Filtered Air Box Seal Gap

I could really use some advice on sizing the gap between the filtered air box (FAB) and the cowl snout for my RV7A vertical induction setup. So far I have trimmed the FAB top plate back to where the cowl will just clear the front edge if I drop it straight down. How large should the gap be between the FAB and the fiberglass snout for the rubber seal strips? The seal material is 1.75 inches wide so I am guessing about 0.75 inch overlap with the FAB for riveting, 0.5 inch gap, and 0.5 inch overlap with the snout? Is that about right?

I am concerned about making the gap too small because of the threads I have read about FAB top plate cracking, perhaps caused by forces put on the FAB at engine startup due to relative motion between the FAB and the cowl. But the instructions don't give a dimension.

Note that I have a Catto 3-blade prop so I am also hoping to make the lower cowl as easy to install and remove as possible. So a related question is has anyone had success attaching the lower seal strip to the cowl snout to make cowl removal easier? It seems like that would work but I don't know if it would compromise the seal with a resulting loss of manifold pressure.

I believe if you search the forums, people have attatched the top strip to the Fab and the bottom U strip around the snout on the cowl with something like 1/4-1/2 inch gap to make cowl assembly removal easier.
I couldn't get mine to seal worth a darn with the fabric on the outside so I slightly reshaped my FAB/cowl snout and clamped/glued my airfabric seal to the outside of the cowl snout. The seal material tucks into the snout and held with tape/fingers while the cowl is raised into position, the seal material then lays into the FAB with the incoming air pushing the seal tighter. Mostly guestimated theory, mine is a taildragger (not flying yet but soon) and the prop isn't installed so we'll see. Pic is of first trail fitting some scrap material but you get the idea.

A Wild Idea

I am wondering if anyone has tried anything like this. Cut the inlet nacelle off the cowl and mold a new flange that would allow it to be reattached after the lower cowl is installed. Then the cowl could be installed without the seal getting in the way at all and the nacelle would be installed with a few screws allowing the seal to slide snugly into the air box snout so the internal pressure makes it seal tight to the walls of the air box just like around the inlets for the engine baffles.

Is this a crazy idea?

John, I wish I had seen your idea while I was originally fitting my cowl, I would have shamelessly copied it! Some people on the forums have made the whole bottom scoop removable.

Bottom Scoop Removal

Cutting the entire scoop off is major surgery. I'm not sure I want to go that far although I guess you could use it as an inspection port too. I'm still hoping to hear if anyone else has tried just removing the inlet port.

Filtered airbox gap with AFP controller

Recently I was up with my safety pilot/CFI shooting approaches. Last one of the day was the best; needles centered, airspeed good, descended to 200 ft. AGL. Life was good! Declared the miss and advanced the throttle. Initially engine RPMs increased for about 2 secs. then fell off to 500 rpm. It is surprising what you can do with 200 ft. when motivated; fuel boost on, switch tanks, advance throttle & mixture. All to no avail. Forgot about alternate air. Proceeded to land without incident. Interestingly when I retarded the throttle, engine RPMs increased. We had enough power to clear the runway and pull off to a run up area. Each time I advanced the throttle, RPMs would decrease. Shut down and got out to take a look.

The FAB (including the al. mounting plate) had dropped off of the AFP fuel controller, and was resting in the lower cowl about 6" aft of its usual position. In retrospect, I think the abnormal responses to throttle inputs was caused by air intake suction pulling the FAB up far enough to blank off the fuel controller air intake.

I had a 1/2" gap between the aft edge of my cowl air snout and the forward edge of the FAB. This may have caused/contributed to the FAB dislodging. Don at AFP likes to see 3/4 to 1" gap. I remedied that, and also installed a brace from the bottom of my engine sump to the FAB mounting plate. Also used some blue Loctite between the fuel controller and the AFP mounting ring per Don's recommendation.

All in all, I feel like we dodged a bullet that day. Over the threshold was the best place possible to lose the FAB. I had flown to Leadville and back the day before. That would not have been pretty. I bought a lottery ticket on my way home, but my luck had run out.

Jim Berry
RV-10