29 Aug 2016

First off, my brother did very well, placing third out of approximately 50 cars with a time of 3:22; the only car that beat him was that 1000 hp Nissan GTR and a McLaren 675LT. He took his GoPro 3 to record his run and not very surprisingly, it screwed him over by refusing to power up even on a full charge. He said he’s completely fed up with GoPros and I can’t say I blame him.

Someone asked why a turbocharged car retains more power at altitude than a normally-aspirated car:

As altitude increases, two things changed in favor of a turbocharged engine. First, lower air density means that the compressor has less load, so it naturally increases speed until it reaches a new equilibrium. Second, there’s also less exhaust back pressure (less pressure the exhaust has to push against in order to exit the exhaust pipe), so the turbine sees a larger pressure differential and spins faster as well. Both factors cause the turbo to adjust itself to a higher speed that self-compensates to the higher altitude. It’s not perfect though because the turbo is now compressing air at a lower pressure to a larger factor (a larger pressure ratio). Compressing the air more raises the temperature and in addition, the intercooler has less cooling air flowing across it, so the air entering the engine is warmer than it would have been at lower altitude. Even so, the end result is that a 400-hp turbo engine (measured at sea level) will still make about 370 hp at 6000 feet (without active control to make more), while an equivalent normally-aspirated engine will make about 330 hp.

There is however a negative aspect of turbocharged engines that canĀ even the playing field: heat-soak, where the intercooler can’t keep up and the air entering the engine gets so hot that the ECU must back off on boost. This is a fairly common issue for lower and mid-market turbo cars, where they simply can’t stay at full boost for longer than perhaps 10 seconds. Most of the time that’s perfectly fine, but put that car on a 3-4 minute hillclimb and whatever power the manufacturer promised likely isn’t there at the top of the hill.

In other news, regarding the intercooler ducting and it possibly becoming part of a roof, I didn’t realize how much I use the roll cage when getting in and out. Getting in without the cage’s help isn’t bad, but getting out is, well, entertaining to onlookers; I’ll have to try some other techniques to see if it’s just a matter of a different habit or if it’s a real issue. I haven’t firmly decided which way to go for the ducting, but the list is getting short. The sightline looking into the rearview mirror passes about 1.5″ above the intercooler, so regardless how the ductwork routes, it’s going to block some portion of the view. The difference between the approaches is how much it blocks. It turns out that having inlets at the top corners of the main hoop are even worse because they’ll completely block the blind spots, arguably more important that the view directly behind. Of course it could be argued that that’s what side mirrors are for, which I don’t have. It’s a fair point; I just want to see how much visibility I can retain with some thought rather than chucking it all out the window right at the start of this. If side mirrors become a necessity, so be it.

A derivation of the above is having the inlets at the upper corners turn the air sharply inward, combining together above and just forward of the intercooler before turning downward. I’ll have to mock it up but already think it’s more work than just having the inlet above the windscreen.

Another suggestion was having scoops down low on the sides of the chassis; it’s what “real” mid-engine cars use, but due to Midlana resembling a Seven, there isn’t room between the engine and main roll hope for large ducts to pass through. Even if it could, then what? It would have to raise up out of the engine compartment to feed air into the intercooler from above. Of course, another approach would be feeding that same air into the bottom of the intercooler and assume there’s sufficiently low pressure above to allow it to flow out. Since there’s no room for ducting however, this approach can’t happen.

Lastly, there’s picking up air from under the car – again, there simply isn’t enough room for large ducts to pass air up past the engine.