Is there advantages to changing cams in a boat?

[scipper77]

I just wanted to learn a little bit more about marine cams. It seems to me that you use a cam that gives max HP at max RPM's for the engine design. Is it really that simple?

Is there any advantage to using a slightly different cam? for example could you use a less aggressive cam (hp peaks at lower rpm's) and get better gas mileage??

I know I personally never run at WOT because the rate of fuel consumption is incredible. Always wondered if you could sacrifice top end that I never use for fuel economy. Who needs to go 45 MPH on the water anyways?? At that speed if I saw a log floating in the lake in front of me I doubt I could avoid it.

I wasn't sure the best place to ask my question, mods feel free to move if this is the wrong place.

 

[WizeOne]

Re: Is there advantages to changing cams in a boat?

.... It seems to me that you use a cam that gives max HP at max RPM's for the engine design. .......

Actually, it is the otherway around. Marine cams, similar to truck and RV cams, are made to keep peak power in the lower rpm range where it is needed most. From an economy standpoint, I do not think you can do any better.

 

[gadget73]

Re: Is there advantages to changing cams in a boat?

For economy, the stock cam is probably pretty darn decent. You'd likely get more results from prop changes than cam changes. Adding a bit more wheel might bring the rpms down at cruise, of course adding too much wheel will cause you to add more throttle to maintain those rpms, negating any economy atvantage.

 

[mylesm260]

Re: Is there advantages to changing cams in a boat?

You need to start reading up on some concepts to fully understand what you are asking.

BSFC - Brake specific fuel consumption: How much fuel does it take to make a specific amount of horsepower.

Peak torque - The RPMS at which your engine makes peak torque.


Now. The vast majority of motors will have the lowest BSFC at peak torque.

A typical 4.3 or 5.7 marine cam will make peak power @ 4400-4800 RPMS and peak torque @ 2800 - 3100 RPMS.

What does this mean? Well, if you can plane at somewhere between 2800 and 3100 rpms, than you already have the ideal camshaft for fuel economy.


Now, if you can plane at 2300 rpms, lets say (I'm betting you can't) then you could run a colder cam (less lift, less duration) that makes peak torque at 2300, and peak hp at say 3900.. That would give you better fuel efficiency.

What would you be giving up? Well lets say you make 200 HP @ 4400... you would now be making 175 or so at 3900... you would loose 25 HP

And if you took that colder cam, and cruised @ 2800, you would be getting worse mileage than if you stuck with your original cam.

So, long story short, the cam you've got is pretty much going to be the best suited for fuel economy, assuming you keep you revs bellow 3500 or so....



Now, on that note, Gen IV small blocks have variable valve timing, and can optimize their valve timing for every RPM range, this results in more TQ, HP and better fuel economy accross the entire RPM range... Too bad mercruiser and volvo have theirs heads in the sand still and don't use Gen IV small blocks yet, even though GM marine makes them, and other boat manufacturers use them (idmar for example)

 

[scipper77]

Re: Is there advantages to changing cams in a boat?

Thank you mylesm260!! I want to hang that post on my fridge You seem to have answered my question and educated me regarding cams at the same time!