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Hi, I was thinking of replacing my MP gauge with a fuel flow/ carb temp gauge in my fixed pitch O 320 RV.Is there any sound reason why I shouldn’t do this ? My plan is to lean to fuel flow( I do still have EGT to monitor). Any thoughts appreciated.Cheers 🐍
 
Python said:
Hi, I was thinking of replacing my MP gauge with a fuel flow/ carb temp gauge in my fixed pitch O 320 RV.Is there any sound reason why I shouldn’t do this ? My plan is to lean to fuel flow( I do still have EGT to monitor). Any thoughts appreciated.Cheers 🐍
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If you have an electronic ignition, it is a good way to identify a manifold pressure leak that would impact your timing.
 
To me the manifold pressure gauge is handy for adjusting cruise setting. It's much faster than waiting for RPMs to catch up.
 
I'm running a FP prop and have a MP gauge, I use it only for HP output calculations and nothing else. If I had to choose I'd take fuel flow over MP for sure.
 
I fly a fixed pitch RV-9A and use MP to set power. My red cube failed a few years ago and I realized that that I did not miss the data, so I removed it. After flying for years with fuel flow I know my aircraft. I always have at least an hour of fuel on board and I use EGT to lean.
 
When I had a fixed pitch prop on my plane I found MP gauge to be useless and when it quit working I never bothered to fix it. in my opinion, trading the MP gauge for a fuel totalizer (or almost anything but a magnetic compass) is a big win.
 
Red Cube

If your red cube failure causes the engine to stop on takeoff(or over hostile terrain) you will wish you never installed a fuel flow. Fixed pitch EAB and I use my manifold pressure all the time as I have in onher fixed pitch airplanes.
 
Fuel flow with carb

Question for those of you with with fuel flow on a carb'd engine: How stable is the flow indication with a steady power setting? The one plane that I have flown in with this set up had the flow rate hunting the entire flight.
 
MiserBird said:
Question for those of you with with fuel flow on a carb'd engine: How stable is the flow indication with a steady power setting? The one plane that I have flown in with this set up had the flow rate hunting the entire flight.
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I have an older EFIS/EMS. The fuel flow number varies as the carb float needle opens and closes to keep the carb bowl at its proper level during cruise. During takeoff and climb, the fuel flow indication is steady. Overall fuel burn numbers are accurate. Newer systems may have circuitry with more smoothing to give numbers that don't vary in cruise. My fuel flow sensor is located after the electric pump and before the engine driven pump, thus giving a higher reading when the pump is on due to pulses from the electric pump.
 
MiserBird said:
Question for those of you with with fuel flow on a carb'd engine: How stable is the flow indication with a steady power setting? The one plane that I have flown in with this set up had the flow rate hunting the entire flight.
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I had a carb for 80 hours and FF reading was very stable. In most phases, the needle does NOT open adn close. It just moves between various open positions to modulate flow and does NOT affect FF accuracy. The same way that closing downa valve on your kitchen faucet reduces water flow. You don't have to constantly open and shut it to get a minimal flow; You just adjust the valve to the position that gives the desired flow rate. Needle/seat in a carb works the same way.

Mounting the Red cube BEFORE the mechanical pump WILL however, create moving FF readings and is not recommended. The mech pump has a very distinctive pulsing with slight reversion on the input side and messes with the way the cube works (counts vanes on a moving wheel. The reversion causes it to intermittently count a vane twice, as it only counts a vane as it moves past the sensor and it has no idea which direction it is moving. It just assumes it is only moving in one direction. Unlike the needle/seat, the pump has a very pronounced start and stop cycle, but only on the input side. The output side is under pressure, which dampens the effect of that cycle.

A red cube mounted between the mech pump and the carb will give very stable readings. It certainly did for me.

Larry
 
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When I had my old RV-4 with a fixed pitch prop, back in the days when fuel flow gauges were rare, MP was invaluable. I could change power setting by 3" with no change in RPM.

These days, in the RV-9A with the Garmin G3X Touch, I set 2200 RPM, EGT just below 1400° and set the throttle to read out 8 GPH directly. Greatly simplifies fuel management when fuel flow is always at 8 GPH.

My instinctive response to the question was MP, of course, because the real reason for knowing power settings was to estimate fuel flow. But since you can read fuel flow directly... who needs MP?
 
Ed_Wischmeyer said:
When I had my old RV-4 with a fixed pitch prop, back in the days when fuel flow gauges were rare, MP was invaluable. I could change power setting by 3" with no change in RPM.

These days, in the RV-9A with the Garmin G3X Touch, I set 2200 RPM, EGT just below 1400° and set the throttle to read out 8 GPH directly. Greatly simplifies fuel management when fuel flow is always at 8 GPH.

My instinctive response to the question was MP, of course, because the real reason for knowing power settings was to estimate fuel flow. But since you can read fuel flow directly... who needs MP?
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I like to be 24" or lower before I lean aggressively. I need MP.
 
I rarely look at the tach on my fixed pitch 6A so I would not want to be without MP.
You own a custom airplane. Why not do a modification upgrade, and add a new hole for an additional instrument?
 
