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Ram air or not for best GPH?

GSchuld

Well Known Member
I understand a ram air intake(well done) increases manifold pressure and improves WOT engine performance.

At 60 or 80% power however, when aiming for maximum long range fuel efficiency, is there a benefit to ram air still at that point? Or is it just as functional getting fresh filtered cold air(or even modestly heated air) in this situation?

Thank you,

George
 
As you noted, ram air is useful at wide open throttle altitudes (>8000’) to gain extra air which mixes with extra gas to gain more power, which lets you go a bit faster, but decreases mpg in most circumstances, since most of us cruise much faster than max range airspeed. It’s only when you are so high that you are below max range speed that ram air will help mpg.
 
Having air for the engine to properly breath is alway good.

Of late I got educated by Dan Horton and Don Rivera (at AFP) on just how sensitive our engines are on air filters as they impact air getting to the engine.

Applying this information for my new RV-10 project (Cold Air Sump horizontal injection) I found many of the current filter options have a significant flow drop. I know Jimmy (the new owner of James Aircraft) is prototyping a new filter box for the James Cowl set up for this engine. Jimmy is underwater with new cowl orders but hopefully he will have a prototype available for flow testing before long.

My point. Ram air is just fine assuming your filter does not present unacceptable flow restrictions.

Carl
 
I should have mentioned that regarding air filtration. Low pressure drop air filters with significant pleating is the ideal.

So in theory, filtered ram air is best at WOT. But filtered non ram air is better for maximum GPH at slow to fast cruise.

Correct?
 
I do not think that is correct.

Consider your engine as a system. A bad ram air filter setup will not perform as well as say a standard snorkel install.

I reported back to Don Rivera after I replaced my AVSTAR injection system with his new FM-150C unit. While the shorten takeoff roll was not totally unexpected, the gain of 2-3 knots in LOP cruise (same exact flight conditions at 9K and fuel flow as with the AVSTAR) was a surprise. This gives you an idea that there other factors associated with your max MPG question.

In other words, ram air or no ram air is not the metric. How much gain you get in MP of your system install is what you should look at.

Carl
 
I guess I should have been more specific.

I should have defined filtered ram air in my theoretical situation as a VERY efficient system, both in effective MP rise and very low pressure drop through the filter at WOT. I realize that this isn’t the standard, but it has been done.

Filtered ram air has reportedly been accomplished with only around .1 MP loss over straight unfiltered ram air at WOT.

This is undoubtedly best for maximum power output and speed at WOT(assuming the desire for the 24/7 filtered induction air)

The question I’m trying to answer is how this compares at say 10,000ft and 75% power, to a non ram air source from an equally large low pressure drop air filter.

Is the ram air at 75% power offering anything constructive?

Would filtered ram air offer any GPH advantage over non ram air (of unrestricted supply) at 75% power at 10,000ft? Same question at 15,000ft…

Thank you for all the responses.

George
 
I have a faded memory of flying a plane many years ago that had a filter-bypass system. Filter was used when close to the ground, or flying through serious dust, but bypassed in normal cruise flight.

Might have been a Mooney, or a Cessna 210..........

Just curious if anyone else has seen something similar.
 
Would filtered ram air offer any GPH advantage over non ram air (of unrestricted supply) at 75% power at 10,000ft? Same question at 15,000ft…

Thank you for all the responses.

George

I think some Mooneys had filter by-pass knobs.
George, you’re asking the wrong question. If you could actually get the same power with or without ram air, then it wouldn’t make any difference. Of course you won’t get 75% power at 10K’ either way, but, let’s just say, you get 65% with ram air, and 62% without. Then the ram air gets you more power, a little higher airspeed, but less efficiency, because we typically fly much faster than max range speed. Only if you approach the service ceiling, where airspeed approaches max endurance, slower than max range, will you see the ram air improve the mpg.
 
All of the pre 201 200hp Mooney’s had RAM air. It allowed the manifold pressure to be increased by 1 to 1-1/2” when the filter was bi passed. That said the filtered system was not very efficient…I think the later models no longer used a bipass as the filtered systems were improved.
 
Sorry, I should have stated 75% of WOT speed available at 10,000ft, not 75% of sea level WOT at 10,000ft. My mistake, sorry.

