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Is my air box costing me performance?

00Dan

Well Known Member
My old -4 which predates the current Van’s FAB has some intake loss issues. The builder apparently decided to hack off the snorkel on the old FP cowl and instead fitted a Cessna style bracket filter box with a K&N filter. I did some testing and found the filter is good for 0.1” restriction, not a big deal. The overall pressure loss is measurable though.

With the filter installed and on the ground, full throttle MAP was 27.7” while ambient was 28.98” (measured by setting altimeter to 0’ and reading the setting). In the air at 7500’ pressure altitude full throttle achieves 21.4” MAP. Sensor calibration was checked with engine off; it indicated 29.0” (it only reads to one decimal place).

Am I leaving measureable performance on the table? I’m debating if retrofitting a FAB or equivalent is worthwhile.
 
In the air at 7500’ pressure altitude full throttle achieves 21.4” MAP. .

On takeoff and climb (full throttle), obviously there's a bit of a loss with your setup. But I think the biggest difference will be if you regularly cruise at full-throttle altitudes (e.g., above 7500'). My newer style induction (RV-10) regularly sees a small ram-air increase in MP over ambient, while your old style shows a definite loss. Whether or not the small (it takes a lot of power just to go a bit faster) speed increase is worth it to you, is for you to decide.
 
Comparing apples with apples

There maybe more to this than meets the eye. If you compare what people are getting with fuel injected engines vs carb etc.
Best to see if another similar powered rv-4 can be used to compare with.
You could end up doing a lot of work for little if any gain.
 
I guess it's costing you some... as any air filtering contraption will...

Assuming I'd build again, I would go for the same idea used 30+ odd years ago: à la Mooney (like the old 201's had). A straight intake pushing the air directly into the throttle body = ram air effect assured :)
Filtering done only during <1000 ft AGL ops, T with an air valve and a hose connected to a K&N conical filter... used also as alternate air.

Remember those "prop driven" intakes, were quite in fashion a few years back...
 

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KiwiPete's point is valid. Carb loss due to a small venturi will be greater than loss through an FI system venturi.

As for filters, it's just a question of media area. Given enough area, the filter drop is tiny. My entire airbox, complete with a 33-2124 filter, measured a bit less than 2" H2O (0.14 Hg) loss when flowing at 1400 PPH, theoretical mass for a 390 at sea level. The OP's 320 would be more like 900 PPH at 7500 ft and 2700 RPM, so for the same airbox, drop would be even less. Note the OP actually measured a filter element loss of 0.1", which I assume to be in Hg units, and determined by a simple with-and-without-the-filter flight test. He could perhaps improve total loss, but the filter isn't a root problem.
 
KiwiPete's point is valid. Carb loss due to a small venturi will be greater than loss through an FI system venturi.

As for filters, it's just a question of media area. Given enough area, the filter drop is tiny. My entire airbox, complete with a 33-2124 filter, measured a bit less than 2" H2O (0.14 Hg) loss when flowing at 1400 PPH, theoretical mass for a 390 at sea level. The OP's 320 would be more like 900 PPH at 7500 ft and 2700 RPM, so for the same airbox, drop would be even less. Note the OP actually measured a filter element loss of 0.1", which I assume to be in Hg units, and determined by a simple with-and-without-the-filter flight test. He could perhaps improve total loss, but the filter isn't a root problem.

Your assumption on units is correct, InHg. My engine is carbureted.

On takeoff and climb (full throttle), obviously there's a bit of a loss with your setup. But I think the biggest difference will be if you regularly cruise at full-throttle altitudes (e.g., above 7500'). My newer style induction (RV-10) regularly sees a small ram-air increase in MP over ambient, while your old style shows a definite loss. Whether or not the small (it takes a lot of power just to go a bit faster) speed increase is worth it to you, is for you to decide.

There maybe more to this than meets the eye. If you compare what people are getting with fuel injected engines vs carb etc.
Best to see if another similar powered rv-4 can be used to compare with.
You could end up doing a lot of work for little if any gain.

