Most showroom inventory four-strokes are delivered in an honest state of tune, however under no circumstances are they fire-breathing dragons that burn gasoline and belch hearth. It’s simple to get extra energy out of them, but it surely takes the experience of a talented engine tuner and a HELOC. What’s a HELOC? A House Fairness Line Of Credit score. Unhappy to say, however the two-stroke was a factor of magnificence when it got here to hop-ups. The routine was easy: port the cylinder, mill the pinnacle and throw a pipe on the factor. Not so with a four-stroke.
Professional Circuit’s Mitch Payton estimates that the swap from full-race two-strokes to four-strokes price his staff about 4 occasions as a lot cash per race bike. It additionally signifies that the variety of newbie and up-and-coming riders that Professional Circuit can sponsor was decreased by a a number of of 4. Why? As a result of within the two-smoke days, when Mitch ported the cylinder and milled the pinnacle himself, the fee was largely labor. As for the exhaust pipe, it was a light metal development that retailed for $250. Enter the four-stroke. Mitch nonetheless ports the cylinder head himself, however he has to purchase valves, pistons, rings, cams, keepers and is derived from outdoors suppliers. Professional Circuit’s price on a high-compression four-stroke piston is bigger than his price for all the pieces he did on a two-stroke. Once you add within the further price of the titanium exhaust system, you get sticker shock.
Welcome to the brand new world order. It thumps and it sucks cash out of your pockets. The MXA wrecking crew sat down with a number of four-stroke tuners to research the ten most simple methods to get extra energy out of a thumper.
NUMBER ONE: HOPPED-UP GAS
The best efficiency mod is to pour VP Racing’s U4.4 race gasoline into your tank. It solely takes just a few seconds and gives immediate rewards (though not as a lot as the sooner model of U4). The rise in octane, power-yielding oxygenates and hydrocarbons (with a larger vitality potential) ends in a gasoline that may produce 6 % extra energy than normal pump gasoline. That’s as a lot as a 3 horsepower enhance on a 450 and a couple of horses on a 250.
NUMBER TWO: EXHAUST SYSTEMS
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With costs operating as excessive as $1000 for a titanium system, efficiency four-stroke pipes have develop into standing symbols. The main components of pipe design are:
(1) head pipe size. (2) head pipe diameter. (3) total tuned size. (4) taper of S-bends. (5) diameter of the muffler core. (6) size of the muffler.
Not like Formulation 1 or street racing exhaust pipes, a four-stroke motocross bike has to develop a broad and usable powerband — as an alternative of most horsepower. With tapered head pipes, step designs, stamped shapes and unique metals (like Inconel) getting used, the costs have nowhere to go however up. Not solely can aftermarket exhaust pipes add a number of horsepower, however they will save as a lot as 5 kilos over inventory methods.
NUMBER THREE: CUBIC CENTIMETERS
For the reason that day that Nicolaus August Otto fired up the primary four-stroke engine in 1876, four-stroke designers have been boring them out. A rise in cubic centimeters is the time-honored technique of creating a four-stroke quicker. There’s one caveat, although: Most four-stroke engines, save for XR50s and TT-R125s, are already on the authorized displacement restrict. Thus, a rise in bore will make your 250 or 450 four-stroke unlawful for AMA racing. The rule, nonetheless, doesn’t apply to the Vet courses or play bikes. Whereas few racers would take into account a big-bore 450, a big displacement 250 might be your final Vet weapon. Anticipate to realize 2 to three horsepower. The concept of a big-bore 250cc four-stroke is the genesis of the KTM 350SXF.
NUMBER FOUR: STROKE IT
Stroking a four-stroke engine additionally will increase the displacement (making the bike unlawful for AMA Professional racing). To stroke an engine, a tuner relocates the big-end bearing additional outboard on the crank halves. Usually, the connecting rod is shortened the identical distance that the massive finish bearing is moved. That is executed as a result of the bigger sweep of the big-end bearing would in any other case ship the piston into the valves. When the rod is shortened, the piston tops out at the usual top-dead-center measurement. At bottom-dead-center, the shorter rod and repositioned big-end bearing deliver the piston just a few millimeters additional down the bore. A piston with a skirt cutaway is often required to supply sufficient crank clearance. A giant-bore offers extra sheer horsepower, whereas stroking a thumper broadens the torque curve.
