Powertrain Rejuvenation: The Rules are the Limit (STX)

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If I’m going to pull the engine for a refresh, there’s no reason I couldn’t replace my flywheel with the stock version and bump down to a more appropriate class for my budget. As such, I wanted to investigate how far I could go with my engine in Street Touring X. Here’s what I compiled from the 1/30/2017 release of the SCCA Solo STX Rules, which can be found here.

Oil Pan/Lubrication

  • Any oil pan is acceptable
  • Any oil pickup is acceptable
  • NO addition of, or modification to, windage trays or crank scrapers

Thermal Control

  • Radiator can be replaced, BUT must be same as stock (or greater in magnitude) in the following areas:
    • Dry weight
    • Coolant capacity
    • Core dimensions
    • Mounting locations
  • Any water expansion tank is allowed
  • Silicone replacement coolant hoses are allowed as long as they’re the same as stock in all dimensions and parameters.


  • Any intake is acceptable
    • This ONLY applies up to the turbo (compressor inlet)… NA cars may replace/modify up to the throttle body
  • NO modification to PCVs, MAFs, etc


Nothing on Fuel other than what’s included in the ECU section. With everything else that’s restricted, there’s not much reason to get into the fuel system here.

Engine Management

  • Traction Control may NOT be altered
  • Stock ECU may be reprogrammed and must retain its original housing
  • A “piggyback” ECU can be added, but can’t be spliced in and must interface between the stock ECU and its wiring harness
  • Signal modifiers may be installed in line with sensors to affect signal output to the ECU
  • Fuel pressure regulators may be used in lieu of electronic alterations to the fuel system


  • Any exhaust, starting at the exhaust ports, is allowed
    • Exhaust outlet may be repositioned per Rule 3.3.3.B.16
  • Heat shields attached to the exhaust may be removed, modified, etc.
  • Any catalytic converter is allowed
    • MUST attach within 6″ of the original unit
    • Multiple cats are allowed
    • The inlet of a single unit replacing multiple units must be located no more than 6″ downstream of the end of the final cat installed in the OEM exhaust
    • Replacement cats must have a minimum catalyst density of 100 cells/in. and a min. substrate length of 3″


Any engine or transmission mount is allowed provided it attaches only to the original mounting points



  • (From Street Stock rules) Cylinders may be overbored to factory first overbore (no more than 0.020″)
  • Accessory pulleys and belts may be replaced with that of the same type (V-belt, serpentine, etc.)… NO modification to the camshaft pulleys.
  • Not really internals, but I had pulleys under internals in my ASP post.

As you can see, this is a much more restricted class than Street Prepared. My engine rebuild will cost much less to remain in this category, and I won’t be so hindered by my budget here. It’ll still be a tough fight against the BRZ/FRS crowd, but with some careful preparation and skillful driving I may still be able to come out on top!

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