Q: The Accord '08 with EX-L pkg has a 4 cylinder engine that produces 190 hp vs the new TSX '09 from Acura that has the same 4 cyl which is tweaked to produce 201 hp. Maybe that's the difference in hp output with premium gas vs the Honda's regular gas. Hmmmm
A: Not sure these are (exactly) the same engines. If they are, then yes, Honda would have deployed a slightly detuned version of what's in the TSX. Actually, the European version of the TSX is called the Accord over there, and it makes 220 hp. The TSX motor is of 11:1 compression, and the Accord's 190 hp 4 cylinder version is 10.5:1. Apparently the Accord is close to experiencing detonation (ignition before the piston is past top dead center position), but not quite while using regular (87 octane). Being able to run the lower octane gas is a big selling point for family cars, and that's what limits the output of the Accord motor compared to that of the TSX. The V6 Accords Gen 6 (1998 - 2002) had a J30A1 motor 9.4:1 compression, and they all ran off of 87 octane. I also believe the Gen 7 (2003 - 2007) J30A2s ran off of the same gas, though they were over 10:1 compression (developed 40 & 44 more hp than the Gen 6 models). The octane rating is achieved by fuel additives such as toluene that increase the flash point (temperature) of the fuel vapor...this refinery additive increases the octane rating. Compressing a fuel/air mixture heats it, so the higher the octane rating, the higher the allowable compression ratio before detonation occurs. Another way of looking at it is that higher octane fuels enable more aggressively tuned engines to achieve their full horsepower potential. Modern engines have microprocessor engine management systems that operating in conjunction with 'knock' sensors, will retard the ignition timing should detonation occur (such as from using too low an octane of gas, or from driving under heavy loads on hot days, etc.). This means you can use lower octane gas in a car rated for 89 or 91/93, but you'll lose some efficiency. Many vehicle user manuals recommend against this procedure for anything other than emergencies.