Mercruiser 5.7 won’t make power

[Scott Danforth]

With a mild cam what vacuum reading would you expect?

Depends where you have the cam set and where the timing is.

15 to 18 inches of mercury is where I would expect at 0 up at idle with 8 BTDC base timing

Bring the base timing up to 10-12 BTDC and you will bring the idle vacuum up.

Did you get a fuel pressure reading while under load?

 

[Oldpos]

Depends where you have the cam set and where the timing is.

15 to 18 inches of mercury is where I would expect at 0 up at idle with 8 BTDC base timing

Bring the base timing up to 10-12 BTDC and you will bring the idle vacuum up.

Did you get a fuel pressure reading while under load?

The fuel reading is steady and does not drop off under load or attempted full throttle.

Vacuum at 8 deg is still around 13 jumping up and down one degree, if I retard timing it will drop to 10 , if I advance it will increase to about 15 , but like I said all reading the needle bounces one degree up and down.

Should vacuum not be steady? I read what the gauge is telling me in the red bands but I’m in between the bands, and bouncing. Before I do something foolish like remove a toon or pull an engine, I want to explore every possibility.

My 13p prop arrives shortly. If I DO NOT see a significant rpm gain, would y’all think more boat design issue with the toon or something internal to the motor?

 

[Scott Danforth]

yes, it should be steady if you are taking readings from the plenum. however you are most likely taking readings upstream closer to a cylinder so you will get pulses.

 

[Oldpos]

yes, it should be steady if you are taking readings from the plenum. however you are most likely taking readings upstream closer to a cylinder so you will get pulses.

I’m pulling off of the large vacuum port on the front of the edelbrock.

 

[dubs283]

I had the center toon modified where the fuel tank is inside of the toon about half way up the boat.

That's a problem. Engine/drive is trying to push/lift all that fuel in front of it located too far away. Think of a lever/fulcrum. Your fulcrum (fuel tank) is too far forward of the lever (engine/drive). Even with a huge diameter, low pitch prop on it you will continue to have issues. The modification is cool but your weight distribution, angle of attack, and dead rise prior to the drive is causing issues no cam, prop, vacuum reading, or ignition system will solve. Unless you supercharge it but that's a waste imo.

The fuel pump should be located in line after the filter, fyi

 

[Oldpos]

That's a problem. Engine/drive is trying to push/lift all that fuel in front of it located too far away. Think of a lever/fulcrum. Your fulcrum (fuel tank) is too far forward of the lever (engine/drive). Even with a huge diameter, low pitch prop on it you will continue to have issues. The modification is cool but your weight distribution, angle of attack, and dead rise prior to the drive is causing issues no cam, prop, vacuum reading, or ignition system will solve. Unless you supercharge it but that's a waste imo.

The fuel pump should be located in line after the filter, fyi

I agree maybe it’s a long run to pull fuel and maybe I should have a second pump at the tank. I debate the weight issue, the tank is located between the marks of 8 and 12 feet from the rear of the boat, which makes it 12 feet from the front of the tank to the bow.
At that point it is a 32 gallon tank and at 7 pounds per gallon that only about 225 pounds. May fat rear is 230, and I have been testing with only two people and no gear on board. Batteries were moved to under the helm vs the rear of the boat so another 100 pounds forward. With the 3.0 I would carry up to 14 people and still make 19 mph. So I’m not sure how the fuel tank placement would affect it.

 

[jimmbo]

On a very long fuel Line/Pipe, the diameter should be increased, just to reduce internal Friction. That 5.7 can Hoover 35 gallons in about 90 minutes at WOT, at least mine can

 

[Scott Danforth]

0.4# of fuel per HP per hour

 

[jimmbo]

That is not cast in stone, but it is a Guide Point

 

[Oldpos]

On a very long fuel Line/Pipe, the diameter should be increased, just to reduce internal Friction. That 5.7 can Hoover 35 gallons in about 90 minutes at WOT, at least mine can

With those figures and gas prices I really wish I stuck with the 3.0

 

[jimmbo]

At the Top Speed that the 3 liter made, the 5.7 will likely be a bit more economical.

 

[Oldpos]

I got the boat home today, I will be pulling the motor in the next few days. Im very interested to see what I find when I tear it down.

 

[Oldpos]

Found it !!!!!!!
For those who gamble and bet on a flattened cam you would have won. Number 4 cylinder intake lifter

I don’t understand how it would have passed a compression check though, and also I checked the lift of the rockers, maybe I screwed up measuring

Another thing , look at the pistons, why are some pistons, you can see the aluminum of the piston ? The risers don’t have rust marks from water.

 

[Scott Danforth]

how does the cam look? if you need a new cam and lifters, suggest roller cam and simply tap the 3 bosses for the lifter spider

 

[Oldpos]

how does the cam look? if you need a new cam and lifters, suggest roller cam and simply tap the 3 bosses for the lifter spider

The lone is fully rounded, I don’t have the cam out but can see it in the lifter boss. It’s a vortec block so it is set of for roller already

 

[Scott06]

The lone is fully rounded, I don’t have the cam out but can see it in the lifter boss. It’s a vortec block so it is set of for roller already

If you still have the donor engine truck cam and lifters that should work for your application

 

[jimmbo]

...if you need a new cam and lifters, suggest roller cam...

With todays Oils, that is the only way to rebuild an Engine. An added bonus, the Rollers can handle steeper Ramps, allowing Cam Profiles that improve breathing without an Increase in Lift and/or Duration,

 

[Oldpos]

With todays Oils, that is the only way to rebuild an Engine. An added bonus, the Rollers can handle steeper Ramps, allowing Cam Profiles that improve breathing without an Increase in Lift and/or Duration,

Is the lift the same between the cams, I found a similar lift roller cam , will it give me the same lope and performance or do roller use larger lifts to achieve the same hp

 

[Scott06]

Is the lift the same between the cams, I found a similar lift roller cam , will it give me the same lope and performance or do roller use larger lifts to achieve the same hp

The profile of a roller cam lobe is at a steeper angle so the valve opens and closes faster - of two cams (on roller one flat tappet) at same lift and duration The roller would have more time where the valve open at a higher lift (even though the max valve lift is the same ) than the flat tappet

This is why a roller will
Breathe better and provide better performance of course costs more but you don’t have break in issues ….

 

[jimmbo]

I wouldn't be choosing a Cam, because of how it makes the Engine Sound. What sounds great in an Auto Engine, isn't the best for a Marine Engine.
Given the Same Duration and Lift, the Roller Cam, has the potential to make better Torque and HP than a Flat Tappet Cam. And as I said earlier, you don't have to worry about the lack of additives in Modern Oils needed to keep wear on the Cam low