Scoop vs snorkel

[RV6_flyer]

I have an O-320 with carb and the standard Van's cowl with FAB.

During flight testing, I ran the following test.

Climbed to 10,000' MSL. Slowed to minimum controllable airspeed. (Somewhere around 40 KIAS in my airplane.) I read and recorded MAP pressure. I advanced throttle to WOT. I waited. After airspeed stabilized for a few minutes, I read MAP and recorded it.

Back on the ground I calculated that there was 0.5" additional MAP at full speed than there was at slow speed. This is only one data point, one altitude, and one aircraft.

IIRC, there was an RV-7 with an IO-360 200 HP engine that had forward facing ram air inlet, valve, and Van's snorkel that did testing of MAP with both. It was a lot of work to get the valve to not leak so that air flow was one or the other. He reported very little difference in MAP with the two different setups. This was RAM air compared to Van's snorkel. The FAB that Van's sells is a divergent duct and in theory should have a pressure increase as the air flow speed decreases. (The opposite of a venturi where the increase in speed decreases pressure.)

[PilotjohnS]

Originally Posted by Bill Boyd

Work in progress. SCAT will be trimmed to length and terminated with a ring of self-fusing silicone tape to mate with the FAB opening.

Are you making a mold ? How do you undo the scat tube to pull the cowl? Maybe i will copy seems like a nice idea. Thanks for sharing.

[wnplt]

I have YIO360-M1B FI no scoop and I get 21.3 MAP at 9500 and 19.9 MAP at 11500.

[DanH]

Originally Posted by wnplt

I have YIO360-M1B FI no scoop and I get 21.3 MAP at 9500 and 19.9 MAP at 11500.

At what TAS and RPM?

[Bill Boyd]

No mold necessary. Piece was cut from the front snorkel, flange was molded into the cutout opening, cutoff wheel kerf was filled with micro to make a tighter fit. PVC was fitted into the air scoop opening and Bondo'd in, SCAT fitted into the PVC stub with Permatex Ultra gasket silicone on outside of tube. Fiberglass over-wrap on the scoop, down into the throat of the intake, bonding the SCAT on the inside surface. This made the piece thicker that stock, so micro was used to build up the corresponding profile on the lower cowl to equivalent height.

FAB was chopped shorter than standard, more schedule 40 was fitted into the opening and a fiberglass upper transition was molded over clay to complete the transition into the FAB body. More fiberglass to hold the PVC stub in place. Sanded a chamfer on the inner edge of the PVC to give a tapered mouth for the SCAT to plug into in service (under slight longitudinal compression is the plan.) . I have more to do here and will post up pictures when done.

The idea was taken from elsewhere on VAF. Fellow named GregMac.

[wnplt]

Originally Posted by DanH

At what TAS and RPM?

2400 RPM

165 KTAS

[RV8JD]

FWIW, here are a couple of data points. RV-8, YIO-360-M1B, 1 Mag & 1 EI, Snorkel, at WOT:

21.5" MP, 2450 RPM, 9500' MSL, 10771' DA, 208 MTAS/181 KTAS.

20.0" MP, 2440 RPM, 11500' MSL, 11985' DA, 205 MTAS/178 KTAS.

[NewbRVator]

Originally Posted by JohnInReno

If you split the scoop intake material so that half is connected to the lower cowl, it makes it much easier to remove the cowl.

That's a great tip. (Fresh on my mind after decowling and recowling today)
I'm thinking it will save me money from the PTSD counseling sessions if I convert soon!!!

[DanH]

Originally Posted by wnplt

I have YIO360-M1B FI no scoop and I get 21.3 MAP at 9500 and 19.9 MAP at 11500.
2400 RPM 165 KTAS

Originally Posted by RV8JD

FWIW, here are a couple of data points. RV-8, YIO-360-M1B, 1 Mag & 1 EI, Snorkel, at WOT:

22.0" MP, 2450 RPM, 9500' MSL, 10771' DA, 208 MTAS/181 KTAS.

20.5" MP, 2440 RPM, 11500' MSL, 11985' DA, 205 MTAS/178 KTAS.

Perhaps some manifold pressure calibration is in order. I have assumed standard day temperatures, but...

Steve, at 9500 and 165 KTAS, static pressure is 20.983" Hg and maximum available dynamic pressure is 0.977. Total is 21.96" and you're reporting 21.3", for a difference of 0.66. At 11,500 and 165 KTAS, it's 19.412" plus 0.918, for a total available of 20.33. You report 19.9, so the difference 0.43.

Here's the thing. Manifold pressure is typically measured at the #3 intake port. There is an intake tract pressure loss between the intake at the front of the airplane, and the measurement point at the intake port.

I've measured it; the MP loss for my horizontal intake is 14.4" H2O, or 1.06" Hg, at 2500 ft and WOT, with one pressure tap in the top of the airbox prior to the filter, and the other tee'd into the standard MP line. Some years ago, Don Rivera measured pressure drop for the airbox and filter on his Superflow at about 5" H2O, so the loss for the remainder of the intake tract (throttle body, intake plenum and intake tube) is about 9.4" H2O, or roughly 0.7" Hg.

An M1B has pretty much the same horizontal intake plenum and tube arrangement as the IO-390. Your tract loss should be very similar. The filter loss should be larger, as a standard Vans-supplied K&N filter for a snorkel has less area.

In addition, it is not possible to convert all the available dynamic pressure to static pressure. A good RV cooling setup might capture 0.8 or 0.85 of it at the snorkel inlet. Bad cowling inlets, sloppy baffles, and poor sealing can push it down around 0.6.

So, manifold pressure only 0.66" Hg less than the absolute maximum available seems unlikely. Best case, you lose 0.2" of the available dynamic pressure to a reasonable pressure recovery coefficient, probably 0.4" to the filter, and 0.7" to the intake tract and fuel control. Carl's numbers look even less likely....thus a suggestion of MP instrument error.

[JonJay]

The early kits 3,4,6, cowling was geared toward vertical induction.... carb, throttle body, etc.... hence the scoop.
Starting with the 8, then 7, we started to see the shift to horizontal induction, primarily fuel injection. You could do either.

My personal preference is the scoop on the older models, and not on the newer kits, although I think both look good.

At the time, almost twenty years ago, it was a $2k delta to go with the horizontal "cold" air induction, vs vertical induction for the same injected set up.
So, for me, I wanted the scoop for the classic look and saving some $$ was ok too.

"Cold Air Horizontal Induction" was a big buzz back in the day claiming an increase in performance from not heating the induction air as it passed through the sump. Not sure if that is still that case for all HZ induction sumps or not today or how that might affect ones decision to go with a scoop or not.