Small-Bore Motocross: KTM 150 SX vs. KTM 250 SX-F - Dirt Rider Magazine | Dirt Rider

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.

MX Start

When 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.

KTM 250 SX-F

All the while never giving up on the two-stroke, KTM was pushing even harder on its four-stroke program, and it shows. An all-new frame with a linkage shock surrounded by all-new bodywork holds a very familiar engine with one significant change: fuel injection. And that change is most likely the biggest advantage this bike has in this duel. As much as the computer-controlled fuel delivery system leaves out one of the biggest mysteries of the four-stroke-jetting-it also adds to an already long power pull, smooths out that power and eliminates any bog in the delivery. It is the one thing the two-stroke did not need stacked against it.Riding the new 250 SX-F feels very similar to the old 250 SX-F with power that goes on forever and a light and agile package. Not as feathery as its 150 brother, but it is light in its four-stroke class. The suspension action felt through the new linkage does not jump right out at the riders; it seemed to take a few tracks and then there was noticeably less grumbling on setting issues. The biggest concern we encountered was a polarity with riders on how the bike turned; some loved it and others weren't so impressed, even with fresh Dunlop Geomax rubber (so we confirmed it was not the tires). The haters (a harsh word, really) also didn't seem to find magic in a particular setup but were also admittedly not totally used to the bike overall.The advantages begin to pile up in the four-stroke's favor when rider comments come in, way more than any of the charts and graphs will point out. There is one thing that rings out: The four-stroke is easier to ride. The power delivery feels so much longer with so much more torque available in the lower ranges of the rpm that every test rider said it was an advantage. It feels so much better that riders also believed they were faster on the four-stroke (some weren't), that the four-stroke puts out more power (it doesn't) and that the bike overall worked better (we can't dispute this, but we can say that more of our riders were four-stroke riders to begin with).In the case of the 2011 KTM 250 SX-F, the company has hit a home run with the fuel injection, though the kickstarting could be better, especially compared to the feather-light and easy kick of the two-stroke. The 250 SX-F's improved power and performance may lack some of the carb-fluttering snap and feel of old, but the rest of the bike is getting tuned for the U.S. market-the linkage works in the mind as well as it does on the track. The entire package, from hydraulic clutch goodness to a six-speed gearbox to brakes that are powerful as well as have great feel, makes this bike ready for a 250F battle royal. Even if it is, in reality, just beating out a two-stroke.

KTM 150 SX Specs
MSRP: $6,299
Weight (tank full): 210 lb
Seat height: 38.4 in.
Footpeg height: 17.9 in.
Seat-to-footpeg distance: 20.5 in.
Sound Test: 93.3 at 5,000 rpm/110.2 dB 2m MAX
KTM 250 SX-F Specs
MSRP: $7,699
Weight (tank full): 235 lb
Seat height: 37.2 in.
Footpeg height: 16.7 in.
Seat-to-footpeg distance: 20.5 in.
Sound Test: 95.8 at 5,000 rpm/111.5 dB 2m MAX

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.

MX Start

When 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.— Chris Green, 5'9"/160 lb/Intermediate

I was very excited to ride the KTM 150 SX, especially since I love two-stroke bikes. During the first day testing at Piru the 150 SX was everything I thought it would be. It ripped through the corners with more command than my 2004 YZ125 and railed up hills with total ease. I fell in absolute love with the bike, and the suspension ate up everything the track threw at it (consequently proving that my YZ125 needs a suspension update very badly). The power of the 150 SX comes on pretty abruptly and is more of a mid to top-end powerband with endless overrev; however, it does have quite a bit of bottom-end power allowing you to rip out of corners like a four-stroke (though I had better luck carrying my speed through the outside of corners). Compared to the 250 SX-F the 150 SX easily dominated at Piru MX. Where the 250 SX-F caused arm-crippling arm-pump, the 150 SX begged to be given more throttle and be revved to the moon. In the air the lightweight 150 SX shined as well, allowing me to change direction and whip the bike around much easier than the heavier 250 SX-F, skipping over the deep ruts left behind by the bigger-bore four-strokes.The next day at Racetown 395 left me with a bit of a sour taste in my mouth for the 150 SX. Everything the 150 SX did perfectly at Piru, it did the opposite at Racetown. I could not get it to track right through the deep ruts. It did not rail over the deep sand like my YZ125 did during the pre-test laps. Keeping the motor in its peaky mid-top powerband ended up tiring me out. On the 250 SX-F I felt better than I did the previous day. This bike is a sand lover! I put the front tire in a rut, and it went right into it and stayed in it (wish I could say the same for the 150 SX). I had a lot of energy left over after a hard ride on the 250 SX-F. On the downside, I wish the bike was easier to handle in the air.— Daniel Guillen, 5'4"/122 lb/Novice

