This month’s column is the short version of an article I wrote for Miata Magazine about a Mazda Miata cylinder head and intake manifold project we were contracted for by Bill Cardell of “Flyin Miata”. Flyin Miata is a Miata high performance specialty house located in Grand Junction, Colorado. The full text version, pictures and dyno charts of this article are in the magazine version out this month. The cylinder head and intake manifold was from a Miata 1.6-liter engine that had undergone a bit of Bill’s turbo add-on magic. The subject in question is a streetcar and would be a daily driver. We would remain conservative here; this is not an all out race engine or car. Still, we could and would help make it stronger.
After receiving the head, we inspected it, cleaned it and I started to think about how I wanted to approach the modifications. I prefer to think about things for a while before I start to work on them. I like to think first and cut second. There is nothing exotic or magic about this head; it is a standard pent roof style four valve lay out. The combustion chamber is smallish and the cylinder walls shroud the valves. Because of the small combustion chamber diameter there is not much you can do about this safely. Because this is a streetcar, we deemed it of primary importance to improve torque and throttle response. We put it on the pressure rack to check for leaks. We also give it a quick check for straightness, cracks, casting flaws, bearing saddle wear or damage. We like to make as sure as possible that we are not working on a damaged head.
After inspection, we blue the intake and exhaust port openings and the cylinder deck surface area. We carefully lay on the gaskets and scribe the openings on to the head. For this cylinder head, Bill requested that we open things up but stick to the stock gasket opening sizes. The widths of the ports were adjusted to the gasket size. I did use my discretion and moved things about slightly. We carefully shaped the ports paying careful attention to the entire port area. We always pay special attention to where the port ends in the bowl area. The bowl area is where the charge tends to collect itself while waiting for the valve to open. The area on both sides of the valve and valve seat, controls the amount of charge that fills the cylinder. The better prepared these areas are the better the port does the work of filling the cylinder.
One thing that became an annoyance was the unevenness and poor quality of the cylinder head and intake manifold castings. You cannot just straighten and/or clean all of this up. Some of the port walls cannot be made perfectly straight without becoming too large and facing a drop in velocity. We were able to straighten this out but it is a lot of work and not something that can be done by an amateur. Some of you may wonder whether an “Extrude-Hone” process could not more easily handle problems of this nature than porting by hand. Well Extrude-Hone works on the theory of hydraulics, which means that the applied pressure is the same in all directions. How then can the media make specific spot pressures adjustments to compensate for specific flaws and unevenness on one side of the wall and not the other in the same area? Everything is treated the same in this process and grit definitely does not know that it is only supposed to move against one wall but not another. I feel that this process has its place but it is not a “cure-all” panacea.

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