F4 Tank - Ram Air

Below is the first completed mockup of the F4 tank after being adapted to the F2 frame as well as the functional ram-air.

Getting the look above took a lot of work in adapting both the F4 tank as well as the F4 airbox to the stock F2 equipment. The first challenge was in getting the F4 tank to fit the F2 frame. While the tanks have the same basic dimensions, the subtle differences in petcock location, mounting flanges and that extra gallon of gas it holds made for some special work.

Shown above are the two mounts. The F2 has rear mount provisions for the F4 tank such that a direct bolt on is possible.

The F4 tank has a slightly larger profile that feels huge if you mount it to the stock F2 frame without any modification. Shown below is how the tank would sit. I wasn't happy with the gaps, petcock elimination and misalignment so I decided to start modifying both the tank and frame.

The F4 tanks front mount is raised up off the frame as shown below. The bump in the stock F2 frame where the stock tank mounts raises it up even more. See below.

For starters, I flattened the tank flange out so as to lower it down and take away some of the height. See Below:

I then cut the side holes off, re-drilled the center of the flange and inserted one of the stock F4 bushings so I could reuse the stock F2 mount on the frame. This lowered the tank just enough to be usefull. See Below:

The rear of the tank posed some more complicated problems. I chose to cut off the stock mount so I could relocate it behind the tank mount. This allowed me to lower the tank almost 2 inches but required some fabrication to recreate a new mount.

As you can see, this brought the tank down to the stock frame but only with the petcock removed as it is directly above the tail frame bar.

For my mod, I needed to cut out the upper tail bars to accomodate the petcock location (which is on the opposite side of the tank from the stock F2 one). I don't have pictures of my X bracing that I welded in around the petcock to brace the tail frame back to the frame (coming soon!)

The stock tank mount was relocated back about 2 inches, along with the battery tray. The upper battery tray lid bolted right back up, but the side battery tray bolts needed new plates added to the tail to re-mount to.  The batter wires just barely reach without any modification. I also cut the stock tail tank mounting plate to create a mounting slot for the F4 solo seat.  It works great.

My F2 had a K&N filter on it as shown in the pic below. The stock F2 airflow comes in from 2 tubes in the lid as opposed to the F4 where the main tubes enter the bottom body of the air box... more on this later in this thread.

Underneath the filter, the stock F2 velicity stacks are aluminum and twist into the aribox.

Since I was keeping the stock carbs, this is where I needed to make some kind of adaption to the F4 airbox. Initially, I wanted to redrill the airbox so it would mount to the carbs directly, but the shape of the bottom of the box, attachment of the F4 velocity stacks and ease of removal required a different aproach.

Comparing the F4 (left) and F2 (right) airboxes, you can see an immediate difference in both design and size. The ram air tubes provide much more air flow than the 2 small feeder tubes on the F2 box, which consequently also pull hot air off the engine and radiator sides.

The next challenge in modification was the front of the F4 box which has two small chambers (touching the f2 box below) that run smack into the coils. I'm wasnt real happy with this decision as I knew it was either relocate the coils or chop off those two chambers. After careful consideration I decided that chopping the chanbers off the airbox was the more gracefull adpation and I would have to keep an eye out for strange power band nuances after completion. Thankfully, nothing resulted from it that I can tell.

Shown below you can see the tubes to those chambers, they almost look more harmonic than vacuum oriented. Reminds me of porting on a speaker box. I foamed up those ports for a sealing. The filter design is substantially different.  I am still running this stock filter but plan on moving to another K&N. Also note the more fluid design of airflow in the F4 box (right).

Cutting off the F4 underside ports game me just shy of 1/4" of clearance that I still needed to find to get the box to mate to the carbs. I located a 1/4" aluminum plate that I decided would be the most elegant way to bridge the gap between the carbs and the airbox. I started by tracing the airboxes to the plate as shown below, also marking the bolt hole locations.

Using only a drill press, I managed to use a 1 1/2" hole saw, various size drill bits and some creative use of a porting bit on my drill press along with the indexing vice to "mill" a clearance in the plate. I'm actually suprised it worked so well!!!

With the place drilled, clearanced and triple checked, I used the factory F2 airbox screws to fasten it to the carbs. Since this was simply a proof of concept, I didnt spend as much time on it as I should have so there are all kinds of flaws in the craftmanship... which is even more suprising that it performed so much better than my F2 box...  got me on that one! 

Below you can see the plugged port holes as well as the velocity stacks mounted up. You can also see the rather crude adaption of the stacks to the ports as they dont entirely line up. My next plate will merge those together in a nice smooth transition. If I get anything above my current performance gain that its just icing on the cake!!!

Finally, you can see its getting dark and I'm working partially without lighting in this area - but the airbox is attached and ready for the tank to be installed. If you are attempting this, make sure you pre-fit the tank after the airbox work is done, so you can properly locate the rear mount for final wleding.

An important thing to note here is that for the ram-air to work properly, you need to plumb the carb fuel bowl overflow vents to the airbox so that both the bowls and the intake side stay equalized together. If you do not, the ram air will force fuel back down your jets over 25 mph and your bike will barely run.

I have a few thousand miles on the F4i airbox now as you can see below by the faded black primer. With the Ram air, I find my typical throttle position to be closed more than open when carving up the hills, especialy when descending through them. If the gas tank is full, the bike will power up to 155 mph without a hitch but at a half tank or less, the pressure cant keep the fuel bowls filled. This would explain the addition of a fuel pump in the F3... which came with ram air. A recently purchased 97 F3 track bike, from which I plan to swap the engine into this bike, will be the next to test with the stock F3 ram-air . From that baseline, I'll run the F4 airbox again on the F3's larger 36mm carbs. Stay tuned...