Dyno Chart #3: Before our fear of boost fully consumed us, we tried a little science. Since we were working and dyno testing at MD Automotive, which has been working on Miatas since they were new, we were swimming in crusty old Miata parts. Unsure if our stock injectors were delivering all the fuel we needed, we swapped in some crusty old injectors from a 1.8-liter MIata. The 1.8 injectors are only very slightly larger, so at part throttle, where the ECU actually listens to feedback from the O2 sensor, the enrichment is mild enough that the ECU has enough authority range to get the mixture back where it belongs. Up on boost, though, it should run a little richer, and the change should either make more power if we were originally too lean, or make less power if we were already too rich.
Turns out it did neither, which must mean we were sitting on a big, wide, good-enough plateau. Since being on the rich side of good enough is less likely to melt something down, we left the 1.8 injectors in and got to work on lowering the boost.
After only a few pulls, we removed the turbo and ported the wastegate, re-shaping the inlet to encourage exhaust to turn the corner into the wastegate, enlarging the opening itself, and contouring the discharge area and relieving the downpipe flange, all in hopes of getting enough wastegate flow to keep the boost down at a more comfortable 5 psi.
Dyno chart #4: The port work did the trick! The gap you see at low rpm is likely just run-to-run variance, but the massive drop in high rpm power is just what we were going for. With boost staying under 5 psi all the way to redline, fuel pressure was finally at a level we could at least hope might survive. You’ve never seen anyone so happy about losing 20 hp.