Mercruiser 3.0 lx sea trial and compression specs.

[alldodge]

All pistons are the same size. To obtain 181 cubic inch displacement (3.0 liters) is by measure the surface area of the piston x the stroke x the number of cylinders.

Radius squared x Pi x stroke x 4 = 181 cu in (3.0L)

The LX comes from the head design and intake/exhaust manifold. The HP difference is minimal. Old motors used calculated HP, newer motors use measured HP at the prop

 

[Who me?]

3lx uses the old 140/3l head
3l uses the old 2.5litre engine head

 

[Guymontag]

Thanks guys! compression test was 150 across all 4 cyl. And it deff will rev. over 4800rpm. I Dont know if this should be a new thread or not. But here goes. I need a new prop for sure mines destroyed but enough there to push the boat 30. I was hoping for 40. The p.o. said 50-55 but I doubt that. I'll be happy in the 40s. Any how here's the numbers on my current prop I don't know what they mean. I'm looking to replace it with the same...or better I'd rather have the top speed . Do I need something diff?

And if my tach is correct I was getting 5000rpm doing 30mph GPS. ( The prop been run over the boat ramp and has major chips out of it)

 

[Guymontag]

The pictures won upload file size to large. The numbers on my current prop are:
18-( or 48 I can't tell it's worn)18120-19 and it has the letter l on it on the oppsosite side.:embarassed:

 

[alldodge]

48-18120-19 is a 14 diameter by 19 pitch. I would just get the same prop or take yours to a prop shop and have it repaired. Aluminum props can be rebuilt

 

[Guymontag]

Would you consider that a prop meant for top end generally speaking? I can sacrifice hole shot for top end..

 

[alldodge]

In order to determine top speed, we first need to know what the drive ratio, WOT rpm and speed using accurate tachometer and GPS speed. Without this info we are only guessing.

If the boat can achieve 4800 rpm, with a 1:94 drive and you have 10% slip with 19 pitch prop, the boat should be at 40 MPH. If slip is reduced to 5 % the your at 42 MPH

If your tach was correct at 5000 rpm, then your motor won't last long, but would also be normal to over rev because the prop is damaged

 

[Guymontag]

How do I find the drive ratio?

 

[alldodge]

How do I find the drive ratio?

If unable to find the drive serial number, then get another person to help.
Pull the coil wire and ground it
Place throttle in forward gear
Mark the harmonic balancer that can be seen easy
Mark the prop
Trim the drive down
Find the slave solenoid and make up a jumper (do not turn the ignition ON)

With one person watching the prop and another watching the harmonic balancer, jump across the top contacts of the slave solenoid. Keep the jumper in place until the mark on the balancer is seen 10 times, then stop.

Should be able to figure it out with those

 

[Guymontag]

Holy sh** ha ha I'll look for the drive number.. the prop is a left right or standard can you tell by the prop numbers? I'm gonna buy the same one in stainless. Anyone got a good recommendation?

 

[Who me?]

vengeance is a good prop i got one most 3 liter engine use a 1.94 ratio if achris is right and the top revs are 4800 then 5000 is ok but check the rev meter not all rev meters are right

 

[Guymontag]

I was thinking about putting a 21 pitch on there I don't mind sacrificing the low end speed. Think that will be too much pitch?

 

[Who me?]

21 will drop your revs about 400 if youre getting 5000 now a 21 will make about 4600 and probly add about 3mph speed sounds like the right prop

 

[alldodge]

A 21 will only drop the WOT rpm by 400 IF a undamaged 19 prop does reach 5000. Right now no one knows what is max rpm

 

[Guymontag]

Ok guys thanks I'm gonna throw the 21 on and see what it does. Might try a 23 after that I'm trying to get 40-45 mph out of this

 

[alldodge]

Might try a 23 after that I'm trying to get 40-45 mph out of this

If that does work out, might try a 30 pitch to reach 60

 

[Guymontag]

Haha ok. I assume you kidding... It's a 16' scarab only weighs 1800. They came with a 4.3 and a 3.0. the 4.3 is said to do 50-55 and the 3.0 is said to do 45-50. Don't think it's gonna happen though. I put a prop I took off 24' Bayliner the numbers were all work so I had to guess. Holding it up to what's left of my 14x19 it looked close it did 35gps at 5500rpm.45mph isn't looking good. Not cool getting passed by a pontoon with a family of 6 when your on a Scarab. Ha ha oh well

 

[alldodge]

assume you kidding.

Yes, your not work with real numbers so ....

at 5500rpm

Now your hitting 5500 with a motor that has 4800 max WOT, if so something else is wrong

 

[achris]

Ok guys thanks I'm gonna throw the 21 on and see what it does. Might try a 23 after that I'm trying to get 40-45 mph out of this

It's about the maximum power. That engine produces maximum power at 4600rpm. Power pushes the boat and if you overload the engine it can't reach the right revs so will not be making maximum power. Put the prop on that allows the engine to spin to the right rev range and you will have the maximum speed. Putting on a bigger prop than that will SLOW your top speed, not increase it....

Remember what a infernal combustion engine power curve looks like... If you have a prop on that is too 'light', and the engine goes over the maximum power revs, you're also producing less power....

Prop to the maximum power and that will be maximum speed. Propping bigger will be slower.

Chris.........

 

[Guymontag]

Gotcha . I think my tach may be off as well. I guess I'll drop the 21 pitch on there and see. What happens from there