Many builders have installed ducts to close out the roof area just after the external roof vent inlet, running it to either an engine intake, or just generally to the engine compartment (as I have done).
There has been some discussion about whether or not the roof vent is functional on the SLC. Some have theorized that it is in just the right place for air to tumble over the top without actually getting into the vent. Others have installed small aircraft-style vents in the duct so air can flow into the interior, hoping that the pressurized (in theory) duct would be a source of cabin fresh air.
This week a couple of SLC builders were chatting about the idea, so we decided to Science It Out, and get some real data.
Or at least as much as we could get, in a lazy, redneck sort of way.
So out came my wife's yarn (I selected a particularly lurid shade of green, hoping for max contrast against the blue paint on my SLC), some tape, and scissors, and in a few minutes, we had our official aerodynamic test rig. I taped up about a dozen 4" strands of the yarn above the vent, just before it on the flat area after the windshield, and also a few just at the top of the windshield in case there was any funny business going on up there.
You'll see the evidence in the video below, but the short version is that at highway speeds (up to around 70), the vent does appear to be functional. At speeds encountered in the test, the air flow, as indicated by the yarn strips, appeared to clearly be flowing into the vent, and in addition, seemed remarkably attached to the top of the body as well- at least as far as the strips were installed. At the top speeds in the test, you could see the yarn strips at the vent opening just beginning to lift.
We don't know what would happen at higher speeds, but suspect that the vent may become more, or even completely occluded. But that's another test. In the meantime, it's clear that it is indeed functional for most street use.
I wouldn't be a Man Of Science if I didn't mention some complexifying factors: my car has a home-made CF Gurney flap, and an enlarged rad opening. So that may have something to do with the results, though I would expect that the generated turbulence from the flap would tend to disrupt the airflow, and make it less likely to remain attached to the body after it flows over the windshield. Another factor is that the test was done with the standard glass windshield. As some builders know, there is a subtle difference in the shape of the glass vs the Lexan windshields, with the glass one being flat along the centerline of the car, as opposed to the Lexan ones which retained the designed-in curve. That, too, should have made the air less likely to remain attached to the windshield, and the upper body where the vent was.
Thanks to Ed Davis, who patiently drove the camera car, took the video, and managed not to hit anything in the process!
Overall, it was an interesting test, and there may be more coming- stay tuned!
For some reason, the video is 5 minutes long- but there really is only content for about 30 seconds...
There has been some discussion about whether or not the roof vent is functional on the SLC. Some have theorized that it is in just the right place for air to tumble over the top without actually getting into the vent. Others have installed small aircraft-style vents in the duct so air can flow into the interior, hoping that the pressurized (in theory) duct would be a source of cabin fresh air.
This week a couple of SLC builders were chatting about the idea, so we decided to Science It Out, and get some real data.
Or at least as much as we could get, in a lazy, redneck sort of way.
So out came my wife's yarn (I selected a particularly lurid shade of green, hoping for max contrast against the blue paint on my SLC), some tape, and scissors, and in a few minutes, we had our official aerodynamic test rig. I taped up about a dozen 4" strands of the yarn above the vent, just before it on the flat area after the windshield, and also a few just at the top of the windshield in case there was any funny business going on up there.
You'll see the evidence in the video below, but the short version is that at highway speeds (up to around 70), the vent does appear to be functional. At speeds encountered in the test, the air flow, as indicated by the yarn strips, appeared to clearly be flowing into the vent, and in addition, seemed remarkably attached to the top of the body as well- at least as far as the strips were installed. At the top speeds in the test, you could see the yarn strips at the vent opening just beginning to lift.
We don't know what would happen at higher speeds, but suspect that the vent may become more, or even completely occluded. But that's another test. In the meantime, it's clear that it is indeed functional for most street use.
I wouldn't be a Man Of Science if I didn't mention some complexifying factors: my car has a home-made CF Gurney flap, and an enlarged rad opening. So that may have something to do with the results, though I would expect that the generated turbulence from the flap would tend to disrupt the airflow, and make it less likely to remain attached to the body after it flows over the windshield. Another factor is that the test was done with the standard glass windshield. As some builders know, there is a subtle difference in the shape of the glass vs the Lexan windshields, with the glass one being flat along the centerline of the car, as opposed to the Lexan ones which retained the designed-in curve. That, too, should have made the air less likely to remain attached to the windshield, and the upper body where the vent was.
Thanks to Ed Davis, who patiently drove the camera car, took the video, and managed not to hit anything in the process!
Overall, it was an interesting test, and there may be more coming- stay tuned!
For some reason, the video is 5 minutes long- but there really is only content for about 30 seconds...
. I would suggest to anybody building a no AC minimalist car to use that roof vent for the passenger compartment fresh air. Due to the aero efficiency the side windows are completely useless for this purpose, that roof vent is an excellent option.