Yoshimura Signature Series Exhaust: Sound, Weight, Real Gains
By Yoshimura R&D of America, Inc. | Published: 2026-10-13
Category: Product Reviews
Compare the Yoshimura Signature Series RS-12 full system and Race Series Straight Cyclone on sound, weight, and dyno gains. See which fits your bike and riding style.
Quick Answer
For riders who want a measurable dyno gain on a specific motocross platform, the Yoshimura Signature Series RS-12 full system for the KX250/X is the stronger pick, adding up to 4.5% peak horsepower and 7.7% peak torque on the supplied catalog figures. If you ride a Honda Dax 125 and care more about weight and a classic up-swept look, the Race Series Straight Cyclone is the better match. Neither is a universal upgrade; each is built for a specific machine.
What the Signature Series Actually Changes
The Signature Series is Yoshimura's platform-specific full-system line. The RS-12 for the 2021-2024 Kawasaki KX250 and KX250X is the clearest example: Yoshimura states it adds usable power and torque across almost the entire curve, with a max horsepower increase of 4.5% at 12,000 rpm and a max torque increase of 7.7% at 6,700 rpm. Those are catalog figures, not independent dyno results, and they describe a full system rather than a slip-on. If you are comparing a Signature Series full system to a stock header-and-muffler setup, the header diameter, collector, and muffler core all change together, which is why the gains are broader than a muffler swap alone.
Sound and Weight: What the Catalog Does and Doesn't Tell You
Yoshimura publishes exact weight for the Dax 125 Straight Cyclone: 5.5 lbs for the system, which is a meaningful drop on a mini moto where every pound affects flickability. The RS-12 listing does not publish a system weight or a decibel rating, so treat any sound or weight claim beyond the Dax figure as unverified. What the catalog does confirm is that the Dax system ships with a matte carbon fiber heat shield, which matters on a bike where the exhaust routes close to the rider's leg. For the RS-12, the practical sound question is better answered by checking whether your track or riding area enforces a sound limit, then calling Yoshimura's sales line at 1-800-634-9166 to confirm current insert or spark arrestor options for your exact application.
Where the Race Series Straight Cyclone Fits Instead
The Straight Cyclone is a Race Series part, not a Signature Series part, and it is aimed at the 2023-2025 Honda Dax 125. It is only offered in classic black and uses an up-swept high-pipe layout. That layout changes the bike's silhouette and moves the muffler higher, which is a styling and clearance decision as much as a performance one. Because the Dax is a small-displacement mini moto, the 5.5 lb system weight and the included heat shield are the headline facts; Yoshimura does not publish a horsepower or torque gain for this system in the supplied catalog information, so do not assume one.
Service Parts Keep Either System Usable Long-Term
Both systems depend on replacement parts that are easy to overlook until something wears out or gets damaged. The Sound Insert Kit INS-37-K is made specifically for the YZF-R7 AT2 Titanium Race Full System and fits only AT2 2.375-inch Titanium Race Mufflers with the big outlet; it will not fit the standard 2.25-inch AT2 outlet. That is a fitment trap worth reading twice. Separately, the left-hand muffler assembly 3901605501 is the kind of part you look for after a tip-over grinds a hole in a muffler. Yoshimura notes that some damage can be repaired through their accident repair service, and sometimes buying a new muffler is the cheaper path. If you cannot find the exact muffler for your application, the catalog directs you to call 1-800-634-9166 rather than guess.
Trade-Offs to Weigh Before You Buy
- Documented dyno gain vs. platform lock-in: The RS-12 publishes a 4.5% peak horsepower and 7.7% peak torque gain, but only for the 2021-2024 KX250/KX250X. That gain does not transfer to another bike. Buy the RS-12 because you own that Kawasaki, not because the numbers look good in isolation.
- Published weight vs. unpublished weight: The Dax 125 Straight Cyclone lists 5.5 lbs. The RS-12 listing does not publish a system weight, so you cannot compare the two on weight from the catalog alone. If weight is your deciding factor, call 1-800-634-9166 and ask for the RS-12 system weight before ordering.
- Sound control vs. fitment precision: The INS-37-K sound insert exists for a specific AT2 big-outlet muffler and will not fit the standard 2.25-inch AT2. Sound management is possible, but only with the correct part number for your muffler. Verify your outlet diameter first; a wrong insert is a wasted order.
Key Product Facts
- KX250/X 21-24 RS-12 Stainless Full Exhaust, w/ Aluminum Muffler: Yoshimura RS-12 Signature Series full system for the 2021-2024 Kawasaki KX250/KX250X, catalog price USD 599.40, with stated max horsepower increase of 4.5% at 12,000 rpm and max torque increase of 7.7% at 6,700 rpm. This is the clearest documented performance case in the comparison, and it applies only to that specific Kawasaki platform.
- YAT DAX 125 2023-25 RACE STRAIGHT CYCLONE HIGH PIPE STAINLESS BLACK: Yoshimura Race Series Straight Cyclone high pipe for the 2023-2025 Honda Dax 125, stainless black, 5.5 lbs, supplied with a matte carbon fiber heat shield. The published weight and included heat shield are the concrete advantages here; no dyno gain is stated for this system.
- SOUND INSERT KIT AT2 RACE MUFFLER (INS-37-K): Sound Insert Kit INS-37-K fits only AT2 2.375-inch Titanium Race Mufflers with the big outlet and will not fit the standard 2.25-inch AT2 outlet. Ordering this insert for a standard AT2 muffler will not work, so measure your outlet before buying.
- 3901605501 LEFT-HAND MUFFLER ASSEMBLY, SS/SS: Left-hand muffler assembly 3901605501, stainless, catalog price USD 207.35, intended for replacing a damaged muffler; Yoshimura notes some damage can be repaired instead. Compare the cost of a replacement muffler against the accident repair service before deciding which route to take.
Which One Fits Your Situation
Best for
- 2021-2024 Kawasaki KX250 or KX250X owners who want a documented full-system power and torque gain
- 2023-2025 Honda Dax 125 riders who want a lighter 5.5 lb up-swept system with a matte carbon heat shield
- Riders who already know their exact bike year and model and can confirm fitment before ordering
Not for
- Riders on a different make, model, or year than the ones listed for each system
- Anyone expecting a slip-on muffler swap to deliver the same gains as the RS-12 full system
- Dax 125 owners who need a published horsepower or torque figure, since the catalog does not state one
