Quote:
Originally Posted by 14pilot
Yes, when the shock wave from the exhaust valve opening travels across the step, it reflects back towards the exhaust valve creating a negative pressure. It's like the waves caused by throwing a rock into water, when the waves hit something like the edge of a pool, they reflect back, except in an engine this happens 50 times per second at 6,000 rpm.
When the step(s) are at the correct length for the application, this wave of negative pressure assists scavenging of the next exhaust cycle by pulling it out into the pressure drop created. The negative pressure could be something like 3-5 PSI at the exhaust valve. This is usually at tuned length of either the HP peak rpm of the engine for a drag car, or the mid-range operating rpm of a street or road race car.
This same effect happens in all exhaust systems. The step in primary diameter just increases this effect. If done wrong, the reflected pressure wave could be positive pressure instead of negative pressure. This would result in pushing some of the air-fuel intake charge back up into the intake track. |
So the distance the step is from the head is very critical and will determine how much pressure drop is created or if incorrect may cause back pressure. So are you only seeing a benefit in scavenging every other exhaust cycle per cylinder?