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FM-150/Stock Snorkel: Power Reduction
So I emailed Don at Airflow Performance and he indicated that ID of the snorkel should match that of the FM-150. So the opening should be 3" and not 2.5". This has been pointed out several times in threads with this topic.
Don guessed that the power loss on the ground would be 2-3 HP and significantly more at altitude. This also will cause the FM-150 to run a bit rich at full throttle. Don said that he can test the stock snorkel if one is sent to him. I asked Van's about this issue and they said that they know about the issue with the snorkel not fitting when using the FM-150. I asked if they would send a snorkel to Don for testing and their response was that they know more people are using the FM-150 and that the engineers will redesign a new snorkel--at some point. No definitive timeline. So apparently they are not willing to send a snorkel to AP--based on their reply. I would like to do this modification tomorrow and plan to start tonight. Therefore I would prefer to not send my snorkel in to AP, since it will take a week to get back--just a guess. Would anybody that will be using the FM-150 and have the snorkel in hand be willing to send it to AP for testing to get hard numbers? Thanks |
Rather than bothering to test the undersized snorkel to get hard numbers, if I were in this situation I would just focus on modifying the snorkel to provide a transition to the proper diameter at the mount flange to the FM 150.
It would be ideal to have the whole flow passage from the 90-degree turn to the flange be bigger, but it would be acceptable to just make a gradual transition of increasing diameter up to the proper size. Just try to keep the transition as shallow an angle and SMOOTH as you can. If the diameter increases over a length of 1.5" or so, with a smooth transition from the original to the larger flange, that would be fine. |
I am modifying the opening. I am bulking up the neck to about 3.3" with epoxy with flocked cotton. Then I will use multiple layers of glass.
I just thought that people here would want hard numbers, if they could be had. |
Bulking up the outside and then trying to enlarge the inside is a very hard road.
Grab some fine grained foam, carve the exact shape desired, incorporating Steve's suggestions. Seal it, PVA it, wax it, and do a 4-ply layup. Done. Not rocket science. Builders have been modifying snorkels since about the third RV-8 ;) |
All that said, it would be cool to have Don test a couple of snorkel-filter arrangements compared to a bower type ram intake.
Dan H and others have emphasized the need for a large enough air filter. I think the general wisdom is that the conical filter in the Bower box is too small. So I am probably suffering a loss of max power on takeoff. I should note that I don't actually have a Bower box. I have the same conical filter, in a similar can, but rather than supply air for the filter side from the lower cowl plenum through reed valves, I have a duct that pulls air from the left cooling ramp. On the other hand, I only use it for ground-roll and initial climb, so it is only field length that suffers. Is the rectangular filter used in the cooling inlet shelf big enough? Are the velocities going into that filter significant enough that it would benefit from a radiused inlet lip treatment? Does the snorkel itself, which has a pretty tight turn with modest flow diameter, produce much loss? - assuming smooth transition into the fuel servo. Or not. |
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You could use the approach Dan suggests. That works great. The one down side is if you already have a nice fit of the orientation of the flange that is just the way you like, cutting it off and re-doing that as part of the reshaped transition may be frustrating, and of course you need to have access to your airplane. Maybe the airplane is at the hangar and you would like to do this re-work at home in your garage. A middle approach would be to surround the existing snorkel neck with some foam, just glue rough pieces on with a hot glue gun. Then shape it, seal it, and glass over it. Then when you grind out the inside wall, you will break through into the foam, which will carve out really easily and leave the nice smooth transition shape that you sculpted onto the foam. Like I said, lots of ways to skin a cat. "When hot glue guns are outlawed, only outlaws will build with composites" |
bulking up neck with "Schmoo" to about 3.30" and then glassing over was uneventful. Keep in mind the desired ID was about he same as the original OD of the neck. SO I really did not get into the schmoo that I put in. I had the glass layers go all the way out to the edge of the flange.
I also used a skim coat of PTM&W ES6247 epoxy between the schmoo and the glass. This is the stuff that Scott, who teaches the EAA fiberglass glass, suggested using. Sticks like **** to everything. It is a great tie coat. Grinding out the inside with a dremel tool with a cutting tool (a kit from china--not sure where I got them, but they worked great) and then a stock sanding wheel took about an hour or so. Should I skim coat the inside of the snorkel where I sanded with epoxy? Or is that a useless endeavor? Thanks Ken |
I am following this with great interest. It would be really nice to get some actual data and comparison. If it yields enough improvement, it will certainly be worth the effort, it also gives us some to do during the down time.
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well too late for me to get the data, since I ground mine out.
Maybe someone else will step up. There has to be someone interested that will be using the FM-150 and not yet to the point of putting on the snorkel. |
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Thinking out loud, I would think that a good set of data would be driven by first having it flying with the stock size, 2.5" and then enlarge it to the FM150 size of 3" and then get the performance data. I am also wondering if the shape of it will make much of difference, although we are only enlarging it by .5" Am I off the mark on this? |
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