Targa Miata
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November 4, 2009 - Based on some things I've read, I've cut out the wall between the plenum and the resonance chamber.
With this setup, the plenum is effectively much larger - but more importantly I now have variable length intake runners. With the butterflies closed, the air will follow the same route as before. But when they open, a short intake path will become available which should benefit high-rpm operation. I also removed the EGR ducting because I don't run an EGR system, and this cleans up the flow near the throttle body. While cutting out the EGR ducting, I did manage to break through into one of the bolt holes, but that was patched up with some good two-part epoxy (ie, not JB Weld). I also modified the gasket that sits between the two halves to match my modified manifold.
There are nice bellmouths on the long runners and ideally I should shape some on the new short runners, but not until I've done a preliminary test to see if the modifications have any effect. It's quick and easy to change the upper half of the manifold. If the testing looks good, I'll go a bit further with cleaning everything up inside both the top and the bottom halves.
Gary at Track Dog Racing has tried removing the bellmouths for the long runners as well as bellmouthing the short runners. He reports power gains on cars with forced induction, but not much without boost. It's worth a shot though.
tags: engine