Quote Originally Posted by CRXer View Post
john,most of the effect is compression,theres always air available to compress,due to idle passages & overlap.
My point is that when the throttle plate is closed there is actually a very limited airflow through the induction system relative to the rpm at which the engine is turning. The flow through the bypass passages and past the slight gap around the circumference of the TP will be very restricted compared to what would be required to even remotely fill the cylinders.

So, overrun will create the highest vacuum (lowest pressure) conditions ever seen in the plenum / manifold. So, if on the overun the majority of airflow into the cylinders is coming from the induction side then there would be relatively little air available to be compressed by the rising pistons, and thus relatively little engine braking effect (at least from the compression of gas).

On the other hand, considering that the atmospheric end of the exhaust system is never closed by any type of valve as the induction system can be, if there is a substantial reversion of gas flow coming into the cylinder through the open exhaust valve, then it may be that most of the gas that is compressed in the cylinders to create the engine braking effectb is actualy entering the cylinders via the exhaust valve..?

I know from personal experience that an engine with a higher CR does create a stronger engine braking effect than a similar engine with a lower CR. A lessening of engine braking effect is also evident with engines that have lost substantial compression due to wear.

An interesting experiment to see if we can get a seat of the pants feel for any difference in engine braking effect relative to just closing the throttle, might be to switch off the engine while the throttle is still wide open (beware steering locks...).