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Increasing a Honda CRF300L’s output by more than 25% with a big-bore kit and other mods

Jun 19, 2023

Folks love Honda’s handsome, capable CRF300L and CRF300L Rally. The dual-sport duo have a lot going for them, and only one significant thing going against them: underwhelming power.

So what happens if you throw some go-fast parts like a full exhaust, big-bore kit, and intake mods at the bikes’ 286 cc single? How much power can you gain, and what difference will it make? 

That’s what one CRF Rally owner, Terence, wanted to know when he reached out to me via Instagram. He’d stumbled across posts I made while putting together CBR300R engines with a big-bore kit I developed back when I club raced Honda’s littlest sport bike, and he wanted to know if the kit would work on his CRF300L. 

AHR 78mm big bore kit for CBR300R and CRF300L
Originally developed for my Ultra Lightweight racing endeavors, this 78mm forged piston kit adds 2 mm of bore and 14.8 cc for a total displacement of 300.8, and pushes compression from 10.7:1 to 11.5:1. The Ninja 400 and RC390 swept the legs out from under the CBR300R in the club-racing scene, but Grom owners are using my piston kit while doing CBR swaps (yes, people are putting hopped-up CBR300R engines in Groms), and CRF riders are starting to show an interest, as well. Photo by Ari Henning.

The CRF’s engine is largely based on the CBR platform so I knew the two-millimeter larger, high-compression piston kit was compatible. The big differences between the CRF and CBR platforms are the airbox, cams, and exhaust, with the CRF’s arrangement favoring broad low-end power while the CBR’s tune is meant to add a little more top-end punch. Emphasis on little, since the CBR only makes about 27 horsepower. Still, CRF owners get just 24 ponies from the factory, so there’s a lot of interest in swapping CBR parts to try to close the gap. 

CBR300R velocity stack being installed in a CRF300L airbox.
Stock CRF velocity stack and airbox-inlet boot on the left, CBR velocity stack and cut-down boot on the right. In general a longer, narrower intake tract is better for low-end power, while shorter, larger-diameter tubes optimize output in the upper revs. You can get the stubbier CBR parts used off eBay for just $30, but they’ll cost you a lot more than that in terms of frustration to install. Photo by Ari Henning.

The plan for Terence’s supermoto Rally was to install the piston kit as well as implement a CBR300R cam swap and stuff the shorter, larger-diameter CBR300R velocity stack and intake boot into the CRF’s airbox. That was all on top of the full exhaust system and high-flow air filter already on the bike. Once everything was buttoned up, I’d haul the Honda out to JETT Tuning in Camarillo, California, to tune the air-fuel ratio with a DynoJet Power Vision module.

CBR300R camshaft for the CRF300L
Honda couldn’t (wouldn't?) provide cam specs, so I sent the CRF and CBR parts to Daniel Crower Racing for evaluation. The numbers were too close to tell a difference on the exhaust cams, but the CBR’s intake cam offers a few thousandths more lift and a few degrees more duration, which helps with cylinder filling. Plus, it’s only $40! Photo by Ari Henning.

I tackled the build in my home shop and had the bike torn down and reassembled in a few afternoons. After some heat cycles around the neighborhood — where the bassy Yoshimura exhaust and the snap of a lighter, higher-compression piston already had the CRF feeling feistier — I hauled the bike up to Camarillo for tuning.

JETT’s John Ethell and I have worked together a lot over the years but hadn’t seen each other since before the pandemic, so it was an enjoyable reunion. As is John’s way, he turned the visit into a learning experience, letting me struggle with my rusty dyno skills and the newfangled Power Vision module before stepping in to teach me how to fish.

John Ethell of JETT Tuning in Camarillo, CA
Don’t let the tank top fool you. When it comes to tuning, John Ethell is all business. John and I first teamed up in 2011 when I was campaigning the then-new CBR250R, and he’s been a mentor ever since. When people ask me how I learned to wrench, I say I read workshop manuals and ask advice from people who know more than me. John is at the top of that list of people. Photo by Ari Henning.

Once the air-fuel ratio was dialed in, I made some power runs. I’d told Terence to expect around 30 horsepower, so I was happy to see a shade over 31. It’s still a small figure, but a represents a huge delta over stock and the engine is no less reliable. My CBR300R race bike was stone reliable as well and made more than 38 horsepower, but that was with high-lift cams, CNC porting, and $25 per gallon race gas. Meanwhile, Terence can get by with mid-grade pump gas. 

Honda CRF300L big bore dyno chart comparison
Blue is is our baseline for a stock CRF300L, and red is the modded bike. Both machines were run with Terence's rear 17-inch supermoto wheel. With more than 31 horsepower and nearly 22 foot-pounds of torque, this modded CRF is stronger than a BMW G 310 GS and hot on the heels of KTM's 390 Adventure. RevZilla image.

From the saddle, the CRF felt transformed, but that’s what everyone who’s modded their bike says. To see what practical difference the mods made, I strapped on a GPS data logger and did a few quarter-mile runs.

My seat-of-the-pants impressions were corroborated by markedly improved performance figures: From a sluggish 16.22 seconds at 73.4 mph to a respectable 15 flat at 81.0 mph, a speed a stock CRF struggles to attain on an open road in still air. For Terence, who rides the bike regularly around Los Angeles, the 2.7-second improvement in 50-70 mph acceleration time is perhaps the most appealing progress. 

Straight-line performance data for the modded Honda CRF300l
Out on the road, the modded CRF's 28% increase in output correlates to big improvements in all aspects of acceleration. For comparison purposes, I swapped Terence’s 17-inch supermoto rear wheel to a stock CRF300L I borrowed from Honda, for dyno runs and straight-line performance testing. RevZilla illustration.

“The biggest difference is the power in third and fourth gears,” says Terence. “It’s so much faster, I love it.”

Folks already loved the CRF300Ls, and with an honest 300 cc and some key performance mods, what little there was curbing riders’ affection for the handsome dual-sport can be corrected.