The Extra Inch...

[bjdecker]

I've been tinkering with various shapes of ducts trying to squeeze a bit more manifold pressure out of (into?) my IO-360-A1B6 equipped RV-7.

My intake manifold is the "standard" VA-132-2 S-duct, pulling intake air from the left inlet ramp. At wide open throttle and various altitudes I am getting roughly ambient pressure downstream of the fuel injection servo (#3 Cylinder port) -- Some examples:

6500ft -> Field altimeter of 30.02" -> 23.6" MAP...
8500ft -> Field Altimeter of 29.95" -> 21.8" MAP...

But I want more; It seems like I could fabricate a scoop of some stripe to convert that hurricane of propeller wash and incoming air into something more meaningful pressure-wise (and mass).

I fabricated a "hood scoop" to sit on top of the air filter...and it didn't work; only gained .1" of MAP and as expected all the CHT's & Oil Temp increased... Uncomfortably...

Back to the TLAR drawing board. I converted the hood scoop into a splitter of sorts, still attached to the top of the air filter, but only occupying the bottom half of the left inlet. It also didn't work; minimal gain in MAP" ~.1", CHT and Oil Temp were unchanged. Until you factor in the reduction in OAT we had yesterday (-10°F), so the efficiency suffered a bit, but well within the operating margin of my plenum/baffle system.

What's next? I think I'll remove the air filter from the system and see what the MAP does...

[Mel]

I remember my 1968 Mooney had a filter bypass that added approximately 1" of manifold pressure.

[bjdecker]

Originally Posted by Mel

I remember my 1968 Mooney had a filter bypass that added approximately 1" of manifold pressure.

So did my '86 205SE and the F1R intake on my last -7. I just don't want to cut holes in the bottom cowl...again...

[Dan 57]

my 1968 Mooney had a filter bypass that added approximately 1" of manifold pressure

Thanks Mel.
The only way to do it... just looking at those snorkel and FAB convoluting routes gimme, and the intake air, some quite severe headache

Filter on for take-off alright, 1000' or outta dust whichever first, ram air... the only way for any intake to properly breathe.

[RV8JD]

There is an RV-8 up here in the PNW that has a similar 'hood scoop'. Don't know who the owner is though. I saw the airplane back in 2005, and saw it again recently at Chehalis, WA. Still had the 'hood scoop'.

[DanH]

Originally Posted by Dan 57

Filter on for take-off alright, 1000' or outta dust whichever first, ram air... the only way for any intake to properly breathe.

Use a larger filter for almost no loss.

[Carl Froehlich]

Originally Posted by DanH

Use a larger filter for almost no loss.

What Dan said!

I suggest a scoop is not the best approach to get a bump in MP.

I did a beta test of a new snorkel before Oshkosh. I did the same data runs with the orginal snorkel and the new one. While I suspect the reason for the snorkel update was far more applicable to the IO-390-119EXP (using the FM-200) than my IO-360-M1B I did measure a MP bump of 0.4” to 0.5”. The stock snorkel provided a bump as well, just not quite as good.

Side note - I’m using the new AFP FM-150C instead of the stock Precision unit. This is a bolt on replacement - and I recommend all IO-360-M1B engine buyers specify the 150C.

I understand Van’s took the beta test snorkel (that I reluctantly sent back to them) to scan to create production molds. Considering all the stuff on Van’s plate of late I would not expect this to be on the front burner. I do note that this new snorkle and the FM-150C was discussed (with photos) by Van’s at Oshkosh this year.

Carl

[Mikeyb]

Confused
I don’t know if it’s right but Looking at the 1 sec log files from my IO360 with the stock filter with roughly 50 hours since cleaning the manifold pressure is never more than 0.4” lower during the takeoff roll than it is with the engine shut down at the end of the flight.

Is there enough dynamic pressure available in the cowl to get another inch of pressure?

[BillL]

Originally Posted by Mikeyb

<snip>
Is there enough dynamic pressure available in the cowl to get another inch of pressure?

That is a really good question. I took air intake data at various times by the Garmin flight data for temperature and baro altitude at WOT. Then, used an on-line atmo calculator to make the comparison to the MAP. It seems my MAP is just slightly higher than the ambient pressure - .5 in-hg IIRC. Air filter drop was quite low, (almost DanH territory) but have no idea for the servo pressure drop.

I need the new Hartzell Composite prop.

https://aerospaceweb.org/design/scripts/atmosphere/

Brian - I think your data just says that you are getting nearly all the benefits of dynamic pressure already.

[DanH]

Brian, there's data out there which says the available static pressure at the snorkel filter location is very similar to the pressure measured with a typical horizontal "ram" inlet. The observation matches theory, as well as measurements from popular research papers.

There are differences between individual configurations of course, but in general numbers, a good cooling system will see a static rise equal to 80~85% of available dynamic pressure. Take a look at the best pressure coefficients in CR3405.

We must slow the flow to convert available dynamic pressure to increased static pressure. The rise for a straight "ram" intake is never 100% of available dynamic, as it's not a closed end system; i.e. like the cooling system, we cannot slow the velocity to zero. However, like the cooling inlet, we can design for internal diffusion or external diffusion and get a static pressure rise. How much depends on details. There is no guarantee it will be better than the intake located in the cooling inlet. However, it is very likely to exhibit less duct and filter loss on the way to the servo.

Specific to Mike's question, yes, the static pressure rise in the upper cowl can easily exceed 1" Hg. As above, the trick is piping it to the servo without filter and duct loss.

How much rise? The plot below is a standard RV-8 cowl (red) compared to a revised RV-8 (blue). The upper plenum values are roughly 12 and 14 inches of water at about 165 knots. That's 0.88" and 1.00" Hg above freestream static. There would be more if the aircraft were flying at max velocity.

I went back to original data for more precision. The reported upper pressure for the stock cowl was 12.7" H20 at 163.4 true, or 0.934" Hg. At 3500 PA and 50F, the available dynamic pressure was 1.144"Hg, so the plenum rise for this particular stock RV-8 was 81.6%.