Hi everyone.
I've been having ongoing issues with fuel pressure fluctuations at higher altitudes and am trying to get them resolved once and for all. I've done everything I can think of to keep the tunnel cool (insulated firewall and floor of the tunnel, pipe wrap insulation around the scat tubes there, etc.), but that hasn't helped. I'm now in the process of moving my fuel flow transducer out of the tunnel and instead between the engine pump and servo as the "red cube" manufacturers suggest to avoid the possibility of the transducer being a source of vapour lock. I've also changed my fuel pressure sensor in case it was faulty in some way. I'm still working on these changes so haven't done a flight yet to see if the situation improves, but while the plane is down am trying to improve the performance of the fuel system as much as possible.
But something which still strikes me as "poor design" is that when cabin heat isn't required, heat from the exhaust muffs is directed to the area between the firewall and engine, right where the engine fuel pump is. I have the stainless cabin heat selector vents as well, which help to prevent heat from leaking into the cabin when it's not wanted, but does nothing to keep the area aft of the engine cool.
The way the heat system is designed is that hot air is always being produced around the exhaust muffs, and all you can do is choose whether to bring it into the cabin or the area aft of the engine. Is there any reason for this? Wouldn't it make more sense to just block the air flowing over the muffs in the first place if cabin heat isn't wanted?
I'm wondering if a "better" method would be instead to have butterfly valves right at the entrance to the heating scat tubes in the engine compartment, and routing them directly through the firewall instead of using the current heat selector vents. Operation with cabin heat on would be the same as now, but I can see two advantages when cabin heat isn't required. First of all, it wouldn't be dumping that heat into the area where the engine pump is. Secondly, there would be more air available for cooling the engine and oil since none would be consumed by the scat tubes. And of course, when cabin heat isn't required, that's when engine cooling is more important.
Not that I'm about to go and make these changes at this point (perhaps it is too much effort for too little gain), but I'm wondering why it isn't done this way in the first place. Or is there some unseen "gotcha" I'm not aware of, like the exhaust muffs getting too hot if there isn't air flowing over them all the time?
Another option I know people sometimes use is a shroud over the fuel pump. But that seems like a "hack" to circumvent poor design in the first place (like running an air conditioner at the same time as central heating on full blast instead of just turning both off). Besides, it looks like to get the shroud on, I'd have to remove and reinstall the fuel pump, and I've heard that can be tricky to do.
Any input?
Thanks! Dan
I've been having ongoing issues with fuel pressure fluctuations at higher altitudes and am trying to get them resolved once and for all. I've done everything I can think of to keep the tunnel cool (insulated firewall and floor of the tunnel, pipe wrap insulation around the scat tubes there, etc.), but that hasn't helped. I'm now in the process of moving my fuel flow transducer out of the tunnel and instead between the engine pump and servo as the "red cube" manufacturers suggest to avoid the possibility of the transducer being a source of vapour lock. I've also changed my fuel pressure sensor in case it was faulty in some way. I'm still working on these changes so haven't done a flight yet to see if the situation improves, but while the plane is down am trying to improve the performance of the fuel system as much as possible.
But something which still strikes me as "poor design" is that when cabin heat isn't required, heat from the exhaust muffs is directed to the area between the firewall and engine, right where the engine fuel pump is. I have the stainless cabin heat selector vents as well, which help to prevent heat from leaking into the cabin when it's not wanted, but does nothing to keep the area aft of the engine cool.
The way the heat system is designed is that hot air is always being produced around the exhaust muffs, and all you can do is choose whether to bring it into the cabin or the area aft of the engine. Is there any reason for this? Wouldn't it make more sense to just block the air flowing over the muffs in the first place if cabin heat isn't wanted?
I'm wondering if a "better" method would be instead to have butterfly valves right at the entrance to the heating scat tubes in the engine compartment, and routing them directly through the firewall instead of using the current heat selector vents. Operation with cabin heat on would be the same as now, but I can see two advantages when cabin heat isn't required. First of all, it wouldn't be dumping that heat into the area where the engine pump is. Secondly, there would be more air available for cooling the engine and oil since none would be consumed by the scat tubes. And of course, when cabin heat isn't required, that's when engine cooling is more important.
Not that I'm about to go and make these changes at this point (perhaps it is too much effort for too little gain), but I'm wondering why it isn't done this way in the first place. Or is there some unseen "gotcha" I'm not aware of, like the exhaust muffs getting too hot if there isn't air flowing over them all the time?
Another option I know people sometimes use is a shroud over the fuel pump. But that seems like a "hack" to circumvent poor design in the first place (like running an air conditioner at the same time as central heating on full blast instead of just turning both off). Besides, it looks like to get the shroud on, I'd have to remove and reinstall the fuel pump, and I've heard that can be tricky to do.
Any input?
Thanks! Dan