Originally Posted by Chip-3Door
Originally Posted by JjCoDeX75
Basically, if I understand it correctly, where the cam separation is increased I guess the overall flow of the engine is reduced, which would lead to increased likleyhood of surge?
JJ
Thats my understanding of it too mate
Remove all the overlap, and increase the duration its open for after BDC and you effectively cap the flow through the engine, and hence move yourself to the left on the compressor map for that amount of boost, by flowing less, which in turn increases the chance of surge.
I suspect there is far more to it than us mere mortals realise though, but its the one subject no tuner on here ever seems prepared to go into depth on from what ive seen!
Okay, so lets have a little recap....
What we know:
The more we can increased the volume of airflow through the engine, the more power that will be produced, subject to the following design constraints;
1. The turbo itself restricts air flow out of the engine. In order to maximise air flow, a larger exhaust housing assists. Notwithstanding this, the larger housing inevitably increases the boost threshold, which is bad.
2. Given that the above statement is true, and
ceteris paribus, the flow the engine itself is generally dictated by the size of the openings (inlet and exhaust), the amount of lift and duration of cams, as well as cam timing.
3. The simple action of changing the duration of the exhaust cam will inevitably lead to the increase of boost threshold, as a greater volume is required to spin up the turbo (?)
4. If you overly increase the inlet size, and do not modify the exhaust cam timing, you are in danger of increasing cam separation, which may lead to turbo surge.
We touched briefly on the benefits of the action of simply porting the engine. Basically increasing the size of the inlets and exhaust ports. This allows a greater volume of air to travel through the engine for a given amount of boost.
However we also agreed that the increased port size will lead to potentially adversely affecting the performance of the car at low rpm levels.
Why is this? Can we discuss this a little further?
JJ