"Tweaked" 801 stock muffler

Tom said:
It depends on the "officials" in your region, but in my opinion it should be absolutely fine; the pre-muffler (catalyst) takes most of the loudness.
But from what I've read the Duke 790 / Pilen 801 engine "likes" a muffler with big volume and some "backpressure"; that's why you'll always loose some performance with the smaller (and also louder) aftermarket mufflers; the stock Duke 790 and Pilen 801 mufflers are very good performers.
With the stock 801 muffler and the small DB-killer you'll have still the same "big" volume and still some more backpressure; I don't know, if it'll make a big difference, if you'll delete the db-killer completely.
I also don't know (especially after my experience with the deformed aluminum spacer) if the coating of the black tip-cover is heat resistant and will take the direct heat of the exhaust gases if you'll delete the db-killer completely.
If you'll give it a try, report from your experiences.
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I'm definitely going to try it and will report back! My bike arrives in 10 days, so I'm just at the planning stage!
 
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Tom thanks for being the first to do this and posting. I just need more people to buy 801s so I can get a stock exhaust to modify.
 
Actionabe said:
Tom thanks for being the first to do this and posting. I just need more people to buy 801s so I can get a stock exhaust to modify.
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You're welcome! (y)
Someone always has to be the first to try something, and (with other bikes) I've also learned a lot from the experiences of others...
 
Great thread Tom and impressive what you are able to make for your bike. You clearly enjoy these type of projects but besides the challenge, why didn't you just opt for a decat from Arrow?
To me it seems that you would still keep the same 'legal' looks of your bike and from what I can find on decat instructions for KTM 790 (dukes/Adventures) it does not seem to be too difficult?
Is there anyone that has already tried this modification on the 801?
 
Stamanas said:
Great thread Tom and impressive what you are able to make for your bike. You clearly enjoy these type of projects but besides the challenge, why didn't you just opt for a decat from Arrow?
To me it seems that you would still keep the same 'legal' looks of your bike and from what I can find on decat instructions for KTM 790 (dukes/Adventures) it does not seem to be too difficult?
Is there anyone that has already tried this modification on the 801?
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To me there are a couple of reasons.
The stock brushed stainless steel Svartpilen exhaust is still my favorite; that's why I went to the relatively big effort to shorten the stock db-killer instead of simply changing the exhaust to an aftermarket exhaust.
At least here in Germany the officals are very sensitiv to decats; at vehicle spotchecks they always check the exhaust; and a decat is imidiately noticeble; while the shortened db-killer is only noticeble if you know it.
With a decat you'll need an addional Coober LM ECU; otherwise you'll get error codes; the Coober LM ECU also helps if you have a problem with the bike running too lean because of the EURO 5 restrictions; and also if you have a problem with stalling.
I never had any problem with the bike running too lean and/or stalling; possibly as I rarely ride in town; about 90-95% of my rides are outside of towns either on the highway or on country roads; and also on small backroads; nearly allways at higher rpms.
Finally the shortened db-killer instead of a decat was and still is the very best option to me; and to me the result is absolutely perfect.
But you're right; I do indeed also like these kind of small, a tad challenging projects. (y)😁

By the way; next to the Arrow decat, there are also some other alternatives with the (necessary) third sensor plug available; for example also from Coober; and also from some other manufacturers.
 
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Tom said:
To me there are a couple of reasons.
The stock brushed stainless steel Svartpilen exhaust is still my favorite; that's why I went to the relatively big effort to shorten the stock db-killer instead of simply changing the exhaust to an aftermarket exhaust.
At least here in Germany the officals are very sensitiv to decats; at vehicle spotchecks they always check the exhaust; and a decat is imidiately noticeble; while the shortened db-killer is only noticeble if you know it.
With a decat you'll need an addional Coober LM ECU; otherwise you'll get error codes; the Coober LM ECU also helps if you have a problem with the bike running too lean because of the EURO 5 restrictions; and also if you have a problem with stalling.
I never had any problem with stalling; possibly as I rarely ride in town; about 90-95% of my rides are outside of towns either on the highway or on country roads; and also on small backroads; nearly allways at higher rpms.
Finally the shortened db-killer instead of a decat was and still is the the very best option to me; and to me the result is absolutely perfect.
But you're right; I do indeed also like these kind of small, a tad challenging projects. (y);)

By the way; next to the Arrow decat, there are also some other alternatives with the necessary the third sensor plug available; for example also from Coober; and also from some other manufacturers.
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I wasn't aware you also require the additional ECU. That immediately makes it very expensive at around 650,- in parts alone.

Here in NL I haven't had any officer spot any removed catalytic converter as they initially check if it's an OEM muffler and wether or not the DB killer is still present.

Your solution is ultimately the nicest and definitely clean looking. If only I had the skills and tools...
 
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I am curious how much volume the 790 engine needs in the can, the arrow ones are a little shorter. I feel like the stock exhaust would look really good if you shortened it by a few inches. I shortened the stock can on my 701 and I feel like it really improved the looks.
 
@Actionabe Here some links that might be of interest to you; especially the third link; it's a direct dyno comparison of the stock 790 exhaust to the smaller Dominator exhaust.

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Here a screenshot from the video; the orange is the stock 790 exhaust, the red is the smaller Dominator (but I don't know if it's with or without db-killer).

Bildschirmfoto 2025-09-27 um 09.35.21.jpg
 
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That looks really good shortened, I might have to buy another stock exhaust and work on it over the winter…

It’s surprising how much they gained just from a tune especially on the top end. I’m not really familiar with tuning on this engine, I’ll have to look into the options. It looks like the short exhaust really doesn’t hurt it too much, although I would be curious to see the torque numbers.

Edit- never mind, looks like they got the cheap power upgrade by starting with the euro 95 hp instead of the US 105 hp version. Still good to see there isn’t a huge drop off from the lack in muffler volume. I think a debaffle and shortened can is the way to go for me.
 
Actionabe said:
Still good to see there isn’t a huge drop off from the lack in muffler volume.
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If you're just looking at the top-numbers, you're right.
But at about 6800 rpm it's about 80 hp with the Dominator, while it's about 88 hp with the stock exhaust.
That's a much bigger difference, than I would have expected.
It would be interesting to know, if it's with or without a db-killer in the smaller Dominator exhaust (back pressure).
 
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Actionabe said:
That looks really good shortened, I might have to buy another stock exhaust and work on it over the winter…
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Someone already did it (it's a webfind); but it's too short for my liking; I would prefer it like on the Kiska sketch (at the custom bike it seems to be a tad shorter).

250927_01.jpg
 
Tom said:
If you're just looking at the top-numbers, you're right.
But at about 6800 rpm it's about 80 hp with the Dominator, while it's about 88 hp with the stock exhaust.
That's a much bigger difference, than I would have expected.
It would be interesting to know, if it's with or without a db-killer in the smaller Dominator exhaust (back pressure).
Click to expand...
Yeah that is a big drop in midrange where most riding is done. curious if it is backpressure, length or volume. Probably a combination of all three.
 
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On the drawing for the custom it says a 10mm shorter exhaust, that looks like a lot more than 10 mm though, more like 50-70
 
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I also need to figure out if there is a way to finesse the fit so that my heel stops hitting the exhaust when I’m on the balls of my feet.
 
I don’t plan on removing the Cat, but if I did I would be tempted to fab an exhaust that is lower like the 701. I think that would clear up the heel issue and look better. Maybe eventually..
 
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