Custom Marine Inc. unveiled its latest R&D project this week—upgrade packages for Mercury Racing‘s naturally aspirated 520 engines. Designed for the do-it-yourselfer to tackle in half a day, the Neenah, Wis., company is now offering 520 Sport Tubes and 520 E-Top Headers for the 520-hp naturally aspirated engines.

According to dyno testing results gathered at Innovation Marine in Sarasota, Fla., on a new 520 direct from the Mercury factory in Fond du Lac, Wis., the 520 E-Tops Headers add up to 31 hp and the 520 Sport Tubes add up to 48 hp (check out the results here).

Pricing for the header packages, which feature CMI’s patented “cool collar” design where the tube meets the cylinder head flange, start as low as $4,900 per pair. Call one of CMI’s three stocking distributors—Offshore Performance Specialties, Performance Boat Center or Teague Custom Marine—for the best pricing and availability.

Mandrel bent and formed from high-quality 316L stainless steel, both designs improve midrange acceleration and torque and come in a polished finish. The 520 E-Top Headers are available in a satin finish as well. Of course some additional costs to keep in mind include adding tailpipes to the Sport Tubes (the E-Top Headers work with existing exhaust outlets) and possibly getting new pitch propellers to take advantage of the extra power.

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