One of the benefits of the Spec Miata concept is that only minimal modifications to the drivetrain are allowed. This prevents preparation costs from skyrocketing. One area where modifications are allowed has to do with the Exhaust. A system that deletes the catalytic converter and restrictive stock muffler will provide real power gains. Take a look at the SCCA class rules to make sure your exhaust system meets "Spec" :
Spec Miata Exhaust system
1. The exhaust manifold must be Mazda OEM, without any material added or removed. No coatings are permitted on the exterior or interior of the manifold. Heat wraps may not be used.
2. The 1999-05 Miatas with California emissions equipment may
substitute the Federal OEM exhaust manifold and ECU for the
OEM CA exhaust manifold and catalytic converter.
3. The post catalytic converter oxygen sensor may be disabled,
replaced, relocated, or removed; the resulting hole (if present)
may be plugged. Original exhaust system heat shields may be
removed.
4. The factory exhaust system beyond the OEM front down pipe
may be replaced, provided the following are true:
a. The replacement system retains the original configuration
(i.e., single tube design) and the tubing is a maximum of
2.25 inches outside diameter.
b. The pipe may end anywhere after the rear subframe.
Forward of the rear subframe, the pipe must follow the
original path of the OEM exhaust system.
c. No expansion chambers. A single muffler may be added.
d. The system meets all event specific sound requirements.
e. A catalytic converter may be gutted, removed, or replaced
with a catalytic converter replacement pipe. The replacement pipe must not exceed 17.5 inches in length and have an outside diameter no greater than 2.375 inches.
