2 bbl to 4 bbl on Merc 5.0 Alpha gen2

[telstar1]

Boat is 2359 Trophy I have the stock 2001 5.0 M/C 230 hp I believe. Wide open is 4200 4300 I cruise at 27 mph at 3700.That scene is ok but Im not happy about my fuel consumption as well as I would like a bit more peak power,not to be used except in heavy going etc.Im struggling and pretty well wide open when I occasionally load up with fuel and people.
I have o experience with 4 bbls so will ask for advice/opinions.
1.At my normal running speed and weight,assumably with only 2 bbls working,will i burn less fuel with a 4bbl?Right now Im doing 9 imp (11 us) gals/hr.
2.The 4 bbl apparently gives 30 more hp. Is this at same rpms ?in which case I guess Id need more prop pitch to use that available hp..Assumably if that is the case Id then cruise at a lower rpm.Maybe this would create a fuel saving in itself?Like overdrive versus drive loiwering the rpms in a car?
3.Where does vacuum come into all this?My old 250 c.i.165 hp m/c had a vacuum gauge which you "kept in the green"ie above I think 5 inches vac.I moved this setup on to my 470 and used it with good results for 20 years. No idea if Im in the green on my 5.0 and wonder what the deal is on vacuum anyway. I always thought it was something to do with conserving valve life.Is this right?Would the 4 bbl improve vacuum at a similar cruise speed?an is it even a consideration?
5. What 4 bbl would be best?Need new manifold?
A lot of questions. Any and all input appreciated.

 

[Don S]

Re: 2 bbl to 4 bbl on Merc 5.0 Alpha gen2

First of all, your 4200 to 4300 WOT rpm is way low, and will absolutely cause you high fuel consumption. What you have to do is find out why your rpm is so low, and then see what your fuel consumption is with things where they should be.
You might want to post a thread in the IO repair forum on your low WOT rpm, but have a look at this thread first. http://forums.iboats.com/showthread.php?t=295070

 

[telstar1]

Re: 2 bbl to 4 bbl on Merc 5.0 Alpha gen2

Im pretty confident I have a strong running unit,clean hull etc. and that to increase rpm I really need to reduce pitch. Don you say this would give me better mileage?Means i would have to cruise at say 4000 rpm which I sort of thought was a bad thing for mileage/fuel consumpt.,but maybe Im just backwards on that thinking?Have never played around with props versus fuel cosumpt. much mainly because prev. boats didnt really burn that much fuel no matter what porop was on them.Havent had the boat that long and have not experimented with any props yet. Currently have a 4 blade 14.5x18p prop which is what was on boat when I got it.

 

[dan t.]

Re: 2 bbl to 4 bbl on Merc 5.0 Alpha gen2

OK,first of all vacuum,it is high with closed throttle,low with wide open throttle, the power enrichment system in the carb works on vacuum. The power valve opening point is calibrated with spring pressure, vacuum closes it. Opening point is generaly around 3 to 5 inches of vacuum.If you can keep the vacuum reading above 5 the power valve stays closed and you will get better fuel economy. Now the prop, bottom line is you need a 350 motor,but to use what you have mabe a 3 blade of the same pitch, Merc makes a 15 1/2x18 with the eliphant ear blades, on my 2452 with a 305 I ran a 48-16440-16 with added cup worked great,you have to play around and see what works

 

[telstar1]

Re: 2 bbl to 4 bbl on Merc 5.0 Alpha gen2

Now the prop, bottom line is you need a 350 motor,but to use what you have mabe a 3 blade of the same pitch, Merc makes a 15 1/2x18 with the eliphant ear blades, on my 2452 with a 305 I ran a 48-16440-16 with added cup worked great,you have to play around and see what works

So with your 2452 and 16 inch prop what speed and revs did you get?Any idea about the fuel consumption?
Im going to hook up a vacuum gauge see what I get.
Thanks

 

[dan t.]

Re: 2 bbl to 4 bbl on Merc 5.0 Alpha gen2

With the 16 prop I ran 36oo rpm at 20 knots for cruise, 4400 wot cant remember the top speed, roughly 10 gph, averaged 2 mpg imperial gallons checked with the trip log on the gps and fuel burned . Averaged over 5or 6 fillups. Its like a different boat with the 350 swap, I am still expermenting with props.