This is something I hope to determine from this thread - are my losses severe enough to make the juice of changing worth the squeeze, so to speak.
 
0320 carb w/ ram air

I owned an older RV-3. It had an 0320 160 hp with a direct ram air intake. On ground opps it had a flap controlled in the cockpit which would cover the air inlet and force air to pass through a filter. The performance was notable on take off in the ram air mode compared to the filtered bypass option. I don’t remember the numbers off hand of RPM or MAP. I do remember well the performance difference.

I now own an RV-9 that i built and have had some issues with cooling on climb out. I spoke to the carb folks about the problem and they suggested that i may have an air filter/intake restriction issue. (Standard VAns airbox with standard K&N filter). They suggested i try a flight with removing the filter and i would get more fuel flow and air flow within the Venturi of the carb. This in effect was the same setup as my RV-3. ( more air also sucks in more fuel through the Venturi)


The first noticeable effect was having to lean when taxiing as the engine now was running very rich. On run up at close to full static rpm, i leaned for best power. Taking off at full rich would not have turned out well.

Result was much more fuel flow and dramatically better cooling and about 1” in MAP. I believe also i may have had higher RPM’s.

I went back to the original Vans airbox/filtered setup as i am not comfortable running without a filter. But i often think about the better cooling and better performance and wonder if there could be a compromise with a larger airbox and filter that is less restrictive. Or having the guts to back to a carb ram setup.

Has anyone else experimented with this? Please tune in. Thanks.
 
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Al, you're saying you saw a 1.0" Hg manifold pressure increase merely by removing the standard round K&N from a Van's vertical induction airbox? If true, it suggests a very poor filter choice (too little area) or blocked media (dirty, too much oil, etc).

On another front, consider carb operating principles regarding mixture. In general, for any given air density, increasing air velocity would indeed increase fuel flow, but the increase would be in proportion. Increasing air density (less filter pressure drop) at the same velocity would make it go lean, while decreasing density at the same velocity makes it go rich. Point is, simply removing a filter should not make the mixture go rich.

Last, increasing fuel flow and leaning to best power mixture nets a power increase, i.e. higher combustion pressures and temperatures. A CHT reduction would be remarkable.
 
This is something I hope to determine from this thread - are my losses severe enough to make the juice of changing worth the squeeze, so to speak.

Not really loss, as the 0.1 filter drop is pretty good. Question is how much dynamic pressure you might potentially harvest as compared to the current setup.

Do you have a photo of your cowl/intake/filter?
 
Not really loss, as the 0.1 filter drop is pretty good. Question is how much dynamic pressure you might potentially harvest as compared to the current setup.

Do you have a photo of your cowl/intake/filter?

See attached. Filter is a K&N 33-2008. On its face I’m seeing approximately a 1.5 inHg loss compared to ambient in both static and flight regimes; it would seem there is room for improvement there.
 

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I’m seeing approximately a 1.5 inHg loss compared to ambient in both static and flight regimes; it would seem there is room for improvement there.

Is that ambient compared to indicated manifold pressure? If the MP gauge is tapped from the #3 cylinder primer port like most, a percentage of the 1.5" is induction system loss...pressure loss downstream of the carb venturi and intake tubes. Reducing tract loss would require (for example) exchanging the updraft sump for something less restrictive. I have hard numbers for the standard Lycoming horizontal intake plenum and tubes. Tract loss is not insignificant.

For now, let's just look at airbox design.

It's not hard to predict dynamic pressure capture for a low velocity ratio engine inlet. I'll need the engine displacement, RPM, true airspeed of interest, altitude, and temperature. We'll look at several different intake areas.

Prediction in hand, we'll need a static pressure measurement taken in the back of your current airbox (see the picture). The difference will be the potential gain.

To make the measurement you'll need to run some vinyl tubing from the cockpit to a piccolo or similar in the airbox, and tap into the aircraft static system. Run a third tap to the MP line if you also want to see the tract loss. You'll use an inexpensive manometer to record the pressure deltas.

