When looking through engines for sale in classified ads, it doesn’t take too long to find one that says the heads and intake manifold have been “gasket matched.” That is shorthand for reworking the entrance of the head ports and the exit of the manifold ports (runners) so that they (hopefully) match. The intent is to give the impression that the ports are truly aligned to aid power production. Two things to note here are: Just because the ports in each of these components have been matched to an intake gasket it is no guarantee that, when installed, the inlet manifold ports are aligned to those in the heads to respectably close limits. Also, you can be absolutely sure that the guy who designed the intake gasket did not use a cylinder head and flow bench to arrive at the gasket’s port shape.
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In reality a gasket match is nothing more than a way to suggest to a potential buyer that the engine is a little more special. Trust me, you often pay for something that is simply cosmetic or out of sight until the next time the engine is torn down. Not only that, but often it actually reduces engine output. Given enough dyno time and engine building experience, you come to realize that we should be calling what we are looking at here as “dyno-developed port mismatching.”
Simple Port Matching
Most intakes are cast with a taper over the last inch or so to facilitate intake matching. Port matching a tunnel ram’s intake runners to the cylinder heads is about as straightforward as it can get. It is usually just a case of looking down the runners to visually check and fix the mismatch. To a degree the same applies to a single-plane intake although usually not all the edges of the manifold’s port can be seen. The first step is to attach the intake manifold gasket to the cylinder heads with sufficient glue (I use contact adhesive) to hold it in place. Do not use so much that you have to scrape gasket and glue off the heads for half a day when it comes time to replace the gasket.
Next, apply a generous amount of heavy grease over the gasket and the mating faces of the intake manifold. At this stage, leave out the lifter valley gaskets at each end of the block. You do this because the head may have been machined to increase compression so it sits lower on the block. If this is the case, the end gaskets aren’t thick enough. If the manifold has been machined to compensate, the use of a regular valley end gasket could be holding the intake off the point it otherwise fully seats.
Place the intake accurately on the block to ensure it is as near as you can get it to being aligned. You don’t want to slide the intake along the length of the block to align it. (You can use the imprint of the gasket on the grease to give you a rough idea where the port runners are in relation to the head’s ports.) Using a flashlight is a good way to check the alignment. Now, with well-greased manifold securing bolts, progressively torque down the intake. Next, blue up a patch on the intake and a head. This is the position you return the intake manifold to each time you reinstall it to check alignment.
Take a look down the ports and assess how much metal needs to come off the runner edges in order to align them. Remove the intake and cut the visible parts of the port to achieve a satisfactory alignment. It is a good idea to have the port in the intake just a little smaller than that in the heads. When you remove the intake, you see an imprint of the runners in the grease. You can use this as a guide to see where metal might need to be added to or removed from the manifold to achieve alignment. It often takes a number of trial installations of the intake to get all the visible edges aligned. Once all the visible edges have been cut and aligned (in most cases, the floor and the sides on the outside turn wall), you can match up the unseen edges. To do this, measure the height and width of the ports in the heads and use this measurement less, say, 0.025 to mark the manifold runners’ unseen edges from the edges you have just cut.
You should now have an intake that is sufficiently well aligned to get the job done.
Complex Port Matching
So much for the easy way of port matching. The following technique is used by most professional head and engine shops that build serious race engines. This procedure is not the fastest but it gets the job done if sufficient care is taken to maintain the best accuracy possible. Essentially, you mark a set of reference lines on the heads so you can transfer them to the intake manifold itself. After transferring the lines from the head to the ends and top of the intake, remove the intake from the engine assembly and ready it for the final phase of the mark up. At this point you should be fully acquainted with techniques to port match any intake to any head.
If you have a dual-plane intake, the complex port match technique is about the only one that works, other than cutting an intake gasket to the exact port shape and transferring an imprint from the assembly grease.
Intentional Port Mismatching
A precision port match may not be what’s best for the engine. Sometimes a precision port match actually cuts power. Why? In effect, power is dependent on how well the manifold runners and head ports manage wet flow. If there is a small step at the manifold/head face junction with the intake runner being slightly smaller, the power is likely be as good as, or better than, an absolutely perfect match.
The straighter the port runner’s approach to the head, the greater efficiency it has. For a tunnel ram intake, I generally attempt to match the roof and sides as closely as possible without making them too large. On the bottom of the port in which wet fuel flow often occurs, I leave a step of about 0.03 inch. For most single-plane intakes, I target a small step on the floor and the edge of the outside turn of the port runner. If you are using a heated dualplane intake, the chance of a wet fuel problem is minimized, but it still does not hurt to do the matches as I have described so you don’t have wet flow issues that foil your realization of maximize power
If you have some idea where the wet fuel flow path is, some redirecting grooves can be cut into the manifold to move the fuel from a low-flow area to a high-flow area. This brings the raw fuel into close proximity to high-speed airflow and the fuel is mostly shredded back into the main airstream.
Written by David Vizard and Posted with Permission of CarTechBooks