Good Question - lets go to the dead sea scrolls
Geo, I liked that answer but I have a question.
Looking at the snorkel thing that takes the air from the left inlet directs it down to the inlet seems like a good way to lose some power. (yep but it does not)
Wouldn't it be better to go with the horizontal induction AND use the lower cowl for the carburetor?
(Of course you would have to take a deduction for looks.)
With this set up, I wonder if you would gain some MP, maybe 1/2" or so from the ram effect. (1/2" of what relative to what)
Thoughts?
How do the French say "Contraire Mon frair" (Bart Simpson says it as well).
From a gut intuitive glance you would think Van's Horz FAB, with that long snorkel and 90 deg turn, would be less efficent, but it is not. It is a tad better then the Vert FAB, only a 1/10 or two MAP (in-hg). That gain is worth a HP or two, which gives a fraction of a MPH increase in speed. No big deal. The no scoop drag reduction is the real gain. The point is the Horz FAB is NOT bad and would be hard to improve upon significantly.
DON'T take my word for it. Go and find the back issue of the RVator with a lengthy detailed article written by Richard Vangrunsven about the development of the Horz Air-box. I don't think Van was certain the Horz Air-box would work at first, but he built and tested it. It works. The cost of a horz plenum on a 160/180 HP angle valve is not trivial from what I recall. If buying a new engine you have the option. If going 200HP angle valve they are all Horz induction. It comes down to personal preference, financial, overall configuration and goals.
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Why does Van's Horz air-box work so well?
First look at Van's
vertical FAB. It slows air (trumpet shape diffuser), air goes past the filter at lower speed & higher pressure, than enters the Carb/FI with a smooth 360 degree flow. Brilliant and not that different than my 1969 Chevy stock snorkel air cleaner housing sitting on the carb.
For the Horz air-box, Van was clever to used the recovery of the exisiting cowl inlet as a scoop. He knew there was good pressure recovery in the first 6"-8" of the cowl, a "WALL" of pressure across the cowl inlet, where the air stagnates. It turns out to be an excellent place for pressure recovery. Often you see oil coolers in this location on some planes. Why not use the cowl inlet for double duty, engine cooling and induction.
One side advantage the HORZ FAB has over the VERT FAB is no leaks. That is hard to seal the Vert FAB and cowl with soft soft seals and relative movement.
Once the air high pressure air goes through the flat K&N filter, it has a large volume box to feed the engine smoothly with out turbulence, a nice diffuser (like a horn) from large to small. YES there's loss with a longer induction. Yes the 90 deg turn at the bottom is not ideal, but some of that is compensated for with nice smooth curved surfaces.
Engine induction flow pulses, start and stop as valves open and close. The long diffuser acts as an air reservoir (eg, hydraulic accumulator). The engine does not suck like a jet, continuously. The air has inertia that ripples back from the intakes ports as the valve closes. You want POS pressure and VOLUME behind the intake valve as it opens. This is where "tuned" induction and tuned exhaust come in, to control these pulses. The air box is an extension of the "tuned intake" which is really an extension of the "tuned exhaust". You want pressure and VOLUME in your air box if you have room.
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Van's Horz air-box has NO scoop, so it wins the min external drag competition. This is the MAIN benefit of the Van HORZ FAB design is no scoop, plus it looks cool.
I just don't think its worth +$1000 to buy the Horz engine induction. I could be wrong about the price premium. If its a IO360 (200HP) Horz induction is standard. If you have a angle valve (horz induction) there is no option from Van. We are talking about 1 or 2 mph more speed with the so scoop. So even if you lost a 1/10 in-hg MAP it would be a plus. As you point out exhaust 4-into-1 is easier with Horz induction.
I went Vertical because I have a carb, but I also wanted a SHORT cowl allowing me to use a standard Hartzell hub prop. I also wanted a Sam James style of cowl (modified stock cowl). So any one of these factors pointed to vertical induction. I will gain at least 6 mph with my SJ style cowl (modified stock), making up for the "ugly" scoop.
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At 175 mph (TAS) cruise you have about (1 ) inch-hg pressure, if you recover 100% of all dynamic pressure. Sadly there aren't many things 100% efficient, including RV scoops and air-boxes. As you know slowing air increase pressure, ie Bernoulli. It takes some room to do that, ideally a gradual change in area to avoid turbulence.
There's about 0.50 in-hg internal engine loss from FI throttle body to the intake valve. If MAP us equal to ambient pressure you need at least 0.50" recovery. Most RV's have a MAP of 0.00 to +0.30 inch-hg relative to ambient pressure. That means total efficiency of the scoop/air-box is around 50% to 75%, which is very good. You're going to have a hard time doing significantly better with the physical limitations.
Higher MAP gains for some RV's are because they're much faster; they have more speed & dynamic pressure to work with. When comparing "RAM RISE" you have to know TAS & density altitude. 210 mph TAS is +1.50 in-hg, another 0.50 in-hg over 175 mph. The angle valve 360 Lyc (200HP) has less internal induction loss I assume. A 220 mph RV-8 with a 200 HP lyc will have more MAP regardless of airbox.
Look at Cessna and Piper performance charts. You're lucky to have minus 1 in-hg MAP in cruise, in part due to lower airspeed, less pressure to recover and a terrible air-box and poor filter location. Cessna plasters the air-filter on the nose bowl. The air hits it at high speed; the filter is like a brick to the air, bouncing off, with little or no pressure recovery.
Notice both of Van's FAB have pressure recovery (slowing air) before the filter. It's not so obvious with the Horz FAB but trust me, the engine cowl inlet is a scoop with good pressure recovery. This is where the Horz FAB filter is.
A note about filters. The K&N AIR FILTER have very little loss,
about 0.05 in-hg at rated flow and CLEAN AIR FILTER. Clean your filter often. Running no filter is not recommended for daily flying in my opinion. Actually air filters straighten out the flow and can be better than no air filter.
How do you slow air down? You need ROOM and a "diffuser", a classic BELL MOUTH shape, going from small to large with a nice gradual curved surface. The mouth of a carb / FI is not large, so you than need to transition back down to the mouth of the Carb or FI. Designing a good air box is not easy.
