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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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That’s why those of us who use the stock IC run 3 chamber shrouds that pull air from the side scoops. The rooftop scoop just isn’t that efficient.
 

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There is a ton of good info in this thread;
 

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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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@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.
It’s cool that your data is similar to the previous study! I also really like your T-top!
 

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2006 Exige
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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 View attachment 1306872
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cooling all that much, but again would appreciate comments from more expert individuals.
The reason you’re results are disappointing is air cannot be compressed until you get well over 200mph.So when you have a big intake with a restriction downstream your screwed.it becomes all about trying to get the entire intake path as large as the input in cross section.Impossible on our cars.
I know this is extremely counter intuitive.My son and I were doing some research while making a wing for a race car.He was reading air can’t be compressed below 200+ mph just as our compressor kicked on.Very confusing moment for me.They were talking about a wing which is in free air.Anyways Google it as there are good explanations there.
 

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Then there is the matter of the air being taken in by the mohawk roof is heated air coming up over the windshield from the radiators.
 
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I think I saw this car and T-Roof at the Secret Car Meet a month back or so in San Diego (Rancho Santa Fe).
Very interesting,
Joel
 
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