M8 lack of back pressure
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Stellar HDF Member
Joined: Apr 2011
Posts: 2,443
Likes: 1,545
From: Central Wisconsin
M8 lack of back pressure
I have a decatted 2019 RGS with MK 45 slip ons. By removing the cat, I now get the sound of a large baloon deflating between shifts and on decel. No popping. Just a lack of back pressure. I am looking at the new Fuel Moto adjustable inserts to see if they will fix it. Anyone else experience this? Anyone have some inserts to sell? I should mention stock engine except intake, slip ons and power vision. Already talked to Fuel Moto but no maps can make back pressure. Thanks in advance.
Road Master
Joined: Apr 2014
Posts: 1,023
Likes: 380
From: Philly Burbs
Originally Posted by h-d wrencher
I have a decatted 2019 RGS with MK 45 slip ons. By removing the cat, I now get the sound of a large baloon deflating between shifts and on decel. No popping. Just a lack of back pressure. I am looking at the new Fuel Moto adjustable inserts to see if they will fix it. Anyone else experience this? Anyone have some inserts to sell? I should mention stock engine except intake, slip ons and power vision. Already talked to Fuel Moto but no maps can make back pressure. Thanks in advance.
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Stellar HDF Member
Joined: Apr 2011
Posts: 2,443
Likes: 1,545
From: Central Wisconsin
Yes I am. I even wrapped them with stainless tape to restrict some flow. By the time the sound sound went away, so did a lot of torque. Honestly, I do not notice much difference with or without the inserts.
Road Master
Joined: Apr 2014
Posts: 1,023
Likes: 380
From: Philly Burbs
Okay, just checking. I didn't have any issues with my MK45s with the db inserts in them and had it dyno'd at 121 hp and 138 ft lbs of torque (i have a 124 big bore kit). I thought it was too loud so I went with the fuel moto inserts and like the sound much better. I plan on getting it dyno tuned again, but I also added a larger TB with ported/polished manifold and level A heads from Fuel Moto.
Extreme HDF Member
Joined: Jan 2017
Posts: 10,790
Likes: 19,651
From: On the move
I have a decatted 2019 RGS with MK 45 slip ons. By removing the cat, I now get the sound of a large baloon deflating between shifts and on decel. No popping. Just a lack of back pressure. I am looking at the new Fuel Moto adjustable inserts to see if they will fix it. Anyone else experience this? Anyone have some inserts to sell? I should mention stock engine except intake, slip ons and power vision. Already talked to Fuel Moto but no maps can make back pressure. Thanks in advance.
I'm using the FM inserts and a dynojet tune from FM. The inserts work and will solve your lack of back pressure issue. If I have just one complaint with the inserts it's that they quieted my exhaust note down significantly.
Intermediate
Joined: Sep 2020
Posts: 31
Likes: 11
From: Melbourne, Australia
I decatted my m8 and running S&S 4" Grand Nationals with the db inserts, back pressure is great and no sound between gears as you're describing. I did take the db inserts out and it literately killed the back pressure. Not being a smart ***, but are the inserts install properly?
Seasoned HDF Member
Joined: Dec 2012
Posts: 6,667
Likes: 502
From: Orange County
I have gone through this too, had the MK45s and noticed the same loss in low end. I went through several muffler setups looking for that sound. I knew the only way to get what I wanted was to put a nice cam in there, I just didnt want to mess with my warranty yet. I now have Cobra Neighbor Haters and these are the best sounding, without being obnoxious and without losing low end torque. I did have the DND BILLET CAT on my bike too and that was a really nice exhaust, sounded really good, maybe a tad too raspy, but then again maybe not; but thats gone and thats another story.
Theres some interesting info on RB Racings website, this guy knows VTwins.
No matter what people say, this is an emissions-designed engine with a lot of compression, locked in place valve adjustment and camshafts that are simply going to give you next to zero in gains for your hard earned money in changing the exhaust. Cam changes need to be made along with a 2-1 exhaust to make any appreciable gains.
Theres some interesting info on RB Racings website, this guy knows VTwins.
No matter what people say, this is an emissions-designed engine with a lot of compression, locked in place valve adjustment and camshafts that are simply going to give you next to zero in gains for your hard earned money in changing the exhaust. Cam changes need to be made along with a 2-1 exhaust to make any appreciable gains.
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Thread Starter
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Stellar HDF Member
Joined: Apr 2011
Posts: 2,443
Likes: 1,545
From: Central Wisconsin
I decatted my m8 and running S&S 4" Grand Nationals with the db inserts, back pressure is great and no sound between gears as you're describing. I did take the db inserts out and it literately killed the back pressure. Not being a smart ***, but are the inserts install properly?
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