Mercruiser 5.7 mpi vs. 7.4 mpi

[Mohabba]

Im in the market for a Bayliner 2855/285.

I see them listet earlier with 7.4 mpi and later 5.7 mpi.

If maintained properly and with low hours, what engine would be prefered in terms of service and performance/fuel?

The systems need to have complete closed cooling or to be refitted. What would a refitting of closed cooling involve?

How much difference in corrosion can be expected from a raw water cooling system in fresh water like Lake Lanier/Hudson River and salt water?

Thank you for replies

 

[Philster]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

Let's see... salt water requires flushing after each use. It also means 1500 bucks worth of exhaust pieces get trashed about every 3-5 years.

5.7 vs 7.4? Well, they both are solid, tested platforms. The 7.4 is 30% larger and will lose the efficiency battle.

I'd go with the 7.4. You will always want more grunt if you throw a 5.7 in that boat. Heck, the 7.5 might be more efficient that a straining 5.7 when you measure ALL the variables.

.

 

[boat1010]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

Like the saying goes."there is no substitute for horsepower" If I have a choice I will always go with the bigger motor. You can always go slower with a bigger motor but can't always go faster with a smaller one. I have the 7.4 in mine and no it isn't very friendly with the gas...But I never even considered fuel economy when I got the boat. I was looking for something that was powerful enough to get me where I was going, and to do the things that I wanted to do. A 28 foot boat is a lot of boat to be pushing around. Mine is a 24 ft and at times I would like to have had the 502 in it.

 

[QC]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

"there is no substitute for horsepower"

This is very true, and in some cases the 7.4L MPI and 5.7 MPI (350 Mag?) are only 10 horsepower apart (310 vs. 300). Annnnd, the 350 is lighter. I would still want the 7.4, but the fact is a 6.2 320 is rated higher than a 7.4 310. Just is. So these decisions are not always as simple as bigger is better, need all the info . . .

Oh, and no need to start the torque discussion. With a Bravo 3, hole shot is not worth discussing, so assuming both setups will plane, I think this is very close to a push. Again, depends on the rating!!!!

 

[bruceb58]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

Take a look at what the torque ratings are at around 3000 RPM because that is what you are going to get stuck at if you can't get up on plane. Then you will see why you want the 7.4L. I have a 7.4L in my 24' boat and don't even need to change props when I go up to Tahoe from sea level.

 

[QC]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

I agree ^^^^^^ IF, there is a problem getting on plane.

 

[Mohabba]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

I see that the Bayliner 2855/285 is common in Denmark with a 6.2 320 option but havent seen any in the US, only later 5.7 300 and earlier 7.4 310.

I think that i would like a 6.2 320

 

[bruceb58]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

I think that i would like a 6.2 320

Why? The torque of the 7.4L is going to be way higher in the RPM that you are going to be operating in.

Are you looking for a used boat? The 7.4 hasn't been in boats for awhile. 8.1 replaced the 7.4L since 2001 or so. The 6.2L is very recent.

 

[QC]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

My 6.2 is a 2001 And I don't agree with the torque comments if these are Bravo 3s . . . Peak torque only comes into play with the throttle at 100%. If you are struggling to plane, then yes, the 7.4 may outperform the 6.2. If not, the 6.2 should be faster top speed and have better fuel economy. In either case we are talking very small numbers, but I do not believe that the 7.4 will deliver any type of tangible performance benefit. Maybe better resale, but only due to misconceptions about marine engine performance. With a Bravo 1 or 2, I might feel diffferently. But again, only if the boat needs 100% throttle to plane. I feel almost the same with the 350 mag.

If you bring the 8.1 or 454 Mag MPI into the discussion, then my opinion changes as those are rated 375 and 385 respectively.

 

[jaxnjil]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

Im in the market for a Bayliner 2855/285.

I see them listet earlier with 7.4 mpi and later 5.7 mpi.

If maintained properly and with low hours, what engine would be prefered in terms of service and performance/fuel?

The systems need to have complete closed cooling or to be refitted. What would a refitting of closed cooling involve?

How much difference in corrosion can be expected from a raw water cooling system in fresh water like Lake Lanier/Hudson River and salt water?

Thank you for replies

try to get B III drives no matter what you get. imho

jill and i just got a 310 dancer with twin mpi 7.4's and have a 7.4 W E

we looked and tried many boats is this range including bl.

the 7.4's will get as good of economy in this range and the smile factor isn't close to the same.

remember back in the day when honda 55 were popular and then the lucky kid on the block got a 90?

 

[Fireman431]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

300 vs 310 HP in a boat on the water? You'll never notice the difference. By the time you factor in the HP to weight ratio of the engines alone, there will be ZERO difference. 10 less HP in an engine that's 150lbs (guessing) lighter is neglible at best.

The torque/HP is going to nose over @ 5250 RPM anyway and big blocks don't like to spin at that speed. Mine are 4300 at WOT.

Resale is going to be better with the big block.

 

[bruceb58]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

Peak torque depends on engine to engine and rarely is as high as 5250.

 

[26aftcab454]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

I sure love my Mercruiser 7.4/ bravoIII
planes quickly- criuse 3500 rpm @ 30mphwot 4600 rpm 43mph

 

[Black as]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

'All other things being equal, a bigger engine makes more power.'

 

[Mohabba]

Re: Mercruiser 5.7 mpi vs. 7.4 mpi

Why? The torque of the 7.4L is going to be way higher in the RPM that you are going to be operating in.

Are you looking for a used boat? The 7.4 hasn't been in boats for awhile. 8.1 replaced the 7.4L since 2001 or so. The 6.2L is very recent.

I havent seen one ad - at all - with a Bayliner 285/2855 having a 8.1
In the US (yachtworld.com) I only see earlyer models with 7.4 mpi and later until now with 5.7 mpi.