Just like fries versus onion rings, Broncos versus Raiders, Chevy versus Ford, the two-stroke versus four-stroke debate is an eternal question that may never be settled. I dig the lightweight feel of the 150, as well as the raw, spastic serving of forward propulsion that pours out of the rear tire when you get the thing to hook up. I also like the sound of the bike, the straightforward maintenance and the significantly lower MSRP. The 250 SX-F gets a thumbs-up for putting out more usable power, being easier to turn and making me more comfortable in less time. On the other hand, the 150 is fast but hard to keep planted, and the delivery has an all-or-nothing feel that takes more work to harness. There's not much by way of lugging, and I felt like my corners were more disjointed than smooth. The 250 SX-F required constant short shifting, the usable power could be considered mellow in spots and the front end had a busyness to it that kept me from feeling good about really pushing at speed. Plus, I'd rather disassemble my remote control than crack into a thumper engine. In my mind, the 150 needs a spark arrestor (not only to make it off-road legal but also to bring the revs down a touch) and a more cornering-friendly suspension setting in order to make my Christmas list. I think the 250 SX-F could benefit from a rejuvenated powerband that requires less shifting, possibly by way of an FMF MegaBomb or similar exhaust system, along with a stabilizer or something to sort out the loose-feeling front end.In the end, I'll take the 250 SX-F simply because I felt more at home on it, but that doesn't mean my personal two-stroke versus four-stroke debate is anywhere close to being settled....— Chris Denison, 5'10"/155 lb/Ramp Kid

KTM\'s four-stroke muffler is designed to pass the 94 dB stationary test and be acceptable a full-noise. It is.

With a new composite wrap mount the two-stroke muffler works. The sound output carries less, but up close it screams.

Chris Green

Nick Evennou

Kris Keefer

The DOHC, four-valve, fuel-injected motor makes up for its complexity with a longer power pull, the four-stroke\'s biggest advantage.

Simplistic and compact, the two-stroke SX is making similar, even more power at a significant displacement disadvantage.

Chris Green

Nick Evennou

Kris Keefer

The expansion chamber may be the biggest wear item (dents) on the 150 SX. The motor is difficult to make more powerful.

In the 250 four-stroke game there is a lot to be gained with modifications, all at a price.

3rd-Gear Roll-On
\nThe third-gear roll-on tells the story of where the powerband pulls and for how long. The lag in the beginning and the early fall-off of the two-stroke shows why the shifting is so important.

MX Start
\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.

The DOHC, four-valve, fuel-injected motor makes up for its complexity with a longer power pull, the four-stroke\'s biggest advantage.

Simplistic and compact, the two-stroke SX is making similar, even more power at a significant displacement disadvantage.

Chris Green

Nick Evennou

Kris Keefer

The expansion chamber may be the biggest wear item (dents) on the 150 SX. The motor is difficult to make more powerful.

In the 250 four-stroke game there is a lot to be gained with modifications, all at a price.

3rd-Gear Roll-On
\nThe third-gear roll-on tells the story of where the powerband pulls and for how long. The lag in the beginning and the early fall-off of the two-stroke shows why the shifting is so important.

MX Start
\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.

Related Content


Latest


Videos