why a large change in fuel flow with ram door open?

[RobertE]

Chaulk this question up to simply trying to understand some of the finer points of our airplanes.

 

I've got a J model that, like all Js, has a pretty efficient air intake system, so the ram door doesn't do much.  If I'm at, say, 20" with filtered air I only pick up another 1/4" by opening the door.  So here is my question.  Shouldn't a roughly 1.25% increase in MAP produce roughly a 1.25% increase in fuel flow?  As it is, that .25" MAP adds about .5 gals/hour, which is more than a 5% boost. 

 

Am I missing something?

[KSMooniac]

I never noticed such a change in FF with my JPI EDM-700 system when I had my ram air system.  When I got my plane painted, I deleted the system and am glad I did.  Not sure how or why you're recording 1/2 GPH change.  

[cliffy]

Could it be 1 of the air diaphragms in the fuel servo sensing the higher MP with the door open and compensating by increasing fuel flow for the added air available.

[RobertE]

My understanding of how the fuel injection system works - and I may misunderstand that system which is why I asked the question - is that having leaned to a particular mixture, the system maintains that mixture by either increasing or decreasing fuel flow in response to changes in manifold pressure.  This is why I would expect the same approximate change in fuel flow as in MP.  Again, maybe I'm just off in my understanding. 

[Super Dave]

If your J has the same RSA 5 fuel injection system that my F does, then yes, the servo senses the change in MP and increases the FF accordingly. I'd agree that .5gph seems a little high for just 1/4" MP. I see about an inch increase in my MP with the ram air open, and I know that FF does not increase anywhere near 2GPH.

 

In my F, the fuel servo impact tubes are directly behind the ram air door, but the MP for the cockpit gauge is taken further downstream. I'm guessing here, but maybe there is some kind of localized disturbance that occurs at the ram inlet that artificially increases what the impact tubes sense?