Vert is fine, Horz is more for aesthetics
tvlawyer said:
can someone 'splain me the difference between horizontal-forward facing induction and vertical induction? why is FF about $2k more and is it worth it?
The splain is this, FF induction cost more and does very little, except looks cool if you use Vans airbox which eliminates the lower cowl scoop. That is it, it does
almost nothing, at least for a RV. Most important you have to decide everything you want up front:
Prop
Airbox
Exhaust
Carb of Fuel Injection
Engine (parallel/angle valve)
Cowl cooling inlets (stock) modified (round)
Brand of cowl, stock, SamJames, Show Planes
Fwd facing induction does not work well with all combos but better with others.
Why FF induction exist in the first place? It was made for light twins that had shallow cowls and nacelles. It allowed tighter cowls, better transition to the wing with lower frontal area. On a RV you have a firewall the is X by Y and you really are not reducing the frontal area. Mooney being the forward thinking folks adapted FF for their single engine planes with some good and bad.
Myth: There is much more "ram" are with the FF induction? Well may be, kinda of, sorta of but not really. Look all the air is going horizontal and all the air has to go vertical and to each cylinder at some point. In other words all the air has to make a 90 degree turn to get to the cylinders at some point. Whether your FI throttle body happens to be vertical or horizontal does no matter much to the throttle body. Of course with a Carb Horz is academic, unless you have a pressure carb, which you should avoid like the plague, expensive and rare.
If you want to "convert" to horizontal with a Vert sump there is a cool adapter tube you can by that makes that +90 turn right at the sump and mounts the FI TB horizontal.
http://www.lazy8.net/intakesystem.htm He has some claims of pressure gain with his system that I have a hard time believing but he is pretty careful in his flight test. The good part is it gives you more room to make an in-line airbox. The down side however is whenever you deviate from the PLANS is you make more work for yourself. So if you use vans airbox and cowl you make less work. Do something different, weird airbox, other cowls (Sam James) you may paint yourself into a corner. Its all good but beware of making a snow balling increase in work. 99% of the builders are best served sticking with known combos.
Overall Vans horz airbox/no scoop cowl is pretty, and you may pick up a MPH or two from not having a lower scoop. (Vans airbox gets air from the left cowl inlet.) It also does petty good in recovering air for induction; more correctly it is has minimal loss and holds it own. Part of the reason is that long Ram horn diffuser (duct) that goes from the flat filter to the FI TB. Cooling is NOT an issue. Those stock cowl inlets are way big and induction air takes very little air. The down side of using Vans Horz airbox is you must USE Vans cowl**. Personally I'm a fan of the ROUND INLETS. Sam James cowl will not work with Vans Horz airbox. However you can put a scoop on the SJ cowl and use Vans Vert FAB just like stock.
**These guys have a RV-8 cowl with round inlets and retains Vans Horz airbox. How they did it not sure.
http://www.showplanes.com/index_1024.htm (If building a RV-8 I would consider this cowl with Vans Horz airbox and FI.) Again you have to think it ALL through.
If going Vertical sump you have lots of options as well, but one thing for sure is you will likely have a scoop on the bottom. Van's and SamJames will work. I think the round cowl inlets and sealed plenum are one of the biggest single GAINS you can get on a RV. What little gain, if any, there is of FF induction is no where near what gain you get from lower drag with the round cowl inlets.
The problem with having straight shot Fwd Facing induction is there is no room to develop a good diffuser (air box to slow the air down) and / or filtration. But Dan the man has a work around on his site that is cool (read below).
Dan the man has a cool set up where he has both Van's airbox and a RAM air induction (by pass of air filer). This adds some complexity and build time and weight but its one way to go. He says he gets more RAM air. Well partly the gain is just from by passing the filter. Of course he adds a scoop into the cowl which may cause some drag? (very little but something).
My opinion is Vertical is pretty good for most builders and $2000 is a stout premium for elimination of the scoop. I like the scoop my self. Also really the scoop does not add much if any frontal area since you still have the cowl and cowl air exit frontal area.
My Preference drove my choice:
I wanted 4 into 1 and Round cowl inlets. Horizontal induction is easier 4 into 1 exhaust routing. With out the Carb/FI on the bottom of the sump the tubing run on a 4 into 1 exhaust is straight forward and you can buy them off the shelf. I went with a Carb because that is the engine I had, a O-360A1A. So I made a custom exhaust 4 into 1 exhaust (more work but cost wise I made out as a bandit, about $600 at the time, see pic below.)
You now can buy off the self 4 into 1's for RV's from several suppliers but most are made around FF induction, but I am sure that has or will change with time and demand. If you have a Carb, 4 into 1 can be done. Even Trikes (RV "A" models) have 4 into 1's available now, which is a good trick considering the nose gear being in the way!
The other MUST HAVE for me was the the round cowl inlets and sealed plenum. One down side of Sam James cowl in the past with a forward facing induction required you use a fixed prop with a longer extension or a special (less available more expensive) constant speed prop with an extended hub (further out from the cowl). With out the longer cowl the SJ suggested airbox did not fit. Its a mess. Sam James I think has changed his cowls so extended props are not needed with FF induction? When I last checked he was working on it, but again, deviate from stock combinations or parts you are asking for more work.
With that said, YOU MUST use Fwd Facing fuel injection IF you want that no scoop look. That will force you down the road of a specific combo of parts. However either Vert/Horz gets the job done. If the no scoop look is your thing, than write a big check. If you want a Carb, do deal, choice is made. If you are flexible consider the Vert just to save money and make it easier to build.
). Lycoming FF sumps, like the one on my angle valve still route the intake tubes thru the oil pan.
