WE RIDE MAXPOWER/RPM’S BIG-BORE KX315

 

WITH TODAY’S METALLURGICAL CONFIGURATIONS, IT IS POSSIBLE TO MAKE AN ENGINE PRODUCE MORE POWER, ACCELERATION, SPEED AND SENSATIONS THAN THE HUMAN MIND CAN PROCESS OR HUMAN BODY MANAGE.

Formula 1 race car designers, from as far back as Colin Chapman, have understood that there is a delicate balance between weight and power. In truth, the never-ending pursuit of power has pretty much ended, and if it weren’t for computer management systems, neither the high-horsepower McLaren F1 car nor humongous-thrust F22 Raptor would make it farther than the first turn. With today’s metallurgical configurations, it is possible to make an engine produce more power, acceleration, speed and sensations than the human mind can process or human body manage.

Nowhere is this more true than in motocross. Unlike in Formula 1 or MotoGP, where the pilot is braced in place, a motocross rider teeters on top of his powerplant. When the horsepower generated begins to exceed human sinew, the motorcycle designer has to back it down and look elsewhere for speed.

Thrust: Torque, not horsepower, is what big-bore engines are all about.

Of course, as a motocross racer, you know all of this. At any point in your career, you could have chosen to go for the big ponies. But, you didn’t, which is why 500cc two-strokes are ancient history and 650cc four-strokes are limited in scope. More power takes more skill, fitness and determination. Brutish acceleration strains the nexus between the eyeball and brain. When objects come rushing at you faster than you can process them, you slow down. And, not surprisingly, many 450cc four-stroke riders go much slower on their 50-horsepower CRF’s than they did on their 44-horsepower CR250’s.

 

IF TEST RIDERS ARE FORCED TO RACE 450FS, THEY ASK TO BE REASSIGNED TO 250FS. IF THEY ARE STUCK IN A FOUR-STROKE PROGRAM, THEY ASK TO BE MOVED TO TWO-STROKES. WE CHALK THIS UP TO THE “GRASS IS ALWAYS GREENER” SYNDROME.

 

Less horsepower is not an evil, proven by the rapid lap times that James Stewart cranked out during his farewell season on the Kawasaki KX125. Bubba proved that power is not the solution to the equation, it is just one factor in a very long formula.

 MXA’s ten-man crew of test riders races every weekend, normally switching bikes weekly to satisfy project assignments. They race a lotand they race a lot of different bikes. Surprisingly, one scenario is played out constantly with test riders. If they are forced to race 450Fs, they ask to be reassigned to 250Fs. If they are stuck in a four-stroke test program for a few weeks, they will ask to be moved to the 250 two-stroke undertakings. We used to chalk this up to the “grass is always greener” syndrome, but that was too simplistic. What we really discovered was that motorcycle racers are in a constant search for the perfect bike. At any given time, they want the pluses of one type of engine, without losing the benefits of their current mount. Unfortunately, that bike doesn’t exist!

Which set the wheels spinning. The perfect bike doesn’t exist, but could we build it by cutting our own swath?We thought we could.

We collected all of our test riders’ wish lists, jotted down the specifications that suited the project, and came to one simple conclusion: They wanted a bike that had the weight and feel of a 250 four-stroke with the power of a 450 four-stroke. We could do that. And we knew just the man to callRick Peterson.

 

OUR GLORIOUS PLAN ONLY HAD ONE STEPBORE AND STROKE OUR KX250F TO ADD SIX HORSEPOWER, SEVERAL FOOT/POUNDS OF TORQUE AND PRODUCE THE ULTIMATE BLEND OF LIGHT WEIGHT AND HORSEPOWER.

 

Rick Peterson doesn’t want you to buy an off-the-showroom-floor bike. He wants you to take your trusty, but rusty, old iron and turn it into something magical; not just a reworked relic, but a bike that bridges the gap between a 450 and a 250. Rick specializes in building big-bore engine kits. If you’ve never heard of him, you’ve probably seen or raced against one of his 134cc, 139cc, 144cc, 151cc or 167cc two-strokes. Or, more recently, his 262cc, 302cc or 315cc four-strokes.

