NGK PFR6B-11B was designed specifically for the TT cylinder head and is generally our most highly recommended plug for the TT. Gap can be reduced for higher boost applications. We normally recommend:

• Stock: ~0.045" (they ship pre-gapped to this setting)
• ~14 psi: 0.035"
• 18-20 psi: 0.030"

For higher boost pressures, you'll likely need to bring the gap down further, but trial and error is often best here. The PFR6B-11B is the only plug on the market which sits flush and centers the electrode in the combustion chamber. NGK Iridiums and similar plugs will still run the car, but can't produce as clean of a burn because they end up sunken into the TT heads.