Race Engine Forum UK

Hi Guy!


When building performance engines, a lot of time and energy goes into modifying the head and manifold to get the best result for a given engine specification.
When this is done - the rest follows......

Regarding the effect of fitting carbs - a lot of unceartainty seem to exist regarding carb size, choke size, ram pipes and their effect on engine behavior. I would very much appreciate it - if You are in posession of such data - if You could post some flow figures from the carbs itself, as it would be helpful for understanding which carb/chokes sizes to use:-)


Best regards

Remi Lovhoiden

It's primarily a question of how much the secondary venturi (choke) will flow rather than the carb barrel (which will tend to be 40, 44, 45 diameter) and typically a 34mm choke will flow 156cfm at 10" depression which is way more than the inlet port on most European 8v heads will achieve. Thus it is not going to restrict, say, a port flowing 110cfm. You will have read, I am sure, about 'controlling sections' and their detrimental effect on massflow.

The other thing to remember, importantly, is that the smaller the choke relative to the barrel the bigger the pumping loss which is why bigger chokes on the same size carb will give more horsepower - though not always more peak torque - given that if you make the choke too big (max on 40s is 34 really, 37mm on 44s and 40mm on 45s) the fuel signal will suffer - in other words the combined depression created by the primary (small) and secondary (big) chokes stacked up one in front of the other will be too weak to draw the fuel out in the right volume.

The information on relative sizes in my book seems to as valid as it ever was.

GC