NUMBER FIVE: HIGH-PERFORMANCE CAMSHAFTS
Efficiency cams can be found with lobes which are timed to open the valves earlier and to maintain them open longer (referred to as dwell). As a rule of thumb, the perfect high-performance cams do their finest work within the vary the place the ability is already concentrated. Probably the most noticeable camshaft change is in the way in which the ability is delivered. Vehicle hop-up outlets have executed a bumper enterprise in cams for many years, and motocross racers are starting to see why.
Not like on a two-stroke, four-stroke porting is finished within the head. There aren’t any ports within the cylinder wall of a four-stroke. To correctly port the pinnacle on a four-stroke requires an understanding of how the air is drawn out and in of the consumption and exhaust ports. Porting and sharpening the cavities of a four-stroke cylinder head can enhance the stream price for extra efficiency. In some circumstances, bigger valves will be fitted to enhance the stream price, however this mod usually solely works on four-stroke engines of a lesser tune.
NUMBER SEVEN: INCREASED COMPRESSION
One of many best methods to get a tougher hit and higher low-to-mid energy is to extend the compression ratio. On a two-stroke engine, compression is elevated by milling the pinnacle (which decreases combustion chamber quantity). On a four-stroke, the compression ratio is elevated by utilizing a piston with a better dome. A high-compression piston usually delivers extra punch at the price of some top-end. Inventory compression is often within the 12:1 vary. It might simply be upped to 13:1 or larger. Watch out, will increase in compression would possibly require higher-octane gasoline.
NUMBER EIGHT: IGNITION MAPPING
Carburetors provided fantastic tuning alternatives. On most small displacement play bikes, like TT-Rs, KLXs and XRs, it’s de rigeur to exchange the inventory carb with a bigger one. The bigger the bore of the carb, which is measured in millimeters, the extra gasoline it may possibly stream. As a rule of thumb, larger carbs enhance mid-and-up energy (at the price of low-end). Why do massive carbs damage low-end energy? With a view to stream gasoline by way of a bigger opening, the engine has to create extra vacuum. The bigger the carb throat, the larger the vacuum required to attract the gasoline into the engine. Most manufacturing unit groups bored their carbs out, tapered their carbs or added aerodynamic velocity stacks. The times of the Keihin carb are over excpet on play bikes. It’s been changed by gasoline injection.
Gas injection will be tuned by remapping the ignition. With the usage of a laptop computer or purpose-built engine tuner just like the GYTR, the ignition timing and gasoline portions will be altered for various throttle openings and rpm. On a hopped-up engine, remapping the engine can produce much more energy. On a comparatively inventory engine, remapping can alter the texture of the powerband (with out really including any horsepower).
NUMBER NINE: CLUTCHES
The additional weight and torquier energy supply makes a four-stroke robust on clutches. If you happen to pump up the horsepower with race gasoline, exhaust methods, big-bore kits, camshafts or flowed heads, the inventory clutch will not be enough. Hinson, Barnett and Wiseco make baskets, friction plates, strain plates and interior hubs that assure extra hook-up.
NUMBER 10: THROTTLE BODIES
Injectioneering gives a throttle physique mod that adjustments the form and opening arc of the inventory butterfly valve (and typically the form of the pulley wheel). This slight change in angle produces considerably extra throttle response. Moreover, Simon Engineering is engaged on a fuel-injection throttle physique that replaces the comparatively inefficient butterfly valves with an old-fashioned slide. Preliminary testing has present that the extra metered opening of a slide (versus the throat blocking motion of a butterfly valve) may end up in vital enhancements. Probably the most unique throttle physique is R&D Racing’s Genius throttle physique. It makes use of a barrell formed valve as an alternative of a butterfly to enhance airflow by way of the throttle physique’s venturi (and it has been used on a number of manufacturing unit street racers). Gas-injector mods are the courageous new world of hop-up artists.
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