FWIW, I’m talking “fast cruise” here, not maximum possible range speed which is quite slow of course.

George
 
Anything restricting the intake lowers engine efficiency. This includes a throttle plate, air filter or anything. Unfiltered ram air increases manifold pressure, so to maintain the same total power, other methods need to be used to ensure the same total power output. Reduce RPM or go further LOP. Otherwise, yes, it's more power (and fuel) on the wrong side of the aircraft's efficiency curve.
 
I did not read the whole thread but....

FREE FLOWING in-take, induction from outside are to intake valve and non restrictive exhaust from exhaust valve to outside is "GOODNESS" (technical term requiring years of engineering school). It will make more HP and better efficiency because the engine does not have the WORK HARD to SUCK and BLOW... as in Suck , Squeeze , Bang , Blow.... 4 Cycles of your Lycoming and all Otto cycle engines. Why do people put on turbochargers and super chargers on airplanes? To increase MAP and fly higher where drag is lower. YES INDUCTION RAM AIR IS GOOD.... always at any power from 100% to 50% and lower. You control your MPG in an airplane with that black knob thing (and the RED and Blue if you have it). In a car it is the skinny pedal.

So what does that mean? You will make more power and have better efficiency. Those are good things you want always. Efficiency means lower fuel burn for given HP. The design of the Van FAB not only does a good job of recovering RAM (the dynamic pressure from doing say 190 MPH) it also has lower airframe drag, especially compared to a typical Cessna.

Van's standard up draft FAB airbox into a Carb or Throttle Body is good for about 0.5 inch (guessing could be better, as much as 1"). It is SO hard to measure this RAM (dynamic pressure over ambient). How I have done it is pull CARB HEAT and block RAM air. I get 1 INCH drop, or one could assume RAM air is good for 1" RAM. Some of the loss is Engine sucking warmer air inside the cowl. Some of that is restriction in the area of carb heat opening, which is smaller than the main inlet. But most of the loss or lower MAP is loss of RAM AIR... I am guessing 0.5" to 1.0 In-Hg. THAT IS FREE BOOST. This is amazing because most planes LOSE MAP due to very restrictive induction system and airbox.

Keep in mind the TOTAL RAM avaiable at 8000 ft at 190 mph is 1.0 Inch/Hg... So it is not realistic to think we gain a full 1" if MAP but we come close. On slower planes there is less RAM to recover.

What about CESSNA with an AIR FLILTER plastered FLAT on the nose bowl of the cowl? Looks OK? Nope. If you do the same flight test I did above on my RV in a Cessna, you will see small loss of MAP with carb heat. (Anyone have a number of MAP with and w/o carb heat in a Cessna). The Cessna air-filter/airbox is already operating with restrictions and nil RAM recovery. It is terrible and even w/o carb heat you have no RAM air induction gain and even lose MAP.... Also a Cessna is much slower (less RAM) and their carb heat is a flame thrower and VERY hot less dense air, so it is hard to draw a straight line between an RV and Cessna. Bottom line aerodynamically the Cessna air filter is draggy and recovers little to no RAM. Van's FAB does better than any Certified Aircraft in the induction/filter/airbox design and performance (in my opinion).

I also estimate Van's airbox efficiency by looking at Lycoming's WOT MAP for a given altitude adjusted for Temp/Pressure and other (certified) Aircraft AFM'a. My RV MAP is higher than what Lyc predicts. Also looking at other aircraft manufactures AFM (say a 180/200HP Mooney), comparing their MAP at altitude charts to mine, my RV has better MAP (and is faster). Later model Mooney's have true RAM with NO AIR FILTER used only in altitude cruise and selectable by pilot from filtered to no filter RAM. Sporty and pilots who leave RAM on in snow and ice have had the engine shut down due to blockage of induction. It is to be used in clear air in cruise. However this system does produce induction RAM recovery and better performance. Does Van's FAB having air filter you can not bypass reduce RAM induction? Yes slightly but the K&N is free flowing and the air velocity has slowed to recover pressure. The FILTER can improve performance, has benefit of smoothing out the airflow into to the induction (Carb or Throttle Body).

