DrQuinn Posted August 18, 2022 Report Posted August 18, 2022 I recently had a new GI 275 configured as an engine instrument. When it was first installed the manifold pressure rapidly fluctuated +/- 2.5 so much so that you had no idea what it was reading. The old gage was rock solid but I was beginning to strongly suspect that it was reading low. I'm speaking a little beyond my technical knowledge but as my mechanic added something onto the line to limit the size of the pressure line which helped bring it down to +/- 1. Later I think he said he pinched a bit of the line a small amount with a pair of pliers which helped a little more however, still, once I apply any decent amount of power it fluctuates about +/- .7. Can anyone explain in layman's terms what we need to do to get it down to a usable level? Quote
Ragsf15e Posted August 18, 2022 Report Posted August 18, 2022 16 minutes ago, DrQuinn said: I recently had a new GI 275 configured as an engine instrument. When it was first installed the manifold pressure rapidly fluctuated +/- 2.5 so much so that you had no idea what it was reading. The old gage was rock solid but I was beginning to strongly suspect that it was reading low. I'm speaking a little beyond my technical knowledge but as my mechanic added something onto the line to limit the size of the pressure line which helped bring it down to +/- 1. Later I think he said he pinched a bit of the line a small amount with a pair of pliers which helped a little more however, still, once I apply any decent amount of power it fluctuates about +/- .7. Can anyone explain in layman's terms what we need to do to get it down to a usable level? A lot of electronic ones need a “snubber” on the line. “Omega snubber air” in Google will get you close. You’ll probably need adapters because the line has flare fittings and the snubber is 1/8”npt. This is from my experience with an jpi, edm 930. 1 Quote
DrQuinn Posted August 18, 2022 Author Report Posted August 18, 2022 26 minutes ago, Ragsf15e said: A lot of electronic ones need a “snubber” on the line. “Omega snubber air” in Google will get you close. You’ll probably need adapters because the line has flare fittings and the snubber is 1/8”npt. This is from my experience with an jpi, edm 930. That sounds like a great place to start, thanks. Quote
Ragsf15e Posted August 18, 2022 Report Posted August 18, 2022 1 hour ago, DrQuinn said: That sounds like a great place to start, thanks. No problem. This is the one JPI recommended to me. It worked wonders. Here’s my caveat(s)… Garmin may have a different sensor than jpi. Also, I never found a snubber with a flare fitting, so I had to put an adapter on either side. Finally, make sure your manifold pressure line isn’t leaking somewhere. https://www.newark.com/omega/ps-8g/pressure-snubber-ss-1-8-npt/dp/31AC5909?gclid=EAIaIQobChMIx7-e2v_Q-QIVevHjBx3MsAbpEAQYAiABEgIZrvD_BwE&mckv=_dm|pcrid||plid||kword||match||slid||product|31AC5909|pgrid||ptaid||&CMP=KNC-GUSA-GEN-PMAX-LOW-ROAS-Shopping Quote
carusoam Posted August 19, 2022 Report Posted August 19, 2022 +1 for use of snubbers when the pressure in the line is always varying/fluctuating… typical of air flowing into cylinders with intake valves opening and closing… The old analog gauges couldn’t react fast enough, so they supplied an average that worked pretty well…. Some use internal snubbers… Often, on mechanical MP gauges there is also a calibrated weep hole…. Primarily to keep fuel from being sucked into the MP gauge on shut-down…. But also adds to the averaging effect… An engine monitor may be collecting data about once every second… it may be showing data from anywhere on a sinusoidal line… depending on how fast the sensor really can read the pressure… Some data collection devices use an averaging technique… the Ceis fuel level instruments use an average of data collected for a handful of seconds to mellow out the waves that occur in the fuel tanks during flight… Its cool having digital accuracy… until it becomes overwhelming… We had @Bob_Belville explore this challenge a few years ago… Bob was an electrical engineer… He made it a challenge to hound JPI until they came up with a proper answer… Unfortunately, Bob is only with us in spirit… Fortunately, Rags has the ability to deliver where Bob left off! Thanks for sharing the details again Rags! Best regards, -a- 1 Quote
DrQuinn Posted August 19, 2022 Author Report Posted August 19, 2022 5 hours ago, carusoam said: +1 for use of snubbers when the pressure in the line is always varying/fluctuating… typical of air flowing into cylinders with intake valves opening and closing… The old analog gauges couldn’t react fast enough, so they supplied an average that worked pretty well…. Some use internal snubbers… Often, on mechanical MP gauges there is also a calibrated weep hole…. Primarily to keep fuel from being sucked into the MP gauge on shut-down…. But also adds to the averaging effect… An engine monitor may be collecting data about once every second… it may be showing data from anywhere on a sinusoidal line… depending on how fast the sensor really can read the pressure… Some data collection devices use an averaging technique… the Ceis fuel level instruments use an average of data collected for a handful of seconds to mellow out the waves that occur in the fuel tanks during flight… Its cool having digital accuracy… until it becomes overwhelming… We had @Bob_Belville explore this challenge a few years ago… Bob was an electrical engineer… He made it a challenge to hound JPI until they came up with a proper answer… Unfortunately, Bob is only with us in spirit… Fortunately, Rags has the ability to deliver where Bob left off! Thanks for sharing the details again Rags! Best regards, -a- Thank you for your enlightening explanation. 1 Quote
DrQuinn Posted August 30, 2022 Author Report Posted August 30, 2022 On 8/18/2022 at 2:20 PM, Ragsf15e said: No problem. This is the one JPI recommended to me. It worked wonders. Here’s my caveat(s)… Garmin may have a different sensor than jpi. Also, I never found a snubber with a flare fitting, so I had to put an adapter on either side. Finally, make sure your manifold pressure line isn’t leaking somewhere. https://www.newark.com/omega/ps-8g/pressure-snubber-ss-1-8-npt/dp/31AC5909?gclid=EAIaIQobChMIx7-e2v_Q-QIVevHjBx3MsAbpEAQYAiABEgIZrvD_BwE&mckv=_dm|pcrid||plid||kword||match||slid||product|31AC5909|pgrid||ptaid||&CMP=KNC-GUSA-GEN-PMAX-LOW-ROAS-Shopping Problem solved thanks to this and it was an easy fit. Thanks again for pointing me in that direction! 3 Quote
carusoam Posted August 31, 2022 Report Posted August 31, 2022 Fantastic follow-up DrQ! Good pirep for the snubber. Best regards, -a- Quote
jamesm Posted September 8, 2022 Report Posted September 8, 2022 so is this air snubber equivalent to accumulator in hydraulic circuit or capacitor in electronic circuit ? Quote
EricJ Posted September 8, 2022 Report Posted September 8, 2022 2 minutes ago, jamesm said: so is this air snubber equivalent to accumulator in hydraulic circuit or capacitor in electronic circuit ? Resistor, actually. The air pressure can be compressed so it is the energy storage element (e.g., capacitor) and the snubber acts like a resistor that increases the time constant of an RC filter and narrows the bandwidth. 3 Quote
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