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Discussion Starter · #1 ·
Living in SoCal allows me to run my S240 without it’s roof most of the time (which makes ingress and egress way easier for me) but I figured that Lotus created the Mohawk for a reason and decided to make a T-top to maintain the ease of getting in and out while hopefully preserving airflow over the intercooler. When I first posted the update on the roof one of the forum members suggested that the Mohawk doesn’t actually do much for the airflow and therefore the T top wasn’t needed. I figured that was a good point so decided to see what the T top actually does, or does not do, relative to no roof. Years ago I had built a crude manometer to help balance the carbs on my motorbike and thought that this could work to measure pressure differentials at the front of the intercooler. The photos show the set up and the results, which were a bit disappointing and a bit odd. I didn’t get a full set of data at all speeds (due not other cars getting in the way) but I did average the results from runs in both directions on a local straight road.
The bottom line is that adding the T-bar Mohawk (I didn’t test it but assume the full S240 Mohawk roof would be similar) increased the intercooler intake pressure by ~20% @70MPH compared to no roof and by ~16% at 90MPH (speed indicated). I was a bit disappointed by these numbers so decided to cut out the rim of the rear intake as I had read somewhere that little lip can restrict airflow. The results were interesting as now the Mohawk resulted in. ~30% gain (compared with no roof ) @ both 70 and 90 MPH. Therefore cutting the lip out increased the pressure differential by ~9% and ~15% at 70 and 90 MPH respectively (There is some rounding error in these numbers but the measurement precision is not all that high so bear with me)
I plotted the data that I had and was surprised to see that the pressure differential didn’t appear to rise exponentially with speed (which according to my very basic understanding of fluids it should as pressure squares with speed).
I’m hoping that forum members more talented than I am can offer some explanations, however I have to confess that despite despite looking quite good and the rising pressure differential numbers I’m not sure that Mohawk does very much as the rising pressures suggest taking there is a restriction in flow at the intercooler. Interestingly if it’s is correct then adding airflow from the side pods probably won’t help intercooler
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cooling all that much, but again would appreciate comments from more expert individuals.
 

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Premium Member
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Discussion Starter · #5 ·
@Me! Thanks for the link, I hadn’t realized there had been 35 pages of discussion on intercooler flow rates already. It was nice to see that my quite crude relative pressure measurement (mm H2O) data lined up with the flow (CFM) reasonably well, in particular the CFM data also linearly increases with speed. Taking an airsaw or dremel to the lip on the intake behind the roof is surprisingly effective in increasing flow rate.
 
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