10 THINGS YOU NEED TO KNOW ABOUT HOPPING UP A FOUR-STROKE

February 26, 2013
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(1) Hopped up gas. There isn’t an easier performance mod than pouring VP Racing’s oxygenated 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 $1500 for a titanium system, performance four-stroke pipes have become status symbols. As with a two-stroke’s expansion chamber, a four-stroke exhaust system is tuned to produce the optimum powerband. The major elements of pipe design are:
      (a) head pipe length,
      (b) head pipe diameter,
      (c) overall tuned length,
      (d)
taper of S-bends,
      (e) diameter of the muffler core and,
      (f) 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. In the future, expect to see tapered head pipes, steps, stamped shapes and exotic metals (like Inconel) being used to deliver more low-end, a broader mid and extra top. Not only can aftermarket exhaust pipes add a horsepower or two, but they can save as much as five pounds over stock systems.

(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 from L.A. Sleeve, Cylinder Works, Thumper Racing or Wiseco big-bore kits.

(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 cams. 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 or dyno or years of know-how. 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) Increase the 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. Be careful, increases in compression might require higher octane gasoline.

(8) Aftermarket 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, Wiseco, Barnett and others makes baskets, pressure plates and hubs that guarantee longer life.

(9) Mapping. The brave new world of electronic fuel injection means that mapping will replaced carburetors, jets and boring out carbs. Every manufacturer has some form of reprogramming software that allow the consumer to change the fuel flow and the ignition timing of the bike. The best use for new mapping is to fine tune the setup as engine modifications are made to the head, piston, valves for pipe.

(10) Gearing. One of the most basic hop-up tools is gearing. Your local track has a distinct set of turns, straights and corners. There is a gear ratio that will work best on this particular layout. But, you have to change sprockets to find out.

 

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