (usually measured at .006 lift on hydraulic cams). It's also affected by the amount of advance or retard of the camshaft. Of course the static compression ratio is a large determining facor in DCR too. There is a complex formula to figure DCR that takes into account all the above specs plus others. That and more detailed info is available for downloading onto your computer by doing a search on the Team Chevelle website for "Pat Kelley" in the performance section. Download "Pat's Page". You must know very detailed specs of your engine to use it though.

Per Mr. Kelly the desirable DCR range for a street engine is 7.5 to 8.5-1, with 8.25 being about the practical limit without aluminum heads and careful tuning. The desirable range for race engines on race gas is 8.8 to 9.0-1 per his article. Aluminum heads dissipate heat faster and generally allow a higher DCR than cast iron heads with all specs being the same.

An example is a stock 396, 325 hp Chevy engine with 10.25-1 SCR. With the stock L35 short duration cam installed with no advance or retard the intake valve closes at 53 degrees after bottom dead center (ABDC). That bleeds off relatively little compression and the DCR would be above 8.5-1. That engine would likely ping under load on 93 octane gas. If the cam were replaced with a larger duration performance cam with an intake valve closing of say 64 degrees ABDC, it would bleed off more compression and lower the DCR to around 8.25-1. Then the same engine in good tune would likely run without pinging on 93 octane. In other words, the intake valve stays open longer after the piston passes bottom dead center and starts back up on the compression stroke. This releases more of the cylinder pressure and lowers the DCR, or EFFECTIVE compression ratio. DCR also directly affects compression tester PSI readings. So the second example with the bigger cam would have a lower compression tester PSI. Of course the bigger cam would shift the engine's power curve upward in the RPM range and cause rougher idle, lower vacuum, etc.

In summary, just because an engine has say an 11.0-1 STATIC (the common spec) compression ratio, doesn't mean it won't run without detonating on pump gas. With a long enough duration cam it should be OK. But the safe bet is to calculate DCR before building an engine or changing cams.