Lyc has a hot oil sump the induction feeds through. This makes CARB ICE less likely but warmer air, the less dense air causes loss of HP. Continental's with carbs are more susceptible to Carb ice issues with cold induction (induction is separate from sump with hot oil).

FI you can get a COLD INDUCTION sump on Lycoming. Colder air is more dense and makes more HP. Just keep in mind ICE/Snow can get into the induction system of FI (fuel injected) planes. If you love flying in clouds, snow, freezing rain) your FI Lycoming needs ALTERNATE air input (not carb heat but just another source of clean warmish air if the air lifter gets blocked by ice/snow)..

Exhaust? Another story. It is not induction but it is the flip side of induction. The IN has to work with the OUT. The exhaust on certified planes are heinously bad. That is why companies like POWER FLOW look like hero's putting their tuned exhaust on GA planes. They get lots of extra HP and thus better performance. Part of the dramatic increase in HP is not because they are quantum leap or new technology (it is tuned exhaust well known). It is the contrast to stock exhaust which is very bad.

No kidding MORE HP is always good... as Tim "Tool Time" Taylor would say, "more power".... Get all the RAM induction boost you can get regardless of the power setting. It is SCICENCE. :D
 
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another angle

Look at this another way.
Typically you'll hit WOT and 75% power at around 7500' DAlt.

If you have an effective ram air inlet, you can still make 75% power at a yet higher altitude, where you'd have less airframe skin friction drag at the higher altitude. Hence faster, thus better miles per gallon than no ram air at the lower altitude? That's my thought but I'm not an Aero Eng.

I wonder if that's in the Mooney performance charts?
 
moving the air through the engine takes energy because of all the flow restrictions in the system. that energy has to come from gas. having a higher air pressure at the inlet from the beginning (due to ram air) should be a good thing.

doing a mind game: if you would attach a vacuum cleaner on your cowl inlet that sucks on your air filter, the engine would have to overcome the reduced pressure and would use more fuel to provide the same amount of power to the propeller.

at least that's my thinking, but i could be wrong.

thank you for asking, it's a very interesting question! :)
 
Usefullnss of RAM air depends on efficiency of the flitered system

All of the pre 201 200hp Mooney’s had RAM air. It allowed the manifold pressure to be increased by 1 to 1-1/2” when the filter was bi passed. That said the filtered system was not very efficient…I think the later models no longer used a bipass as the filtered systems were improved.

The 201 had RAM air too, but the new filter and air box system was so much more efficient, that the increase of manifold pressure was only about an 1/2". In theory it's useful above 8000 ft and WOT, but in my '89 Mooney I see no measurable difference in speed. Mooney omitted the RAM air system in 1990.
 
I have a faded memory of flying a plane many years ago that had a filter-bypass system. Filter was used when close to the ground, or flying through serious dust, but bypassed in normal cruise flight.

Might have been a Mooney, or a Cessna 210..........

Just curious if anyone else has seen something similar.

Rod Bower’s ram air option does bypass the filter when ram air is selected.
 
Ram air

I had a Ram air setup on my RV-3. It had a bypass to filtered air for ground opps or dusty conditions. It was a very fast plane. Light with an 0320 160 hp. At altitude 8k running at 2500 rpms. I could easily keep up (often faster) than my 0360 buddies in their Rv-6, 7, 8. When we landed to top off i had always much less in fuel burn.

The construction of the Ram air was very simple with a fiberglass constructed tube direct from the snorkel with a 90 bend to a fiberglass flange bolted to the bottom of the carb. Above the tube a small round filter box. When the carb heat flap was closed off at the entry of the tube, air would bypass to the filtered air.

I have often thought of converting to a similar setup in my Rv-9, 0320 160 hp.
One thing i have learned by experimenting removing the air filter is that the engine will run very rich with greater air flow due to the Venturi effect. Leaning to best power prior to take off is critical. Much cooler cylinder temps on climb as a result due to higher fuel flow.

Im very curious if anyone has tried this? Or knows anything about early Rv’s that had a similar setup.

The bottom line is filtered air is restricting the airflow and the performance of the engine. Do you really need filtered air at cruise?
 
...
The bottom line is filtered air is restricting the airflow and the performance of the engine. Do you really need filtered air at cruise?

