Now that we've got a good understanding of air-flow, we can move on to cylinder head porting. If you haven't yet read our article on the basics of cylinder head porting and air-flow, I'd suggest you do so now as it provides the foundation for understanding what we want to achieve with the actual cylinder head porting.
Preparing the Cylinder Head
Before we can get started, we need to strip down the cylinder head; remove the camshafts and camshaft pedestals, then remove the valves, valve springs and valve stem seals. You should also remove all manifold studs. With everything stripped, you need to inspect the cylinder head for cracks. It's no good porting a cracked cylinder head, though a cracked cylinder head may still be good for experimenting on, so don't throw it away! The most likely areas where cracks will appear are between adjacent valve seats, and around the valve seats, especially around the exhaust valve seats. You may need to some emery cloth to remove any carbon deposits to do a thorough check.
If you don't see any cracks, have the cylinder head thoroughly cleaned in a chemical bath. You can dip a cast iron cylinder head in a hot caustic solution but don't dip an aluminum cylinder head in it! Caustic solution will react with the aluminum and give off an explosive gas! For an aluminum cylinder head you should use Trichloroethane. If you don't have access to a chemical bath, you can use engine cleaner and a stiff brush to get oil and gasket pieces off. Once the cylinder head is clean and dry, use a sand blaster or a wire brush to clean off any stubborn carbon deposits. Once that's done, do another thorough check for cracks.
If you don't see any cracks, have the valve seats replaced and the valve guides removed by a reputable engineering shop. Replacing the valve seats are not crucial as long as they're in a good condition. However, you must have the valve guides removed.
WARNING: Take care when working with a grinder. Adhere to the following safety precautions when porting cylinder heads and using a grinder in general:
- Wear eye protection when working with a grinder; goggles are advisable but a full face visor would be better.
- Wear a dust mask or a respirator; inhaling metal filings is harmful.
Porting the Cylinder Head
We'll get to enlarging the port in a while when we discuss gas flowing; but for now we'll focus on the main aim of cylinder head porting, which is to smooth and straighten out the ports. If this is your first attempt at cylinder head porting, I'd suggest you try to master that first. Starting on the intake ports, use a flame-shaped carbide and attempt to remove any obvious bumps and crevices in the port without removing too much metal, then try to straighten the post so that it has a consistent size from the mouth to the point where it curves into the valve throat. Remember to move the carbide all the time and don't hold it on one spot as it will quickly create a hollow that will be difficult to remove! Once you're happy that you've got your first port nice and straight you can use a grinding stone to smooth it if it's a cast iron cylinder head, or a sander band if it's an aluminum cylinder head. Now try to replicate your work on the other ports. Use an inside caliper to make sure all the ports are the same size.
Now, working from the valve throat side, use an oval carbide to blend the short side radius. Again, try not to remove too much metal. Also remember that you want a smooth flow through the valve throat area and that you want a consistent port size through the length of the port. Once you have blended the short side radius, turn you attention to the long side radius where the valve guide boss is located. Use an oval carbide to flatten the valve guide boss until you have a consistent port size from the manifold face to the valve seat.
Now all that's left is to smooth the port with a flapwheel or a fan grinder; then use a vernier caliper to measure the height of the valve guide boss through the hole for the valve guide. Measure the height of the valve guide boss on its shortest side and on its longest side. Then replicate your porting work on the other ports until all the ports are identical.
Once you're happy with the intake ports, turn your attention to the exhaust ports and smooth and straighten them out in the same manner, without removing too much metal and retaining the squarish shape of the exhaust ports.
Matching the Manifold
The intake manifold and exhaust header are integral to the efficient air-flow in and out of the engine. Getting a smooth flow from the manifold to the cylinder head ports, especially in the case of the intake manifold, is crucial for good air flow and power. Remember that air-flow doesn't like sudden changes in direction or tube size. As we mentioned in designing and building performance exhaust systems, the exhaust port can be slightly smaller that he exhaust header to help prevent reversion.
Start by making a cardboard template of the intake and exhaust manifold faces, and cut out the port openings and the stud holes accurately. Fit the template to the cylinder head, taking care to match the cylinder head side of the template to the cylinder head. You will have to insert the manifold studs or manifold bolts into the cylinder head to line the template up correctly. Once the template is lined up accurately, scribe the outline of the template onto the cylinder head. You can also use a manifold gasket to mask out the port sizes if the port openings on the gasket fit the manifold accurately.
Now you can enlarge the ports gradually until the intake port matches the port openings on the intake manifold, and the exhaust port is slightly smaller than the port openings on the exhaust header. Remember that the aim of cylinder head porting is to create a smooth straight port that has a consistent port size from the manifold face through the valve throat area. Also try to keep the ports walls on rectangular exhaust ports as straight as possible. You can use an engineer's square to scribe straight lines that can serve as guides for your porting on the port walls.