MXA’S WE RIDE YOSHIMURA’S RMZ450

John Lennon: Imagine the two-stroke-style handling of the RM-Z450 mated to the engine know-how of Yoshimura. It’s no longer a dream.

IF YOU THREW A STONE OUT OF YOSHIMURA’S FRONT DOOR, YOU WOULD BREAK THE WINDOW IN SUZUKI TEAM MANAGER ROGER DECOSTER’S OFFICE. TEAM SUZUKI AND YOSHIMURA AREÿ LITERALLY A STONE’S THROW APART.

Yoshimura is unique to the hop-up business in that it is so specialized, so pigeonholed, so categorized, that when you say “Yoshimura,” you automatically think “Suzuki.” The bond between the two companies is so tight that if you threw a stone out of Yoshimura’s front door, you would break the window in Suzuki team manager Roger DeCoster’s office. Team Suzuki and Yoshimura are literally a stone’s throw apart.
This is cool from a performance point of viewit is obvious that Yoshimura has the inside line to Suzukibut from a marketing standpoint, it narrows their consumer base. Their competition doesn’t have the same narrow focus. Even though Pro Circuit runs Kawasaki’s KX250F race team, the majority of their exhaust and performance hop-ups are for Hondas. FMF is the official pipe sponsor of Team KTM, but the acronyms FMF and KTM are not joined at the hip. Yoshimura is grafted to Suzuki in our psychesalthough Yoshimura makes a full line of pipes for the green, red and blue bikes.

THE RM-Z450’S CHARM IS THAT IT GETS THE JOB DONE WITHOUT USAF THUNDERBIRDS-STYLE ACCELERATION.ÿ THE ALLURE OF THE RM-Z IS BASED ON THE FACT THAT IT IS EASIER TO USE THAN THE COMPETITION’S ENGINES.

Yoshimura’s Suzuki bloodline extends back decades via road racing. In the two-stroke days, Yosh’s AMA Superbike credibility didn’t do them much good in the ring-ding world of motocross. But, with the advent of the new breed of high-performance, four-stroke motocross bikes, Yosh’s years of milking mega-horsepower out of four-cylinder Suzuki road racers paid off. How so? Almost every modern four-stroke motocross bike got its start in the road race departments of the Japanese manufacturers. It’s no secret that one cylinder off of an AMA Superbike is a good jumping off point for building a four-stroke motocross bike. In fact, Ilmor Engineering of Formula 1 fame once offered to give ATK’s Horst Leitner the cylinder head off of one of its race cars to build a motocross engine from.

Once the four-stroke motocross wave kicked in, Yoshimura was ready. They do the RM-Z250 engines for Broc Hepler and Davi Millsaps and the RM-Z450 powerplant of Sebastien Tortelli. It was with this knowledge that the MXA wrecking crew decided to have Yoshimura build us a full-race, flat-out, high-performance Suzuki RM-Z450 (borrowing as much know-how as possible from Sebastien’s factory bike).

Make no mistake about it, our Yoshimura Suzuki RM-Z450 is as close to a Suzuki works bike as you can get (albeit not outfitted in Team Makita garb). This isn’t just a stock bike with a pipe and graphics. This is the engine package that Sebastien Tortelli races.

Color blind: When Yosh’s RM-Z450 rolled off the showroom floor, the only black parts on it were the tires. They fixed that.

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YOSHIMURA’S ENGINE PACKAGE MADE IT UNNECESSARY TO HANG ON LONG AND HARD. THIS BIKE COULD PULL THE NEXT GEAR WAY BEFORE THE TEXTBOOKS SAID TO SHIFT.

In stock trim, the Suzuki RM-Z450 is a little passive off the bottom (worsened by overly generous gearing choices at the factory), very strong in the middle and adequate on top (although a little flat in the higher rev ranges). The RM-Z450’s charm is that it gets the job done without violent hit, locomotive chug or USAF Thunderbirds-style acceleration. Instead, the allure of the RM-Z is based on the fact that it is smoother, broader and easier to use than the competition’s engines. The mellow bottom-end, which takes its time from gear to gear, is highlighted by an amped-up midrange and a metered pull into a top-end that seems to back itself down as the revs near the limiter. If we found the RM-Z’s genie-in-a-bottle, we’d spend one of our three wishes on more thrust to help the RM-Z450 close up the gaps in the four-speed tranny.

Beyond a shadow of a doubt, Yoshimura R&D’s full-race engine fulfilled our wishes. Power delivery was immediate and intense. Lucky for us we had an SDG seat cover on, or we would have been blown off the back the first time we wicked it on. As you can guess, it has an abundance of low-end power, which expanded tenfold as it revved into the midrange. Using just the low and mid, it was possible to handle every situation that reared its head.

