WE RIDE THUMPER RACING’S BIG-BORE YAMAHA YZ310F

So, what makes the Thumper Racing YZ310F different? The devil is in the details. This isn’t a do-it-yourself big-bore kit that calls it a day after wedging in a bigger piston. The MXA wrecking crew has nothing against basic big-bore kits; we have tested some phenomenal big-bore engines that cost less than $750. We would, however, be remiss if we said that they ran like mini 450s, because they didn’t. Why not? Because you can’t go as big as necessary with just a piston kit. The bigger the piston (in relationship to the stroke), the more sluggish the powerband gets. A very big piston produces a “chug engine.” What’s a chug engine? It is an engine with a flat top-end, very little free-rev, and a power output that chugs up and chugs down the rpm range.

That was the last thing that Thumper Racing wanted to build. They were looking to build a big-bore 250 four-stroke that would run with the aplomb of the original YZ400 (and weigh less). The problems are obvious. Adding 60cc of displacement within the compact limits of a 250cc engine is easier said than done. Thumper went to great lengths to try to achieve the perfect engine setup. The result wasn’t cheap (and Thumper Racing wasn’t looking to add a few horsepower; they were looking for a full-race powerplant). All the parts and services are available from Thumper, but unless you are a Trump, Hilton or Murdock, you had better have deep pockets, because this is not a cheap mod.

There are some key lessons to be learned about engine tuning from Thumper Racing’s YZ310F?largely about manipulating the power characteristics. And, nothing makes a lesson hit home like getting out in the dirt and feeling the results for yourself. So, without further delay, here’s what the MXA wrecking crew learned about Thumper Racing’s Yamaha YZ310F.

SHOP TALK: TO GET POWER, PUT IN THE HOURS

The team of factory engineers who designed your OEM motocross engine juggled dozens of parameters to achieve power, durability and efficiency with your engine. All of the stock components were mixed into a calculated balance of conflicting factors. Thus, when an engine tuner changes the dimension of one engine part, there is a ripple effect on all the other parts. It is easy to ruin the delicate balance that the factory engineers were aiming for. For example, you can easily produce more power, but the character of that power might be unpleasant. Every modification comes with consequences. The goal of a good tuner is to see each modification as an opportunity to make the whole better than the sum of its parts.The bigger the changes, the more engine parameters are affected. Jumping from a 250cc engine to a 264cc displacement might cost a little top-end, but that compromised size can be lived with by running a little less compression. Going up to a whopping 310cc, however, presents much bigger challenges.

The stock YZ250F bore and stroke is 77.0mm x 53.6mm. Thumper Racing’s YZ310F has a bore and stroke of 84mm x 56.6mm. The 3mm of extra stroke were achieved with a Hot Rods stroker crank. To get the extra 7mm of bore, Thumper installed a custom sleeve from Advanced Sleeve. The new sleeve extends 8mm farther down into the cases (toward the crank) to support the piston throughout its longer stroke. Thumper utilized Millennium Technologies cylinder plating. Additionally, Thumper does their own mod to the outside of the sleeve adjacent to the water jacket. They cut grooves in the sleeve to increase surface area and aid heat transfer.

In the name of keeping the engine balanced, Thumper deviated from stroker kits that shorten the connecting rod to make up for the increase in stroke. Thumper reasoned that with a shorter connecting rod and a decreased rod-to-stroke ratio, there would be excessive side loads on the piston. These side loads would lead to increased wear and loss of power. Another negative of a short rod is that piston speed is increased near top dead center. This limits how high the engine can rev before flattening out. To solve these problems, Thumper used a 2mm-longer Carillo rod. The MXA wrecking crew has some experience with the longer Carillo rod (from when we built a long-rod YZ250F engine). The long rod had very positive effects on the stock YZ250 powerband. On the YZ310F, the rod ratio was actually slightly higher than on the stock engine.
 
In order to make room for the longer rod, Thumper had a custom CP piston made. The piston’s wrist pin is 1mm smaller, and it has a different piston compression height, meaning that the rod sits farther up in the piston. The compression ratio of the YZ310F was reduced from 13.5:1 to 13:1. Dropping the compression ratio offers three advantages: (1) It better copes with lower-octane gas (and declining pump-gas quality). (2) It helps achieve a free-revving feel. (3) The setup is more durable (although the smaller-diameter wrist pin requires servicing an estimated 10 percent sooner than the stock wrist pin).

Keeping the engine in balance doesn’t just refer to the interplay of the power, torque and rev, but, more literally, the balance of the crankshaft. Thumper Racing drills holes and adds tungsten weights as needed to get the proper balance, which is important to performance and especially to durability. An incorrect balance factor, even as small as 4 ounces, is mass that will be multiplied when it starts to reciprocate. At 2000 rpm, that 4 ounces becomes 4 pounds of force. At 13,500 rpm, an out-of-balance crank can tear the engine apart.

To start our test, we compared the Yamaha YZ310F project bike back to back with our exhaust-equipped stock YZ250F. The stocker hit harder off the bottom and felt on par with the YZ310F power at low rpm. Once the YZ310F got to the midrange, however, it quickly distanced itself from the stocker and kept going into the upper reaches. Even though the exhaust greatly helped the anemic top end of our stock YZ test bike, it couldn’t hold a candle to the eager over-rev of the YZ310F. For a rider wanting to keep it humming, the tall gearing made it easy to hang high without excessive shifting.

Thanks to the larger countershaft sprocket, which translates to about three fewer teeth on the rear, the powerband was really broad. The very tall gearing suited faster riders on faster tracks, but a better overall setup would have been the equivalent of two teeth taller than stock. This would help riders get through the powerband quicker and get from second to third gear sooner, which was a little tough on fast, steep uphills.

Test riders had few complaints with Thumper’s YZ310F. After resolving a small clog issue in the carburetor on day one, the YZ310F ran trouble-free. A bigger engine naturally meant that a little more force was necessary at the kickstarter and a little more vibration resonated through the chassis.

A lot of expertise was put into Thumper Racing’s YZ310F big bore, and that translates into “mucho dinero.” A sizable wad of cash was burned in turning the YZ250F into a YZ310F. How much? Not including the exhaust pipe or clutch parts, $2949.86. When you tack on $2000 for a titanium exhaust system and premium clutch package, your are looking at a $5000 engine kit.
 
Can the price be justified? Not by everyone, but maybe by you. There is no doubt that the Thumper Racing YZ310F breathes fire. It vastly surpasses the typical big-bore 250 four-stroke in terms of overall powerband. Not only did it love to rev, but it was more forgiving of mistakes. Best of all, it weighed about the same as a typical YZ250F. It was more like a YZ400 than either a YZ250F or YZ450F. Mission accomplished. It was a great engine, but at this price, it should be.