I replaced the supertrapp mufflers on my 90 ultra with some nice, correct oe take offs.
bike runs much better...but what kind of mods are there for these mufflers? They're quiet, and I like that...but there could be a little more noise from them, and I'd not complain.

 

Your bike only runs better on the stock mufflers because, I suspect, it wasn't properly tuned to run with the SuperTrapps! Put them back on and tune your bike to run right? Having owned my 1990 Glide from new, the 'wisdom' of the day was to drive a spike into the rear of the mufflers, but having done that the bike needed to be adjusted to run right. So you are stuck in a cleft stick! Whichever you do you need to make adjustments, or find a man who can. There's no substitute for a local indy with experience of tuning 'em just right.....

 

Bike is properly tuned, that's for sure. With either set of slip ons.
I have a few issues with the supertrapps...they sound like crap, i don't like the way they look, and ive never liked messing with those rings and caps to adjust back pressure. On a car, or bike.
i wasnt sure if there was some sort of mod done to the oe mufflers back in the day, since I'm thinking folks were probably limited with aftermarket slip-ins for those bikes in the 80s and early 90s .I dont want to cut the flanges off my head pipes to use current aftermarket mufflers, so I'll stick with these on a good running bike

 

The flared stock pipes, if you still have them, could be troublesome and were a pain! The flared part at the end of each pipe, where they join the mufflers, tended to fracture and fall off in small pieces, reducing the degree of support at that join, also introducing leaks. I had the good fortune to pick up a near new set of pipes, which kept my bike running with my modified stock mufflers. Any set of stock pipes up to and including 2008 will fit your bike, enabling you to use any of the many aftermarket mufflers currently available and that is what Evo owners have been doing for many years. Your stock pipes greatly limit what is available to you! My Supermeg 2-into-1 system is for a 2008 bike. Back in the day it was routine to cut off the flared pipe tips, to use alternative mufflers, then when the later plain stock pipes came out, in the mid-1990s, that gathered pace and I did so with my second set of flared pipes. Decent sets of discarded later stock pipes became popular with earlier Evo owners!

 

I have have the stock flared pipes, not sure if they're factory or an aftermarket set, they look a little different at the split for the left side than factory, seem to be a bit heavier gauge and have some nice gusseting around the "y"
I think i will look for a later set of head pipes, so I dont have to cut these up, and go with different slip ons, if I get to where i dont enjoy the silence of the factory cans

 

Don't believe I've ever heard that saying before.... But I don't get out much....

OP: I ran a set of SuperTrapp slipons on my 1993 FLHS for a lot of years until the head pipes finally cracked beyond repair. I ran 16 disks and the bike sounded/ran great with the stock CV carb tuned, SE air cleaner, and EV27 cam.

I'm now running a Thunderheader, which is a bit louder than the slipons I had. But it was cheaper to go that route than attempt to go OEM duals.

 

16 disks on each muffler? The ones on my bike had I think 6 on each muffler. Sounded like...well, mt friend said...do you like the way those things sound? I knew what he meant, cause I was thinking the same haha.
I took all the discs out...felt like it ran better, but I'm not a dyno. With the stock mufflers, it really does feel better. Seems to have more low and mid grunt. Again, not a dyno, but it's noticeable.

 

When looking for a later set be aware that some '07 models came with O2 ports, which will need blanking plugs to fill them, same with all the '08 pipes. Otherwise they fit and function as those without the ports.

 

Only 6 discs each? Poor bike must find it hard to breath! That's a very stingy number, unless they also have the open end cap instead of the closed one that normally comes with them.

 

I need to correct that, it was 12 disks per side. IIRC, the max (with optional extended bolts) was 16 per side, and SuperTrapp recommended starting with 8. (All this was 24 odd years ago, so I may or may not recall the exact figures.) I would think 6 disks per side would sound close to OEM mufflers that have been 'punched out'. Without re-tuning the carb, there are going to be some performance issues.

I really did like the look/sound/performance of those slip-ons However, the overall cost of replacing them, along with the head pipes, was way more than I was willing to shell out. Like I posted, the Thunderheader works good. EVO's seem to really like 2-1 exhausts. It's a bit louder than my slip-ons were, but I've gotten used to it. I do miss the look of dual mufflers on a touring bike, but I wasn't willing to install the 'falsey' Thunderheader just to appease my vanity. I also feel it's a bit better performance wise (butt dyno).