Our plan took shape rather quickly. The MXA gang agreed that we would start with a box-stock 2006 Kawasaki KX250F. We love this bike. We think it is the best all-around 250 four-stroke made. We knew that it would be the perfect platform for our perfect bike project. Our glorious plan only had one stepwe were going to bore and stroke our KX250F engine out to 315cc. That would add about six horsepower, several foot-pounds of torque and produce the ultimate blend of light weight and horsepower.

What was it going to cost? $2785. That’s a lot of money, but for a rider caught between a 450F and 250F, it’s cheaper than buying one before realizing that you would rather have the other. We could have done it for less money, but we would have had to compromise by only increasing the displacement to 262cc. Although that is a healthy increaseit results in a faster KX250Fwe wanted a totally new style of enginea blend of 450 and 250. For that we needed to go as big as possible.

What would our $2800 buy? The Rick Peterson-designed MaxPower/RPM’S 315cc kit includes an Ice Cube cylinder, piston kit (with piston, rings, wrist pin and circlips), O-ring head gasket, all gaskets and a stroked crank. It is best to send your complete engine to MaxPower to get the work done (drain the oil and water). They will split, inspect and match the cases before installing an 81mm Wiseco piston and CNC-machined Ice Cube cylinder. The crank will be stroked by 8mm, resulting in a 315cc displacement. No head mods are required.

 

IS BIGGER ALWAYS BETTER? AS A RULE, YES. BUT, THERE ARE EXCEPTIONS. THE MXA WRECKING CREW HAS RACED BIG-BORE 250 AND 450 ENGINES THAT HAVE BEEN TUNED TO A STANDSTILL.

Our goal was to go as big as possible, without wrecking the best aspects of the KX250F powerband. Is bigger always better? As a rule, yes. But, there are exceptions. The MXA wrecking crew has raced big-bore 250 and 450 engines that have been tuned to a standstill. Although they are bigger, they aren’t always better; the power is too brutal, too short, or it winds down slower than it winds up. As with any engine tuning exercise, a good big-bore is a balanced combination of bore, stroke and compression.

Although the old adage claims that there is “no replacement for displacement,” that isn’t true if the specs are butchered. Which leads us to the inevitable question: How far can the stock KX250F cylinder be bored? In our experience the stock KX250F cylinder shouldn’t be bored more than 2mm oversize (that’s approximately 262cc). Any more and reliability takes a serious nosedive. The cylinder walls become too thin and tend towards heat failure. In truth, we wouldn’t overbore a stock Honda CRF250 cylinder at all. The CRF250 cylinder had breakage problems from the git-go, and the factory fixes made it acceptable for the stock bore, but nothing bigger.

The conundrum for the MXA wrecking crew was that we needed to go way over a 2mm overbore to get the 81mm piston to fit (that is a 4mm overbore). That couldn’t be done with the stock cylinder. No problem. Rick Peterson makes his own cylindercalled the Ice Cube. By CNC machining a heat-treated, billet, 6061-T6

aluminum cylinder and having it plated with a Nickel-Silicon-Carbide liner, the Ice Cube cylinder offers more room for the overbore, which also translates into a larger water jacket and more rigid structure (for a truer bore).

Additionally, on a stroked engine, like the KX315F, the Ice Cube cylinder can be machined taller to eliminate the need for a spacer plate and the requisite extra gaskets.

 

IS A BIG-BORE ENGINE HARDER ON THE LOWER END? YES. IT’S SIMPLE PHYSICS; MORE POWER AND MORE TORQUE MAKE THE RUNNING GEAR WORK HARDER.

 

 

Boring and stroking is a simple process, but it does beg a few questions about the aftereffects.

