Mercruiser 357 Magnum VS 350 Mag MPI

[300sflyer]

I need to re-power a twin screw 31 foot Chaparral, [bravo 1 drives] with new engines. Long story....

I am looking at 2- new 357 Magnums [325 HP witth 4 barrel carbs] vs 2- new 350 Mag MPIs [300HP]. The 357s are significantly less expensive, however I have been told they will consume about 25% more fuel than an MPI engine of the same size.

Is there really that much difference in fuel economy? Are there any other big differences I should be aware of before deciding which way to go?

Thanks,

 

[Bondo]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Long story....

Ayuh,... I would Guess that you're inta this hull for the Long haul,..??
If so,...
I'd go with the MPI....
If you're gonna walk away in a few years,... Go with the Carbs....
Although, the MPIs might pay-off at selling time,..??

Btw,.... We just Love long stories,....

 

[300sflyer]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Btw,.... We just Love long stories,....

The short version...

Just bought the boat knowing one engine is junk... [1995 5.7 250 HP] risers and manifolds are toast, which caused the engine to fail. The other engine is an almost new 2008 250HP TKS 5.7, with just 12 hours on it. Yes... 12. I'm thinking I could sell this engine for a half decent price...

As I understand, these TKS engines are being phased out...

I need to replace the one engine for sure... but don't want to have 2 out of date engines... and would also like a bit more power, as the previous owner said the top speed was only about 30 mph.

Your thoughts?

 

[TilliamWe]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

I've never heard of a 357 Chevy. So is that motor remanufactured or something?

MPI is the way to go, in my opinion.

That said, the previous owner was doing something very wrong if the boat only went 30mph. My dad had a Chap Signature 300, with twin 5.0 EFIs at 240hp each, with Bravo 2 drives, and it went 40.

Me, personally, I'd buy one 5.7TKS to match the one you have now, and go boating!

 

[300sflyer]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

I've never heard of a 357 Chevy. So is that motor remanufactured or something?

http://www.mercruiserparts.com/357_magnum.htm

 

[John_S]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

How about Mercruiser's 383 MPI? They were factory remans, but haven't heard much about them in awhile. I bet you could use the extra torque they would provide.

 

[TilliamWe]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

http://www.mercruiserparts.com/357_magnum.htm

Okay, thanks. It's a bored out 350. And it's a remanufactured engine, not brand new. It's also A LOT less expensive than the new 350 MAG MPIs.

Before i bought 350 MAG MPIs I'd buy the 383MAG MPI (which is priced in between the 357 and 350MAG). My dad installed one in my sister's boat as a 454 MAG replacement and it does an excellent job.

But I repeat, the best and most cost effective thing to do it buy one 5.7TKS.

 

[300sflyer]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Cost effective in the short run... Yes I agree. They are about 7 K for the 5.7 TKS.

In the long run considering the better fuel economy of an MPI, I'm not so sure. I plan on keeping the boat for at least a few years.

 

[TilliamWe]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

You won't use the boat enough to save $5000 in gasoline.

Come on, do the math. Even if the carb costs you 25% of fuel economy, you are talking about getting 1mpg vs 1.25 mpg. At $4.00 a gallon, you would have to use an additional 1250 gallons of fuel to spend that $5000 difference.

How much gas do you use in a year?

 

[QC]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Oh and the 357 is crank rated and not merc. There is no more power there so do not let that fool you. The MPI is rated at the prop and those drives eat around 30 hp . . . Tilliam is right about payback. Maybe the resale gets it done. The MPIs will be waaaaaay sweeter than anything else you are considering, but make that decision more because of coolness and hassle (starting/maintenance) than because of economics.

Edit: Correction . . . that 357 IS factory Merc. Be careful with the chart below that I found at BAM (referenced site above). They show HP at the prop on all but those that I have bolded. Those are flywheel horsepower and, again, you need to subtract as much as 30 HP to compare to the list above. I consider this a major oversight and MANY engines are mis-marketed this way. It causes a LOT of confusion, expense, dissatisfaction etc. etc. etc.

Model HP

3.0L 135

4.3L 190

5.0L 220

5.7L 260

4.3L MPI 220

5.0L MPI 260

350 MAG MPI 300

350 MAG Seacore 300

MX 6.2 MPI 320

496 MAG 375

496 MAG HO 425

357 Magnum 325

383 Magnum 350

400 Scorpion 400

 

[TilliamWe]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Thanks for the heads up on cranshaft versus propshaft HP, QC.

And thanks for backing me up on the fuel savings taking a LONG time to realize.

 

[300sflyer]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Thanks for all the info and advice guys!

Gas is currently just shy of $1.00 per litre here in Canada, [almost $4.00 per gallon] and the summer before last we hit $1.40 per litre. [almost $6.00 per gallon]

Who knows what it will be next summer, but I would think $1.25 per litre or $5.00 per gallon, is a safe bet at the marinas.

The tank holds close to 600 litres. At a 25% savings with MPI engines, that translates to about $180 worth of extra mileage, with each fill up. After 28 fillups [Lots of boating!!] , the extra $5000 is covered, so I see where you guys are coming from.

On the other side of the coin, how much do MPIs add to the value of the boat at resale time? Hard to say I guess...

 

[TilliamWe]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

The tank holds close to 600 litres. At a 25% savings with MPI engines, that translates to about $180 worth of extra mileage, with each fill up. After 28 fillups [Lots of boating!!] , the extra $5000 is covered, so I see where you guys are coming from.

On the other side of the coin, how much do MPIs add to the value of the boat at resale time? Hard to say I guess...

Do you boat that much?

MPIs won't add $11000 to the boat's value, I know that for sure.

 

[haulnazz15]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Most boat purchasers wouldn't know the difference I'm sure. Doubt it adds anything to the value to go with MPI's. I'd stick with the 357 Mag.

 

[300sflyer]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Pulled the bad motor today, and here are a couple of pictures.

 

[300sflyer]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Update:

I went with the 357s.

Took the boat out for its first run, and they ran very well! With just 2 hours on the engines I have been real easy on the throttle, and have not gone above 4000 RPM. The manual says I can got to WOT for brief periods, after 10 hours.

Here are a few pics.

 

[Juniinc]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

good lookin engine hoist, nice work man.
bet those twin 357s love to drink some gas.

 

[QC]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Cool! Would love to see WOT data when you can get it.

 

[fixation]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Mike,
I am VERY interested in seeing how things are going with the new motors. Here is my situation:
My name is Mike- Your name is Mike
I have a 1996 31 Chaparral w/twin 350 mag mpi's (one new in 2006, one original 1996 motor- Cracked Block-needs replacement)- looks like you have been through the same thing.
I am looking at maybe selling the 2006 350 Mag MPI (only 125 hrs) and replacing both engines with either a 357 Mag carb or only one 350 mag MPI to match. PLEASE RESPOND with your opinion. I laughed when I saw your post as it is almost identical to my situation.....and your name is Mike !!
thanks,
mike

 

[Don S]

Re: Mercruiser 357 Magnum VS 350 Mag MPI

Best to start a new post with YOUR question, post a link to reference this thread.
Even if it is similar. After a reply or two to your question, it would then be invisible to and not replied to anymore.