HONDA TRX450R HOP-UP
Horsepower isn’t something you buy—you have to earn it.
Anyone can throw money at a quad, and, occasionally, the results might be favorable. But, in most cases, those kinds of gains are modest compared to the results that are possible with a little effort and a lot of testing. In other words, buying your quad a silver ankle bracelet on Valentine’s Day isn’t as effective as writing it a love poem. Write that down.
The most common illustration of this comes with the installation of an aftermarket exhaust pipe. Common sense tells us that a stock quad is somewhat bottled up. The EPA and various state agencies require that even high-performance ATVs be very quiet and very clean. So, the inevitable search for power leads us first to the exhaust department. But, horsepower is much more elusive than that. The exhaust is just one piece in a larger puzzle that includes air, fuel and volumetric efficiency. If you install a 45-horsepower exhaust pipe on an engine that was tuned to make 30 horsepower, the result isn’t going to be that great. It’s possible that you might even lose power until you bring the rest of the motor up to the same level.
RUN NUMBER ONE: ALL STOCK
To get a good baseline, we ran the Honda in perfectly stock form. First, there are a few things you should know about horsepower and dynos. When a quad is tested on a typical dyno, it is tested with the throttle wide open. When you ride a quad, the throttle is almost never wide open. Studies have shown that even top pro riders have the throttle maxed out less than 5 percent of the time. The dyno only tests the main jet of a carbureted motor, whereas the rider experiences the slow jet, the needle and all the other circuits that affect performance. The only reason we test the motor with the throttle wide open is for the sake of simplicity, but you need to understand it’s only part of the picture.
RUN TWO: SLIP-ON PIPE
RUN THREE: PIPE, AIRBOX LID REMOVED
The easiest way to let the Honda breathe is to remove the airbox lid. On most ATVs and dirt bikes, only part of the total noise is produced by the exhaust pipe. In order to pass a ride-by sound test, intake noise must be squelched as well, so Honda (and other makers) has designed a very restrictive airbox. It’s matched to the stock exhaust, so once that balance is disturbed, the motor is no longer tuned perfectly.
RUN FOUR: PIPE, HEAD PIPE, LID REMOVED
In our search for more power, we could have gone in many different directions from here. The most lucrative change, we decided, would be to change the head pipe. It was the most obvious, remaining restriction in the intake/exhaust equation. The stock exhaust was designed as a single unit. The most restrictive part might well have been the tail section, and that makes economical sense, especially considering that the Motoworks slip-on sells for $209.95. But a full system eliminates the next weakest link as well and still sells for under $400.
RUN FIVE: PIPE, HEAD PIPE, NO LID, RICHER JET
The next logical step is to get the air-fuel mixture right. When we opened up the exhaust and the airbox, we did nothing to add fuel. That meant the motor was very lean. The Motoworks lab has access to an oxygen sensor, and that sent us to a 168 main jet. Here’s where that business about testing with the throttle wide open becomes important. We got away with only changing the main jet on the dyno. In the real world, it’s not that simple. You would have to change the needle as well as the slow-speed jet, and maybe even the leak jet.
RUN SIX: PIPE, HEAD PIPE, NO LID, RICHER STILL
RUN SEVEN: MORE REVS
We know a secret. Virtually all the top Honda tuners do the same trick; they cut a green wire that comes out of the ignition’s black box. This changes the rev limiter and allows more rpm on top. The rev limiter isn’t there to prevent damage to the motor, but it does allow the TRX to pass that important sound test.
For our final run, we cut the wire and expected to see another big horsepower increase. Imagine our shock when we found that the motor lost almost a full horsepower at peak! Our new max power reading was 38.8 horsepower. The motor did indeed rev past the 9800 rpm barrier by another 500 rpm, but we caused other changes in the spark-advance curve that we didn’t anticipate—and that no one at Honda could explain. It seems that most of the 450’s testing was done in Japan a number of years ago, and now those engineers have moved on to other projects.
At a higher state of tune, the increased rpm would be much more beneficial. Revs become a more effective means of increasing horsepower when all the low-hanging fruit has been picked. Just as we found when we installed a slip-on pipe with no other mods, the change in the rev limiter would require other changes to make sense. Perhaps headwork might hold the key, or perhaps a cam, but those kinds of changes exceeded our mission’s goals. We were looking for cheap, lost power and we found it.