REMEMBER WHEN…: POWERVALVE FOURTRAX 250R | CT Racing brings in motocross technology
**Editors Note-This story originally ran in Dirt Wheels April 2000
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Several years ago, CT Racing developed the first aftermarket aluminum cylinder for the Honda 250R engine. Called the Pro-Cross (originally Pro-X), this new cylinder allowed ATV racers much more leeway with motor modifications than the stock 250R offered.
In the motorcycle motocross world, engines with various types of power valves have been standard equipment for well over a decade. Virtually every 250cc and 125cc race bike has some sort of power valve now, but there has never been a production ATV with one.
The next step for CT Racing was obvious—develop a power valve modification for their Pro-Cross cylinder. Power valves in the various motocross bikes all accomplish the same goal, but have different designs; for example, Kawasaki’s KIPS is different from Honda’s Powerport which also differs from Yamaha’s Powervalve.
CT Racing has adapted a power valve system used on Rotax water-craft and snowmobiles motors, called a RAVE. Before we get into any details, let’s first talk about what a power valve is.
A power valve is a moving mechanism that changes the exhaust port timing as the rpm gets higher. The basic theory is that a low exhaust port provides more bottom end power, like on a woods motor, and a higher exhaust port gives you the optimum top-end performance, like what you would use for a drag motor.
Without a power valve, engine builders have to compromise somewhere in the middle if they want good horsepower throughout the entire powerband. A power valve gives you more horsepower at both ends of the powerband since it changes the port timing according to the needs of the engine at that moment. This is why today’s motocross bikes are able to leap over triple jumps with such small takeoff runs.
CHANGING PORT TIMING
Most motorcycles drive the power valve system from the crank shaft. The Rotax RAVE system is fairly simple in that it consists of a diaphragm which is operated by exhaust gas pressure which then moves a valve that changes the port timing.
Way back in 1992, CT Racing used this same system grafted onto a Honda 250R cylinder to win a TT National. It was certainly a success at the time, but there were a couple of problems that kept it from taking off.
Customers wanted this modification, but it was too labor-intensive to be practical for sale to the public. The other problem they experienced had to do with the cylinder. Due to the extensive amount of welding required, the stock Honda 250R cylinder became distorted and some of them developed serious leaks during testing, and had to be repaired. Again, not a good thing as a production modification.
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The modification CT performs with the RAVE power valve on the Pro-Cross cylinder requires no welding and eliminates that problem. The other update CT is doing for their 250 race motor is Nikisil coating of the cylinder bore. This is also borrowed from current motocross motorcycle technology.
The cylinder has no sleeve and is essentially a hard chrome down to the aluminum. The advantage to this is it transfers heat away from the piston at a much higher rate than on a sleeved motor. Steel sleeves retain some heat. The disadvantage to a Nikosil motor is it cannot be bored—if you damage the bore it has to be re-nikisiled or sleeved.
There are many other upgrades associated with the Pro-Cross cylinder, compared with the stock Honda cylinder.
Again, the Pro-Cross cylinder draws several features from current MX motorcycle technology. The cylinder does not use a head gasket but instead utilizes an O-ring. The cylinder has two auxiliary exhaust ports which allow for more port area without sacrificing durability. A super-wide exhaust without auxiliary ports causes the rings and piston to wear faster.
Having auxiliary ports also allows motor builders to stagger the port timing to develop a broad power curve. Larger transfer ports with flatter rooftop angles are also featured in the Pro-Cross cylinder. The intake has CT boost ports and large Boyeson holes. Compared to the stocker, the Pro-Cross cylinder has double the water capacity, and most importantly, the water jacket wraps around the exhaust tunnel, cooling the cylinder where it needs it most.
One of the unique things about this engine kit is the utilization of a ’98 CR 250 Honda ignition. We’ve seen CR ignitions used by CT and other motor builders over the years, but this one is different. The CR 250 ignition has wires that go to the carburetor, which has an electronically controlled solenoid that operates a power jet.
It receives input from the ignition system which opens and closes the solenoid. This particular CR ignition has an aggressive curve—running the timing very advanced in mid range—that tends to make the motor run hot.
