1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

[silver_power]

Hello, to everyone.
i was wondering the following...
My old engine (1992) 350lx 4bbl with 260 crank hp@4600rpm. This means with simple calculations that the engine torque was 397Nm@4600 (at crank).
My new engine 2009 350Mag Mpi has 300 prop hp@5200 , so lets say about 330 crank hp@5200 which means 446Nm@5200 (at crank). So i can know the toque at the wot but i can not know the torque at 4600rpm so to campare it with my old engine. My point is that if my new engine has the same torque at 4600rpm with the old one.....then my boats top speed will be axactly the same...even if the new engine has +65 nominal hp at prop. How much torque do you believe the 350mag can have at about 4600rpm?....more that the old one or the same?.....Is the any torque diagram of these engines?
Thank you very much.

 

[wire2]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

I'm not totally clear on what you're asking, but keep in mind that a gasoline engine's torque typically peaks and drops off before the HP peaks.

You can calculate one from the other for a given rpm with the formula;
Power (hp) = Torque (ft-lb) * RPM / 5252

 

[John_S]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

Max torque has always been at lower rpms than max hp. Don't know if it is of value at the max hp rpm.

Finding a torque curve for the LX may be hard. On the GM marine engine web pages, I think there is one that should be close to the 350MAG.

What size and weight boat? What prop and gear ratio? WOT GPS speed on the LX?

Torque is important to get on plane, but the HP will be important for wot top speed.

 

[dirtyoldman]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

Hello, to everyone.
i was wondering the following...
My old engine (1992) 350lx 4bbl with 260 crank hp@4600rpm. This means with simple calculations that the engine torque was 397Nm@4600 (at crank).
My new engine 2009 350Mag Mpi has 300 prop hp@5200 , so lets say about 330 crank hp@5200 which means 446Nm@5200 (at crank). So i can know the toque at the wot but i can not know the torque at 4600rpm so to campare it with my old engine. My point is that if my new engine has the same torque at 4600rpm with the old one.....then my boats top speed will be axactly the same...even if the new engine has +65 nominal hp at prop. How much torque do you believe the 350mag can have at about 4600rpm?....more that the old one or the same?.....Is the any torque diagram of these engines?
Thank you very much.

Your current top speed is probably limited by torque as you suggest, but power output will drop very significantly after a point, and that's limited by things such as your valve train and carburetor. Your new engine will not have some of the same limitations, so the engine will continue producing torque and horsepower higher in RPM's than the old one did - which may be where the extra HP comes from.

You are correct that torque is what makes it go...but incorrect is that torque is no constant throughout the RPM range. Your current engine loses torque at about the current top speed, and the new one will continue to produce for a few hundred RPM more. So you will have more top speed with the new engine.

You really need the torque and horsepower curves of the engine to get mroe information. I bet mercruiser published them.

 

[John_S]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

You really need the torque and horsepower curves of the engine to get mroe information. I bet mercruiser published them.

If they did publish them, they must be internal only or supplied to boat builders with NDA. They are not found in any of the over-the-counter technical data I have seen. Just max HP.


Here is the GM data (see last page):http://www.gm.com/experience/techno...specialized/marine/REV_F_2011_5700_Marine.pdf

 

[45Auto]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

My point is that if my new engine has the same torque at 4600rpm with the old one.....then my boats top speed will be axactly the same...even if the new engine has +65 nominal hp at prop.

Power is what determines the top speed of your boat. Every boat has a "Power Required versus Power Available" curve. When the amount of power required exceeds the power available your boat won't go any faster. If your boat would previously go 50 MPH at 4600 RPM, that means it took 260 HP to make it go 50 MPH. To go 51 MPH might have taken 270 HP, but your motor only had 260 HP available. Your new motor will also have about 260 HP available at 4600 RPM. However, it will keep accelerating past that RPM (probably to 5000 RPM or so) until it reaches the point where your boat requires more HP than it has available. That should mean an additional couple of miles per hour in top speed.

See this thread for more of an explanation:

http://forums.iboats.com/showthread.php?t=225803&highlight=secrets

 

[silver_power]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

I agree with all of you...that is correct
So lets talk with numbers
With the old engine, my boat had on gps 55-56mph@4550-4600.For now my new engine has anly 3,5 operating hours so i can not push it till wot, and so find out what is the rsult. But to be honest, there is a BIG difference in time to plane. The new engine has a lot more torque at low revs and so pushes the boat in plane VERY quicker.
What do you you think will happen with my new engine (350 mag mpi) concerning top speed?
I used and i am still using the same alpha drive (1,48:1) and 22p Enertia and the boat is a Wellcraft 202 Nova Spyder.

