INTO THE CHAMBER
Often there is considerable flow to be gained in the shape of the combustion chamber. The chamber de-shrouding cut made as part of the initial valve job machining is a big help in most instances. Hand blending these cuts into the chamber often yields a flow gain. Further work often involves recessing the chamber adjacent to the intake valve leading to the spark plug boss and often in working the plug boss itself.
Here is a nice example of a reworked production cast iron chamber, fully detailed for flow
POLISHING FOR POWER
Once the heavy cutting is done with the carbides, the ports and chambers can be polished, using a combination of sanding attachments, flap wheels, or cartridge rolls. Often, the polishing process will uncover minor surface irregularities, bumps, and dips that are hard to see but typically present in the rough carbide-cut surface. Boundary layer airflow physics tells us there is little flow to be gained from surface finish, but in practice, significant flow gains are sometimes seen from polishing the ports. The polishing process does remove metal, making the port slightly bigger, and the removal of minor irregularities probably accounts for the balance of any improvements.
Beyond the potential for flow gains, commercially ported heads need to have a nice uniform
With the stock small intake valve smog heads, and nothing more than a good valvejob to hel
SHOW ME THE NUMBERS
Just what kind of gain can be had from practicing the porting tips we're preaching here? We ported a set of lowly 360 Mopar factory smog heads to put theory to the test. We did not take these heads all the way to polished gems, but simply had the machining done as discussed, and carved on them with an assortment of carbides to dramatically improve the port shape. With basic porting we achieved a gain of about 30 percent in intake flow and a 45 percent improvement on the exhaust side.
For our next trick, we heaved on our freshly ported heads. The 360-cube test engine seemed
The flow numbers themselves were impressive enough, but we wanted to see if it would stack up to more power on the dyno. We had a fresh 360 Mopar short-block that had been rebuilt with stock internals, and fitted with .030-inch-over Federal Mogul 10:1 hypereutectic pistons. A custom Comp solid lifter cam with FL272/FL276 lobes on a 108 degree lobe separation provided around .550 lift at the valves with our 1.6:1 Probe roller-tipped rockers.
Our "stock" heads were a set of production smog #587s, same as the ported versions, with the original 1.88-inch intake/1.60-inch exhaust valves. The "stock" heads did have nice Serdi-cut performance valve seats, but no additional massaging. Running this much lift meant that the tops of the guides had to be cut down to clear the retainers, and we needed more spring than will fit at the stock 1.65 inches installed height. A set of Engle dished retainers and #993 single springs took care of that. We were ready to run with an Edelbrock Torker II intake and 750 Speed Demon bolted on top, and open 1 5/8-inch Hooker headers mounted to the other side.
|INTAKE PORT FLOW |
|.050" ||27||31||31||31||31||32 |
|EXHAUST PORT FLOW |
ALL TEST WITH CLAY RADIUSED ENTRY
Test 1: Stock port with fresh Serdi valvejob
Test 2: Bowl blend
Test 3: Blended/profiled shortside turn
Test 4: Deep pocket porting/profile guideboss
Test 5: Intake pushrod pinch opened; Exhaust roof kink blended
Test 6: Chamber mods
*PEAK OBSERVED FLOW WITH 2-INCH 90DEG FLOW PIPE