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I will try to answer what I can here...
Yes, on take off, with the valve closed, the air is coming from the hot side of the engine. Depending on your cooling system, it may well be +100 degrees when the engine is hot. There would be less ram air effect at the low airspeed at any rate. If your airplane is under powered, this may be a problem. I am based at 7650 ft msl, so I routinely lean for takeoff, and the warmer air does not affect my standard setting (back side of the lever is set to the "x" in mixture on the quadrant, purely airplane specific of course). When I spent a month in FL this spring, I took off with the mixture set full rich, and didn't see any difference, but then I had so much extra performance down there it was already scary.
I have been flying this system for 18 months and 150 hours, including a Sun 100 and an AirVenture Cup race. I have not destroyed my engine with detonation even though it has 10:1 pistons.
I believe the reason for the MP rise is that the air has inertia, and it would rather continue going straight into the servo than make a 90 degree turn and go out through the filter. I am sure that some of it does make it out the filter, but the MP readings I took are very repeatable with a Vision Microsystems digital MP gauge, taken off the port at the intake valve on the right rear cylinder.
As I tested the system I learned two interesting things that I didn't expect. First, that the temperature on the middle of the firewall only runs about 30 degrees F above ambient (my cooling system only). No doubt it would be hotter under the cylinders. Second, that the air in the bottom of the cowl is about .3 to .4 "Hg higher than ambient at cruise speed. So, not all the ram air gain is lost through the cooling fins.
Full disclosure...I am flying an RV8 (that I built) with a parallel valve IO-360 with 10:1 pistons, dual Lightspeed, 4 into 1 exhaust. Even at 7650 msl, and with the power loss from the warm air through the filter, I am off the ground in 1000 ft at gross, so it hasn't been a problem.
I have been lurking on this list and Legacy dreaming for some time now. I do like the technical knowledge that resides here, and appreciate the civility that reigns as well.
John Huft
Hal Woodruff wrote:
I may not be understanding the setup with this system, but it appears that filtered air is gained from inside the hot end of the cowling. With this arrangement I’d be most concerned with the effect on the power produced when you need power the most – at takeoff. I have no idea what the temperatures might be but it seems as though they’d be significantly higher than ambient. 50°, 100°, 150° more? This has got to have an adverse effect on the power output. Some other questions that come to mind is: 1. what about the possibility of detonation with really “hot” induction air when taking off on a hot day? 2. What is the effect on the mixture ratio when inducting hot air ? Wouldn’t you have an extremely rich mixture for takeoff?
I may be way off base, but the effects during takeoff of this system could be catastrophic. I personally would want to get some more information before committing to this induction setup.
To fix this, possibly, the filtered air could be baffled to be in the cold air section of the cowl. Maybe put a chamber around the filter and duct cold air from the cold side of the cowl. Just thoughts...
Hal Woodruff
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