What about the top-end? Although the stocker is relatively flat on top, the punched up horsepower and increased torque of the Yoshimura engine package made it unnecessary to hang on long and hard. This bike could pull the next gear way before the textbooks said to shift.

The power was astonishing. The weakest point of the stock RM-Z450 powerband is the bottom end. The tall gearing of Suzuki’s four-speed tranny needs a lot of juice to jump it up to the next gear ratiojuice that the stocker is lacking. The
Yoshimura-ported head and TRC exhaust pipe fixed that. It was a gear better in every situation. Things we did in second on the stocker, we could pull off in third with the Yosh engine. Every MXA test rider loved the way the Yoshimura RM-Z450 ran. It was stronger than the stocker at every point on the powerband and, best of all, it delivered the extra power with a snappy staccato.
All the power in the world doesn’t mean doodly-squat if the bike’s suspension can’t handle it. To achieve a better feel, MB1 modified the RM-Z450’s Showa suspension components. The MB in MB1 stands for Mike Battista, who most recently tuned the suspension of Team Honda before going out on his own to start MB1. Racing the stock RM-Z and the MB1-equipped 450 was an eye opener. Most MXA test riders were satisfied with the stock Showa settings until they tried Mike’s settings. In motion, the fork and shock were plush and fluid.

ONE TOUCH THAT WE STOLE FROM THE YOSHIMURA PARTS SHELVES WAS A $129 ALUMINUM OVERFLOW CATCH TANK. THESE TANKS ARE NORMALLY FOUND ON PAVEMENT RACERS.

The rest of the bike pulled itself together quite naturally, starting with a powder-coated black frame offset by a $69.95 Yoshimura graphics kit.
The exhaust system was a Yoshimura TRC Pro Series titanium pipe. It retails for $745 and is available with aluminum or titanium muffler cans. Yoshimura also has its own cylinder head porting department and in-house cam profile. The Stage R camshaft set retails for $595, and there are two levels of cylinder head porting: Stage 1 and
Stage 2. Every Yoshimura engine is dyno tuned to insure that it lives up to factory specs.

Building a full-race RM-Z450 requires an abundant supply of aftermarket parts. For our bike, we opted for Applied triple clamps, Tag Metals handlebars and grips, Motul lubricants, ARC levers, Galfer brakes, Dunlop tires, Twin Air filters, SDG seat covers and an RK chain.

One touch that we stole from the Yoshimura parts shelves was a $129 aluminum overflow catch tank. These tanks are normally found on pavement racers to keep radiator fluid from spilling on the track. Since SoCal has a lot of concrete starting pads, we connected it to the radiator overflow tube to keep antifreeze from dripping on our starting spot. The only downside was that we had to empty it every now and then.
ÿYoshimura’s parts shelf has more to offer than just exhaust pipes and engine service. We did a supermarket sweepstakes through Yosh and came back with Lightspeed carbon fiber fork guards, case guards, a glide plate, disc guards and a chain guide. We also added Yosh’s $19.95 aluminum crank and $17.95 aluminum top-dead-center inspection plugs, CNC-machined rear brake clevis ($50), oil filler plug ($18.75), axle blocks ($48.95), steering stem nut ($14.95) and billet clutch cover ($199).

THE ONLY ABSOLUTELY, POSITIVELY MUST-DO ITEMS ON OUR WISH LIST WERE THE ENGINE MODS, EXHAUST SYSTEM AND SUSPENSION IMPROVEMENTS.ÿ WE CAN’T RANT AND RAVE ENOUGH ABOUT HOW WELL THEY WORKED.

We admit that our Yoshimura RM-Z450 is overkill. The only absolutely, positively must-do items on our wish list were the engine mods, exhaust system and suspension improvements. That was our main focus and we can’t rant and rave enough about how well the engine and suspension worked when employed in unison. It’s rare that a bike works so flawlessly. The suspension absorbed everything the track threw at it. We experienced no headshake whatsoever. The bike cornered like a charm. No berm was too deep for the powerband and no double too far.

This is truly one of the most powerful RM-Z450’s we’ve ever ridden. A fact that was proven when Yoshimura’s Brant Russell called us towards the end of our two-month test and asked if he could borrow the bike back for the Glen Helen 250 National so that Larry Ward could ride it. Then, a few weeks later, Brant called and asked if he could borrow the bike again.

“What for?” we asked.

“Aaron Yates wants to race the AMA Supermoto Championship in Reno, and we think your test bike would be perfect for him,” said Brant.
“Of course it would be,” we said, “because it already has an overflow catch tank.” o

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