FMF Gnarly and Turbine Core for Yamaha YZ250X

What it is

• FMF's pipe and mufflers for the new Yamaha YZ250X. Some not available.

Here is what you really need to know. That Yamaha YZ250X needs it’s own pipe and muffler if you want proper performance. How do we know, it wasn't because FMF told us. We actually tested some stuff and found this out. If you want to lose a little bit of power and performance just bolt on YZ motocross pipes and mufflers on and you’ll be sorry.

FMF took the time to develop and refine their Turbine Core 2 muffler to specifically match the different compression ratio and powervalve timing as well as keep the off-road power style the YZ is so good at producing.

The Gnarly pipe focuses on increasing the lower RPM horsepower and torque and works on the YZ250X unlike the Fatty. FMF is looking to add four-stroke tractor like torque to your two-stroke. Designed to withstand the harsh elements it is made from 18-gauge steel construction and a nickel coating for rust-free looks. And the O-Rings are included.

The Turbine Core 2.1 is the newest FMF spark arrestor using the turbine core internal and a larger aluminum body with a stainless steel end cap. It has more packing volume yet is claimed to be a half-pound lighter than stock. But FMF did not put this pipe into production after we tested an early version.

The Turbine Core 2 is the standard for slip-on spark arrestors. It is made from an aluminum extrusion with the proven turbine core spark arrestor and then a perforated tube surrounded by packing capped with a stainless steel end cap. This is currently being produced as a S/A application for the YZ250X

*This evaluation was updated 9/13/2016 to reflect current information.

How it works

• Definitely some options for tuning the power.
• Not as narrow as the well-designed stock pipe.
• We really liked the look and the sound coming out of the new Turbine Core 2.1. Too bad you can't buy one!

* This test has been updated to reflect that FMF never put the TurbineCore 2.1 into production and the TurbineCore available for the YZ250 is not what we tested here.

Like we said, don’t expect much if you slip on YZ250 motocross pipes or mufflers and expect to get performance gains. We did and the bike lost bottom, lost torque and even lost top-end pull and power. From a simple little pipe and muffler change showing how important it is that your package is tuned. Yamaha went to great lengths to get the new X to run like it does and the pipe is one of the key components of the package like it is on any two-stroke. Obviously they did a really good job as pipe makers quickly found out that the stock pipe was hard to beat.

First off in testing we did a number of different tests and switching around combinations of pipes (stock and FMF) and mufflers (stock and FMF) to test every combination. All of our testing was done at near 2000 Ft. elevation and in roughly 60-degree weather. We were basically a clip position and a main jet size leaner in jetting on all of our setups for normal riders. Slower riders who did not rev the bike as much could have easily dropped a pilot jet size and another on the main jet with the FMF pipe.  

Individually the Gnarly pipe does most of what FMF claims. Though the pipe alone needs to have the leaner jetting to work properly, with the stock muffler it was still a little rich on the bottom and loses some of the very low-low end crispness of the stock system. Then when everything cleans up and starts functioning there is more torque which means less stalling when a gear high. It pulls with slightly more authority and runs through the powerband making more power at similar RPMs. Up top it feels like it makes the same power but does not rev out as far as stock. It prefers to be shifted early and use the meatier mid-range compared to the stock pipe where you can ride it any way you like. One other thing the FMF pipe does is resist dents better though it’s thicker build but it also sticks out farther on each side compared to the stock pipe where Yamaha went to painstaking measures to tuck the shape of the stampings in for tighter side clearance. On a performance alone stance the pipe is better for those who like a more robust mid-range and meatier torque feeling.

The Turbine Core 2 muffler alone on the stock pipe also causes a slight drop in crispness across the range and really likes the leaner jetting. If anything it runs mostly the same with a slight decrease in throttle response through the range. But the bike did pick up on top end power and the length in which it revved out. The exhaust note was very close to stock still but at least now the bike was spark arrestor legal for when it needed to be. The Turbine Core is 2.9 pounds which is a half-pound lighter than stock.

Onto the pre-production Turbine Core 2.1 (It had a smaller outlet opening than the YZ250 one does) and this one was better. On the stock pipe it kept the crispness of the stock straight through muffler even with the spark arrestor. It felt like the bike actually started pulling earlier and also revved longer. Power wise we could not feel any increase and if anything in the middle it felt like there was just a bit less. But the bike was easier to ride and on top it pulled the gearing better. On the noise side is where the 2.1 shines taking the bark or sharpness out of the noise and especially to the rider the bike sounds quieter. Weight wise it is identical to stock at 3.5 pounds.

Now we tested the FMF Gnarly and the different S/A silencers. Mated to the Turbine Core 2 the pipe starts to come alive. It still isn’t as crisp as the stock/stock setup but it does start pulling down as low. It has a torquier feel and the mid-range pull is enhanced quite a bit over stock. Now the bike really likes gear-high riding. The overrev is not as crisp as stock but the length of the pull comes back with this muffler.

Even better for most of us was the pre-production Turbine Core 2.1/Gnarly combination. Mostly because it mirrored the stock pipes crispness and smoothness with just a little more mid-range and then a long and smooth overrev. There felt a little more power everywhere in the spread except on top where it felt the same as stock. Not too bad for a S/A setup that was also quiet. This really made us (and pipe makers trying to do better) aware of how much work Yamaha put into the tuning of the X. Too bad it is not for sale.

Riders who wanted more of a snappy and motocross feel preferred the Turbine Core 2 and riders who liked smooth and seamless power delivery, or were more what we’d call trail rider category preferred the pre-production Turbine Core 2.1 that is not for sale. The FMF parts are priced very competitively and actually seem like a bargain when compared to what a four-stroke system costs. With the two-stroke pipe sticking out and just asking for dents, this is a good thing and FMF has accomplished the very tough task of making a better pipe for the YZ250X.

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