Small-Bore Motocross: KTM 150 SX Vs. KTM 250 SX-F – Dirt Rider Magazine
The hottest question you can pose anywhere in dirt bike land is: “Which is better, the two-stroke or the four-stroke?” Depending on the participants in the conversation you will get a million different answers. OK, two, but a million different reasons why. The topic is impassioned, interesting and confrontational.
Both engine types have valid advantages, as well as dark little secrets. So which is better? Well, we’re not going to tell you. No, it isn’t a secret. It isn’t a government cover-up.
And the fact that we can’t tell you will only fuel that fire, but some choices-blond or brunette, slim or curvy, moto or off-road, NY or CA and two-stroke or four-stroke-are decisions only you can make. What we will do is give you the facts from some very methodically conducted testing (of these two bikes with more matchups to come soon) for you to make your own decision.
Then you can tell everyone which is better.We chose to start this series with a pair of small-bore motocross bikes, a matchup that seems pointless until you look at it a little closer. In amateur racing, in most organizations, you can race even a 250cc two-stroke because the class is just called 250cc. Some are more specific and set the two-stroke limit at 144cc or 150cc against the 250cc four-stroke.
But in the real world the 125cc two-stroke was the first and most obvious victim of the 250cc four-stroke. Two-strokes responded with big-bore kits, and KTM produced the 144 and now the 150 models specifically to meet the challenge (the engine displacement never changed from 143.6cc). For fairness and to start off right we chose the 2011 KTM 150 SX and 250 SX-F to begin this experiment.
Both bikes were tested box-stock with the only modifications being what could be done with clickers and spring-preload adjustments. We used riders of all weights (within reason for these sized bikes), sizes and ability levels as well as some who were exclusive to only two-strokes or four-strokes prior to riding these test bikes.Our testing was completed at three locations on three radically different tracks.
Milestone’s jumpy and rutted pro track, Piru’s hard-packed, technical and hilly main track and Racetown 395′s wide-open and sandy track. Our first day was a setup and break-in for the bikes and to see if they were on the same playing field before going any further, and they were. The next two days were arranged to run motos, with the bikes going head-to-head in paired rider groups all the while being timed.
Then we allowed riders to try the bikes after the motos to see if anything they felt while not racing them was particularly odd or spectacular. We ran them in front of a radar gun to find out if radio waves showed anything more. Finally, the bikes were run on FMF’s dyno to let a machine tell us exactly what the motors were doing.
KTM 150 SX
If you say the two-stroke is dead, we say look at KTM and realize that you’re wrong. For 2011, the Austrian manufacturer has brought out a whole slew of new two-strokes, and the 150 SX is just the start. Although the motor isn’t radically changed from the past few years, the revisions it has benefited from make it about as mighty as it can be in its current configuration.
So much so that when prodding about modifications to make the engine perform at a higher level, internal KTM race team tuners said there was little to do other than maybe a pipe and muffler change and that would only shift the power’s location in the spread. Porting, compression and intake tract setup are pretty much as good as they can get right from the dealer’s floor. This while running on pump gas and your favorite two-stroke oil at 32:1; we ran Rock Oil Synthetic for the duration of this comparison.
On the chassis side of things KTM went all-out with a new frame and suspension setup, but it did not go to linkage on the two-stroke motocross bikes while the four-strokes did. Why? KTM feels two-strokes are about simplicity and easier maintenance, and the linkageless system is just that. The advantage the orange engineers found with the linkage, the ability for a bike to work better at more tracks with less suspension changes, did not affect the two-stroke as much.
But the frame’s design now has the shock tower isolated from the main spar of the frame and allows the use of a longer shock. All of the bodywork is paired to the new frame as well as the airbox for better flow.Alone, this bike is a standout performer and KTM has worked some serious magic into the motor. Pumping out 38.9 horsepower (4.1 more than the 250 SX-F), KTM gets a lot more tug out of the two-stroke than seems possible.
The pull is impressive, especially on mid through to the top where this screamer really needs to live to be competitive. To go fast there is no being lazy, and your shifts cannot be mistimed; good thing the hydraulic clutch and perfectly spaced transmission make it easy. The 150 pulls hard off of jumps that take just that little bit extra that the 250 SX-F will not give.
But in the torque game it is hard for the two-stroke-it simply isn’t making as much for as long (see the charts)-hence the shifting and sifting. The downfall for messing up is a bog, and it takes a high skill level to ride above the bog 100 percent of the time.The two-stroke’s advantages lie in the weight of the bike and what the bike feels like when you’re riding it. Light!
No matter the rpm or the condition of the track, the 150 SX stays agile, light on its wheels and a rider can make the bike go exactly where they want much easier than on the 250 SX-F. Although both bikes steer similarly, the lack of compression braking on the 150 allows riders to get into turns faster, or gives them fits with needing more rear brake control which has some implications on how the bike feels in the turns.
Most riders were able to adapt to it with time or set the bike up a little different to get comfortable. There were minimal issues with turning, overall it rates excellently. The suspension is set for 165-pound riders but pulled duty from 130 to 190 without many issues. There were hardly any comparisons drawn between the linkage and non-linkage systems, which was surprising to most riders as both suspension systems did great jobs.
If anything, the two-stroke was a little bouncier in the chop going into turns and would use more stroke in rollers and jump faces. The rear also bottomed harder when slammed into stuff or was over-jumped while the front worked identically to the 250 SX-F on hard hits.The verdict? Well, for a high-level rider the 150 is and can be a competitive race machine, but stock for stock, there is no rest for the 150 rider.
The problems arise when it is much easier and rewarding to modify the 250 SX-F, which is already easier to ride fast. For a novice racer, mistakes on the 150 are costly and frequent, but we feel over time that rider will improve faster on a two-stroke. The initial cost advantage is significant, and the maintenance requirements are about equal in cost until there is a true problem.
Then the two-stroke wins out big by being simple.
KTM 250 SX-F
Average Lap Times:
Averaged over 16 three-lap motos. Track conditions were not a factor since bikes were on the track racing at the same time during all timing. *Lap times were very inconsistent (especially on the 150) but averaged out similarly.
nWhen the bikes are kept at peak power and shifted properly they are about dead even until the extra horsepower of the 150 starts to play out. In reality the bikes are a tie off the start line.
These two-stroke and four-stroke brothers are very impressive machines. The KTM 150 really got my attention with its ability to overtake the KTM 250 SX-F down long straights in the upper gears. The 150, being a two-stroke, is low on torque which makes corner speed a challenge. This is a huge advantage the 250 SX-F holds over the 150.
The question is, what’s more important, top speed or corner speed? I tend to lean toward corner speed because anyone can go fast in a straight line. Riding the 150 really makes you work harder in corners, but I see the extra challenge as a way to stay aggressive and keep up your momentum.
The 250 SX-F offers a wider variety in line choices. It has the power to keep speed through soft berms and muddy ruts, as the 150 needs to stay on the main line to avoid loss of traction. However, I honestly can tell you that neither of these bikes has much of an advantage over the other.The first thing I would do on the 150 is gear it more toward acceleration to put more power at the bottom-end.
The 250 SX-F felt a little lacking on the top-end, so I would do some minor motor work to turn up the heat and keep the power from flattening out too soon. Aftermarket exhausts work wonders on four-strokes, so to me it’s a must-have. Daniel Guillen
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