Most showroom stock four-strokes are delivered in a decent state of tune, but by no means are they fire-breathing dragons that burn fuel and belch fire. It is easy to get more power out of them, but it takes the expertise of a skilled engine tuner and a HELOC. What’s a HELOC? A Home Equity Line Of Credit. Sad to say, but the two-stroke was a thing of beauty when it came to hop-ups. The routine was simple: port the cylinder, mill the head and throw a pipe on the thing. Not so with a four-stroke.
Pro Circuit’s Mitch Payton estimates that the switch from full-race two-strokes to four-strokes cost his team about four times as much money per race bike. It also means that the number of amateur and up-and-coming riders that Pro Circuit can sponsor was reduced by a multiple of four. Why? Because in the two-smoke days, when Mitch ported the cylinder and milled the head himself, the cost was mostly labor. As for the exhaust pipe, it was a mild steel construction that retailed for $250. Enter the four-stroke. Mitch still ports the cylinder head himself, but he has to buy valves, pistons, rings, cams, keepers and springs from outside suppliers. Pro Circuit’s cost on a high-compression four-stroke piston is greater than his cost for everything he did on a two-stroke. When you add in the additional cost of the titanium exhaust system, you get sticker shock.
Welcome to the new world order. It thumps and it sucks money out of your wallet. The MXA wrecking crew sat down with a host of four-stroke tuners to analyze the 10 most basic ways to get more power out of a thumper.
NUMBER ONE: HOPPED-UP GAS
The easiest performance mod is to pour VP Racing’s U4.4 race gas into your tank. It only takes a few seconds and offers instant rewards (although not as much as the earlier version of U4). The increase in octane, power-yielding oxygenates and hydrocarbons (with a greater energy potential) results in a fuel that can produce 6 percent more power than standard pump gas. That’s as much as a 3 horsepower increase on a 450 and 2 horses on a 250.
NUMBER TWO: EXHAUST SYSTEMS
With prices running as high as $1000 for a titanium system, performance four-stroke pipes have become status symbols. The major elements of pipe design are:
(1) head pipe length.
(2) head pipe diameter.
(3) overall tuned length.
(4) taper of S-bends.
(5) diameter of the muffler core.
(6) length of the muffler.
Unlike Formula 1 or road racing exhaust pipes, a four-stroke motocross bike has to develop a broad and usable powerband — instead of maximum horsepower. With tapered head pipes, step designs, stamped shapes and exotic metals (like Inconel) being used, the prices have nowhere to go but up. Not only can aftermarket exhaust pipes add several horsepower, but they can save as much as 5 pounds over stock systems.
NUMBER THREE: 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, though: 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 2 to 3 horsepower. The idea of a big-bore 250cc four-stroke is the genesis of the KTM 350SXF.
NUMBER FOUR: 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.
NUMBER FIVE: 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, and motocross racers are beginning to see why.
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 an understanding of how the air is drawn in and out of the intake and exhaust ports. Porting and polishing the cavities of a four-stroke 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.
NUMBER SEVEN: 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 easily be upped to 13:1 or higher. Be careful, increases in compression might require higher-octane gasoline.
NUMBER EIGHT: IGNITION MAPPING
Carburetors offered 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 of thumb, bigger 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. Most factory teams bored their carbs out, tapered their carbs or added aerodynamic velocity stacks. The days of the Keihin carb are over excpet on play bikes. It’s been replaced by fuel injection.
Fuel injection can be tuned by remapping the ignition. With the use of a laptop or purpose-built engine tuner like the GYTR, the ignition timing and fuel quantities can be altered for different throttle openings and rpm. On a hopped-up engine, remapping the engine can produce a lot more power. On a relatively stock engine, remapping can alter the feel of the powerband (without actually adding any horsepower).
NUMBER NINE: CLUTCHES
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, Barnett and Wiseco make baskets, friction plates, pressure plates and inner hubs that guarantee more hook-up.
NUMBER 10: THROTTLE BODIES
Injectioneering offers a throttle body mod that changes the shape and opening arc of the stock butterfly valve (and sometimes the shape of the pulley wheel). This slight change in angle produces significantly more throttle response. Additionally, Simon Engineering is working on a fuel-injection throttle body that replaces the relatively inefficient butterfly valves with an old school slide. Initial testing has show that the more metered opening of a slide (as opposed to the throat blocking action of a butterfly valve) can result in significant improvements. The most exotic throttle body is R&D Racing’s Genius throttle body. It uses a barrell shaped valve instead of a butterfly to improve airflow through the throttle body’s venturi (and it has been used on several factory road racers). Fuel-injector mods are the brave new world of hop-up artists.