Most showroom stock four-strokes are delivered in such a high state of tune that it isnt easy to get more power out of them. It takes the expertise of a skilled engine tuner to even try–and they dont always succeed. But, as we careen towards a four-stroke future, the four-stroke hop-up business will solidify. The MXA wrecking crew sat down with a host of four-stroke tuners to analyze the ten most basic ways to get more power out of a thumper.
1. HOPPED-UP GAS
There isn’t an easier performance mod than pouring VP Racing’s Ultimate 4 race gas in the tank. It only takes a few seconds and offers instant rewards. The increase in octane, power yielding oxygenents and hydrocarbons (with a greater energy potential) results in a fuel that can produce 6 percent more power than standard pump gas. That’s a three horsepower increase on a 450 and two horses more on a 250.
2. EXHAUST SYSTEMS
With prices running as high as $800 for a titanium system, performance four-stroke pipes have become status symbols. As with a two-strokeÕs expansion chamber, a four-stroke exhaust system is tuned to produce the optimum powerband. The major elements of pipe design are: (1) head pipe length, (2) head pipe diameter, (3) overall tuned length, (3) taper of S-bends, (4) diameter of the muffler core and (5) length of the muffler. Unlike Formula 1 or road racing exhaust pipes, a four-stroke pipe has to deliver a broad and usable powerband–instead of maximum horsepower.
3. CUBIC CENTIMETERS
Since the day that Nicolaus August Otto fired up the first four-stroke engine in 1876, four-stroke designers have been boring them out. An increase in cubic centimeters is the time honored method of making a four-stroke faster. There is one caveat, however. Most four-stroke engines, save for XR50s and TT-R125s, are already at the legal displacement limit. Thus, an increase in bore will make your 250 or 450 four-stroke illegal for AMA racing. The rule, however, doesn’t apply to the Vet classes or play bikes. While few racers would consider a big-bore 450, a large displacement 250 could be your ultimate Vet weapon. Expect to gain two to three horsepower. Rick Peterson Motorsports makes 302cc versions of all the popular 250 four-strokes.
4. STROKE IT
Stroking a four-stroke engine also increases the displacement (making the bike illegal for AMA Pro racing). To stroke an engine, a tuner relocates the big-end bearing further outboard on the crank halves. In most cases, the connecting rod is shortened the same distance that the big end bearing is moved. This is done because the larger sweep of the big-end bearing would otherwise send the piston into the valves. When the rod is shortened, the piston tops out at the standard top-dead-center measurement.
At bottom-dead-center, the shorter rod and repositioned big-end bearing bring the piston a few millimeters further down the bore. A piston with a skirt cutaway is usually required to provide enough crank clearance. A big-bore gives more sheer horsepower, while stroking a thumper broadens the torque curve.
5. HIGH-PERFORMANCE CAMSHAFTS
Performance cams are available with lobes that are timed to open the valves earlier and to keep them open longer (known as dwell). As a rule of thumb, the best high-performance cams do their best work in the range where the power is already concentrated. The most noticeable camshaft change is in the way the power is delivered. Automobile hop-up shops have done a bumper business in cams for decades. Motocross racers have not embraced different cam shapes, but this will change.
6. HIGH-FLOW PORTING
Unlike on a two-stroke, four-stroke porting is done in the head. There are no ports in the cylinder wall of a four-stroke. To properly port the head on a four-stroke requires a flow-bench. A flow-bench measures how much vacuum it takes to draw air out of the intake port and exhaust flange. Porting and polishing the cavities of a four-strokes cylinder head can increase the flow rate for more performance. In some cases, larger valves can be fitted to improve the flow rate, but this mod normally only works on four-stroke engines of a lesser tune.
7. INCREASED COMPRESSION
One of the easiest ways to get a harder hit and better low-to-mid power is to increase the compression ratio. On a two-stroke engine, compression is increased by milling the head (which decreases combustion chamber volume). On a four-stroke, the compression ratio is increased by using a piston with a higher dome. A high-compression piston typically delivers more punch at the cost of some top-end. Stock compression is typically in the 12:1 range. It can be upped as high as 13:1. Be careful, increases in compression might require higher octane gasoline.
8. BORED-OUT CARBS
As with a two-stroke engine, carburetors offer wonderful tuning opportunities. On most small displacement play bikes, like TT-Rs, KLXs and XRs, it is de rigeur to replace the stock carb with a larger one. The larger the bore of the carb, which is measured in millimeters, the more fuel it can flow. As a rule, big carbs increase mid-and-up power (at the cost of low-end). Why do big carbs hurt low-end power? In order to flow fuel through a larger opening, the engine has to create more vacuum. The larger the carb throat the greater the vacuum required to draw the fuel into the engine. At low rpm settings, many four-stroke engines donÕt produce enough air velocity to get the fuel flowing–thus, low-end power is lost until the air flow is sufficient (at higher rpm). Conversely, smaller carbs flow fuel more easily, start better and have better throttle response.
The extra weight and torquier power delivery makes a four-stroke tough on clutches. If you pump up the horsepower with race gas, exhaust systems, big-bore kits, camshafts or flowed heads, the stock clutch may not be sufficient. Hinson makes baskets, pressure plates and hubs that guarantee longer life.
10. WET SUMP
The Yamaha YZ250F and YZ450F can have their lubrication systems switched from dry sump to wet sump with aftermarket kits. These kits eliminate the YZ-Fs oil tank and, in the process, save two pounds of oil and oil lines. Wet sumping doesnÕt necessarily make a bike faster, but it does make it lighter. Wet sump YZ-Fs will need to have their oil changed more frequently.
In the future, a new breed of four-stroke oil will increase horsepower. These new oils will have the lubricating qualities of a 30-weight, but the viscosity of a 0-weight (comparable to water). These low viscosity oils will reduced drag on the mechanical parts and improve everything from throttle response to top-end.