Motocross Action – The Rocket KTM SX350F
Job number one when hopping up a KTM 350SXF is to beef up the power below the 12,200 rpm peak. Rocket Exhaust’s engine mods did just that.
Don Leib has been around the motocross industry his whole life as a racer, exhaust pipe builder and father of Grand Prix rider Michael Leib. When he started Rocket Exhaust, he had a vision of offering the most affordable exhaust systems possible. To implement this, Don sourced a manufacturer in China to build exhaust pipes to his specifications. Even with shipping from China, Leib hoped to bring his Rocket Exhaust systems in for under $500. On paper, it was a brilliant idea, but in practice, China was too far away for Leib to guarantee sufficient quality control (in both construction and specifications). There is nothing worse than unloading a container of exhaust pipes that don’t fit the bikes they were intended for. Don Leib didn’t give up—although he did give up on China. Rocket Exhaust opened its own facility in Southern California. He still had the dream of offering low-cost, high-quality exhaust systems. To keep the germ of his original philosophy, Rocket would forego titanium in favor of high-tech stainless steel and aluminum systems that combined a very wide range of manufacturing processes: forgings, billet parts and billet extrusions, plus die-cast and stamped parts. With a USA based manufacturing facility, Rocket could also expand in to engine work—doing engines for AMA Pro riders Bruce Rutherford, Killy Rusk and Ronnie Goodwin. Rocket Exhaust asked the MXA wrecking crew if we would like to test their full-race KTM 350SXF. We were interested, but there was a catch. Rocket had to borrow the KTM 350SXF that they had done for Dr. Chris Alexander. Alexander is an orthopedic surgeon who treats many SoCal motocross racers and works with the Asterisk Medical Crew at the AMA Supercross National races. In essence, this test should really be called “We Ride Dr. Alexander’s KTM 350SXF.” Rocket went through the good doc’s 350SXF with a fine-tooth comb to make sure that it highlighted all of their workmanship—and to make sure that it didn’t blow up. As for the MXA wrecking crew, we treated Doc Alexander’s bike as we would any test bike, which means we rode it hard, raced it often and didn’t worry about blowing it up.
In Stage 1, Rocket ports the cylinder head and performs a matching five-angle radius valve job. Stage 2 is a new squish.
At $499, the stainless steel Rocket Exhaust is affordable. The aluminum muffler’s colored end cap is strangely alluring.
The only fork mods were stiffer fork springs and an SDI spring seat. Very simple and very good.
SHOP TALK: PIECE BY PIECE
Although we are familiar with Rocket Exhaust, we haven’t actually tested one since October of 2005.
The 2005 review never made the magazine, because Don Leib got cold feet midway through the KX250F exhaust test. He feared that his pipe would get a bad review and it would hurt his business. When he called to ask us not to print the test, we agreed—never telling him that we liked the pipe and would have given it four stars. The MXA wrecking crew is used to companies being afraid to have their products tested because they lack confidence in their workmanship.
Six years later, Don Leib got his nerve back up and called us again. The MXA gang agreed to test his handiwork, but made a pact among ourselves that no matter the outcome, we would print the test. Here is the work that Rocket Exhaust put into the doctor’s KTM 350SXF.
Stage 1. The Rocket Exhaust KTM 350SXF power plant has Rocket’s Stage 1 and Stage 2 engine modifications. In Stage 1, Rocket ports the cylinder head and performs a matching five-angle radius valve job to increase air/fuel flow through the cylinder. After the five angles are cut into the valve seats, Rocket rounds off the corners by hand to get the smoothest radius that they can. Rocket likes to use a Vertex high-compression piston ($229.60) to get the most out of the Stage 1 mods.
Stage 2. Rocket’s Stage 2 mod is $120 extra. Stage 2 involves modifying the squish to improve combustion efficiency. Squish is defined as the space between the top of the piston and the bottom of the cylinder head.
While being keenly aware of piston clearance, engine tuners can reduce and reshape the squish to improve performance. Rocket decked the cylinder and head, milling a few thousandths from the edges where the gasket sits. Rocket can set the squish to different clearances to accommodate local racers or AMA Pros. Rocket exhaust. Part of Rocket’s marketing strategy for their exhaust systems is to cater to the needs of engine tuners and customers by offering custom-length mufflers. Customers can select their desired dimensions to work specifically with their engine setup. They can also choose among various noise-reducer nozzles, spark arrestors and different-colored end caps. You can order a Rocket muffler to meet FIM or AMA sound rules. MXA wanted to test an off-the-shelf Racer Series system, which only costs $499. For the cash, you get a head and mid-pipe manufactured from thin-wall stainless steel. Rocket uses this material not only for durability and cost, but for its heat properties. Rocket claims that by retaining more heat than titanium, stainless can make more power. The muffler canister is aluminum with distinctively colored end caps. Optional setup. We also agreed to race and test two different configurations of the KTM 350SXF setup.