BTW, separate issue, but the current setup surely leaks dynamic pressure around the perimeter of the filter, adding pressure to the lower cowl volume, thus reducing deltaP across the cylinders....in other words, it is detrimental to cooling, and a drag loss too. Exactly how detrimental I don't know, but it's not as good as it could be.
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Is that ambient compared to indicated manifold pressure? If the MP gauge is tapped from the #3 cylinder primer port like most, a percentage of the 1.5" is induction system loss...pressure loss downstream of the carb venturi and intake tubes. Reducing tract loss would require (for example) exchanging the updraft sump for something less restrictive. I have hard numbers for the standard Lycoming horizontal intake plenum and tubes. Tract loss is not insignificant.

For now, let's just look at airbox design.

It's not hard to predict dynamic pressure capture for a low velocity ratio engine inlet. I'll need the engine displacement, RPM, true airspeed of interest, altitude, and temperature. We'll look at several different intake areas.

Prediction in hand, we'll need a static pressure measurement taken in the back of your current airbox (see the picture). The difference will be the potential gain.

To make the measurement you'll need to run some vinyl tubing from the cockpit to a piccolo or similar in the airbox, and tap into the aircraft static system. Run a third tap to the MP line if you also want to see the tract loss. You'll use an inexpensive manometer to record the pressure deltas.

BTW, separate issue, but the current setup surely leaks dynamic pressure around the perimeter of the filter, adding pressure to the lower cowl volume, thus reducing deltaP across the cylinders....in other words, it is detrimental to cooling, and a drag loss too. Exactly how detrimental I don't know, but it's not as good as it could be.
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Dan,

This is very helpful. I’m always up for actual testing. I am indeed reading off indicated manifold pressure from the #3 tap, so the loss number is an all-inclusive value. What sort of values have you seen for tract loss?

Any issues if I gather test data and then work with you to get the predicted values using the test condition for a direct comparison?

How sensitive to X location will the piccolo tube be? The rear most I can get while still being forward of my carb heat/alt air flapper valve will be just slight aft of the carburetor intake.

As far as the airbox/cowl mating, would just some silicone baffle strips be sufficient?
 
I am indeed reading off indicated manifold pressure from the #3 tap, so the loss number is an all-inclusive value. What sort of values have you seen for tract loss?

WOT, 2500 MSL, 2700 RPM, about 14.4" H2O (roughly 1.0" Hg) for the whole intake tract....filter, FM200, plenum and tubes. Airbox, filter and FM200 is about 5" total on the flow bench, so call it 9.4 (roughly 0.7 Hg) from the FM200's flange adapter to the primer port. It will be fun to see total drop for a carb on an updraft sump.

Any issues if I gather test data and then work with you to get the predicted values using the test condition for a direct comparison?

Works for me.

How sensitive to X location will the piccolo tube be? The rear most I can get while still being forward of my carb heat/alt air flapper valve will be just slight aft of the carburetor intake.

Whoops, my suggested location was brain dead. More forward is fine. Alternate to a piccolo might be a flush tap in the side wall...you know, a static port. Just make sure nothing can come loose and get sucked up into the carb, or block the flapper. Safety first.

As far as the airbox/cowl mating, would just some silicone baffle strips be sufficient?

If you can somehow arrange them so pressure blows then shut. Worry about it later. This airbox may get a flotation test.
 
I'm sure 00Dan will be along later, after contemplating options. For now, a quick follow up regarding measurements.

Test setup; one line tapped into the aircraft static system, one line tapped into the manifold pressure instrument line (i.e. the #3 primer port), and one line connected to a static pressure tap in the side of the airbox. The tap was fabricated by drilling a small hole in the airbox wall, then gluing a barb fitting to the box exterior. See photo.

The lines required restrictors to stabilize the electronic manometer readings. A tiny notch was filed in the edge of an AN426-3 rivet head, then the rivet was pushed into the tube. See photo.

Test conditions were 2500’ PA, 67F, TAS 191 MPH (166 KTAS), at 2700 RPM.

