The Vortec combustion chamber is worlds ahead of the old-school SBC designs. Very little w
Now the intake valve is on the move. After creeping open with a slow and steady opening ramp, the valve now moves at maximum velocity to get to maximum lift. The piston is moving down the hole rapidly and by the time it is around 73 degrees ATDC, it is moving at maximum piston speed of an incredible 4,000 feet per minute when this little 383 is turning 6,500 rpm. That’s 45 mph folks. Let that little tidbit sink in real quick. The piston just went from 0 to 45 mph in .002 seconds. Awesome. Back to the intake valve though. It is chasing that piston down the hole while there is a growing vacuum in the combustion chamber drawing fresh air and fuel in. The piston rounds BDC and starts to come back up but the air blasting down the intake runner is moving so fast and with such energy that it continues to come in all the way until the intake valve closes around 40 degrees after BDC when the true act of compression, dynamic compression as they say, begins. Then the cycle starts over.
Though the engine was designed only to run to 6,500 rpm, valvetrain control was still deem
The Ferberts, of course, know all this. And they use this knowledge in every step of designing their runners and valve jobs. For that exhaust port, they wanted something that was unrestrictive, yet not so big and blown out that it wouldn’t create good velocity. It is, in effect, an extension of the header tube bolted up to it and as we have seen so many times in the past, a moderately sized header pipe can make significantly more torque than a huge header pipe.
Choosing a valve job for the exhaust, the brothers considered that they wanted to ensure maximum flow with minimum restriction while the piston was on its way up, but didn’t want to waste too much of the still-expanding gasses during blowdown. They decided a 50-degree valve job would satisfy those needs. The 50-degree valve job traditionally trades some low-lift flow for a gain in higher-lift flow. Picking the right exhaust valve opening and closing points gets a little trickier when veering away from the traditional valve jobs and flow curves.
If designing the exhaust side was a little tricky, the intake side gets downright treacherous. When that intake valve was just cracking open during split overlap, the boys wanted to keep the reversion to a minimum but still allow the valve to start moving open or it would never get enough lift in time to take advantage of that incredible vacuum in the chamber at max piston speed. To quell some of that reversion, a 50-degree valve job was performed on the intake as well.