Please help me solve the problem of my boat losing 4 mph in top speed over a couple of weeks time. This spring I upgraded my 06’ 496 mag engine (375 HP) by adding a Raylar BP-203 camshaft, new AC Delco spark plugs, Whipple 496 mag HO computer reflash, replaced the stock fuel pressure regulator with an adjustable one (set it to 50 PSI WOT per Larry at Raylar), and some other small maintenance items done to it. I boat on a fresh water lake in Michigan and keep my Shockwave Tremor 25’ in the water in a slip during the boating season and I take it out to clean it and get it out of the water periodically. When I first launched the boat in the water and ran it was running at 72 MPH with 3/4 tank of fuel and 2 people and gear on board. The boat was very peppy and had lots of zip and It probably would have gone faster but I was running out of lake real quick at that speed. After about 2 weeks of being in the slip the top speed of the boat is now 68 mph with the RPMs showing the same as it was when the boat was doing 72 mph top speed. I cleaned the bottom of the boat with my brush on an extension handle this past Monday and then took it out for a high-speed run and it seemed a little faster getting up to full throttle but I’m still getting 68 MPH for the top speed with ½ tank of fuel in it???? I have no alarms going off when the engine is running I haven’t made any changes to the boat (same prop, same gear load, etc..) I don’t have an engine scanner and I haven’t had it scanned yet to see if any fault codes are being registered by the computer, but as I said I have no alarms going off on the engine at all. The only thing that has changed since I first put the boat into the water is the temperatures have gotten warmer here but when I have been doing the testing of high speed runs I have been doing it on days with low humidity and 82 degrees and under.
I would like to know where the diagnostic port is located on my engine and if there’s a way that I can check for codes without a fancy diagnostic scanner? I have seen Youtube videos of a guy using a paper clip and a multi-meter set on continuity to hear the code beep sequences registered by the computer / smartcraft. The video is an older engine and so I’m sure that the diagnostic port is configured differently then what I have. I still have the same prop hub in the prop as I had prior to doing the engine upgrade (flo torque II) and I’m now wondering if the prop could be slipping some at WOT but it doesn’t feel like it is at least at the lower speeds. I will probably switch over to a solid flo torque hub just to see if that is causing the problem. The engine and drive oil levels look fine. Please let me know what things that I should check and try as I want to get this issue resolved as fast as possible.

Please help me solve the problem of my boat losing 4 mph in top speed over a couple of weeks time. This spring I upgraded my 06’ 496 mag engine (375 HP) by adding a Raylar BP-203 camshaft, new AC Delco spark plugs, Whipple 496 mag HO computer reflash, replaced the stock fuel pressure regulator with an adjustable one (set it to 50 PSI WOT per Larry at Raylar), and some other small maintenance items done to it. I boat on a fresh water lake in Michigan and keep my Shockwave Tremor 25’ in the water in a slip during the boating season and I take it out to clean it and get it out of the water periodically. When I first launched the boat in the water and ran it was running at 72 MPH with 3/4 tank of fuel and 2 people and gear on board. The boat was very peppy and had lots of zip and It probably would have gone faster but I was running out of lake real quick at that speed. After about 2 weeks of being in the slip the top speed of the boat is now 68 mph with the RPMs showing the same as it was when the boat was doing 72 mph top speed. I cleaned the bottom of the boat with my brush on an extension handle this past Monday and then took it out for a high-speed run and it seemed a little faster getting up to full throttle but I’m still getting 68 MPH for the top speed with ½ tank of fuel in it???? I have no alarms going off when the engine is running I haven’t made any changes to the boat (same prop, same gear load, etc..) I don’t have an engine scanner and I haven’t had it scanned yet to see if any fault codes are being registered by the computer, but as I said I have no alarms going off on the engine at all. The only thing that has changed since I first put the boat into the water is the temperatures have gotten warmer here but when I have been doing the testing of high speed runs I have been doing it on days with low humidity and 82 degrees and under.
I would like to know where the diagnostic port is located on my engine and if there’s a way that I can check for codes without a fancy diagnostic scanner? I have seen Youtube videos of a guy using a paper clip and a multi-meter set on continuity to hear the code beep sequences registered by the computer / smartcraft. The video is an older engine and so I’m sure that the diagnostic port is configured differently then what I have. I still have the same prop hub in the prop as I had prior to doing the engine upgrade (flo torque II) and I’m now wondering if the prop could be slipping some at WOT but it doesn’t feel like it is at least at the lower speeds. The engine and drive oil levels look fine. Please let me know what things that I should check and try as I want to get this issue resolved as fast as possible.

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What type speedometer are you using?

Hi I'm using my boat speedometer which I have verified with Garmin GPS on multiple runs and different days a to be very accurate ( I was very surprised that it was dead on). I have cleaned the Bravo 1 internal pickup at the sandbar as well just to be sure that there was nothing causing it to give bad readings. I used the Garmin GPS on the first couple of high speed runs after putting it into the water this spring and it was showing 72.4 mph on it I meant to bring the GPS unit with me to the boat on Monday to use after I cleaned the bottom but I forgot it at home.

Before I went any farther I'd recheck with the GPS.

Was the engine turning the same rpms or was the rpms lower?

Yep...................[After about 2 weeks of being in the slip the top speed of the boat is now 68 mph with the RPMs showing the same as it was when the boat was doing 72 mph top speed].

We don’t have a performance boat but do have a 7.4 mpi Bravo3 226 Cobalt. If there are 2 things that effect our speed is weight and water and air temperature. I know for fact when it is cool out in the high 60’s we get the most speed in fact the best has need 63.5 mph. In this hot weather we struggle to get anything over 60. Another thing that you may want to consider is the oil. If it is doing it’s job the lubricity could be decreasing. Start adding all these variables together and 5 mph could easily need compromised.

im not a mechanic

3 pointstar