The three pressure lines were connected to an electronic manometer in turn, with the manometer's second port open to cockpit pressure. The static port deltaP was 0.16 inH2O. The manifold pressure tap measured -21.80 inH2O. The airbox port measured 2.61 inH2O. Indicated manifold pressure was 25.6 inHg.

Given freestream 0.16 above cabin pressure and airbox at 2.61 above, the airbox rise above static is 2.61 – 0.16 = 2.594. We'll round to 2.6” H20 above static.

Pressure at the primer port was negative, so –21.8 + 0.16 = 21.64. We'll go with -21.6” H2O below static

Induction loss, airbox to primer port, is thus 21.6 + 2.6 = 24.2” H2O, or 1.78” Hg, and that value does not include the filter. No surprise. It’s a carb on an updraft sump, and both are known to be restrictive. For comparison, under similar conditions the horizontal sump on my 390 with an FM-200 had a 1.0” loss, including filter. That value should reduce even more with the manifold used on the 390-119, or possibly one of the non-Lycoming manifolds.

The above is simply pressure loss in the intake tract between the airbox and the intake valve. The original question ("Can the airbox be improved?) largely revolves around how well it converts available dynamic pressure into increased static pressure...the ram effect which allows most of us to see a rise in manifold pressure with speed rise.

The combustion air inlet is not a closed pitot, but rather, has an exit (the engine), so for a simple low Vi/Vo inlet, available dynamic pressure is based on the difference between the inlet and freestream velocities. Volumetric efficiency for a stock angle valve Lycoming is close to 0.9. Although it's surely less for the updraft parallel valve, here I'll stick with 0.9 as it won't make a large difference, and said difference would improve the ram value.

Assume the existing setup is replaced with a round, external diffusion inlet of 3” diameter feeding a large volume airbox. Given 2700 RPM and a 0.9 VE, intake volume is 6480 in^3/sec, thus inlet velocity is 45.3 knots. Aircraft velocity (the freestream) was 166 knots, so velocity for available dynamic pressure is 121 knots. So, at 2500 feet and 67F, the dynamic pressure rise should be 0.63"Hg, or 8.6" H2O, somewhat better than the 2.9"H20 measured above. Airbox total pressure for this displacement, RPM, velocity, altitude and temperature should be 27.95” Hg, so after a 1.78” intake tract loss, the indicated MP should be 26.17”Hg, or 0.57” higher than the 25.6 indicated with the Cessna airbox.

We can roughly cross check that gauge indication using the measurements. Static pressure at 2500PA and 67F should 27.315"Hg. The induction loss was 1.78, so 27.315 - 1.78 = 25.72. The slight difference between 25.72 and 25.6 indicated could reasonably be seen as the filter loss, 1.6" H2O, as the filter was not included in the measured intake tract loss. The manifold pressure gauge is probably accurate.

A 2” inlet ring is not nearly as good, with an airbox pressure rise of only 0.1768’Hg. If a 2" inlet was desired for some other design reason, it would require internal diffusion. A 4” ring would bump ram pressure to 0.85", but the additional 0.2 (0.85 - 0.63) may also net a bit more external drag, as the housing would have more wetted area. Note the pressure gain is shrinking in proportion to each additional increment in diameter. All design is the art of intelligent compromise.

Anyway, the old school GA airbox appears to be costing a potential half inch of manifold pressure. It's also leaking ram pressure into the lower cowl, deteriorating engine cooling and increasing drag. If it was my airplane, it would indeed get a flotation test in the nearest body of water. 00Dan may decide different, and that's fine too.
 

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I want to start by thanking Dan for his help with guiding me through the what and how of these measurements. It’s great to have some hard numbers to work with. Dan provided me with some reading material on intake design and I’ll be following up with a more thought out decision once I’ve processed that. For now, the airplane is of course perfectly usable and airworthy as is. My first inclination is to perhaps make some cowling modifications and change the intake geometry more than the airbox itself; details on this are TBD.
 
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