When did Spec Miata cylinder heads come out?
We began the experiment by flow-testing three examples, two straight heads with minimal core shift from 1995 production cars, and one from a crate motor sourced from Mazda late last year. Everyone in Spec Miata knows that the 2015 rule set underwent some changes in the offseason.
Are there rule changes for Spec Miata in 2015?
Everyone in Spec Miata knows that the 2015 rule set underwent some changes in the offseason. The rule changes themselves weren’t major, but the upheaval that led to them was, particularly in terms of what modifications were permitted on the cylinder heads.
What’s the compression ratio on a Spec Miata?
The rule reads like that because there is more power to be had by blending the cuts and by opening up the radius a little, and it’s easy to check with go-no-go tooling. Spec Miata rules also allow for resurfacing of the head to achieve the factory compression ratio so long as the head is no shorter than 5.245 from the deck to the cam cover rail.
What are the radial relief cuts on Miata cylinder heads?
Both heads did get the radial relief cuts on the combustion chamber walls, .7600 on the intake side and .6750 on the exhaust. This cut, which is circular and concentric to the valve guide “unshrouds” the valves, which has long been known to improve flow and unlock more power. The cuts allowed by the rules are about as large as you can make them.
What causes core shift on Miata cylinder head?
When the cores move out of place, you get a “core shift,” which means that you get too little metal on one side, and too much on another side of the core. On a Miata head, you can see it best in the throats beneath the valve seats.
How tall is the intake valve on a Miata?
No NA or NB Miata has more than .350 inches of lift on the intake or exhaust. “When your valve only opens .350”, port volume, size and shape really don’t have much effect on it,” McKenzie said. “You’ve got a cylindrical volume that’s .350” tall that’s capped by the valve and the seat, and that’s why getting it around the valve is so critical.
What should the idle RPM be on a Miata?
Factory spec for the Miata specifies standard timing as 10° before top dead center (10° BTDC) ±1° at idle speed(850 rpm). If your timing is advanced more than this, you may experience pinging under some conditions. Dirty spark plugs Metallic burr in cylinder
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What should the gap be between spark plugs on a Miata?
As spark plugs wear, the gap between the electrodes tends to get a bit wider. The standard gap on the Miata’s spark plug should be 1mm. A wider gap may result in misfiring due to the voltage not being quite high enough to cause a spark across the electrodes.
What should the ignition timing be on a Miata?
The piston is already beginning to travel back down the cylinder when the combustion occurrs. The piston is therefore travelling a shorter distance under power which translates to power loss. The timing on a stock Miata should be set to 10° before top dead center (10° BTDC) ±1° at 850 RPM idle speed.