Stealth Head Mod // 2025-2026 Arctic Cat ZR / RIOT / M 858 C-TEC2 Catalyst
Our Stealth Head Mod with billet dome inserts is the key to unlock maximum power and performance from the 858 C-TEC2!
Speedwerx Stealth Head Mod // 2025-2026 Arctic Cat ZR / RIOT / M 858 C-TEC2 Catalyst
Our Stealth Head Mod with billet dome inserts for the Arctic Cat 858 adds a huge amount of power and performance! The chamber design is completely different from stock- we switch back to a true conventional dome shape chamber, and change the width and depth of the squish band. The results are 5 HP gain on premium pump gas at sea level with our 12.75:1 domes and 8+ HP at higher elevations with increased compression ratios. Cylinder to Head O-Rings sold separately- see related products below.
Preliminary testing has shown a 18hp gain with our Pipe, Y-Pipe, Stealth Head, and Air Intake with at high elevation! We will be offering our Exterminator Kits for a complete package that include these parts plus an ECU Tune and Hypershift Clutch Kit for maximum power and performance in a bolt-on kit. Update 2/28: an ECU Tune is required with the Pipe/Head/Intake combo- see related products below.
Available Compression Ratios - Recommended Elevation Usage with Non-Oxygenated 91+ Octane Premium Pump Gas:
12.75:1 - 0-3000’
13.25:1 - 3000’+
13.75:1 - 4500’+
14.25:1 - 6000’+
14.75:1 - 9000’+
If you wish to use pump gas at varying elevations, order for the lowest elevation you regularily ride at. Higher compression heads can be used at lower elevations with high octane race fuel or a race/pump blend, however we do not have specific fuel recommendations for these cases.
About Speedwerx Stealth Heads:
What we do is machine / gut your stock head so that you can run our interchangeable billet domes. These domes drop right into place under the modified stock head cover without anyone even knowing it! Not only does this add some serious power to your machine, but it is a cheaper alternative to a complete billet head kit set-up. Kit requires modification to the stock head, and purchase of two billet domes and o-rings for the top of the domes. Inner and Outer O-Rings for the Cylinder to Head are available separately as well- see related items below.
Note: Base price is with exchange of your stock head!
Core Options:
1. Send us your stock head core and we will send you the Stealth Head once we receive it.
2. Purchase the Stealth Head outright and we will send it out right away. You can keep your stock head, or ship it to us and we will credit you back when we receive it!
TECH NOTE: Power Valve cables- we have seen a large number of sleds that the power valves are not set properly from the factory or they have moved, this dramatically effects the overall performance of you machine. Using AC tool # 836-00005 you need to confirm these are set right and if you have issues check them again. In our opinion, this is needed on every single 858 out there even in bone stock form. There is also a Technical Advisory regarding the Servomotor Bracket Alignment issued 12/5/25- your dealer may have already contacted you about this done this as part of the pre-delivery inspection, but they can help you with these adjustments if you are not comfortable doing them yourself. You can purchase the Adjustment Tool Kit here- https://speedwerx.com/apv-adjustment-tool-kit-858-c-tec2-models.html
| Manufacturer | Speedwerx, Inc. |
|---|---|
| NOTE | Base price is with exchange of your stock head! |
| Prop 65 | ⚠WARNING: CANCER AND REPRODUCTIVE HARM — SPEEDWERX, INC. PROP65 WARNINGS OR WWW.P65WARNINGS.CA.GOV. |
WARNING: Cancer and Reproductive Harm- www.P65Warnings.ca.gov See Proposition 65 Information for more on this Consumer Product Exposure Warning for residents of California.
These items are intended for closed-course competition only. Alteration of emissions-related components can constitute violation of federal and/or state laws. Substantial fines and penalties can result if used for other than closed-course competition use.
