lowrider ST making Less HP after big bore install.
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Stage I
Joined: Apr 2026
Posts: 1
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From: SoCal
lowrider ST making Less HP after big bore install.
Hey, new to the the forums and thought maybe post about a issue Im having
Ive got a 23 lowrider ST.
I recently had 131 cylinder and pistons done on the bike. Prior to that I had a CR483 cam, 5.5 injectors, S&S billet ported manifold, tear drop intake, and S&S superstreet 2 into 1 catted exhaust done and dyno tuned with a thundermax. It made good gains. The cat ended up dislodging itself from the collector (Ive been told its a common thing? And I know one other person that it has happened to.) and I ended up pulling it out. But it ran different. I went back to my tuner and decided to go bigger cylinders/pistons. I wanted to go 128 but they were backordered/out of stock. Tuner said he can grab SE 131 pistons/cylinders, gaskets.
Said it would be good with stock heads since Im running same cam. The 131 cylinders were installed and dyno tuned, it runs fine but I am currently making 2 less HP than before installing the cylinders, but I did gain about 4-6 in torque. He also did have to replace the exhaust rockers as there was sign of wear.
Tuner says its the exhaust since the cat popped out and recommends swapping that out for a more solid power curve. A friend of mines tuner says its cam and injectors. And another friend says Im bottle necking intake flow with stock heads. Im not sure what the issue would be.
I guess Im looking for help/advice to improve gains. The bike runs fine, and it does pick up quick if I want it to. But having that work done to it to make less than is bothersome.
any insight or advise is welcomed and would be helpful.
Ive got a 23 lowrider ST.
I recently had 131 cylinder and pistons done on the bike. Prior to that I had a CR483 cam, 5.5 injectors, S&S billet ported manifold, tear drop intake, and S&S superstreet 2 into 1 catted exhaust done and dyno tuned with a thundermax. It made good gains. The cat ended up dislodging itself from the collector (Ive been told its a common thing? And I know one other person that it has happened to.) and I ended up pulling it out. But it ran different. I went back to my tuner and decided to go bigger cylinders/pistons. I wanted to go 128 but they were backordered/out of stock. Tuner said he can grab SE 131 pistons/cylinders, gaskets.
Said it would be good with stock heads since Im running same cam. The 131 cylinders were installed and dyno tuned, it runs fine but I am currently making 2 less HP than before installing the cylinders, but I did gain about 4-6 in torque. He also did have to replace the exhaust rockers as there was sign of wear.
Tuner says its the exhaust since the cat popped out and recommends swapping that out for a more solid power curve. A friend of mines tuner says its cam and injectors. And another friend says Im bottle necking intake flow with stock heads. Im not sure what the issue would be.
I guess Im looking for help/advice to improve gains. The bike runs fine, and it does pick up quick if I want it to. But having that work done to it to make less than is bothersome.
any insight or advise is welcomed and would be helpful.
Tourer
Joined: Dec 2022
Posts: 297
Likes: 174
From: Alexandria, VA
Hey, new to the the forums and thought maybe post about a issue Im having
Ive got a 23 lowrider ST.
I recently had 131 cylinder and pistons done on the bike. Prior to that I had a CR483 cam, 5.5 injectors, S&S billet ported manifold, tear drop intake, and S&S superstreet 2 into 1 catted exhaust done and dyno tuned with a thundermax. It made good gains. The cat ended up dislodging itself from the collector (Ive been told its a common thing? And I know one other person that it has happened to.) and I ended up pulling it out. But it ran different. I went back to my tuner and decided to go bigger cylinders/pistons. I wanted to go 128 but they were backordered/out of stock. Tuner said he can grab SE 131 pistons/cylinders, gaskets.
Said it would be good with stock heads since Im running same cam. The 131 cylinders were installed and dyno tuned, it runs fine but I am currently making 2 less HP than before installing the cylinders, but I did gain about 4-6 in torque. He also did have to replace the exhaust rockers as there was sign of wear.
Tuner says its the exhaust since the cat popped out and recommends swapping that out for a more solid power curve. A friend of mines tuner says its cam and injectors. And another friend says Im bottle necking intake flow with stock heads. Im not sure what the issue would be.
I guess Im looking for help/advice to improve gains. The bike runs fine, and it does pick up quick if I want it to. But having that work done to it to make less than is bothersome.
any insight or advise is welcomed and would be helpful.
Ive got a 23 lowrider ST.
I recently had 131 cylinder and pistons done on the bike. Prior to that I had a CR483 cam, 5.5 injectors, S&S billet ported manifold, tear drop intake, and S&S superstreet 2 into 1 catted exhaust done and dyno tuned with a thundermax. It made good gains. The cat ended up dislodging itself from the collector (Ive been told its a common thing? And I know one other person that it has happened to.) and I ended up pulling it out. But it ran different. I went back to my tuner and decided to go bigger cylinders/pistons. I wanted to go 128 but they were backordered/out of stock. Tuner said he can grab SE 131 pistons/cylinders, gaskets.
Said it would be good with stock heads since Im running same cam. The 131 cylinders were installed and dyno tuned, it runs fine but I am currently making 2 less HP than before installing the cylinders, but I did gain about 4-6 in torque. He also did have to replace the exhaust rockers as there was sign of wear.
Tuner says its the exhaust since the cat popped out and recommends swapping that out for a more solid power curve. A friend of mines tuner says its cam and injectors. And another friend says Im bottle necking intake flow with stock heads. Im not sure what the issue would be.
I guess Im looking for help/advice to improve gains. The bike runs fine, and it does pick up quick if I want it to. But having that work done to it to make less than is bothersome.
any insight or advise is welcomed and would be helpful.
First, if the only thing you changed was the big bore kit, you shouldn't be expecting a big increase in HP. You just added 14 cubic inches to a bike that makes less than 1HP per cubic inch. So, all else being equal, you should expect an increase, but not a lot. (By the way, it sounds like you had a pretty nice build on your 117; how did your compression ratio change with the 131 kit?)
Second, Stop thinking you are making less horsepower. If you put your bike on the dyno again today, you will almost certainly get different numbers. Dyno results will vary based on the ambient conditions like temperature, humidity, and even the way the bike is mounted on the dyno. So, a 2hp difference is nothing. I'm not saying the numbers don't matter, but relatively small variations are just background noise. The HP and Torque curves tell you more than the actual numbers for a single run.
Third, the new extra cubic inches won't do much if they can't breath. So, while the cam and heads that worked really well for your 117 are probably OK at 131, they might be less than ideal with the extra displacement. As an example (according to Fuel Moto), while the Wood 22xe is a great cam that works really well on a 117, the Wood 77x handles the larger displacement better. (That's what I went with, and it works great on my bike.)
And last, a quick compression check is easy and is a cheap way you make sure that the new cylinders, rings, and gaskets are really doing their job. (Your builder should have done that when he put the engine back together, but check to be sure. And even then, I would go back and check again, now that everything should be fully seated.)
That's it, you said you bike is running fine, enjoy it!
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