Unless you plan to ingest a bird, I'd say you do not need a filter in cruise. The challenge is the installation of a reliable system to switch from filtered to unfiltered air. Builders have been successful at doing this, but each installation is unique. If there were a "Van's Aircraft-approved" solution then I think many would adopt it.

It would be great to see a true A-B testing on the same aircraft between the standard snorkel and a "ram air" intake with other variables held constant to see if there is value in the ram air solution. Perhaps Van's has done this and decided there was not enough improvement to warrant the investment to develop this.
 
Leaning to best power prior to take off is critical. Much cooler cylinder temps on climb as a result due to higher fuel flow.
?

I don’t understand how leaning results in higher fuel flows.
Engines are set up to be over rich at 100% sea level power for a reason.

I notice many posters define ‘efficiency’ as getting more power out of their engine. But the OP specifically asked about fuel burn rate. Very different things.
 
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I grew up with a '65 Super 21 Mooney. It was the second model year with the angle valve 200 IO-360-A1A

Yes, it had a filter bypass which was good for an inch of MP. So now we know the filter loss of the Mooney designed system of the day.

I cannot tell you how long that was a feature on the angle valve Supers and Executives but it went well into the 70's

To get back to the efficiency question, a couple have already stated the correct answer.
Low MP loss will allow higher MP at higher altitudes resulting in higher cruise speeds than available to a craft with a restrictive filter. Assuming both are running same power and fuel flow at different altitudes, the faster craft will have higher fuel efficiencey.
 
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Yes, my post was regarding GPH fuel efficiency at cross country cruise speeds.

But regardless, the thoughts here are welcome.

George
 
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If someone wants to cruise an IO-360 at 110hp and 7500', that can be done at full throttle, being 22" and 2400rpm (made up numbers for example sake). Now if ram air can deliver an extra bit of manifold pressure and give 22.5", we need to reduce RPM to maintain 110hp. so lets assume 2350rpm.

We're still at full throttle, now with a higher manifold pressure and lower RPM for the same 110hp. The efficiency different would be minimal, perhaps immeasurable, but theoretically there.
 
I don’t understand how leaning results in higher fuel flows.
Engines are set up to be over rich at 100% sea level power for a reason.

I notice many posters define ‘efficiency’ as getting more power out of their engine. But the OP specifically asked about fuel burn rate. Very different things.

Bob. Without a filter you cant even taxi the plane unless you lean. The unrestricted airflow sucks in an excess of fuel from the Venturi effect in the carb.
You have to lean for best power at run up otherwise the engine would be de- powered by running to rich. The unrestricted airflow on climb sucks in more fuel thus has a very noticeable cooling effect.

Ram air would have the same effect. The carb would need to be adjusted or leaned if using Ram air.

I am no expert with this. But i have done it as an experiment for cooling on climb by removing my filter.
I had an excess heating problem and no longer do as i upgraded to a higher flow rate carb (with a filter).

Regarding the OP fuel burn question. Ram air from my understanding results in better engine performance and efficiency.
 
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You have to lean for best power at run up otherwise the engine would be de- powered by running to rich.
.

Most stock engines are in fact ‘de-powered by running too[sic] rich’ at takeoff settings. Leaning to best power at sea level can place them uncomfortably close to detonation limits.
 
Most stock engines are in fact ‘de-powered by running too[sic] rich’ at takeoff settings. Leaning to best power at sea level can place them uncomfortably close to detonation limits.
There is no dangerously close to detonation when below 75% power.**

** That is assuming, good fuel octane, timing standard (typ 25 deg BTC) and standard compression (not racing high comp pistions).

You can lean to your heats content below 75% power as long as CHT is not excessive.
 
There is no dangerously close to detonation when below 75% power.**

** That is assuming, good fuel octane, timing standard (typ 25 deg BTC) and standard compression (not racing high comp pistions).

You can lean to your heats content below 75% power as long as CHT is not excessive.

True. But my quote says ‘at takeoff settings’. And the prior poster who I quoted said he leaned for best power for takeoffs.
 
Leaned

Yes!!! When the filter was removed!!! You have to.

If you run with Ram air the carb will need to be adjusted to not run to rich.
 
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