Slater Skins Air Box Cover (Yamaha YZ250F/450F)

Remember Slater Skins? Yeah, the Blue Buffalo Supercross Team that had the complete signage/coverings across their bikes was John Slater’s brainchild, but most recently he has started to offer air box covers for late model YZ-F’s that increase airflow. We all know more air equals more power, but how does it relate to the track? I took one of Slater Skins air box lid covers out with me on a test day, to see how each performed on a 2020 YZ250F, as well as a 2021 YZ450F and found the results interesting. 

Now you might be asking yourself, why the hell do I need more power on my YZ450F? You don’t if you’re a mere mortal like me! However, if you’re looking for slightly more midrange pulling power on softer tracks as well as faster tracks with hills, this Slater Skins cover will give you a little more in those areas. Where I really liked the Slater Skins cover was on the YZ250F. The Slater Skins cover not only has front vents (with screen coverings) for increased air flow, but also more volume to allow more air inside the air box. By doing this I can really feel more bottom end pulling power out of the YZ250F as well as better low end RPM response through corners. This Slater Skins air box cover can make your YZ250F have just a little more snap out of the hole along with a slightly snappier mid RPM punch in third gear. I notice most of the mid range RPM increase while accelerating out of a corner (in third gear on the YZ250F) when the track has nasty square edge or bumps. Trying to “hop” over some of those bumps with the stock cover makes the bike feel a little less reactive to your throttle hand and heavy. With the Slater Skins cover the YZ250F felt crisper and allowed me to “pop” over bumps with a lighter more exciting feel. The difference between the stock lid and the Slater Skins lid is NOT MIND BLOWING, but it is noticeable (albeit mostly on the YZ250F). If you look at a 2021 YZ250F air box cover you will notice Yamaha put holes on the back side of the air box lid so that tells you that even the in house Yamaha R&D team was looking for increased air flow in the stock cover. The difference is that Yamaha put their vents near the back of the lid and Slater Skins incorporates the front part of the lid for increased ventilation. 

The next question is, what about the noise Keefer? Yes, it is a little louder, but to me it sounds very race bike-ish. The suction or increased noise you’re going to hear from your air box is not going to deafen you. However, I tested some other Slater Skins covers with larger vents and those were too loud and to me the power increase didn’t warrant the noise. If you’re coming off of another brand of motorcycle onto the YZ250F with the Slater Skins cover then it could be nuisance, but if you already ride a Yamaha, you should not notice a huge amount of noise. The suction noise/does get louder as you open your throttle. 

If I was needing a little more bottom end pull as well as RPM response the Slater Skins $80.00 air box lid offering is a good deal. The cover comes in white/black, but If you're looking for a blue cover, John has a blue sticker kit he can install on a black cover if need be. Blue will be available shortly. The cover comes with a washer clip for your stock dzus fastener and are available by reaching John over at slaterskins@gmail.com.   

Notes: I tested the Slater Skins cover with a stock air filter cage/screen. I have also tried it with a Twin Air Power Flow open cage/filter with even better results, but also a little louder. FYI…

Just note that your filter will/can get dirtier with the vents being up front of the lid. The screen that covers the holes doesn’t allow a ton of dirt through, but it can get much dirtier in sandier/silty conditions. FYI…

Yoshimura RS-9T Full Titanium/Carbon Muffler System

The 2018 Honda CRF450R is a refined version of the 2017, albeit a better one I may add. If you listened to the muffler shootout podcast you would know that this is a great muffler to add to your new Honda CR450R. If you haven't listened do yourself a favor and click on the podcast tab and look for the "Honda CRF450R Muffler Shootout Podcast". Once I got my hands on one and I took it through the ringer of the shootout process, I knew Yosh had put some R&D time into this system. We know Yoshimura works closely with the Factory Honda team here in the states and have seen the RS-9T muffler on Cole Seely and Ken Roczen’s new bikes so it's no surprise that they work pretty damn well. The Yoshimura RS-9T system only comes in a dual can set up (no single mufflers are available from Yoshimura) and is a three-piece slip fit design. Installation of the system was painless and took 15 minutes to install, which was nice to do when at the track swapping mufflers.

Once installed and on the track the Honda’s exhaust note turned from high pitch 250F’esq to a deep throaty more traditional 450 factory race bike sound. The crack of the throttle (or RPM response) is slightly smoother and less crisp, but bottom end pulling power is increased over the stock muffler. Rolling out of corners, in second gear, feels like the rear wheel has even more traction than stock and was more controllable while accelerating out of hard pack corners. The slightly smoother RPM response takes away the Honda’s excitement feel ever so slightly, but helps you gain a little more control coming out of corners. The mid range pull is healthier than stock and while the stock system had a tough time pulling third gear in tighter corners (without a gearing change), the Yoshimura system will give you an easier time rolling third gear corners. With just the flick of the clutch lever (in third gear) the Honda will be in the meat of the power once again and have you down the straight in a hurry. We noticed top end and over-rev was as good as stock (which is great). It pulls adequately down the straights in second and third gear and there wasn’t a time where I thought to myself  “I need more top end pulling power”. Ehhhhhh. No. You could tell Yoshimura was focused on bottom to mid-range pulling power when they designed this muffler system and they succeeded in doing so. It is also impressive that they didn’t lose any top end and over-rev in the process.

