VTC and dynamic compression ratios
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The effect on the dynamic compression ratio would be the result of simply changing the intake valve closing in relation to the pistons position in the bore as well as a result of vtc's effect on overlap.
It is hard to speculate exactly how and when though without much further calculation due to the fact that so many other variables exist in terms of dynamic compression. The cam design is only part of the picture, as your exhaust and intake design will have direct effect as well.
Basically state though, I believe it would be safe to say that dynamically speaking torque production is your indicator. These engines well tuned exhibit a very broad and almost dead flat torque curve which is saying alot in terms of volumetric efficiency and in turn dynamic compression. The torque these motors produce over such a wide powerband is certainly the result of the VTC function.
Closing the intake valve earlier at lower rpm's, when intake and port velocity is lower, aids in cylinder filling by reducing overlap and thereby reducing intake reversion and/or simply the presence of exhaust gas (which takes up space). It also closes in a period when the cylinder is closer to BDC exposing more cylinder volume for filling (this is especially effective with a taller high compression piston). This all aids in dynamic compression at lower rpm.
As rpms increase though, the intakes move towards overlap and later closing lends itself more to utilizing the inertia of the mixture to "cram" the cylinder, as well as evacuate the cylinder of combustion by-product from the previous cycle. Once again increased volumetric efficiency is being achieved and your dynamic compression ratio is rising with it.
The overall effect here is somewhat dependent on the exhaust and intake design and it's resonant rpm. If the exhaust and intake system are designed hand in hand then manipulation with VTC should be very successful in creating as broad and flat a torque curve as possible.
It is hard to speculate exactly how and when though without much further calculation due to the fact that so many other variables exist in terms of dynamic compression. The cam design is only part of the picture, as your exhaust and intake design will have direct effect as well.
Basically state though, I believe it would be safe to say that dynamically speaking torque production is your indicator. These engines well tuned exhibit a very broad and almost dead flat torque curve which is saying alot in terms of volumetric efficiency and in turn dynamic compression. The torque these motors produce over such a wide powerband is certainly the result of the VTC function.
Closing the intake valve earlier at lower rpm's, when intake and port velocity is lower, aids in cylinder filling by reducing overlap and thereby reducing intake reversion and/or simply the presence of exhaust gas (which takes up space). It also closes in a period when the cylinder is closer to BDC exposing more cylinder volume for filling (this is especially effective with a taller high compression piston). This all aids in dynamic compression at lower rpm.
As rpms increase though, the intakes move towards overlap and later closing lends itself more to utilizing the inertia of the mixture to "cram" the cylinder, as well as evacuate the cylinder of combustion by-product from the previous cycle. Once again increased volumetric efficiency is being achieved and your dynamic compression ratio is rising with it.
The overall effect here is somewhat dependent on the exhaust and intake design and it's resonant rpm. If the exhaust and intake system are designed hand in hand then manipulation with VTC should be very successful in creating as broad and flat a torque curve as possible.
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