How to Port & Polish an Intake Manifoldby Richard Rowe
Of all the modification that one could perform on an engine, there's just something about porting and polishing that feels sort of special. The idea of removing material from intakes, heads and exhaust ports to increase performance, using nothing but a few cheap tools and your own ingenuity, certainly makes for a warm, fuzzy, "I beat the system" kind of feeling. But approach with caution -- beating the system means being smarter than the engineer who designed your engine and more skilled than the robot that built it. It also means doing a little more than you might expect.
Remove the intake from the head or heads, being careful not to damage the intake gasket. If you damage the gasket, you'll need a new one. Once you have the intake off, you need to port-match the intake ports on the cylinder head. Don't even consider porting the intake until you've port-matched the heads. It's OK -- even desirable -- to have a manifold port that's slightly smaller than the head port, but you never want it the other way around. That would create a "step" in the intake tract, which would kill intake flow. See the eHow article "How to Port Match Cylinder Heads" for directions on this first step.
Flip your manifold over on its back, with the engine mating surface facing up. Align the old gasket to the manifold, making sure all of the bolt holes are exactly aligned; you might even consider installing a few short bolts and matching washers and nuts to align it and hold it in place. If you do so, you can skip this part: coat the exposed manifold port material inside the gasket port holes with machinists' dye. Alternately, you can trace around the inside perimeter of the gasket holes with a permanent marker, or simply spray the entire area with a few light coats of black spray paint. When you remove the gasket, you'll have a colored ring around the ports. This is the maximum amount of metal you'll want to remove.
Plan your approach. If you've already port-matched the heads -- which you have -- then porting the intake runners is much the same in terms of approach. However, you're removing less port material, and tapering the port enlargement about two inches down into the passage. You'll also have to work around the injector hole or bung, if present. Unlike the head port, you don't want to exactly match the manifold port to the gasket; you want the finished port at least 1/16-inch smaller than the gasket hole. First, for safety, just to make sure you don't accidentally make it larger than the head port. Second, because a small lip on the intake side can create an "anti-reversion step," which can potentially increase volumetric efficiency. And, even if it doesn't do much, saying that sounds really impressive when you tell your friends later.
Soak several medium-sized, rough, conical grinding stones in machinists' oil or penetrating oil for half an hour prior to starting. Most non-plastic intakes are aluminum, which clogs up grinding stones like mozzarella cheese in sandpaper. You could use a carbide bit for speedier material removal if you're very skilled and have a steady hand, but carbide bits will cut through aluminum like a hot knife through the aforementioned cheese. It's safer, if a bit slower, to use a grinding stone and plenty of machinists' oil to keep it from clogging up with aluminum. Keep several stones of every type handy; you can clean them out if they do get clogged, but you don't want to stop every five minutes to do it. Before you start, place a drain pan under the manifold's throttle body opening to catch the used oil for re-use.
Full a two-cup measuring cup up with machinists' oil. Fit the conical, oil-soaked grinding stone to your die grinder, and set it to medium speed if it's adjustable. Brace your arm on the table, and start gradually removing material around the port opening. Every 15 seconds, pick up your cup full of oil and flush the area and the bit with about an ounce. A steady flow of oil is best, but regular flushings are acceptable. Work slowly, lightly around the port, removing a very small amount -- no more than a 1/16-inch -- of material on every pass. A slightly sharp bevel of about 45 degrees is acceptable for initial port shaping; focus on shaping the port at the surface along the ring you marked with the dye or marker. You may need to stop periodically and refit the gasket to double-check your work.
Shape all of the ports, remembering to keep them at least 1/16-inch smaller than the port opening in the head. Don't concern yourself too much with small divots and irregularities; you can smooth them out later. When you've shaped all of the ports, fit a similarly oil-soaked cylindrical grinding stone to the die grinder. Now work on blending that hard, 45-degree bevel down into the intake runner. Start by holding the grinder so the side of the bit lays on the sharp edge of the bevel, and begin removing material. It's a good idea to move the bit up and down slightly, so you don't overly wear the cylindrical stone in one spot. It's crucial that its sides remain flat to ensure a smooth blend. If it does go pear-shaped, you can straighten it out by holding the side of the stone against a flat scrap piece of iron or steel.
