exposed carb trumpets - airflow and performance?
Discussion
This has been bugging me a while but I cant figure this out. I have seen numerous cars both fast road and race with exposed carburettors that have trumpets at 90 degrees to the airflow across them when in motion. good examples are '7' inspired cars which have Weber 40/45s that come horizontally through the side of the bonnet with/without an air filter or some DFV powered racers with vertical trumpets again at 90 degrees to the airflow when at full chat.
I have two thoughts on this.
One is that perhaps the fast flow over the lip of the trumpet, at the leading edge at least, does draw air into the carburettor and aid airflow. I dont think this outweighs the disturbed airflow factor though?
The other thought is that - like blowing across the top of a milk bottle - the flow of air at speed actually means the air pressure actually drops in the inlet spoiling performance?
Either way I cant see that having the trumpets arranged in such a fashion is efficient? I would have thought that the trumpets or some sort of cowl facing into the airflow would provide a 'ram air' effect which would be much better?
Can someone shed light?
I have two thoughts on this.
One is that perhaps the fast flow over the lip of the trumpet, at the leading edge at least, does draw air into the carburettor and aid airflow. I dont think this outweighs the disturbed airflow factor though?
The other thought is that - like blowing across the top of a milk bottle - the flow of air at speed actually means the air pressure actually drops in the inlet spoiling performance?
Either way I cant see that having the trumpets arranged in such a fashion is efficient? I would have thought that the trumpets or some sort of cowl facing into the airflow would provide a 'ram air' effect which would be much better?
Can someone shed light?
It's fairly easy to calculate the ram air effect, although I haven't done so recently. The gain at 100mph is around 0.2 - 0.5%, if that.
With the trumpets inside an air filter (who would race without a filter?), and the inside of the filter quite close to the end of the trumpets, the direction of orientation will make no difference. Exposed trumpets will give worse performance.
With the trumpets inside an air filter (who would race without a filter?), and the inside of the filter quite close to the end of the trumpets, the direction of orientation will make no difference. Exposed trumpets will give worse performance.
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