Posted: Mon Feb 02, 2009 1:13 pm
2 different types of Mivec you're talking about is the problem.
The older version of Mivec you find in an FTO motor, is a hydraulically actuated, mechanical system. Meaning, its a fixed system, the only change you can make in it, is adjusting the cutover point to the larger cams (Mivec cams). Its a system that was designed specifically to give a naturally aspirated engine more power, over a longer range. As such the engines were designed with compression ratio's of 10.5:1 to compliment that system.
The newer versions of mivec are computer controlled in a much more finite way. The computer can not only adjust the cutover point, but the inlet valve timing itself can be altered at any point during the rev range - according to external factors - such as torque load, air density, fuel type & most importantly, boost. Hence you can now get "turbo Mivec" engines these days. 10 years ago, there was no such thing. Whats important with the difference between the 2 styles of engines is cams. Traditional turbo cams are a short lift, long duration lobe design, where as a naturally aspirated cam profile has a high lift & short duration. Hence, that if you can effectively "change your cam profile while driving" (ie the computer altering the valve timing) then you can effectively have a motor the drives like it naturally aspirated, but when you put your foot down, its effectively turbo charged as well.
The only disadvantage of the newer systems, is that the adjustable timing only occurs on the inlet timing. Where as the older system did both inlet & exhaust. Although, I am told, some of the newer EVO's have the ability to do both.
The older version of Mivec you find in an FTO motor, is a hydraulically actuated, mechanical system. Meaning, its a fixed system, the only change you can make in it, is adjusting the cutover point to the larger cams (Mivec cams). Its a system that was designed specifically to give a naturally aspirated engine more power, over a longer range. As such the engines were designed with compression ratio's of 10.5:1 to compliment that system.
The newer versions of mivec are computer controlled in a much more finite way. The computer can not only adjust the cutover point, but the inlet valve timing itself can be altered at any point during the rev range - according to external factors - such as torque load, air density, fuel type & most importantly, boost. Hence you can now get "turbo Mivec" engines these days. 10 years ago, there was no such thing. Whats important with the difference between the 2 styles of engines is cams. Traditional turbo cams are a short lift, long duration lobe design, where as a naturally aspirated cam profile has a high lift & short duration. Hence, that if you can effectively "change your cam profile while driving" (ie the computer altering the valve timing) then you can effectively have a motor the drives like it naturally aspirated, but when you put your foot down, its effectively turbo charged as well.
The only disadvantage of the newer systems, is that the adjustable timing only occurs on the inlet timing. Where as the older system did both inlet & exhaust. Although, I am told, some of the newer EVO's have the ability to do both.
Haven't seen any of the boys around for a while thou..