(1) Pump gas. The basic setup was tested and dynoed with pump gas and the stock ignition. (2) Race gas. The second option was to test the bike using Renegade’s MX4 race fuel ($15 per gallon) and a Vortex programmable ignition. The Renegade MX4 fuel is not AMA legal because of its lead and oxygen content. The Vortex ignition map was devised by Rocket’s own testing. You can buy Rocket’s map for your Vortex ignition for $100. Engine accessories. Other engine aids included a Twin Air air filter with a KTM two-stroke filter cage, which doesn’t have a restrictive backfire screen. The used a Hinson pressure plate, inner hub and clutch cover. Finally, Torco T-4R four-stroke synthetic blend motor oil was the lubricant of choice. Suspension. SDI, aka Suspension Direct, was given the nod for the suspension duties. The only fork mods were an SDI spring seat kit ($99.95) and stiffer springs. SDI’s spring seat helps control damping from the mid stroke on. The advantage of tuning via the spring seat is the ability to reduce bottoming without making the forks harsher initially. The rear suspension was modified with a longer SDI shock linkage ($199.95). The SDI link lowers the rear end and stiffens the initial shock stroke to reduce wallowing and make the rear shock more stable. SDI also installed their shock bladder kit ($119.95) to help the rear wheel follow the ground better over smaller stutter bumps.
Rocket’s engine work and exhaust system pumped up the power by three full horsepower. Best of all, it was usable.
Chassis. To help the handling, Rocket’s KTM 350SXF was outfitted with Ride Engineering’s 20.5mm off set triple clamps ($479.90) and HPSD steering stabilizer bracket ($149.95). The Honda stabilizer itself costs $297. Ride also supplied bar mounts and fork adjusters. The chassis rode on Dunlop MX51 tires.
Miscellaneous. Other aftermarket goodies included Tag Metals front and rear sprockets (51 teeth on the rear). Tag also supplied XT1 handlebars ($109.99) and Rebound Technology dual-compound grips ($12.99). The chain was an RK GB520MXZ gold ($104.83). Light Speed’s carbon fiber glide plate ($199.95) protected the cases. Moto Hose supplied the radiator hoses ($109.99).At the controls were Sunline V1 MDX brake and clutch levers for$69.95 apiece. Rocket collaborated with 180Decals to design a “Rocket Exhaust” full-bike graphic kit with the rider’s name and number for $250. Ride Engineering’s shiny oil-filler cap, timing-plug brake and master-cylinder covers contributed to the look.
TEST RIDE: WHERE’S THE REV LIMITER? Before putting the Rocket Exhaust KTM 350SXF on the dyno, the MXA test crew consulted the dirt dyno. We started out on the Rocket Exhaust 350SXF equipped with pump gas and the stock ignition. Since almost every engine tuner under the sun has taken a shot at trying to find the perfect balance of power on the KTM 350SXF, we didn’t know what to expect. The stock KTM 350SXF is basically a top-end engine. It makes its peak horsepower very high in the rpm range (12,200 rpm) and has average midrange. If we could have anything out of the KTM 350SXF, we would want more power situated lower in the rpm range (around 10,000rpm instead of 12,000 rpm).The first thing we noticed about Rocket Exhaust’s KTM as we grabbed gears up the start straight was the smooth, free-revving power band. The hopped-up and geared-down machine made enough torque to get through the first 8000 rpm much quicker than the stock engine. At eight grand, the Rocket Exhaust KTM 350SXF made one horse more than the stocker. That extra horse was noticeable and useable. Test riders didn’t have to wait around for the bike to get going. The extra low and mid power made it easier to stay in the optimum power range, but that range was still high on the tachometer. At 10,000 rpm, the Rocket Exhaust 350SXF produced1-1/2 more horses than the stocker. This was useable power that thrust the 350SXF out of corners and up big hills much quicker than the stocker. By the time both engines reached peak, the Rocket engine had a three-horsepower advantage over the stock KTM 350SXF. The Rocket engine was better every where, but significantly better from8000 rpm to sign-off. Generally, on the stocker, we had to “wait to shift, ”because shifting too soon meant that we weren’t getting full power from the engine. Not so with the Rocket Exhaust KTM 350SXF; it made more power than the stock KTM from the moment the engine reached 8900rpm. Everything after 8900 was icing on the cake. On the Rocket Exhaust 350SXF, we could wait if we wanted to get max power, but even if we shifted 3000 rpm sooner, the bike was making more horsepower than the stock KTM made at peak. Good stuff.
RIDING THE OPTIONAL SETUP Between test sessions and races, the MXA wrecking crew drained the gas tank and changed to the Renegade race fuel and the Vortex ignition. The improvement with the fuel and yellow Vortex box was difficult to perceive from the saddle. The bike felt crisper from mid and up, but very fluffy down low. Additionally, the engine would not smooth out when cold and had to be warmed up for five minutes before it would clean out. In the end, we were surprised to discover that the race gas and Vortex ignition didn’t really bring very much to the party. Given their druthers, every MXA test rider said that they would rather run the stock ignition with pump gas than the optional setup. On the dyno, the difference between the two Rocket Exhaust options was visible, but not what you would have expected. At 8000 rpm, the race-gas-fueled bike only made one-third of a horsepower more. At 10 grand, the difference was only two-tenths more, and at peak, it made one tenth of a horsepower more. It definitely wasn’t worth $15 a gallon.
VERDICT: WHAT DO WE REALLY THINK? Rocket Exhaust’s KTM 350SXF is a very expensive, highly modified race bike, but all of the work paid off. The “Rocket” name was fitting. Rocket Exhaust didn’t turn the 350SXF into a mini 450; they just made it into a broader, faster KTM 350SXF. Plus, Rocket has Stage 3mods (camshafts, DLC-coated buckets, titanium valves, stiffer valve-spring kits, bronze valve seats and a higher rev limiter) that we didn’t even try. Somewhere in SoCal, there is an orthopedic surgeon carving up the track on one sweet Katoom. For more information, go to www.rocketexhaust.com.