TRX250R Airbox Install on ATC250R

[Red Rider]

I got a TRX250R airbox off of eBay about a year ago, with the hopes of swapping out the ATC's airbox in favor of it. Why do I want to do this, you ask. Because I believe the TRX's airbox is a better design, and more importantly, less restrictive.

In the first pic, I have both airboxes side by side. Note how the TRX airbox has a fairly straight, unobstructed path from the airfilter to the carburetor, while the ATC airbox has that nasty 90 degree bend in the airbox-to-carb boot.

To see if I can get the airbox high enough to clear the swingarm, I'll need to cut off the coolant overflow bottle's rear bracket from the frame. Talk about the point of no return! Believe me, I took plenty of measurements to ascertain whether or not this airbox would work before I broke out the cut-off grinder. I will also need to relocate 2 items to fit this airbox in the ATC's frame:

1. Rear shock reservoir is interfering with the airbox-to-carb boot, so it will be relocated to the back of the frame near the rear grabbar, similar to the TRX's mounting location.

2. AC Regulator bracket is interfering with the airbox, as well as the new location for the rear shock reservoir, so it will be relocated to an, as of yet, undetermined location.

In the remaining pics, I removed the rear shock spring so I could swing the rear suspension through it's full travel to make sure the swingarm wouldn't contact the airbox under full compression. It looks like it should clear just fine.

[jrsqlc]

Looks real good from here. Any difference in the performance??

[3wheelrider]

Is it mounted permanently yet? Hows it held now?-I've always thought of somehow doin that straight shot to the carb without ditchin' the airbox... great idea.
Off topic but- Is that footbrake pedal homemade? Any more pics of it? Also what swingarm is that?

[Red Rider]

Looks real good from here. Any difference in the performance??

Thanks, nowhere near complete yet, so I don't know. I'm not expecting a whole lot, but every little bit helps right?

Is it mounted permanently yet? Hows it held now?-I've always thought of somehow doin that straight shot to the carb without ditchin' the airbox... great idea.
Off topic but- Is that footbrake pedal homemade? Any more pics of it? Also what swingarm is that?

No, it is not mounted permanently yet. In the pics, it was being held in place by some twist ties & 2 strategically placed screwdrivers. That of course was only temporary to check the clearance during the suspension swing. When I get it finalized, it will utilize the 2 factory airbox mounts on the TRX airbox, and a custom rear mount. The rear brake pedal is an aluminum number that I made a few years ago & the swingarm is an 88-89' TRX250R swingarm that I lengthened 3". There are a few pics of both items on my website, just click on Red Rider's Sand Machine at the bottom of any of my posts.

[Red Rider]

Once I was sure the TRX airbox would fit in the frame and clear the suspension, I went ahead and cut off both forward airbox mounts, the AC regulator bracket, & 1 wire harness stay. All of the residual bracket material & weld was then ground down to the tubular frame. I used the thin-walled tubing from the old airbox mounts, that were removed, and had them welded to the frame in the new locations. After a little sanding, priming, & painting, the frame is protected from the elements once again.

[Red Rider]

I got the relocation of the rear shock reservoir finalized last night, so I installed the carburetor, carb-to-airbox boot, airbox, & reservoir. Now I can take some more pics, and more importantly, measurements for the new, longer shock reservoir hose. Once I get the new hose & install it, the rear shock will be sent out for a long overdue rebuild. Since the TRX airbox's rear mount was incompatible with the ATC's frame, I had to remove it. Pic #1 shows the new rear mount that I fabbed up.

[Red Rider]

There hasn't been a whole lot of progress here lately, it would seem. Well, actually there has been progress, but it was stuff that I had already done. I realized that the airbox was dangerously close to hitting the rear shock spring under full suspension compression (not very obvious when you've removed the spring from the shock in order to swing the suspension through it's entire range of travel. Out of sight, out of mind I guess). An easy fix though, just raise up the rear of the airbox a bit, which means trashing a perfectly good rear bracket & making another one. The new one is the same style as the first one, but it now holds the airbox at the proper level.

I finally decided on the new location for the AC regulator also. It will be in a similar spot as before, but will hang on the side of the airbox, rather than from the frame. I made a bracket for that, and wasn't happy with the results, so I made another one, which I was pleased with (In this case, the 2nd time was the charm).

I also got a prototype shock-to-reservoir line fabbed up, so it should be easy for Race Tech to duplicate when they do the rebuild on the shock, which was shipped out today.

Today's pics show the relocated reservoir with the prototype line installed & the airfilter's view looking straight down the airbox-to-carb boot.

[Red Rider]

While I await my rebuilt shock's return, I've been accomplishing little odds & ends on this project. The TRX airbox is quite a bit wider than the ATC unit, and it was going to interfere with the stock mounting location of the coolant overflow bottle. I suppose I could have ditched the bottle entirely, but I really didn't want to do that. I also thought about mounting the stock bottle, or a different bottle, in another location, but that didn't seem feasible. I ended up keeping the bottle in the stock location, which required a hole to be cut in the side of the new airbox, so the overflow bottle could protrude inside about 1".

