I have a relatively new to me 2006...

[Charles_III]

I have a relatively new to me 2006 Sundancer 40 w/ 8.1 S Horizon MerCruiser gas engines @ 370 HP each and am concerned that the engines may be under performing - new fuel pumps, refurbished fuel injectors, replaced all heat exchangers, upgraded spark plugs, tune-up, annual service, new bottom paint, new shaft seals/cutless bearings, etc. Cruising: 19 kts @ 4200 rpm. WOT: 25 kts @ 5000 RPM. And the starboard engine burns ~ 15% more fuel per hour than the port. Your personal experience, feedback and insights would be very much appreciated. Thank you!

[wingless]

Welcome to the forum.

It may be that a more detailed basic engine examination is required. Start w/ the correct factory service manual (FSM) for the engine.

My suggestion is to check everything, all very basic stuff. Here is a partial list:

Rotor
Distributor Cap
Timing
Spark Plug Condition (Insulator Visual to Indicate Burn, Ceramic Cracks)
Spark Plug Gap
Compression
Fuel Pressure (Multiple / All Test Conditions)
Engine Fault Codes
All Vacuum Hoses Clean / Clear (Mine are Cleaned Annually)
Flame Arrestor Clean (Mine is Cleaned Annually)
New Fuel Filter(s) (Mine is Replaced Annually)
Remove / Inspect / Clean / Replace Intake Plenum (Mine is Cleaned Annually)
Engine Alignment
Propeller Tune / Balance (Mine are Always Prop Scan Class S)

[Lar]

Those engines are rated to turn 4200-4600 rpm max. 5000 is to high. If your tachs are accurate I would check the props for correct size.

[Charles_III]

Thank you Lars! Upon inspection, we learned that the prop stamp is 21 x 20 vs 21 x 22 per the parts manual. We are having the props re-pitched as we speak!

[cbruck]

I have a 2010 390 DA with the same engines but I have sterndrives and at 4200 rpm I run about 34 mph. Mine redlines at 4500 rpm. The 496 were made for torque and unless they are well balanced internally I would be very cautious about running them above 4500 rpm. Hopefully your props are the answer as it sounds like you have done most everything.

[Big_mike1]

U have one prop at 21x22.5 and starboard at 21x22

[Big_mike1]

That’s the way Searay did it

[Ooh_La_La]

I’m replacing my props on Monday, 20x20 on my 454, 400 EC. I’ll let you know if I get back some knots

[Shark]

We had a 1992 499 express and I replaced the props with 20x20 on our 454's and gained 4 miles per hour at cruise (3400rpm). I could cruise at 24 to 25 mph at 3400 rpms. It was a great improvement because before at 3400 rpm we only did 21 mph. The correct props make a world of difference! These were 4 blades props by the way. Our wop rpms were 4000.

[greatscott]

you have a cruiser (key word) if you want the motors to last keep it at 3200 rpm ,remember its a big boat w gas motors 16 to 17 mph is all u should be doing just saying