Dynon:Too Much Information?

[N223JH]

This is a request for a layman's explanation for the variation of engine fluid readouts. Specifically, why do fuel pressure indications vary, sometimes inverse to what would be expected, even in cruise flight?

I have an RV-12 Rotax with the fuel return line installed per plans and the Dynon 180 pinball machine screen. Sometimes throttle application results in higher fuel pressure indication (huh?), sometimes lower--as would be expected. The sensor has been bench-tested as accurate to Dynon's current standards. Level cruise variations range from 4.4-2.75. Green arc starts at 3 PSI. Variations from normal 3.7-3.9 last from momentary to a minute or more. The Dynon downloads on Excel confirm these numbers.

I have been watching the fuel pressure because of suspected vapor problems operating in hot weather on winter grade car gas. I have been through the fuel system from tank vents to carb bowls and no anomalies found. Fuel flow numbers seem in line with power.

I have been told not to worry about variations as long as they are in the green. But I sure would like to know what's going on. TIA

Jim
RV-12 N233TX

[Ron Lee]

I am not familiar with your configuration but my RV-6A with O-360 Carbureted engine has fluctuations in fuel pressure. I cannot characterize it during flight.

However, fuel flow jumps up and down such that I have to watch it for a minute or so to get a feel for the average (actual) fuel flow.

That has been discussed here often and is not a problem for my aircraft.

[N223JH]

Yep, I found several good threads on the fuel pressure problems on Lycs. They were exactly like I experienced with my 6A.

I was hoping by posting on this forum to get some of the more experienced engine systems guys to speculate on the Rotax/fuel return line/boost pump on all the timeconfiguration. Since I can't hit the boost and get a few pounds like in my 6A, the low pressure indication is more problematic.

I have already shrouded and blast-tubed the gascolator and seen measurable improvements in fuel pressure, suggesting that vapor formation may be a factor. The mechanical fuel pump is up front but I may need to run some scat tubing there, too.

I suspect avgas would be at least a partial fix but the Rotax doesn't like it, especially at lower RPMs.

[Peterk]

I have already shrouded and blast-tubed the gascolator


Jim, can you be a little more explicit about what you did around the gascolator? thx.

Pete

[N223JH]

Pete, I fabricated a stout fibreglas shroud with a snoot at the top to take 1 1/4 scat tubing, bottom open. Two AN3 bolts/nutplates thru firewall. Hinged section at rear to get around fuel port. Scat secured to engine to RH "porthole" at nose where f-glassed boss holds scat with hose clamp. Real ugly but seems to help.

Send me a PM with your email and I'll send you pics of the install. Now to figure out something for the fuel pump. I think this car gas is going to be a challenge in summertime South Texas. Jim