Fuel flow issue

[wbolan]

I am having trouble getting the correct fuel flow for my O-540. I am currently getting 26 GPH at a 19 degree attitude, I can get about 31 in a 3 point attitude. I removed the fuel flow meter and took out the one 90 fitting that came out of the gascolator and installed a 45 and that has gotten me to where I am now. I am going to try and redo some of the lines from the Tee from each tank to the gascolator and make the lines somewhat straighter but not sure if the is going to get me much. If there are any idea's that I missing let me know.

[alaskabearhawk]

What are you aiming for? 26gph is amazing…

[Bcone1381]

Some ideas.....

• Experiment with flow rates at all your fuel system components to understand where the flow must improve.
• measure fuel low by draining fuel into a bucket on a scale for 1 minute.
• If flow slows below your target you know where to increase the line diameter.
  • For example, if it flows out the gascolator above your target but not into the carburetor, then increase that line to 1/2".

I curious if 1/2" lines after the TEE's might put you over the top. I also am curious if the gascolator can be modified to increase the fitting sizes into and out of it.

[wbolan]

According to the FAA I am looking for around 35.75 (260HP X .55 X150% / 6 = GPH) I have seen others results and they were a lot closer.

[jaredyates]

I can totally feel your frustration with this, having gone through it once recently and once several years ago. You'll encounter people who say they built it like you did and got 45 gallons per hour, which I just don't understand how it's possible. But apparently they do. I found that in a gravity feed only system, the flow rates you're getting are pretty typical for 3/8th lines. If you're going to run a 540 without a pump, you might need to increase the line size from the gascalator to the firewall to half inch, which would also require using a larger gascalator. Before you go to all that trouble, it's probably worth taking some measurements further back upstream in the system. For example, disconnect the left or right line coming from your tank to your selector and measure your flow rate through each one of those. And then connect them together and measure again, etc. See how much each part of the system is restricting your flow. I think it's unlikely that your elbows upstream are causing as much of a problem, but perhaps the only way to know for sure is to start testing. If you can find a flow meter that works without pumps, please let us know.

[Bearhawk Aircraft]

Are you carbureted or injected? 150% for Carbuerated installs and 125% for injected (pump) engines.

§ 23.955 Fuel flow.

(a) General. The ability of the fuel system to provide fuel at the rates specified in this section and at a pres- sure sufficient for proper engine oper- ation must be shown in the attitude that is most critical with respect to fuel feed and quantity of unusable fuel. These conditions may be simulated in a suitable mockup. In addition—

(1) The quantity of fuel in the tank may not exceed the amount established as the unusable fuel supply for that tank under §23.959(a) plus that quan- tity necessary to show compliance with this section.

(2) If there is a fuel flowmeter, it must be blocked during the flow test and the fuel must flow through the meter or its bypass.

(3) If there is a flowmeter without a bypass, it must not have any probable failure mode that would restrict fuel flow below the level required for this fuel demonstration.

(4) The fuel flow must include that flow necessary for vapor return flow, jet pump drive flow, and for all other purposes for which fuel is used.

(b) Gravity systems. The fuel flow rate for gravity systems (main and reserve supply) must be 150 percent of the takeoff fuel consumption of the engine.

(c) Pump systems. The fuel flow rate for each pump system (main and re- serve supply) for each reciprocating en- gine must be 125 percent of the fuel flow required by the engine at the max- imum takeoff power approved under this part.

[Tyler]

I had the same issue with the same setup. I converted to 1/2" lines and fitting coming out of the fuel selector, got the new gascolator (steves), and ran 1/2" all the way to the carb. Prob solved and got over 40gph with that setup. I have an O-540, Model 5. The steve's gascolator did not require much work to mount in place of the Bob model.

[wbolan]

Did you use steve gascolator with the 3/8" NPT fittings? And the one that doesn't require a bracket? I was looking at those thinking the 3/8 NPT fittings would help instead of the 1/4 NPT fittings

[Tyler]

Yeah man. I got the one with 3/8" NPT for 1/2 lines and no bracket. I used aluminum angle to mount to my existing gascolator mount plate. Using Bob's and switching the fittings was an idea on the table but it still didn't eliminate the 3/8" port in the gascolator itself. The swap out of a few items was so simple and resulted in such a drastic improvement in fuel flow, I can't imagine not doing it. The peace of mind is wonderful, knowing you have above acceptable fuel flow for your engine at all attitudes, with no need for any additional pumps, electronics, switches or protocols for takeoff/landing.

[Bcone1381]

A -7 fitting will secure a 1/2" line to Bob's gascolator. It would not flow as well of course as the -8 fittings with their 3/8" NPT female thread. but it could be tested. Just an idea, and you asked for ideas.

I am also curious if Bob's gascolator has too many barriers to have its female fittings upsized from 1/4 NPT to 3/8 NPT?

[Battson]

So, this is an O-540, which drinks less than 90 L/h at full power at sea level (23 USgal / h). So 150% of that fuel flow, the amount required for a carb install, would be less than 34.5 USgal / h.

Your install looks great, and it should easily deliver over 100%, 125%, and hopefully 150%... Often that last little bit seems to get tricky.

Have you checked for obstructions in the lines?
This is a common problem, swarf, spider nests, cobwebs over the fuel screens in the tanks, rubber grommets accidentally created by manufacturing the flexible fuel lines - these are all real problems I have seen first hand, just with our small fleet in New Zealand. They cause huge blockages and reduce flow a lot.

Personally, I would look into reinstalling the fuel flow unit as that is super useful. If you had to install a pump and use it for full power, I would do that....