11 July 2016

I forgot to mention yesterday that I did some quick “hand sensor” airflow experiments about where to get air for the intercooler. There are three possible locations: each side of the main roll hoop, above the top edge of the windscreen, and above the main roll hoop.

Each side of the main roll hoop

I was unable to reach back to feel what the airflow is to the sides of the main roll hoop. I would like to retain rear visibility and some might wonder why I care. Try tipping your rear view mirror up in your commuter car and driving to work that way; it may be a little thing but can be a nuisance. Granted, side mirrors can be added to handle losing the rear view mirror, but it would be nice not to. Retaining rear visibility seems to argue for side inlets but there are two strikes against that approach. First, there’s only a couple inches above the intercooler before it intrudes into the rear line of sight. Also, just having side scoops completely block the center mirror’s view of the car’s blind spots, so going with side inlets isn’t an automatic winner. In fact, depending upon the size of the side scoops, they may even block rear visibility of the side mirrors as well. Before writing it off entirely, airflow needs to be measured.

Above the top edge of the windscreen

The airflow is strong and very smooth as expected and is the clear winner airflow-wise but means running a large duct back over my head and down to the intercooler. Visually it’ll be a big deal, but having a wide scoop along the front edge means it could be fairly low profile. I’ll have to do a mockup to see how wide it can be before it gets in the way of getting in and out. Of course, a shift in perspective is to consider it a feature – a roof. Depending upon shape, it could serve to cut down on both wind flow and perhaps even as hard points for mounting future doors. It would take the most fabrication and time though.

Directly above the main roll hoop:

Airflow was also good though about 6″ higher due to turbulence from the upstream chassis tubes and windscreen. Right below the main roll hoop the airflow was flowing forward which wasn’t surprising, but it was still impressive how airflow could be opposite just inches apart. Anyway, this is the most direct and shortest (and lightest and least labor-intensive).

Regardless where the inlet is placed, a good first step would be to use manometers to measure pressure differential. Take a length of clear tubing, staple it to a board in a U-shape, fill it slightly with water and then run each end of the tube to the test points. Those points could be both sides of the intercooler, or for an even more direct comparison, measure differential pressure between the three inlet points, looking for the highest pressure of the three.

For mock up purposes, cardboard can serve well, cut and taped to the intercooler with plastic drier ducting serving to route inlet air. It’ll look odd but I’m used to that 🙂 Or just go straight to fabrication, making it from aluminum sheet or composite.

What I don’t know is how large the inlet need to be; obviously the bigger the better but at some point it’s a case of decreasing returns. I haven’t found any equations or rules of thumb for how large the inlet should be relative to the frontal area of the intercooler.

Lastly, air to the air filter needs to be picked up from somewhere other than the engine compartment – I just haven’t gotten around to it. As it is now, at the end of the test drive yesterday, inlet air temperature was 46 degrees C (with no boost) so it needs to change. It’ll have to come from either the same ducting feeding the intercooler or a separate inlet to the top of the engine cover.


27 Feb 2009

The gas tank is tentatively done, in CAD at least. By moving one side a little, capacity is now 16 gallons! This is enough for just about any drivetrain builders might want to use and gives the option of cutting it down if not as much capacity is needed. Still waiting for the fuel level sensor, and have to place the various external hose fittings, but it’s getting there.

I had a discussion with a race car designer who feels that the flat diffuser is better. The Katz book (figure 6-39 if you have it) shows that downforce increases with a diffuser angle of up to 12 degrees, while drag actually drops at angles up to about 4 degrees, then increases up to 12, but remains lower than if there was no diffuser. McBeath, in, Competition Car Downforce, discusses curved diffuser design but doesn’t provide data (which is understandable since aero design doesn’t transfer easily between vastly differing cars.) I don’t have a big-ass wing helping to suck air out through the diffuser, though the Katz data does not either which is good for my application. So, since Katz has actual quantifiable data, and because Kimini felt very stable at high speed, I’m going back to the flat design.