I have MP and FF on a FP O-360-A1A. I like having both, but FF is king for me because I fly LOP.
 
I had MP as one of the inputs to my flight data recorder to calculate %HP. Data computer died, I had extra panel hole not needed for anything else, and had a MP gage sitting on the shelf so I put it in. I still use MP and tach to determine %HP but have to do it in my head.

I think of the MP gage on my fixed pitch airplane just like having a tach in my BMW with automatic transmission. Both give me somewhat non-required information.
 
I have a 6 with 0-320 carbureted and FP. I have the high quality original Vans branded guage. Kidding, I don't trust these much between the knock off quality of the guage and the cheapie sending units. I have replaced the "Vans" Tach, Oil Pressure, and now I am considering replacing the MP with a FF computer like the one vans sells https://store.vansaircraft.com/flight-data-systems-fuel-flow-gauge-ie-fc-10.html
I fly out of a 4500 MSL airport and with the warmer than standard temps in Northern UT the MP (which isn't terribly accurate in my ship) doesn't really do much for power settings because a throttle position change is accompanied by an RPM change. Even with the course pitch prop I can only get "Square" power settings up to around 7500MSL in the winter (This is backed up in the Lycoming manual). Whats interesting is the manual shows RPM, ALT and FF for power settings and not MP. I think I would rather have FF (and the other nifty functions) than MP. With a constant speed prop I would drill another hole in the panel and have both;).
 
No, of no real value. The "data" it provides will not make any useful difference. Leave it out, keep the thing simple, eyes outside, have fun flying the airplane (not operating it).
 
I have a FP Catto and O-320 with Red Cube and G3X system.

I know I probably should but I don’t remember ever looking at the manifold gage. I use the lean assist on the G3X, EGT, FF, and % power to manage the engine. I hope that is reasonable!
 
I have a fixed pitch O-360 in my RV-4. I love the MP gauge. I use it exclusively for power settings, as the RPM varies with airspeed. If I could only have a MP or a tach, I would rather have the Manifold pressure gauge. (Although legally, the tach is required equipment) If I were you, I would find a way to add your fuel flow gauge yet keep the MP. As for carb temp, that’s my least referenced instrument, I wouldn’t miss it..
In a draggy Cessna, then I would use the tach for power settings, but with a slippery RV, the rpm varies with airspeed too much and it’s so much more precise to use MP.. if you haven’t been flying that way, give it a shot!
 
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So there you have it.
Some that say it is useless and some that say they use it almost exclusively over looking at the tach.
For those that find their way to this thread in the future…. Consider carefully the explanations they give for having a MP gauge. Once you understand them, you may find it valuable as well.
 
rvbuilder2002 said:
So there you have it.
Some that say it is useless and some that say they use it almost exclusively over looking at the tach.
For those that find their way to this thread in the future…. Consider carefully the explanations they give for having a MP gauge. Once you understand them, you may find it valuable as well.
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I can see I should learn more about the MP gage and what info it provides that my FF and % power dont give me. Thanks for the nudge.
 
rockwoodrv9 said:
I can see I should learn more about the MP gage and what info it provides that my FF and % power dont give me. Thanks for the nudge.
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MP doesn't give you any needed information that you can't get elsewhere. It's just that RPM, FF, and % pwr all take a few seconds to stabilize whereas MP indication is immediate.
 
Help me to understand this guys, with a fixed propeller load, what is MP going to tell me? I've been up and down the Lycoming Manual this morning and it isn't mentioned anywhere for the FP engines. Back to the OP's question one of the things I found interesting in the MP vs FF discussion is that FF can be used to set power and (Per the Lyc manual) is one of the accepted leaning methods. I know my plane pretty well and I know what it will burn in many cruise settings and altitudes but given a choice I think I would take the FF instrument.
 
IMHO, with a FP propeller I find MP to be a better indicator of power than RPM's are. Not a big deal if you just go around the pattern or go to only a few places, but this can be significant if you fly to very different parts of this world.

As an example, I have flown my O360 / FP prop airplane to the Caribbean (SL), Central America (Mountains), Canada, Texas, Florida, Arizona, Nevada and many places in between. While doing this the airports can be at very different altitudes and depending on temperature, greatly different density altitudes. At a low DA a specific RPM produces a certain amount of power while at a high DA the same RPM's produce a different amount. To me SUPER important when approaching to land at an airport you have never been to. So I use MP settings in the pattern which compensates for airports at different DA giving me similar performance. For my airplane as a baseline I use 19" MP on downwind, 16" MP for base and 12" MP for final no matter what airport I am landing at. Then I adjust accordingly for the actual landing being done. Those are just the numbers I use and another airplane's numbers will probably be different.

I attached images of my EMS showing MP, HP and %HP at the same RPM's but at different altitudes and DA. In the 4,500ft image the MP is 22.8", HP is 128 or 69%, while in the 6,500ft image the MP is 21.5", HP is 120 or 65%. Both instances my FP prop is at 2,480RPM's and the only difference is altitude which affects engine power and how the aircraft performs.

My 2 cents and FWIW. ;)

20240624_4500ft.jpg 20240617_6500ft.jpg
 
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