What compression ratios are recommended for 4v iron and Al heads using 91 octane?
Original Post
Mark,
the answer to your question is very dependent upon which cylinder heads and what camshaft you are using, there is no one answer.
The standard answer is to run whatever compression is dictated by your camshaft supplier. A longer duration camshaft which normally has more overlap and has rougher idling characterisitics will call for more compression, a shorter duration camshaft with less overlap and smoother idling characteristics will call for less compression.
To some degree aluminum heads will tolerate a little more compression than iron heads, all other things being equal. However, combustion chamber design plays a bigger factor in this than the material of construction. An open chamber head will tolerate the least amount of compression, a closed chamber head mounted on an engine block that has been "zero decked" will tolerate more compression, and the latest crop of alloy cylinder heads featuring high swirl combustion chambers will tolerate the most compression.
Compression ratio, ignition timing, carburetor calibration and fuel quality are very interactive criteria. The closer you push the limit on the compression ratio the more critical it becomes to have a consistent quality of gasoline, and properly tuned ignition and carburetion. Since gasoline quality is anything but consistent these days, my advice is to go conservative on the compression ratio.
Gear ratios, the type of transmission and the weight of the vehicle also have minor roles in determining the best compression ratio for a motor.
I know that's not the precise answer you were hoping for, but its all that can be safely provided without having more specific information about the vehicle & motor in question.
cowboy from hell
the answer to your question is very dependent upon which cylinder heads and what camshaft you are using, there is no one answer.
The standard answer is to run whatever compression is dictated by your camshaft supplier. A longer duration camshaft which normally has more overlap and has rougher idling characterisitics will call for more compression, a shorter duration camshaft with less overlap and smoother idling characteristics will call for less compression.
To some degree aluminum heads will tolerate a little more compression than iron heads, all other things being equal. However, combustion chamber design plays a bigger factor in this than the material of construction. An open chamber head will tolerate the least amount of compression, a closed chamber head mounted on an engine block that has been "zero decked" will tolerate more compression, and the latest crop of alloy cylinder heads featuring high swirl combustion chambers will tolerate the most compression.
Compression ratio, ignition timing, carburetor calibration and fuel quality are very interactive criteria. The closer you push the limit on the compression ratio the more critical it becomes to have a consistent quality of gasoline, and properly tuned ignition and carburetion. Since gasoline quality is anything but consistent these days, my advice is to go conservative on the compression ratio.
Gear ratios, the type of transmission and the weight of the vehicle also have minor roles in determining the best compression ratio for a motor.
I know that's not the precise answer you were hoping for, but its all that can be safely provided without having more specific information about the vehicle & motor in question.
cowboy from hell