Aprilia Exhaust Derestriction

(Covers Mille models to 2003, Tuono models to 2004)

As mentioned here, the Gen 1 Aprilia ECU was designed with a hidden high-performance map built into it that could be activated by clipping a particular wire in the wiring harness. Using the high-performance map required removing a baffle from the stock muffler. This page explains how to remove that baffle.

I'm a sucker for building special tools when it comes to motorcycle repair, maintenance, and general messing around. This particular tool was designed to remove the front baffle from the 2-1 single-pipe Aprilia models. If you spend enough time with a Dremel tool, this puller is not really required but where is the fun in that?

If you remove the pipe from the bike and then stare down the inlet, you see something like this:

There is a baffle in the inlet pipe that starts out with a fairly wide diameter, but funnels down to a narrow outlet just before entering the muffler itself.

The red circles in the picture identify a pair of spot welds that hold the baffle in place. The first step is to get a Dremel tool and grind the spot welds away. If you are lucky, you can reach down to the far end of the baffle with a long hook-shaped tool and just yank it out.

If you are not so lucky, you can build yourself a special baffle-remover tool. You will need:

  • about 16 inches of 3/8 all-thread rod

  • a piece of 3/16 mild steel 1 inch by 1.5 inch

  • three nuts to fit the all-thread rod

The idea is to construct something that slips through the smaller hole on the baffle at its far end, which then acts as a puller to pull the baffle towards the front.

I started with a piece of scrap 3/16 mild steel 1 inch by 1.5 inch to be used as the puller end of a special tool. I rounded the ends with a file, and drilled a 1/2 inch hole in the center. I hand filed a bevel onto the top right side of the hole and the bottom left. The "right" and "left" don't matter, as long as the bevels are on different sides for the top and bottom.

The whole point to the bevel is to allow the puller to sit on the all-thread at an angle. This allows the puller to slide through a smaller hole than if it were sitting perpendicular to the all-thread. A picture should make it obvious:

After going through the hole, the puller tab will tend to sit flat when pulled against the back of the baffle, which makes it too big to fit back through. The picture (below) demonstrates how that works using a baffle that has already been removed.

So with jam nuts on the end as in the previous pictures, lower the puller into the baffle and work the puller tab through the hole. Lift on the puller so that the tab sets against the back of the baffle. Drop a large piece of metal that has a hole in it over the top of the puller, then spin down the third nut with a washer under it. The piece of metal is not critical. In this case, I grabbed a die from my tubing bender since it was a nice, thick piece of metal with a hole already drilled in it. The cardboard is just to keep the end of the pipe and the bender die from marring each other.

At this point, you just wrench on the top nut, which causes the back end to get pulled on because the front end isn't going anywhere. If your spot welds were not fully ground away by the Dremel tool, there will be a nice 'pop' at this point as they finally break. The baffle will come out with a minimum of wrenching after the welds break.

Objective Differences

So what does the baffle do? My measurements show that the inside diameter of the exhaust header is about 64mm. The inlet end of the baffle is about 48mm in diameter. The outlet end is 36mm in diameter. Applying some Pi*R2 math indicates that the you have just increased the unrestricted flow area at the smallest point from about 1000 square mm to about 3200 square mm. Basically, you have tripled the size of the opening heading into the muffler.

Subjective Differences

Honestly? Not much. I'm not sure I could even notice a difference in sound level. Maybe if I were able to do an A/B comparison of a modified bike against an unmodified bike, a difference might be apparent. I can say with certainty that it didn't become antisocial in any respect.