How to Port and Polish Big Block Chevy Headsby Richard Rowe
Cylinder head porting isn't what it used to be. This procedure was seen as something of a black art, best left in the hands of the greatest of gearhead alchemists. Two myths arose from this era of ignorance regarding port flow. The "bigger is better" approach espoused by old-school mechanics actually can cost performance at anything but maximum valve lift, which is why modern head porters prefer to reshape rather than to hog out their subject ports. The second myth regards polishing, which does nothing to enhance flow; it only makes the port pretty and the expensive, ported heads easier to sell.
Gasket-match the intake and exhaust ports. This process is one of the simplest and most important porting procedures, which can add another five to 10 percent of flow at any valve lift. Place an old intake gasket on the head's intake port surface. Spray it with a light coat of white paint. Remove the gasket; you'll see a white ring around each port. Use your cylindrical carbide bit to cut this white ring away. Blend the area about an inch into the intake port. You can gasket-match the exhaust port as well, but leave a 1/8-inch of white ring around it. This process will enhance exhaust gas expansion into the manifold and increase port flow.
Eliminate the intake-side pushrod pinch. The pushrod pinch, the narrowest part of your port, represents an enormous disturbance to flow. Use your biggest and most aggressive carbide cutter to cut the pinch all the way down to the port wall, exposing the inner cylinder where the pushrod goes. Create a new pushrod channel by hammering in a piece of thin copper or chrome-moly tubing. You can braze the tube into the head with a small torch and some brazing rod. Be careful not to overheat and crack the head. You also can smooth out the bump on the inside of the port with standard two-part metallic epoxy. However, A-788 marine epoxy will better resist gasoline and methanol.
Remove about 3/32-inch of material from the apex of the short-side turn (where the port starts to bend downward toward the valve) on both the intake and exhaust sides. Continue to smooth the rest of the short-side turn into the port to about 1/2-inch on either side of the turn. Replace the old valve guides with new bronze units; worn valve guides can cost about 15 horsepower. Cut the valve guide bosses in the bowl area (behind the valve) down until they protrude no more than about 3/16-inch onto the port roof. Smooth them into the port roof to enhance flow below 0.200-inch lift. Continue smoothing the ridge around the bowl area, opening the bowl up to blend it into the valve seat. Consider replacing your valves with stronger stainless steel or chrome-moly units to compensate for the shorter valve guides.
Unshroud the area around the valve heads inside the combustion chambers until the valve heads are no closer than 1/8 of an inch to the combustion chamber walls. Carefully smooth the casting ridge around the valve seats in the combustion chamber. Have your machine shop perform a three-angle valve-job on the seats. Unshrouding the valve heads in this way can have an enormous impact on port flow at under 0.200-inch lift.
Items you will need
- Popular Hot Rodding: Porting Cylinder Heads -- Common Head Porting Mistakes
- Popular Hot Rodding: Cylinder Head Porting Basics
- Corvette Fever: Brodix HVH Big-Block Heads --Where Smaller Ports Are Better for Your Big Block
- Off-Road Magazine: Build a 740hp Big-Block Chevy in Your Driveway
- Drag Cars: Cylinder Head Porting
- Large piston aircraft engine image by Andrew Breeden from Fotolia.com