 

[dirtyoldman]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

I agree with all of you...that is correct
So lets talk with numbers
With the old engine, my boat had on gps 55-56mph@4550-4600.For now my new engine has anly 3,5 operating hours so i can not push it till wot, and so find out what is the rsult. But to be honest, there is a BIG difference in time to plane. The new engine has a lot more torque at low revs and so pushes the boat in plane VERY quicker.
What do you you think will happen with my new engine (350 mag mpi) concerning top speed?
I used and i am still using the same alpha drive (1,48:1) and 22p Enertia and the boat is a Wellcraft 202 Nova Spyder.

I would expect top speed to be higher by the amount of the difference in RPM that you can run. If the new engine is rated up to 5000 rpm and can produce enough power in that range (which I assume it can), then your top speed will probably be about 7-8% higher.

 

[John_S]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

You probably have a decent shot of hitting 60mph with tuning to the right prop. You will have to patiently wait for the break-in, so you can start doing the test runs.

 

[silver_power]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

My new engine is rated up to 5200rpm.
I believe that i hill gain about +6-7mph on gps.
Math sais that since i had 56mph@4600 , i will have 63mph@5200.......
The big question is ....... will I reach 5200rpm with +65hp@prop...?????
600rpm for 65hp?...Is it reachable according your experience?
The only problem is that the only truth will be clear after break -in .....and i can not wait.....!!!!!!

 

[45Auto]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

I believe that your 1992 260HP engine was rated at the prop just like your new 300HP engine is. That means that you'll have an additional 40HP. A rule of thumb is that it takes 10HP to gain 1 MPH, so hopefully your boat will be about 4 MPH faster than before.

 

[silver_power]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

I believe that your 1992 260HP engine was rated at the prop just like your new 300HP engine is. That means that you'll have an additional 40HP. A rule of thumb is that it takes 10HP to gain 1 MPH, so hopefully your boat will be about 4 MPH faster than before.

My old engine was definately rated at crank.
The specifications are 260hp@crank , 235hp@prop and wot 4200-4600rpm.
350 mag mpi 300@prop , wot 4800-5200rpm .
That is why i am speaking for +65hp@prop.

 

[John_S]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

My new engine is rated up to 5200rpm.
I believe that i hill gain about +6-7mph on gps.
Math sais that since i had 56mph@4600 , i will have 63mph@5200.......
The big question is ....... will I reach 5200rpm with +65hp@prop...?????
600rpm for 65hp?...Is it reachable according your experience?
The only problem is that the only truth will be clear after break -in .....and i can not wait.....!!!!!!

From my experience, it will take more hp than that. Depending on hull, it is generally not a linear function at these speeds. If you check your service manual, I believe your LX is rated for 250hp at prop. Given the very noticeable improvement in plane times, I think it is possible to hit the 60mph mark. But, maybe not. Many a v hull has had challenges crossing that mark.

It is unlikely to find someone with your exact hull doing the exact upgrade you did. Many people do not cough up the extra $ for a mpi.

My advice: Spend some time on the water and get the break-in over as soon as possible.

 

[silver_power]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

1992 350 magnum 280hp@crank and 250hp@prop (wot 4400-4800)
http://www.piranha.com/motor_specs.php?id=2283


1992 5.7LX 260@crank and 235@prop (wot 4200-4600) http://www.piranha.com/motor_specs.php?id=2281


These are the numbers for sure according the mercruiser manual that i found in my boat when i first bought it. It was with all the other papers that the factory put inside the boat.
Until 1995 ,i think, mercruiser used to announce both the crank and prop hp. But now only the prop hp.
Anyway, i will keep you posted with final results after break-in....
Thank you very much.

 

[Mischief Managed]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

Drag is a function of the square of speed. If your boat goes 56 MPH with 235 HP at the prop, math says it will go 63.27 MPH with 300 HP at the prop, assuming prop slip and drag remain equal.

If you are going 56 at 4600 RPM, 5200 RPM (again, with the same slip and drag) will yeild 63.3 MPH. Amazingly, you might be perfectly propped right now.

The math:

new top speed = the square root of ((56x56)/235)x300.

new top speed at 5200 RPM = (56/4600)x5200

 

[silver_power]

Re: 1992 350 LX 4bbl vs 2009 350MAG MPI torqe question

I hope to reach 62mph.......
we will see......