The Vert FAB is one of the best air-boxes avail
Van's FAB for the vertical isn't very efficient at all. Put together right, the fwd facing setup is more efficient even without the ram...
That's not true my friend. In fact it is very efficient air-box, especially for Carb fed engines. Van wrote several articles in the RVator in 1991 and than there where later articles about the development of the RV-8 Horz FAB. The Vert and Horz FAB where compared in Vans RVator articles. There was no major difference. When comparing them to factory planes they often have get 1 or 1.5 inches of HG more MAP. When comparing the Vans Vert and Horz FAB's, they are close. I recall the Horz pulled 0.2" or 0.3" more at top speed sea level. It's all documented (RVator) and tested. That is a one or two HP more and top speed would change little just from that MAP bump. The key or benifit of the Horz FAB is the less drag of the no scoop set-up, good for a MPH or two.
The problem with tractor engine set-up is it's right up front, there's not enough room for a diffuser or air-box plenum to take the fast air and slow it down. However both Vans Vert FAB and Horz FAB takes the air slows it down (expanding volume or diffuser). The filter in the Vert FAB filter actually straightens the air out which has benefits especially for carbs. The even airflow into the Carb venturi is goodness. If you look at the MAP we can pull compared to any Cessna or Piper (even if we slowed to their speeds) it's very good. James Aircraft cowls have a special air-box that uses a cone shaped filter. Some think that is better because the cone shape filter has more area. Filter area does not make much difference, since the resistance of the K&N filter at very high flows is tiny. 1/2 of nothing is little change, so larger filters do little for performance, except they take longer to get dirty.
What is better? Well if you go Horz induction and a straight fwd facing intake tube that is shaped like a toilet paper roll, it has no "diffuser" or plenum (also no filter). It's just a straight shot and the air does not slow down and get converted into pressure. However its a straight shot which as some advantage; its not great from a pressure recovery standpoint but than internal loss is small. Externally a straight shot tube is draggy because air spills out over the external scoop. It can only suck so much. The engine starts and stops induction wise, so the air is not flowing in one continuous motion. So a larger "reservoir" volume is advantageous (aka Vert or Horz FAB). The down side is there is no room to develop truly large volume duct/plenum/diffuser.
Vans Horz FAB solution goes up to feed air, very cleverly. The Horz FAB is the one with the filter in the left cowl engine cooling inlet, horn and a 90 degree turn into Horz FI throttle body. To go forward, there's little or no room for an air-box of any kind. Vans Horz FAB is a little better than the Vert FAB, but not by much. It does not mean the Vert FAB is bad. It just does not have the same volume. What makes the Horz FAB better is that long HORN or plenum. It takes the air from the left cowl inlet through a big flat filter. That HORN makes a "reservoir" and larger volume of air to feed the engine. The gradual plenum shape or "diffuser" slows the air down and converts it to pressure. Two necessary trade offs in the design are the 90 degree turn at the FI TB, second having the filter right of the bat. The filter deal would be draggy and inefficient if it was facing the full +200 mph airflow, however it works because the filter is in the cowl inlet. There is some pressure recovery from the upper cowl. The side benefit is no external scoop which is good for a MPH or two more, just from less external drag? Volume of air in the Horz FAB "horn" or Vert FAB "trumpet" is of great benefit as a reservoir. Engines don't breath continuously like a jet, the suck and back up.
The nice thing about the Vert FAB is the air is slowed down before going into the filter. However it would be nice if it was longer and larger, but again with a tractor engine set-up there is little room to develop a good air-box diffuser with lots of volume.
We have talked about this in other threads, but the air has to GO 90 DEGREES SOMEWHERE. Whether the air is making the turn in the air-box or making the turn or inside the sump/plenum, it has to go vertical into the engine cylinder intake ports at some point.
The real reasons in my opinion to go Horz induction are the ease of exhaust installation with 4-into-1 designs and second the NO scoop set-up. The no scoop deal is more for looks. Is $2000 worth it? That is up to you. Speed gain for no bottom cowl scoop of 1 or 2 mph is a guess, which seems reasonable. That that is what Van guessed in his RVator article I recall. The extra 0.20 inch-hg might give a hp or two more? There are high performance COOL plenums and better induction tubes, so you may make more hp than just 2 or 3? However I don't have any data on that. I think they cost more than $2000.
If going with the James Cowl that is another set of criteria and issues and options regarding induction direction.
