Since the target of most criticism regarding the TSX has been the motor and its relative lack of thrust, we'll start by examing the changes underhood first. At the 2003 US media launch for the TSX, Mr. Sakuji Arai (the chief powertrain engineer for Honda 4-cylinder motors whose credits include the F20C and K20A masterpieces) was quite visibly proud of the TSX's K24A2. In fact, he asserted that the K24A2 was Honda's finest 4-cylinder ever - a claim which makes you think a little bit about what the engineering goals were. While there's no doubt the TSX's 2.4L DOHC i-VTEC inline 4-cylinder is one of the best in the business, we (and many others) felt that it wasn't quite up to the task of pushing around nearly 3300lbs of sedan with any real sense of urgency.
It's not that the original K24A2 motor is a huge letdown, it just doesn't seem to quite fit the playful spirit effused by the TSX. Thanks to the undersquare, long stroke design, there's plenty of grunt, even at low rpms and part throttle application. The biggest problem is that there's not much grunt left in reserve for those times when you drop the hammer to scoot past slow moving traffic on a two lane or for when you just want to open it up on your favorite backroad. This is where you're left wanting for more.
For 2006, that problem has been addressed - to an extent. The actual extent is a little unclear, as on paper the difference in power amounts to an extra 5hp*. Of course, the asterisk indicates that things are more complicated than they appear. Thanks to the 8/04 revision of the SAE's J1349 standard, Honda has re-rated all of their motors for 2006, so we can't directly compare the output of the '05 K24A2 to the '06 version based upon published specs. Of course, we've asked what the actual difference is and Acura claims that the revised motor would have been rated at 210hp using the older SAE method, so this motor should be about 10hp stronger than last year's.
Other than taking their word for it, the only way we have to verify the difference is with a chassis dyno, and sure enough, that's what we've done. You can read the full results a little bit later in this article but we can assure you that the claimed improvement is legitimate.
What have Honda engineers done to extract this extra power from the motor? To summarize, they basically improved flow through the motor. The changes are fairly comprehensive for a mere 10hp improvement, which is beginning to make me believe that this 2.4L iteration of the K-series motor maybe didn't have quite the potential I initially thought (and hoped) it would. Starting from the intake side, the diameter of the inlet pipe leading to the air resonator has been increased from 70mm to 80mm. The throttle body has been enlarged from 60mm to 64mm. These changes are said to be responsible for an intake airflow capacity, helping the motor gulp as much as 110 liters/second (up from 95 liters/second). To allow the motor to injest this extra 15 liters per second, the intake valves have been enlarged by 1mm (to 36mm). A revised high-rpm profile on the intake camshafts results in a 0.9mm increase in valve lift and 14 degrees of extra duration (opening 2 degrees sooner, and closing 12 degrees later). Obviously, if you're burning more air and fuel, you have to do something with the spent gases. So to deal with the added volume of exhaust, the head pipe (the pipe below the 4 primaries on the header) has been enlarged from 60mm to 65mm. The catalytic converter has been spec'ed up to a higher flow rate, and behind the cat, the remainder of the exhaust system has been enlarged by just under 3mm pretty much the entire way back. The changes to the exhaust system result in a reported increase in exhaust capacity from 115 liters/second to 125 liters/second.
Now these changes might seem sufficient to account for the 10hp gain, but Acura's engineers weren't quite finished. Inside the block, they utilized an old racer's trick and have reduced some of the cylinder-to-cylinder pumping pressure by drilling vents in the crankcase. They also spec'ed stiffer connecting rods, a stronger crankshaft with additional balance weight and the pistons now have deeper valve reliefs to accommodate the higher lift profile of the new camshafts. Interestingly, while the motor gained horsepower and torque, it actually managed to pick up 1mpg on the EPA City cycle for the 6MT model (now up to 22mpg). Also interesting is the fact that even with these fairly significant revisions, the motor is still stamped with the K24A2 designation.