The firewall forward area, and how the builder organizes/installs everything on the firewall itself, is part of the art of building one of these planes. The goal of my firewall layout is: 1) efficient layout (short hose runs, logical routing), and 2) serviceability (the ability to get at everything for removal/repair). Also, special attention must be paid so that everything can withstand the vibration and heat of the fwf environment. Firewall design on the RV-3 is especially challenging due to the smaller firewall, but especially because the back of the engine is so close to firewall, much closer than any other RV model. Wow, it's tight back there!!


Firewall recess
In order to make room for the prop governor I had to install a stainless steel recess into the firewall. I used the recess kit made for the RV-7/9 available from Van's, carefully ascertained where it needed to be, and cut my firewall. It is riveted in place with ProSeal smeared all around so as to prevent any leaks. Even with the recess installed I had to modify the prop governor cable bracket because it wouldn't also fit in the same recess with the governor.

Oil filter adapter
Most of the flying RV-3s I've seen do not have an oil filter, just the Lycoming screen suitable only for preventing small chunks from circulating in your oil system. If you want to install a normal oil filter on the back it will hit the firewall, therefore an angle adapter such as that made by B&C will be necessary. B&C makes three different spacers to locate the angle adapter rearward to clear whatever engine mount tubes or whatever else might be in the way, in this case one of the engine mount tubes does interfere. When I ordered my engine I guessed and ordered the .75" spacer. It turned out the 1.4" was needed for my installation. I've seen two other RV-3s that worked with the .75" spacer but they both had the older four point engine mount. The newer six point mount has a different shape to the upper bars that requires the 1.4" spacer. B&C oil filter adapters can be found here... www.bandcspecialty.com

RV builders know that a FAB is a "filtered air box". Van's makes a FAB for both the O-320 and O-360 Lycomings with carburetors or updraft fuel injection servo bodies. The two different FABs are quite different in size with the O-360 version being a fair amount larger in all dimensions. I'm installing an IO-320 and using the new Silver Hawk EX fuel injection. It turns out that the servo body is taller than the standard O-320 carburetor by 7/8". There's very little space between the cowl's intake scoop and the the FAB so this turned out to be a problem. At first I thought I could make this combo work by making a modification to the FAB-320. Having installed a FAB-360 in my RV-8 I was familiar with how they are constructed. As I examined the FAB-320 I noticed that the filter element does not contact the bottom of the fiberglass bowl as it does on the FAB-360, but rather is suspended with an aluminum plate on the bottom. Not using the bottom plate and modifying the box to retain the filter as it does on the 360 version would get back some of the height and clearance taken by the taller servo. Turns out though its not enough. I ended up having to order a special lower cowl from Van's with an O-360 intake scoop. For further details on the cowl check here. Further, the O-360 intake scoop cowl has it's inlet farther forward and is a larger diameter, therefore I also needed to order a new FAB-360 turning my FAB-320 into scrap. If you are going to install fuel injection you can save a bunch of money by special ordering the correct cowl to start with!!

Alternator mount
Be sure and shorten the alternator mount arm so as to allow the alternator to nest fairly close to the engine case and provide good clearance for the pulley to the cowl. You do this by simply cutting off the arm and drilling a new mount hole. You will then need a shorter belt than Van's calls out, I ended up with a 35.5" belt.


Heater box
For improved fire resistance I'm using the stainless steel version of the Van's triangular firewall heater box made by EPM.AV Corp., details here. Because I want to duct the heat up to a vent in my center console I opted for the version with the flange on it where it penetrates the firewall. Then I have a 90° elbow attached with scat tube on that. I located it behind the right rudder pedal where it's in close proximity to the heat muff thereby using short runs of scat tube.