After weighing both the stock and Yoshimura RS-9T systems you will be saving 1.9 pounds, which is a nice chunk off of a already heavy (on paper) motorcycle. This is a significant weight loss but for $1,499.00 it is a very pricey bolt on modification. If you are looking to save a little money Yoshimura offers the stainless steel/carbon version for $977.00, but you will not be saving much weight (only half a pound). I really like the craftsmanship that went into the Yoshimura system. The welds are flawless and the mufflers tuck up inside the side number plates for a stealthy, compact look. If you’re looking for al little added giddy up and bling from your new 2018 CRF450R, the Yoshimura RS-9T system will help you on the track, but your wallet will be not as fat. This is the price we pay for getting some extra HP and a better-looking muffler right? This is one of my favorite systems for the 2018 Honda CRF450R and it has held up great over time (over 20 engine hours). You can head over to Yoshimura-rd.com to get yours or call them at 800-634-9166.

If you have any questions about this test please feel free to email me at kris@keeferinctesting.com 

Dubach Racing (DR.D) 2018 Yamaha YZ450F Slip-On Muffler

Before I start this review I am going to go on a little rant here, so bare with me. Mufflers are always a hot topic when it comes to horsepower, especially when you get a new bike. I know a lot of riders out there get a muffler system before they even ride their new bikes! Ride your new bike and break it in before purchasing any muffler please! See what you're working with and purchasing, so you know if its better than stock. Because if it’s not better than stock, what is the reason you purchased it in the first place? To look cool. To drop some weight. Those two statements would be the only answers that are acceptable when someone asks you why you have a new muffler on that bike that has an hour meter that reads 0.1 hours. Ok moving on…….. 

When it comes to Yamaha motorcycles, Doug Dubach is a name that comes to mind immediately right? Doug has been a Yamaha test rider for 87 years and is almost that old, but don't tell him I said that. Seriously, Doug knows the ins and outs of the 2018 YZ450F like the back of his hand, so I was anxious to try his slip-on muffler when I reached out to him. However, instead of trying a full system I wanted to do something different and just try a slip on muffler system. A slip on system is only the rear section that keeps the stock headpipe and mid-pipe intact. I installed the DR.D stainless muffler system (that saves just over one pound compared to stock) in only 3.5 minutes. 

I am very picky when it comes to mufflers and how they deliver the power to each bike I evaluate. The DR.D slip on keeps the stock feeling YZ450F bottom end pull intact, but maybe softens the low end RPM response just a tad. This is only noticeable on super loamy style tracks, but helped rear end traction feeling on hard pack style tracks. Losing a little RPM response and gaining some rear wheel traction is not a bad thing on this beast of an engine. I want to note how hard it is to build a muffler system that keeps the low end pulling power feeling of a stock 2018 YZ450F as I have tested several full muffler systems that had less bottom-end than stock. Where most of you will feel the difference between the stock muffler and the DR.D is mid range pulling power. The mid-range meat that the DR.D system delivers is impressive and gives you a better pull in second gear and helps the third gear transition when shifting. With the stock Yamaha system (with stock gearing) it was hard at times to carry third gear through corners, so shifting to third immediately after the corner was a must. With the DR.D system I have the freedom to leave the Yamaha in second through the corner and shift early without the blue machine falling off the power much. Or if you choose to run third gear in corners it picks up the power slightly earlier and helps lazier/lug heavy riders get out of the corner quicker with more rear wheel traction. Top end is also slightly increased over the stock muffler as you are able to hold each gear (well, to be honest the only gears I used was first through fourth) slightly longer. If you're looking for more over-rev this isn't the muffler you are looking for because the DR.D system keeps all of the stock feeling over-rev on the track and doesn't gain anything extra. This isn't a bad thing as the standard YZ450F power character revs out just fine. 

The DR.D slip on stainless system is the best slip on system that I have tried on the 2018 Yamaha YZ450F and will only set you back $395.00. Not to mention that you’re not losing any horsepower and gaining some mid-top end, while getting some added rear wheel traction. The DR.D muffler system comes with quality packing, has a nice sound to it without it being obnoxiously loud, is easy to re-pack on your own and doesn't blow out quickly. If it comes time to re-pack and you don't want to do it yourself, Dubach has an exhaust packing service where you can send in your muffler and he will re-furbish it for you for a small fee. If you're wife is having a conniption fit about you spending more money on your new bike you can tell her you're saving around $600.00 by not going to a full system. You’re actually compromising and doing her a favor! You're welcome fellas! You can check out all of the mufflers that DR.D makes over at dubachracing.com. 

If you have any questions about this test, please free to contact me at kris@keeferinctesting.com