The Underseat Exhaust System – aka: the Holy Grail of motorcycle exhausts.


I guess because of the (few) similarities with the Ducati 916, it seemed natural that a Hawk GT would be the ideal candidate for fitting an underseat exhaust.  I’d seen a couple of underseat systems on Hawks, but none of these had the “finished” or street look I was after.  You have to admit – with this exhaust the slim lines of a Hawk are greatly accentuated – no muffler can hanging off the side!  Poser value goes off the chart.

Hey, it may not make power like an M4 but this isn’t a race bike trying to eek out that last 1/4hp.


Several obstacles have to be overcome to fit an underseat pipe and muffler.  First, for a street bike you have a battery to deal with.  The OEM location consumes a lot of real estate that the pipe could use as it snakes it’s way past the swingarm and up under the seat.  2nd is the tire clearance.  Stock, the Hawk has about 4” (maybe a tad more).  Not much room for a muffler.  That brings up another point – I did a TON of looking but was unable to find a suitable muffler that would fit between the stock subframe rails, not be too tall, be the right length and have a single inlet and dual outlet design.  Nothing – NADA!  More custom work….

I don’t want to discourage anyone wanting to do the underseat limbo, but it sure as heck ain’t no bolt-on!


Solving these problems one at a time – additional clearance was provided be the Greer tail, allowing the muffler to tuck closer to the subframe and yielding the needed 4” of wheel travel without hitting.  It was still necessary to fit a longer shock – the early model CBR900RR, which itself had to be modified (in addition to the “standard” mods required for fitting on a Hawk) in order to limit total travel to near Hawk length.  Between these two, there was enough clearance for a muffler about 3.5” tall, with a little room left over for a heat shield on top.

Next was the pipe.  I decided to start out with a Jim Davis left exit pipe because it already was designed to start moving the exhaust up under the seat area and it was stainless steel.  That meant I could remodel it and then just repolish for and everlasting finish.  It uses the OEM Hawk SS headpipes, which I smoothed and polished also.  Cutting and sectioning the JD pipe and adding a few new SS sections got the exhaust up under the seat to attach to the muffler.  This was a tedious process of fitting one section, tack welding, fitting the next section and tacking, and so-on until the full pipe was completed.  Then, after being satisfied all was well it was fully welded up, flap wheeled and polished.  Looks simple enough, eh?




Next is the muffler.  My first attempt was at using a custom muffler made at a Hot-Rod shop in Florida.  This was an all stainless steel muffler made to my dimensions, inlet and outlet design with their internal “muffling” design.  It fit and looked OK, but didn’t muffle too well.


Here it is on the bike:






It sure looked sweet – and was a lot of work getting to this point.


Well, after trying to modify it for improved sound control, I finally decided to start over with an all new design.  The new muffler was going to be mostly aluminum construction, with stainless steel end caps and full-length perforated tubing inside.  I was trying to equal the overall sound/noise level of a previously fitted D&D can – which was basically a straight through perforated core.










Having two outlets and one inlet, I made Y shaped internal core design with removable end caps making it rebuildable.  The new muffler was quite a big quieter than the previous version, but still a little too loud for my taste.   The final configuration was to reduce the outlet tip size to reduce the noise.  Ended up with 1” tube outlets inside the larger SS flared outlet tips, this being fed from the 2” inlet and 1.5” perforated cores.  Here’s a few pics of the pieces:




















And on the bike:  (the aluminum can is painted with hi heat black paint)




Now she sounds good and looks killer – just the thing to finish off the GTR!