no margin of error, there are small drops in volume @ .050, and .100 - after the new valve job I'm hoping to gain some of these #s back
To keep it as simple as I can for a wide audience.... Part of the bowls shape, and part of the bowls/ports size, as well as the original valve job "helped" the .050, and .100 lift #s, but the first 2 seriously restricted volume. In essence the original port/bowl shape was to "extend" the 3rd angle(throat angle) of the valve job to an extreme to maximize port velocity, at the expense of volume and the 7AF has very good port velocity - just find a dyno print out of a stock 7A and you will see it makes a nice torque curve. But @ 115hp it leaves volume on the table.
The goal here is to increase cylinder filling, and while I know some of the velocity has dropped do to porting, the drop is minor compared to the gains. From a bit of digging (3rd post down - http://www.wheelsjamaica.com/wheels_forum/index.php?topic=165509.0 ) The 2nd gen camshafts are 224 duration overall.
2nd GENOverlap: 8.00 degreesIntake Duration: 224.00 degreesExhaust Duration: 224.00 degreesIntake Installed Centerline of 106.00 degrees ATDC.Exhaust Installed Centerline of 110.00 degrees BTDC.
We can do a bit of math, and a bit of guessing( duration @ .050") 18 degrees of ramp(36 total - up and down) are common on 4AG cams, so we can assume this, and then subtract 36 degrees to find the duration @ .050" for the 4/7AF cams: 36-224=186. ASSUMING the ramp is correct, or even close we are @ or near 186 degrees duration @ .050". Now we are given the centerline @ 106, so there is half of our 186 before 106, and the other half after it. Subtract 93 from 106 and we make an educated guess the cam is open .050" @ 13 degrees after top dead center, 13 degrees off TDC the downward moving piston is not yet at peak draw. At this point the cam lift is opening faster now, in the next 93 degrees, the lobe will open to .338". And this is where the volume advantage will fill the cylinder better, for the cylinder will be at its peak vacuum, when the valve is open higher.
I hope that wasn't too overwhelming.........
For those who like numbers: add up all the volumes at all the lift points except .350" as the cams don't lift to here, now double the result. Now compare before vs after.... Before 1236.2 cfm total, vs after 1354.2 cfm