Thanks Chris!! Your input is very much appreciate and makes me feel much much better. I will check and see if my cruise psi goes up after a bit. I guess I was sweating so many beads from my forehead that I was only concerned about looking for places to land. Luckily, it ran fine all the way home. It comforts me to know the psi minimum for my engine is .5 psi. The only thing that makes me wonder is that this recently changed. When I first flew, the psi 's were 4-6 but then again, that's when it was 25F outside and I was flying around the airport at less than 75% power - so I think at 70F and cruising power could be why it changed but I just wanted to be sure before I try to go somewhere and lose the engine. I just spoke to Lorn Olsen for 1/2 hour on the phone who told me to check the vents and make sure they are not
blocked. He also said his indicator reads zero psi on climb out. Oooohh, that would stress me out but he also explained why - which had everything to do with the carb and how it is set up and where the fuel pressure sender is located. Mine is right on the engine driven fuel pump. People tell me to move it to the firewall with a flex hose because it will fail prematurely with all the engine vibration. Again, thank you so much for your ideas and help!! I will keep you posted. We definately have the oil temp problem 100% solved. It's almost too cold now. In the fall, I will add a restrictor that Van's sells for the Stewart Warner coolers. After I solve or understand this fuel pressure deal, I will probably look into your diffusers and plenum. Right now, my CHT's are running 325-350F. Thanks Chris!
Matt
marv@lancair.net wrote: Posted for "Zavatson, Christopher J (US SSA)"
:
Matt,
You have three different issues here:
1. Performance characteristics of diaphragm pumps naturally produce
higher pressure at low flows rates and decreasing pressure at higher
flow rates. You should be able to see this during take off and initial
climb at low altitude. Pressure will drop from 6 to possibly 1.5-2 psi.
At idle, fuel pressure should be at its highest, which you observed
during landing.
2. Fuel tank venting. Some positive pressure is good, but be careful.
You could theoretically pressurize the wing with full ram pressure
~1psi. At this pressure the wing skins will bulge. You will be able to
see all the rib locations when the light is right. I observed this on
a
flight after an unrelated change that lowered the vent farther into the
slipstream. I measured the tank pressure and found almost full ram
pressure. Some filing on the vent tube brought the pressure down.
Vents can only contribute +/- 1 psi so that isn't the whole story
either.
3. You are not alone in your observation. After observing the same
thing primarily on long, high altitude flights, I discussed this with an
A&P here at work. He used to work for Piper and they saw the same
thing. He tells me they spent lots of time instrumenting the system
trying to figure out the root cause of this occasionally pressure drop,
but they came up empty. As you saw, the boost pump does not eliminate
this odd behavior. This points to something inside the carburetor.
Does your pressure come back up after a time in constant cruise?
Chris Zavatson
N91CZ
360std
www.N91CZ.com
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