From VitalMX:
Vital MX - With Honda releasing a new CRF450R in 2017, we all knew that meant they would follow it up in 2018 with some sort of revision to the CRF250R. And due to the spy pics from Japan this year, we also knew that it would be even more "new" than the 450 last year. While we expected it to get the new chassis, bodywork, suspension and down-draft intake layout of the 450...what was unexpected is the change from the Unicam, which the CRF250R has used since its debut in 2004, to a dual overhead cam and finger follower actuated valve system on their latest engine. This means the CRF250R has an all new engine for this year. For those that want to get things in a video they can listen to while on the go, we snagged one of the project leads, Bill Weppner, and recorded this piece for you all to listen to. (click play below) For those that want to read on, keep scrolling!
Now the cams weren't the only thing to get doubled up for 2018, as the new CRF250R also features twin exhaust ports leading to twin head pipes. This leaves the new model with two separate exhaust systems, but with head pipes that are unique from one another. The right side is longer, features a different taper and has a resonance chamber, while the left model is shorter and doesn't have the chamber. Beyond this, the new engine also features a shorter stroke and larger bore when compared to the 2010-17 model engine...this coupled with the larger valves, down draft intake, finger followers and twin headers all equal to more power which is aimed at the mid to top of the RPM range. In short, Honda has aimed for more than a 10% power increase, which they were able to nail on the CRF450R last year.
Beyond this, the CRF250R had one more little surprise as it will feature electric start only. While the CRF450R came with a kickstarter last year before switching the optional electric start kit as standard, the CRF250R doesn't even have a kickstarter area on the engine case. So it's committed to the button only for the foreseeable future. What it does share with the CRF450R is a new chassis and swingarm, along with the new Showa "A-Kit" type spring fork and shock.
All-in-all, the CRF250R has taken on the finger follower and right side cam chain design we see on the KTM, the downdraft intake from the Yamaha (albeit with a standard cylinder layout), and the twin head pipes we've seen aboard the TM 250F and combines it all together with some of their own unique features to try and reclaim their spot at the top of the 250F class. With the well received chassis the 450 gained last year and the switch back to spring forks, we could see it get up there. The only real thing that looks to be holding the new bike back is the weight gain, with about three pounds coming from the switch to spring forks from the previous SFF TAC, and another four pounds or so from electric start, battery and engine layout changes.
“We’re excited to show this all-new, no-compromise model to our customers, and it’s appropriate that we do so with our family of Red Rider amateur motocrossers here at Loretta Lynn’s,” said Lee Edmunds, American Honda’s Manager of Motorcycle Marketing Communications. “The CRF450R has become the top-selling 450cc motocross model, and now that the CRF250R shares many of the same performance-focused updates, we’re confident it will enjoy similar success in the market and on racetracks everywhere.”
ENGINE
In order to achieve their powerplant development goal of increased power (particularly at higher revs), engineers gave the 249cc engine an all-new dual-overhead-cam design, using a finger rocker arm with Diamond Like Coating (DLC). The titanium intake and exhaust valves are larger and have a higher lift and narrower angle, while the valve springs’ coils now have an oval cross-section to keep engine height as low as possible. The engine has more oversquare dimensions, the cylinder offset has been increased, and the piston now features a bridge-box design–a first for a production Honda motocrosser. The intake system is now of a downdraft design for maximum efficiency, there are twin exhaust ports, and the dual exhaust features smoother routing and optimized pipe length. An electric starter is standard, powered by a small, lightweight lithium-iron phosphate battery. The end result of the copious changes is an engine with more power across the rev range, especially impressive at peak rpm.
http://www.vitalmx.com/product/guide/Bikes,6/Honda/CRF250R,15698#product-reviews/799