WHAT IS IT? It’s an exhaust system with two muffler cores. It was only available for Hondas and Suzukis in its 2011 debut year, so the KX250F system is brand new.

WHAT’S IT COST? $1095.00 (carbon), $1045.00 (titanium).

CONTACT? or (800) 634-9166.

Here’s a list of things that stand out with Yoshimura’s 2012 KX250F RS-4D exhaust system.

(1) Construction. Yoshimura offers two different types of mufflers, the RS-4 and RS-4D. The “D” stands for “dual core.” A traditional muffler canister stair-steps down to the pipe diameter, but the Yoshimura has a sleek transition. This isn’t for looks; there are tricky things happening in there. Both the RS-4 and RS-4D have a dual-stage baffling and resonance chamber to help control flow and sound. Just aft of the baffle, the RS-4D pipe splits into two perf cores.

(2) Why dual? Having two perf cores in the muffler means more internal volume, more outlet area and the opportunity to play with a greater number of dimensions to achieve the desired sound and power. On the negative side, the design adds complexity and weight. In the end, with more stringent sounds rules, Yosh felt they could get a better response and more power to the ground with the dual-core design.

(3) Header. For 2012, Yoshimura’s R&D time yielded an improved head pipe design. The money-saving thing to do was to only offer the new head pipe on systems that fit 2012 model-year bikes. Yoshimura went the extra step and made the new head pipe design retroactive. If you buy the 2010 or 2011 KX250F system, you get the new head pipe.

(4) Installation. The Yoshimura system fit our KX250F well and stayed in place. The only potential trouble is losing any of the fasteners over time, because the system uses its own proprietary special spacer, springs and grommet. Most other pipes use longer springs, so if you lose one of Yosh’s short springs, you will have to look hard to find a replacement.

(5) Performance. Swapping from the stock system to the Yoshimura RS-4D yielded an immediate improvement in throttle response everywhere in the curve. Total low-to-mid power was only improved modestly, but it felt big because of the added sensation of the hit. That snappy response made the bike really jump out of turns and rev into the meat of the powerband. The best power improvement came in the upper-mid to top end of the powerband. Right in the sweet spot that most riders get to about 50 feet down a straight, the RS-4D pulled like a team of racehorses. Overrev was a little better than stock, too, but close in performance.

WHAT’S THE SQUAWK? We had two complaints: (1) The RS-4D is not available as a slip-on or in stainless/aluminum materials. (2) The titanium- and carbon-only choices make the exhaust system cost a sizable chunk of change.

It’s easy to make a slow bike faster, but the 2012 KX250F is not slow. To Yoshimura’s credit, the RS-4D made the all-good KX250F powerband even better. The power was good, but power-per-dollar ratio, not so much.

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