Reaching out to see if others have made similar observations and/or have thoughts/suggestions. My narrative will be somewhat lengthy, but want to provide as complete of a picture as I can.
I have an RV-10, IO-540-D4A5, Precision Airmotive Silverhawk (EX-5VA1, Bendix-Type) Fuel Injection, GRT PFD/MFD/Engine Monitor and Sensors. There are approximately 550h on the engine since new. In trying to reduce the spread in Fuel Flow between the first and last cylinder to reach peak EGT, I conducted several GAMI lean sweeps and with guidance from Don at AFP iteratively replaced injector restrictors, as well as changing the flow divider spring from 4 lb to 2 lb. The GAMI spread improved to 0.4 gph, but not the 0.2 gph that I was targeting. Bottom line, the engine runs fine. Induction leak tests have been negative.
When in cruise, I note that I get some fluctuations in indicated FF, without making any mixture adjustments, e.g. if cruising at WOT, 2300 rpm and 10.5 gph (moderately LOP), indicated FF may fluctuate +/- ~0.2 gph, seemingly at random. I should note that FP, on the mechanical pump runs around 21 or 22 psi. On the ground, I have plumbed in an analog gauge to confirm FP readings—they match. Turning on the boost pump increases the pressure to 27 or so. I’ve had the plane since 2018 and a review of engine data indicates these pressures to have been consistent throughout; in other words, while the pressure on the engine-driven pump is a bit lower than what seems typical (Lycoming’s min. spec. is 14 psi), I’m not seeing evidence suggesting the pump is starting to fail.
I’ve tracked each fuel addition since purchasing the plane, comparing the predicted fuel needed to fill the tanks v actual. Even when using 45 gal or so before refueling, the numbers are consistently within ~0.4g, always “predicting” a slightly higher need than actual. I could probably tighten that up further, but haven’t tweaked the flow sensor—if there’s going to be a slight error, I prefer it on the conservative side. Additionally, FFs on takeoff (boost pump on) are in line with expectations at various density altitudes, e.g. roughly 1 gph/10 hp. My home field is just under 3,000’ MSL, with DA’s often 6,500’ or so, so I don’t make the full 260 hp.
In reviewing engine data with Savvy analysts, they’ve mentioned that for reasons unknown, they often see FF fluctuations in IO-540s similar to what I’m experiencing. My first question to other RV-10ers is whether that fits with your experience?
It gets stranger from here… My flow sensor is in the tunnel, a foot or so forward of the boost pump, with an in-line filter in between. I understand that placing the sensor forward of the mechanical fuel pump as well, as near as practical to the throttle body is considered ideal, but my arrangement seems reasonably common. At any rate, I started to wonder of my sensor placement was contributing to the (mild) fluctuations in FF (since fuel was being drawn through the sensor rather than pushed) and whether absent those fluctuations the GAMI spread might actually be better than it appeared. As a potential test, I ran a couple of mixture sweeps with the boost pump on. Not only was pressure increased, but indicated FF was as well. Without the boost pump, I could lean down to ~8.6 gph before the engine would start to run a little rough. With the boost on, the onset of roughness began at about 12.4 gph. If I turn the boost pump off at that mixture setting, I’m still just at the onset of roughness, and indicated FF drops to ~8.6 gph—right where I would have expected it to be if the boost pump had been off throughout. That makes no sense to me—the engine should run the same on a given FF, with or without the boost pump. The only thing that comes to mind is that indicated FF is somehow significantly “off” at relatively low FFs when the boost pump is on, perhaps due to pulsation of some sort (recall that as noted above, the sensor is just foot or so forward of the boost pump). I contacted both Savvy and Precision Airmotive. Neither had an explanation. Question 2: Have others on VAF experienced something similar? Thoughts on alternate explanations? At the end of the day, it’s not really problematic—10.5 gph gives me 160-165 KTAS with enough range to make most of my trips without a fuel stop--but seems quite odd.
I have an RV-10, IO-540-D4A5, Precision Airmotive Silverhawk (EX-5VA1, Bendix-Type) Fuel Injection, GRT PFD/MFD/Engine Monitor and Sensors. There are approximately 550h on the engine since new. In trying to reduce the spread in Fuel Flow between the first and last cylinder to reach peak EGT, I conducted several GAMI lean sweeps and with guidance from Don at AFP iteratively replaced injector restrictors, as well as changing the flow divider spring from 4 lb to 2 lb. The GAMI spread improved to 0.4 gph, but not the 0.2 gph that I was targeting. Bottom line, the engine runs fine. Induction leak tests have been negative.
When in cruise, I note that I get some fluctuations in indicated FF, without making any mixture adjustments, e.g. if cruising at WOT, 2300 rpm and 10.5 gph (moderately LOP), indicated FF may fluctuate +/- ~0.2 gph, seemingly at random. I should note that FP, on the mechanical pump runs around 21 or 22 psi. On the ground, I have plumbed in an analog gauge to confirm FP readings—they match. Turning on the boost pump increases the pressure to 27 or so. I’ve had the plane since 2018 and a review of engine data indicates these pressures to have been consistent throughout; in other words, while the pressure on the engine-driven pump is a bit lower than what seems typical (Lycoming’s min. spec. is 14 psi), I’m not seeing evidence suggesting the pump is starting to fail.
I’ve tracked each fuel addition since purchasing the plane, comparing the predicted fuel needed to fill the tanks v actual. Even when using 45 gal or so before refueling, the numbers are consistently within ~0.4g, always “predicting” a slightly higher need than actual. I could probably tighten that up further, but haven’t tweaked the flow sensor—if there’s going to be a slight error, I prefer it on the conservative side. Additionally, FFs on takeoff (boost pump on) are in line with expectations at various density altitudes, e.g. roughly 1 gph/10 hp. My home field is just under 3,000’ MSL, with DA’s often 6,500’ or so, so I don’t make the full 260 hp.
In reviewing engine data with Savvy analysts, they’ve mentioned that for reasons unknown, they often see FF fluctuations in IO-540s similar to what I’m experiencing. My first question to other RV-10ers is whether that fits with your experience?
It gets stranger from here… My flow sensor is in the tunnel, a foot or so forward of the boost pump, with an in-line filter in between. I understand that placing the sensor forward of the mechanical fuel pump as well, as near as practical to the throttle body is considered ideal, but my arrangement seems reasonably common. At any rate, I started to wonder of my sensor placement was contributing to the (mild) fluctuations in FF (since fuel was being drawn through the sensor rather than pushed) and whether absent those fluctuations the GAMI spread might actually be better than it appeared. As a potential test, I ran a couple of mixture sweeps with the boost pump on. Not only was pressure increased, but indicated FF was as well. Without the boost pump, I could lean down to ~8.6 gph before the engine would start to run a little rough. With the boost on, the onset of roughness began at about 12.4 gph. If I turn the boost pump off at that mixture setting, I’m still just at the onset of roughness, and indicated FF drops to ~8.6 gph—right where I would have expected it to be if the boost pump had been off throughout. That makes no sense to me—the engine should run the same on a given FF, with or without the boost pump. The only thing that comes to mind is that indicated FF is somehow significantly “off” at relatively low FFs when the boost pump is on, perhaps due to pulsation of some sort (recall that as noted above, the sensor is just foot or so forward of the boost pump). I contacted both Savvy and Precision Airmotive. Neither had an explanation. Question 2: Have others on VAF experienced something similar? Thoughts on alternate explanations? At the end of the day, it’s not really problematic—10.5 gph gives me 160-165 KTAS with enough range to make most of my trips without a fuel stop--but seems quite odd.