Work downward into the port, blending the taper to an inch or two in. For this step, a metal straight-edge stuck down into the port can help you to identify high spots on the inside of the port wall. Keep working down until you have a smooth, straight taper to an inch or two into the runner. Don't worry about the surface finish at this point; just keep flushing with oil to keep the stone clear, and focus on getting the perfect taper. At some point, you may have to work around an injector bung. If you like, you can use a conical stone and extension to carefully remove some material from it to make it a bit smaller, and profile the front and back edges to a knife-like "wing" shape. This is a bit overkill, and you run the risk of making the bung casting too thin. But some porters opt to aero-shape the injector bung just to know it was done.
Remove the manifold from the area and clean it out thoroughly with dish detergent, water and a few rags. Push the rags all the way through the runners with a flexible extension while the manifold is submerged in soapy water. You're working "dry" from this point, so you need to remove all traces of oil. Rinse the manifold out with clean water, then wash it again in fresh, soapy water. Rinse it thoroughly, and dry it with rags and compressed air.
Fit a 180-grit flap-wheel to your die grinder, using a mandrel extension if necessary. Use the flap-wheel to do a bit of final shaping and smoothing in the port, along the taper, and around the port opening and the injector bung. Blow the area out with compressed air and wipe it clean to check your work. Now comes the easy part: polishing. Fit a scotch-type abrasive wheel to the end of a flexible extension, and fit that extension to a drill. Drills tend to work better here because you need higher torque and lower speeds. Set the drill to medium speed, and work the abrasive wheel into the port opening until it fits snugly inside. Hold the drill steady, turn it on, and start working the abrasive wheel up and down inside the runner, much as you would hone a cylinder. Work all the way from the plenum chamber back to the port opening, if possible.
Remove the wheel and stop to check your work every 20 seconds or so. You might be tempted to use the abrasive wheel to remove material and make the runner larger, but that isn't the aim here -- and it can be disastrous on square ports, because the wheel will remove much more material along the flat sides than in the corners. Experienced machinists and porters know exactly how much material they can get away with removing to increase the runner volume; you probably don't. The goal here is just to smooth the walls of the runners, eliminating casting marks and roughness. This alone will increase runner volume slightly. If you have square ports and see a concavity developing on the flat walls, stop immediately and move on to the next port.
Smooth and clean up the central plenum chamber if you'd like. This area is often rife with protrusions and casting marks, and there's often some room for enlargement or improvement. Obviously, you'll need different stones to work end-on in the chamber; flat-faced sanding discs and spherical stones will generally prove the most useful. Remember to use plenty of oil as you did on the runners, and smooth it afterward with the abrasive wheel. Often, the easiest way to do this is to push it all the way through the runner from the port side, and smooth the floor of the plenum from the inside. If you have a carbureted engine and see a "waffle" pattern in the bottom of the plenum, don't smooth it too much; this is present to enhance fuel atomization. Some machinists opt to either cut a notch in center dividing walls or cut them out completely, effectively turning a low-rpm "dual-plane" intake into a higher-rpm "single-plane." But you do this at your peril; best to leave the dividing wall alone unless you know exactly what effect it will have on the engine.
Wash everything out, dry it and take pictures of your fine work. Frame them, put everything back together, and install a set of headers to capitalize on your now free-breathing intake.
- If you're planning on running a larger throttle body, this is as good a time as any to open up the throttle body bore in the intake manifold. Use more or less the same procedure as above, but use your new throttle body gasket as a template. If you have a round bore opening, you can keep it round by working in circles around it with a cylindrical grinding stone. Go in the direction the stone is spinning, in fairly rapid revolutions -- about one trip around the inside of the bore every two seconds. A bit of final honing with a larger abrasive wheel should get it into useable shape if you've sent it slightly out of round. In this case, it's better to go big; get as close to the gasket opening size as possible to avoid a flow-disrupting step.
Things You'll Need
- Ratchet and socket set
- Old manifold gasket
- Duct tape
- Machinists' dye, permanent marker or black spray paint
- Machinists' oil
- Measuring cup
- Drain pan
- Die grinder
- Head porting kit with grinding stones, flap wheels and mandrel extension
- Metal straight-edge
- Dish detergent
- Clean rags
- Flexible extension
- Scotch-type medium abrasive wheel, 320-grit equivalent, sized slightly larger than the intake ports
Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.