(1) Do big-bore engines work the valve train harder than a stock engine? Yes and no. Our KX315F uses the same head and valves as the stocker, but because it makes more peak power at 1000 rpm less, the valves should last about the same duration as the stockers. That said, MXA test riders almost always install stiffer valve springs in the stock KX250F, and that would be a smart move on the KX315F.

(2) Is a big-bore engine harder on the lower end? Yes. It’s simple physics; more power and more torque make the running gear work harder.

 

GOING BIG ON A 250CC FOUR-STROKE IS LOW-TECH HORSEPOWER. IT IS HAMMER AND CHISEL TECHNOLOGY, BUT THAT’S ITS CHARM. BIG-BORE ENGINES MAKE THE MOST POWER WITH THE LEAST THOUGHT.

 

(3) Is a big-bore engine harder on the clutch? Yes. If you use the clutch a lot, the 40 horses will eat plates faster than the stock 34 horses. We recommend a complete Hinson Clutch and Pro Circuit clutch springs. However, if you are one of those riders who rarely slips the clutch, you could make do with the stock stuff.

(4) Do big-bore engines rev as readily as stock engines? The stock answer is no. Bigger pistons weigh more, push a larger volume of air and, as a rule, rev slower and lower. It’s a fact that the KX315F doesn’t rev as far in the power pulse as the stock KX250F engine. While both engines will rev to the limiter, the 315 makes its peak 40 horsepower at 9800 rpm, while the KX250 makes 34 horsepower at 10,800 rpm.

(5) Wouldn’t it be better to have the stock cylinder displacement hopped-up with porting instead of going big? No. For a privateer, going big is the quickest, fastest and cheapest way to get “works-style” power. The bored-and-stroked KX315F pumps out 40 horsepower, considerably more torque, and is easier to ride than a legal-displacement, but breathed-on, factory bike. Plus, it doesn’t need race gas.

(6) Is this the wave of the future? Noexcept at the personal level. In what could read like a slap, going big on a 250cc four-stroke is low-tech horsepower. It doesn’t require grinding down cam followers, special titanium widgets or sizzling black boxes. It is hammer and chisel technology, but that’s its charm. It’s simple. And because it is so basic, it is almost foolproof. Big-bore engines make the most power with the least thought. It is almost like cheating (and depending on what class you try to ride it in, it could be actual cheating).

(7) Isn’t it a cheater engine? No. It could be used by cheaters, but not very smart ones. Cheaters beware! The Ice Cube cylinder looks nothing like the stock KX250F cylinder. The quickest way to be caught cheating is with an Ice Cube cylinder or spacer plate under your stock cylinder.

 

A 450 MAKES 50 HORSEPOWER, THE KX315F MAKES 40 HORSEPOWER. IF THE GAME WERE ABOUT HORSEPOWER, EVERY VICTORY WOULD GO TO THE 450.

BUT, THEN THE SPORT WOULD BE CALLED “DRAG RACING.”

 

The MXA test crew spent considerable time racing the MaxPower KX315F, but we never raced two motos on it back-to-back. Instead, the test riders would race the KX315F in one moto and the stock KX250F in the next moto. This round-robin approach to racing led to a slam-dunk victory for the KX315F.

Surprisingly, the Rick Peterson-designed 315cc engine wasn’t all that different from the stocker in powerband. Yes, it did make peak power a little earlier, but the overall feel was much broader. It pulled hard out of the corners, and it was literally mistake proof.

But, let’s not get too misty about a mid-size motocross engine. While it is possible to beat a 450-mounted rider on the KX315F, the victory comes as a result of the bike’s lighter weight, more agile handling and less abrupt power. A 450 makes 50 horsepower, the KX315F makes 40 horsepower. If the game were only about horsepower, every victory would go to the 450. But, then the sport would be called “drag racing.” In motocross, it’s not the man with the most power, but the man who makes the most of the power he has.

As for the MXA wrecking crew, the MaxPower engine kit bridges the gap between chasing 450s and harassing them. For more info, contact MaxPower at (608) 224-2524.

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