Vacuum Tuning

Sir,

I suspect the chart relates to mild hydraulic cammed motors. I did some checking on my old Payback #2 thread and that thread relates to the motor before it was completely rebuilt. Even back then idle vacuum was jumpy. More so of course. That motor was in sad shape. Loose timing chain. Worn rings and valves. The only common denominator is we used the same cam. New lifters and springs of course, but the cam itself was undamaged. So the cam is what is causing the fluctuation. Makes sense. On the exhaust stroke with the overlap the intake is opening as well and that would in effect cause a slight stall in the intake charge to that cylinder which would cause a short drop in vacuum. As RPM's go up the air mass coming down the runners in the intake has increasing velocity which allows it to overcome the exhaust back pressure and of course the exhaust charge is also increasing in velocity which acts as a purging factor. I bet if i installed full length headers the fluctuation would drop even more.