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What performance to expect from Merc 351 233hp 2bbl

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wet_wonder

Contributing Member
"I'm trying to get our 197

"I'm trying to get our 1975 Trojan Express F-25 to the right performance. It has a '77 Mercruiser 351 233hp, 2bbl, and I just put on a 13x10 prop, which improved the rpms. After switching out the prop, I took it out on calm water and got WOT at 4,000 rpm and about 16 knots on my GPS at best trim (18.5 mph). I was hoping that fully open it would get 18 to 20 knots, and that I could achieve a 15 knot cruising speed. I'll be swapping out plugs and checking the timing this weekend.

Anyone out there have or know about this config, or can speculate based on this config? - What kind of performance should I expect, what RPM WOT, what cruising rpm and speed?"
 
"Do you know what the original

"Do you know what the original prop was? Do you know what the WOT RPM Range is for your engine?
Hows clean is the hull? What's the boat weight.
When was the last time the engine compression was checked, when was the last tune up.

You may want to think about a 4bbl Carb & manifold. What type of exhaust manifolds do you have."
 
"Thanks Brian,
I've learn

"Thanks Brian,
I've learned that the 255hp 351 is designed for 4bbl, and the 233hp is designed for a 2bbl. Hull is clean, weight is moderate. I don't know the specs for RPM.
Just looking at my setup, what would you expect from it in general?"
 
"Well I've been trying to

"Well I've been trying to find the WOT ( wide open throttle) range for your engine, I did find on an old post somewhere that someone mentioned 4200RPM as a minimum, if that's true then you have a little too much prop and won't be going any faster with your current setup. I know that Mercruiser 305 & 350ci engines run 4200-4800rpm
so if you had that and your WOT was 4000 then you either have engine issues, too much weight or just too much prop by at least 2" of pitch.

Now, as for the 255hp being "designed" for a 4bbl, I wouldn't personally say it that way. Most likely the only difference between the 2 engines is a manifold and carb change/upgrade. The Camshaft, Cylinder heads, pistons, crank, bore & stroke are most likely the same.

So you may have some dwindling HP issues or if you really want to go a bit faster (totally understand) you'd need to either upgrade your engine or replace it.

You can expect a 4bbl carb to go for $100 to $400 depending on condition and brand and who you buy it from. An intake manifold is in the same range but can vary with type, as there are many types of 4bbl manifolds for a 351 Ford Engine that you can use in a marine environment.

What's you gain if you upgrade those 2 items? well.. maybe 4-6 MPH.

I do think your prop is a little small for that boat, I would expect a 14"-15" (being a larger boat) and a 15p-17p Prop at least, unless your running in Utah or anyother high elevation >2500+

Have a compression test and then a leak down test done on the engine just to see where it's at before you go much further."
 
"It had a 14x10 on there when

"It had a 14x10 on there when I bought it, and could barely get above 3k rpms and 13 knots. I switched to a 13x10 and picked up to 4k rpms and 16 knots.

I believe the 233hp 2bbl was a standard engine offered with the Trojan F-25, so I was curious as to what performance was expected given this setup. 4k and 16 knots may be what I'm supposed to get, I don't know."
 
"i could be wrong, but a 25 f

"i could be wrong, but a 25 ft boat at 3000 lbs give or take with 230 hp should be at least a 30 mph boat all day. i would suggest that you look for some very basic issues both in the hull and motor area. something really simple like , " are you getting full throttle at the carb ?" and what do the plug colors look like."
 
"It's a pretty big 25'

"It's a pretty big 25'. More than 3000 lbs, more like 4,500 lbs. I think.

I just pulled the plugs and they look tan. Thought I may be running too lean so I'll check the timing this weekend. I feel like I should be getting another 500+ rpm from somewhere. I'll also check the fuel filter I suppose."
 
i just punched your numbers in

i just punched your numbers into a " boat speed calculator" which admittedly is generic but has been uncannilly accurate in the past... 230 hp. 4500 lbs heavy deep v is calulated at bewteen 45 and 50 mph allowing for a huge prop slip... so i would suggest that there is something fundementally wrong somewhere.
 
"The tan plugs is good, the co

"The tan plugs is good, the color is more of function of fuel quantity ( too rich or lean ) and quality.

What type of drive system do you have?


