beermanpete

Streets of Willow - February 2010

Certainly the per cylinder displacement and the RPM are the primary factors in determining the carb size. Motorcycles with a similar cylinder displacement (and stroke) to the 240Z (260Z?, 280Z?) will have a similar RPM range and therefor a similar carb size what we need. Cycles with a smaller cylinder displacement but a higher RPM "might" have a suitable carb size. This is where one must be savvy. I need to determine the size needed for my application and then go look for "it" rather than look around and say "I hope that will work".

Kiwi, The Weber, Mikuni/Solex and Dellorto carbs typically used on the Z cars are 2 barrel carbs. Typically used in sets of three on 6 cylinder engines. This yeilds one venturi per cylinder. The typical set-up does not cross-connect one cylinder to another, thus it is called an "Isolated Runner" system. The size required depends on the displacement of the engine and the RPM of maximum output. In the book "Weber Carburetors" by Pat Braden there is a chart provided by Weber that shows the carb size required based in individual cylinder displacement and RPM of maximum output for a race engine (for an IR system). This chart indicates that for a 2400cc 6 cylinder engine that has maximum output at 6000 RPM the carb VENTURI should be 33mm. This is for a "race" engine. Street engines will likely want to be a bit smaller. To temper this there is also a chart for a single one-venturi carb on an engine with maximum output at 5000 RPM. In this case the carb indicated for the 2400cc engine is 31mm to 35mm dependent upon application. Regarding the motorcycle carbs, it does not matter that they come from a motorcycle. The size is the size and they will flow a similar amount of air as a comparable quality car carb. Also, since they are constant velocity (as are the Hitachi/SU carbs) it is not as much of a penalty if they are too big (compared to fixed venturi designs).

Almost all bikes have IR carb setups. They generally don't require constant attention, only when the valve adjustment is changed. Most of the 4 cylinder bike carbs might be bit small for the Z engines. The carbs from a 1000cc twin would certainly be large enough. I have been wondering if using 6 of the Hitachi/SU carbs would be worth the effort. Has anyone done this?