To seal up the giant hole in the side of the airbox, I fabbed up a piece of 5052 aluminum sheet metal with a bunch of complex bends & angles that allow it to wrap around the portion of the bottle that protrudes into the airbox. I painted it black, riveted it into place, and then sealed up the edges with black silicone. Since the upper mount for the overflow bottle was in the way of the airbox & was removed from the frame, I made up a new mount & riveted it to the airbox as well. Hindsight being 20/20 as it is, since I ended up cutting the TRX airbox for the overflow bottle, I guess I could have left the stock upper mount on the frame instead of removing it. Pic #1 shows the coolant overflow bottle indention & new upper mount.

I also got the airbox intake sorted out. The TRX airbox uses a separate intake tube, routed to the right, front side of the airbox, and the ATC airbox uses the frame's backbone routed to the center, front side of the airbox. To connect the frame's backbone to the TRX airbox, I used the stock ATC's frame-to-airbox boot, but I clamped the airbox end onto the frame instead, and rotated the boot off to the right. This got the boot close to lining up with the TRX airbox intake, but not quite. To make it line up perfectly, a new hole was cut about 1" higher & towards the center. The remaining portion of the old hole was sealed up with a scrap of plastic from the hole that was cut in the right side of the airbox. To connect the ATC's frame-to-airbox boot to the TRX's intake boot, I made a 2 1/8" OD tube from some 5052 aluminum sheet metal, welded up the seam, sanded it, & of course polished it. I also ventilated the airbox lid a bit too. Pic #2 shows the completed intake plumbing.

The final pic shows the AC regulator mounted in it's new location. I am now anxiously awaiting the return of my rear shock.

[fabiodriven]

I must admit, I kinda like this mod.

[Red Rider]

I must admit, I kinda like this mod.

Fabio, thanks, but it almost sounds as if you are "ashamed" to admit it though.

Good news, my rear shock is done & on it's way back home.

[Red Rider]

There hadn't been much progress on this until today. My rebuilt shock came back about a month ago, and looked brand new. Unfortunately, the reservoir line was bolted to the shock at an incorrect angle, and the bend in the banjo fitting was a bit too extreme, which was causing the stiff, braided stainless steel line to "kink up" when the shock & reservoir were mounted correctly.

After a 30 minute phone conversation with Race Tech, who were very understanding, as was I, we determined that the best course of action was to send them my prototype line again. Once they had received the line, they forwarded it to their custom line builder who made a new line with straight banjo fittings this time. The new line was then sent to me so I could attach it to the shock & reservoir at the exact angles. The rear shock was then removed from the bike, boxed back up, and sent back to Race Tech so they could recharge it with oil & nitrogen once again.

I got the shock back a few weeks ago now, and finally got around to putting it all back together. Now I need to go riding to test out the new rear suspension, and see if I notice any gains from the airbox switcheroo. It sure sounded nice in the garage when I was running it. Ask my neighbors, I'm sure they'll agree.

[Red Rider]

I guess I never updated this thread after the first ride years ago. Well first off, the rear suspension feels much better after being freshened up and re-sprung/re-valved for my weight, riding style, and longer swingarm. As for any power improvements due to the TRX airbox, it's hard to tell, as my butt-dyno isn't calibrated precisely enough. Maybe there would be some measurable gains on a real dyno, or maybe not.

Recently, I decided to clean up the airbox intake boot design. I replaced the frame-to-airbox boot with some 2.25" ID silicone hose. The new boot is comprised of 2 pieces of silicone hose, with 30 degree bends, cut to the proper length, and clamped together in the center, for a smoother transition from the frame to the airbox.

What else can I say, but I was bored.

[Aulbaugh]

That was a great read Jeff! It looks great and great write up!

[Hedbngr18]

Looks like everything turned out really well. My question is, with that much work required wouldn't it have been about the same to fab up an aluminum air box? Just curious. I have toyed with these same ideas for my trikes. But as you said earlier, you were doing it more because you had the part lying around. Not so much as you thought it would give you massive power gains.

[Red Rider]

Looks like everything turned out really well. My question is, with that much work required wouldn't it have been about the same to fab up an aluminum air box?

Thanks. Rather than fabbing up an aluminum airbox, I probably would have just bought an aluminum airbox for a TRX250R. Then it probably still would have needed to be modified to make it work. Instead, by purchasing a stock plastic airbox, I was able to get everything else I needed at the same time.

I have toyed with these same ideas for my trikes. But as you said earlier, you were doing it more because you had the part lying around.

The part was lying around, only because I had this project in mind when I purchased it. It just took a bit of staring at the part in the garage everyday for awhile before I got motivated enough to undertake it.