Fuel flow sensor
One of the issues builder's face when laying out fwf components is where to locate the fuel flow sensor for the fuel computer. Most fuel flow computers (often incorporated in a full engine monitor system) use a fuel flow sensor purchased from Floscan. Electronics International however makes their own improved version which they call the "red cube". Advanced Flight Systems, makers of the EFIS/EMS box I've installed, uses the E.I. sensor as well. Regardless of the sensor variety the issues are the same. The sensor makers want it located as far downstream in the fuel system as possible. The reason for this is that they don't want fuel pulled through it thereby creating a lower pressure condition. When hot this can cause a vapor condition in the fuel line sooner, commonly known as "vapor lock". The higher the pressure the higher the temp at which the fuel vaporizes. Therefore they advise you to locate the sensor after the aux fuel pump, after the engine driven fuel pump, and in the case of a fuel injection system, even after the servo body. This means installing it in the line that goes from the servo to the flow divider or "spider" on top of the engine. Further, they instruct you to install it horizontal with the wires facing up and 6" of relatively straight fuel line leading into it. Meeting all these conditions can be difficult but can be done. Pics of my installation show that I've got it located where they specify but have not complied with some of the other parameters. When I asked the folks at E.I. "which was more important, locate it downstream of the servo or have it horizontal with the wires exiting upward", they responsed to have it after the servo. So, that's the way I installed mine. This installation works very well so far. It is accurate, doesn't change flow readings when the aux pump is turned on, and the fuel system has never even hinted at vapor lock so far.


Oil cooler


Due to the shape of the cheek cowl on the RV-3/4 the oil cooler must be positioned quite low on the back of cylinder number 4. If you bolted it right to the baffling as depicted in the plans the cylinder fins would block more than half the inlet to the cooler. Also, due to the close proximity of the hot fins to the oil cooler, I would think there would be a fair amount of heat transfer. I therefore constructed a small "mini-plenum" that positions the oil cooler about an inch back from the cylinder and provides a small air chamber, or plenum, for the air to fill before entering the oil cooler. The mini-plenum is constructed as a separate piece which then is riveted to the baffle. Thanks to RV-4 builder Rion Bourgeois for the idea, I copied his installation almost exactly. I then installed an oil cooler shutter on the back to help keep oil temps up in the winter. It would be more effective on the front but I would have had to construct my mini-plenum differently which I didn't want to do. Pic of the shutter coming soon.

Baffling and overall fwf layout
The RV-3 is especially challenging in laying out the fwf area because the engine is situated so close to the firewall. In addition to having logical functional routing of fluid lines, control cables, and wires you really want to keep service accessability for everthing in mind, difficult in such a tight environment. Since "one picture is worth a thousand words" really applies in terms of seeing how everything lays out and works together I've posted pics of my fwf area from several angles. These are large pictures so you can see detail, feel free to download them and reduce them in your viewing software of choice.


Engine cooling
As I built my -3 I talked to as many owners of both RV-3s and RV-4s as possible and paid attention to any trends in the way their systems cooled, both cylinders and oil. From my informal survey it seems that the cheek-cowled RVs (RV-3 and RV-4) cool more effectively than the newer designs. In fact many, especially the O-320 powered ships, report overcooling of both cylinders and oil. I've found that with my stock Van's baffled system my cylinders are perfect: they run in the 330-370° range during summer cruise and only under a strenuous hot climb will my hottest cylinder hit 400°. The oil however does overcool. During high-power summer operation the highest oil temp I've seen is 175°. 55-65% cruise yields 165° so I know I'll have a problem in the winter. I have a Stewart Warner oil cooler which does indeed cool more effectively than the others, which I probably didn't need to spend the extra money on. I will shortly be experimenting on ways of getting the oil temps up, more as it develops.


Oil cooler shutter
Due to my overly cool oil temps as noted above I bought one of the oil cooler shutters from Van's. Now that I'm past the engine breakin stage I finally got around to installing it. I'm pleased to report that it works great! Initial testing shows that it will increase my temps by around 35°. Note that the shutter needs to be installed in FRONT of the oil cooler for maximum effectiveness. Other builders have installed them on the back of the oil cooler and they typically show only a 5-10° temperature change. In the temperate NW where I live I have found I need to leave it closed fully in anything other than warm summer conditions.



Cowl heat shield
Having had some heat damage to the cowl on my RV-8 I learned that one needs to protect the cowl where the exhaust pipes get near it. Van's now sells some adhesive-backed aluminum that works well, the pic to the right shows how I installed mine. Having heard from several other builders about heat damage to cowls I highly recommend adding some sort of heat shielding. It does add some weight but I think it's worth it for the long term benefit of the cowl.