Engine RPM 4000
Gear Ratio 1.5:1
Propeller Pitch 10
Propeller Slip 15%
Speed MPH 21.5"
 
"Something else to think about

"Something else to think about - that engine is 34 years old (assuming it's original) and was not designed to run on today's ethanol/gasoline.
That alone will reduce the HP from it's original output. Also none of the rubber fuel system components will survive long with ethanol. Old engine, large boat, gained weight over the years,
may have developed a "power hook" if the stringers have grown soft. By today's standards it was marginl power to begin with. Like stated above, need to do a compression check, complete tune up, think about a new engine if you plan to keep for a long time. Another thought - check to see if the fuel tanks are made of fiberglass.
Ethanol will eat 'em up and clog the entire fuel system"
 
"Yeah those are good thoughts.

"Yeah those are good thoughts. Fuel tank is metal. Motor was a full rebuild from 2004. I suppose I'll just do what I can, including checking the fuel system. Thanks."
 
"Dave:

I found your operati

"Dave:

I found your operating specs. WOT is 3800-4200. Is the prop a 13" x 10P prop or vice versa? If it's a 13" prop increase the blade to 14"-15" or as close as possible to get more thrust. The RPMs may drop a little. A four blade 14.25" X 10P will help with thrust.

Your engine is running fine based upon the plug color. Plug gap is .030", timing is 10 deg. BTDC, point gap is 0.016"-0.019" and dwell is 28-31 deg. As stated a 4 BBL carb will increase HP and RPMs @ WOT. You need to decide if the cost is worth the HP and few knots it will give you."
 
"I had a 14x10, and could only

"I had a 14x10, and could only get up to 3k rpm. I just switched this past week to a 13x10, and can now get it to almost 4k.

I'm switching out the plugs this weekend. Picked up MR43Ts. I don't think the old plugs are gapped at .03. They look more like .045, but I didn't measure. Is .03 the right gap for the new plugs?

I don't think my timing is at 10 deg either. I took a quick look and it looked like it was dead center. I'm checking that this weekend as well."
 
"Is .03 the right gap for

"Is .03 the right gap for the new plugs?"

YES; 0.030" plug gap is per SELOC tuneup spec. chart.

10 deg. BTDC timing is also per SELOC tuneup spec. chart.

Note: all MR43T plugs are shipped w/0.045" +/-gap which is for post 1990 engines.

Check the distributor weights to make sure they move.
 
I just checked the plug I had

I just checked the plug I had pulled. It's an automotive R45TS gapped at .035. So I'll be swapping in the marine MR43T gapped at .03.
 
Dave:

My pleasure. You mig

Dave:

My pleasure. You might try that old 14x10 prop when you finish the tuneup.
 
"That 25 foot Trojan must be a

"That 25 foot Trojan must be a awful heavy boat. I have the exact same 351W , 233 hp setup on a 24 foot fibreform that weighs well over 4000 lbs dry and I get 4000 WOT and about 38 mph. I cruise at 3200 and 28 mph and am currently running a 16", 16P prop. I had a 15.25",17P on before and was getting about 30 mph at 3000 rpm but was a bit sluggish out of the hole and motor seemed to be labouring a bit at cruise.
I am guessing that your Trojan must be quite a bit heavier and actually is a displacement hull rather than a planing hull."
 
"I just found some specs and p

"I just found some specs and performance info. Weight is around 5,000 puunds. It has a big tall cabin. The curve is 14 knots at 2500rpm, 20 knots at 3000 rpm, and 28 knots at 4000rpm. So I've got something incorrect. The motor is a pretty new rebuild, with less than 300 hours on it. So it must relate to how its adjusted. I have to talk my brother-in-law, former truck mechanic in the army, to come and save the day."
 
"Can you trailer it to a Truck

"Can you trailer it to a Truck scale? then go back without the boat and weigh the trailer, then you'll know if you got heavy.."
 
"Update:
I bought a Pertronix

"Update:
I bought a Pertronix electronix ignition and new coil. I was about to install it today, and when I pulled the old plate from the distributor, I found that the springs were both broken and that top plastic piece was broken. So I guess it wasn't advancing the spark? Is that correct? Would this account for the boat not able to get above 3k rpm (it's rated for 4200rpm)?
Also, should I have the distributor rebuilt, or, in the context of the pertronix ignition I'm installing, get a new distributor to best match with it?
Thanks"
 
""Posted on Thursday, May

""Posted on Thursday, May 07, 2009 - 02:23 pm
...Check the distributor weights to make sure they move."

You could have found that problem a month ago!
uhoh.gif


Due to the dist. age; get a reman. if available."
 
Ask us; that is why we're

Ask us; that is why we're here. Glad that you found the problem. Enjoy your boat.
 
"springs you might be able to

"springs you might be able to get a spring kit from a local auto store, plastic peice at the top? Rotor ??? Can also find distributors on Ebay or used marine parts suppliers.."
 
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