To control the heat, they added the power jet which shoots extra fuel in for cooling. The power jet is closed at low RPM so the motor carburates very cleanly; it then opens in the mid range to control heat and closes at high RPM when the ignition retards and lets the motor make extra power.
CT likes this setup because they get more bottom from the added timing advance and more top because this ignition also has more retard than the standard FourTrax engine setup. On the dyno they get significantly more power everywhere, compared to stock. The only downside is that the ignition will not run lights, and it’s expensive. A special backing plate is also needed to bolt the ignition up to the engine.
Having all the motor in the world won’t win races unless you have a great-handling chassis to go along with it. For our test session, CT brought out their R&D race machine. This particular quad is not their full-on racer for 2000, which will be using a Lone Star chassis.
The quad we rode uses an Arens chassis from Michigan because it’s built to Honda stock geometry. This allows for the testing of A-arms, swing arms, shock linkage components and suspension parts that are designed to fit on a stock FourTrax 250R frame.
An advantage to the Arens chassis is it’s made of Chromoly and much more durable than a stock frame. It also has a removable subframe making it easy to access the rear shock when making changes. Though it’s officially CT’s R&D machine, this quad is certainly race-ready at the Pro level.
CT has been extensively testing the new TCS FourTrax suspension system. The TCS shocks were set up for +2” A-arms and a stock-length swingarm and stock rear suspension linkage. Like other modern high-performance racing shocks, these TCS units run with no pre-load.
If you move the pre-load adjusters it just raises or lowers the quad ride height. With this system you can get maximum wheel travel without having your quad sitting high up in the air. Our TCS fronts have adjustable compression damping and just two springs.
The rear features adjustable compression and rebound damping. TCS only uses two springs where some companies use more. They use the soft spring for ride height and a main spring for stiffness and roll control.
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It also uses an internal valve that has a shim stack for tuning. With this system, hydraulics control the shock’s performance. With a shim stack they have internally tunable low- and high-speed compression damping; rebound damping is controlled the same way.
Many of the high-performance shocks on the market don’t have internal tuneability so they use springs to do the work. With low- and high-speed dampening and the SCS spring setup, you end up with a quad that offers maximum travel. It also has a low ride height, which makes for very plush performance in the small, braking bumps, as well as handling the big Pro-level jumps.
Even though the plastic is stock Honda, this machine gets its good looks from the Zing graphics and seat cover. CT now offers the Zing graphics kit for the R in both red and blue.
RACE TRACK TEST
We did our testing at the Glen Helen motocross track in southern California on a day when not much was going on. The good part about this is we had the place to ourselves. The downside is they don’t water the track so it gets pretty dry and slick.
In these conditions you don’t get to hammer the throttle in the turns to take advantage of the awesome power; you’ve got to use finesse. This motor has such a useable powerband it was an easy task.
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But when you get off the corner and onto the straights is when the fun begins. Pin the throttle and this thing hauls! Craig Peterson from ITP was out when we were testing and without riding—only seeing and hearing—claimed he had to have one.
Craig said “the tone of the exhaust does not sound like a normal national motor, it sounds like a factory works motorcycle”. We had several test riders, including White Bros. R&D technician and four-time National MX champion Gary Jones. Gary said, “This thing’s got power everywhere, and it’s easy to go fast.”
Dirt Wheels test rider Adam Campbell said, “This motor has the broadest powerband I’ve felt on a race quad. It comes on strong right from the bottom and just keeps on pulling to a top-end that I could never open up fully on this track. This should be the motor to beat at next year’s Nationals. The chassis and suspension were also top-notch, certainly ready for pro-level action. It turned great, fit my style great, and landed super-smooth off the jumps. There’s not much else you could ask for from a racing quad.”
Our test session went flawlessly with no problems and lots of laps run by everyone who wanted to ride.
HOT MODS FOR QUADS
The bottom line is, CT Racing has upped the technology bar and we’ll probably see a lot of scrambling in the race industry to follow suit. The Pro-Cross cylinders have already proven to be a dominant force in some types of racing, and it looks like we may see it invade the National Pro 250 class.
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