Dubach Racing Development NS-4 Full System SS/AL

Tester: Chris Siebenhaar
Bike: 2020 Kawasaki KX250
Mods: Completely Stock
Weight: 7 lb - 4 oz
Weight Savings: 1lb - 3 oz

Whether Dr. D means “Doctor Dubach” to you, or “Dubach Racing Development”, one thing is for certain, the Doctor has a long list of first-hand racing and testing accomplishments that few in the industry can match. All that knowledge has been distilled into Dr. D products, from his ingenious engine relocating kits (mainly for Yamaha’s), to exhaust systems for every current motocross manufacturer.

Recently, I was able to test their NS-4 exhaust on a 2020 Kawasaki KX250. While I had quite a bit of knowledge of the brand and the man behind it, I had zero experience with any of the products.

Unboxing

Upon opening, I found everything neatly tucked inside the box, bubble wrapped for safe handling.

• Stainless Steel Header
• Stainless Steel Mid-Pipe
• Aluminum Muffler w/ Magnesium endcap 
• Necessary Hardware
• Spark Arrestor
• “USF Approved” sticker to place on your exhaust if you run the S/A\

Installation

Install was 99% a breeze. That 1% comes from the fact the fitment of the pipe pieces (header to mid-pipe, mid-pipe to muffler) was very snug. Which, is not a bad thing, this just means it minimizes the chance of exhaust gasses escaping before they were intended to. I had a little anti-seize laying around, so with a slight schmear around the joints, they slipped together with relative ease. Other than that, all the mounting points were spot on and required no pushing, pulling, or coaxing to get into place. 

Start-Up

The sound of the NS-4 system was a nice change from the OEM exhaust. It had a deep, tight tone to it, with a healthy bark when you cracked the throttle. 

Testing Grounds

I chose four very different tracks to test the system; Cahuilla Creek MX, Glen Helen, State Fair MX, and Fox Raceway. Each has its own nuances that help to test the characteristics of performance products. Cahuilla Creek is fast and flowing with lots of elevation changes and usually rather sandy. Glen Helen is well known for its massive power-robbing hills. State Fair has a very flat layout with deep ruts, typical of most “fair ground” style tracks. Fox Raceway while a large track, has lots of jumps right out of corners that require a broad powerband to really put consistent laps together (especially on a 250F).

Stock Performance

One thing to keep in mind about the 2020 KX250 for performance tests, is that for 2020 Kawasaki brought over their superbike engine developer in hopes of getting more power from their 250 engines (if you have ridden a ZX-10R Superbike you know that the bike is an absolute rocket ship). In doing so, they found a lot of horsepower up top but sacrificed a bit of the low to mid-range to get there, thus leaving you to keep the rpm’s up in order to get the most out of the new engine. 

Cheeks in the Seat

Track Performance

While the overall presumptive feeling heading onto the track was that there were gains right from the bottom (mostly due to the sound), on track there was a little difference to be felt down low. But this is more of an engine trait rather than an exhaust shortcoming. Where the exhaust started to show signs of performance gains was coming into the mid-range. Here, there was a healthy bump in power that carried through the mid-range and into the top end. Peak Power/Over-rev didn’t seem like there was too much change, but again, that’s not a bad thing so long as you don’t lose power. With Kawasaki pumping so much into the top end, what the bike really needed (in my opinion) is a stronger mid-range to help you get there with more steam and better utilize each gear. 

At tracks like Glen Helen and Cahuilla Creek that have multiple long uphill’s with momentum-robbing turns right before, the added midrange helped the drive out of the corner and many instances reducing or eliminating the need to slip the clutch to get into the power. This is a huge plus as it can save your clutch components in the long run and reduce the risk of frying a clutch while riding, something I always try to be cautious of on 250F’s. Additionally, Glen Helen notoriously gets rough, everywhere; turns, straights, entrances and exits, you’re just doing work, all the time. With rollers and pockets that develop right out of corners, the added mid-range helped to loft the front wheel up in an attempt to smooth out a track that can deteriorate from lap to lap. 

At Fox Raceway the benefit of added mid-range was much appreciated for the multiple jumps right out of a corner as well as the deep sand in the corners which have a habit of developing soft spots that seem to reach out hold on the bike as you exit. 

State Fair MX, like most fairground tracks that need to maximize track layout with limited acreage, is on the tighter side. Lots of 90 and 180 degree turns in order to add length and save space. In a layout like this, having a bump in mid-range power was welcomed at a significant amount of time is spent around the area of the power. Really only winding out a gear for a second or two before you’re back to the next turn. Riders that ride tracks with this format will love this system as amplifies the zone you will be spending most of your time in. 

Off-Road

Due to the circumstances that the world has been in lately, this left majority of our southern California tracks closed for around a month (give or take), so in order to make due, I went out and did some off-road riding figuring that it would make the test that much more well-rounded. The area I rode in was made up of fire roads and lots of single and two-track, and in this setting, I thought the NS-4 system was fantastic! Being that nearly all of my time was spent in and around the mid-range, the improvement in power was a breath of fresh air. The quick jolts between trees or through the bushes or popping over a tree root or rock was made that much easier with newfound gains.

DFI Couplers

I ran both the White Coupler and the Green Coupler depending on the track, as each proved beneficial. The white coupler while adding to the mid-range and the throttle response, signed off early and left the top-end feeling flat. I preferred this one at both State Fair MX and Fox Raceway, again while Fox Raceway is a long track with fast sections, the benefit of having the extra punch in the mid-range was more important for certain sections. This coupler emphasized the boost by giving the stock lack-luster mid-range a “hit”. 

The green coupler was preferred at both Glen Helen and Cahuilla Creek which had long straightaways and uphill’s that require you to stretch out each gear into higher rpm. Overall the green coupler made for a very smooth and linear power curve, you lost some of the hit the given by the white coupler, but you gained significantly more throughout the powerband. 

Overall a really great exhaust system. It increases performance where the bike needs it the most, has robust construction, great fitment, is easily rebuildable, improved looks over stock and sheds a little weight while it's at it, all for minimal price (in the 4-stroke exhaust world). 

It's cons come from what it's not trying to be; exotic. So don't read that as a bad thing. For just shy of $640, it's hundreds of dollars less than some of the Titanium systems on the market, so expect the difference in price to be reflected in terms of shear beauty (think Akrapovic) and weight. Also I did keep catching my finger on the resonance chamber when reaching for the kickstarter, more of an operator error, but something that should be kept in mind as I now have a few gloves with extra ventilation.