What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

FM-150/Stock Snorkel: Power Reduction

Stockmanreef

Well Known Member
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.
 
Last edited:
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.

Ken, with composites, there are lots of ways to skin a cat. The approach you describe will work, but grinding out all that flocked resin (we call it schmoo) will not be fun. It is hard, durable stuff. You will have a hard time telling when you have reached the end of the schmoo and have started cutting into the fibers of the new outer shell. You will have a hard time sculpting a smooth transition shape working on the inside.

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"
 
Last edited:
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.
 
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.
 
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.
Can you please elaborate on this and what is it involved in getting the data?

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?
 
Can you please elaborate on this and what is it involved in getting the data?

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?

Mehrdad,

The idea would be to send samples to Don Rivera at Air Flow Performance and have him test them on the flow bench. He would measure the pressure drop across various snorkel configurations.

The shape of the transition between the 2.5" and 3" bores certainly matters a lot. Nothing worse than an aft-facing step -- that is, having the tube abruptly expand from 2.5" to 3" diameter going downstream with a square step.

Ideally, go farther upstream in the snorkel where the diameter is bigger, and make a smooth transition without any restriction/reduced diameter. Second best is to gently expand out from the smaller diameter to the 3" diameter over as much length as you can.

The need for the gentle transition is that you are making a diffuser that is slowing the flow down from the higher velocity in the smaller diameter section, and you want to effectively slow it down with no flow separation to get the full available pressure increase. What happens with the aft-facing step is that the flow separates and you just have a jet of the higher velocity going into the larger diameter tube. The jet breaks down through turbulence so that you end up with the slower velocity, but no pressure increase. The available pressure increase was wasted in viscous dissipation.

At the same time, I would love to have Don test with and without the filter, and with the filter with a small radius lip around the perimeter to help the flow turn over the corners and down into the filter.
 
Last edited:
I have 2 snorkels

I have 2 snorkels and live in SC .. willing to send one in for test. I?m faced with the same issue anyway with A FM-150. Just learned about this on Saturday.

Rob
 
Results

Got the results back from Don today.

I sent him an unmodified stock snorkel.

“An airflow test was performed on the subject air duct to compare pressure loss through the duct when attached to a standard RSA-5 and FM-150 fuel control. Calculating airflow for an IO-390 the maximum rated airflow would be no more than 1400 PPH. The following charts show airflow through the respective fuel control without the duct installed and with the duct installed. Flow numbers do not account for air filter loss as this part was not supplied. Carb loss numbers are in inches of water. Airflow numbers are for air at standard sea level air.

WIDE OPEN THROTTLE FLOW TEST WITHOUT AIR DUCT INSTALLED

Airflow PPH - 600 - 800 - 1000 - 1200 - 1400
RSA-5. 1.5 2.6 4.3 6.4 8.8
FM-150. 1.5 2.5 4.0 5.9 8.0

WIDE OPEN THROTTLE FLOW TEST WITH AIR DUCT INSTALLED

Airflow PPH - 600 - 800 - 1000 - 1200 - 1400
RSA-5 2.0 3.4 5.3 7.6 10.4
FM-150 1.8 2.9 4.7 6.8 9.5

Flowing the duct at 1400 PPH airflow with nothing attached to it gave a loss of 6” H2O. The duct has an opening at the flange that would attach to the inlet of the fuel control of roughly 2 5/16” diameter. The FM-150 has an opening at the inlet of the fuel control of 3” diameter. Taking the area change of 2 5/16 and opening it up to 3” without accounting for any other flow losses in the duct, the loss through the duct would go from 6” to 2.1”. Doing this on the installation using an FM-150 would more than likely eliminate the added loss from the duct. This would be the equivalent of an additional 8% airflow to the engine.”


I plan on sending a modified snorkel to Don once we get one completed for comparison.

Hope this helps the group!

Rob Arduini
 
Last edited:
Sorry looks like the formatting got messed up once it posted. the 2 digit numbers should be under each of the PPH. Ex. 600 should have 1.5 under it, 800 should have 2.6 and so on.

Let me know if it?s not clear and I?ll try to fix it.

Rob
 
Raven31

....Doing this on the installation using an FM-150 would more than likely eliminate the added loss from the duct. This would be the equivalent of an additional 8% airflow to the engine.

I plan on sending a modified snorkel to Don once we get one completed for comparison.”

The math is understandable, but there's nothing like an actual test. Did that ever happen? If so, would you mind posting the numbers? Decision time for me. Expanding the base of the snorkel isn't to tough, but the transition on the inside is another story. For 8%, I'd do it. For less than 5% I'll probably leave it alone.

With no additional data, I'll stay with the existing/standard configuration as it has a nice transition on the inside.

Thanks,

Fred
 
Back
Top