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MP gauge in a fixed pitch RV ?

Python

Member
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 🐍
 
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 🐍

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.
 
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.

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.
 
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.

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?
 
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?

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.
 
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