23' Performance 5.0 vs 5.7 Mercruiser

Hooligan · #1 01-08-2014, 12:07 AM

I am wondering if anyone has any comparison numbers for me: a 23' with either a 5.0 or 5.7. I am looking for Cruise speed, WOT speed and accomodating RPMs.

I know quite a few 23's around had the 5.0 and just looking for comparison against the 5.7.

Turns out my motor (that I purchased in the spring) has good compression on only 7 cylinders and has been pulling one along for the ride. 185 psi compression in 7 cylinders and only 40 in the weak cylinder (#5).

I could never get the boat over 4250 rpm... tried three different props and got some good speed but I'm still missing atleast 800-1000 on top.

Time to replace the motor or rebuild, trying to decide which way to go: carbed 5.7 or MPI 5.0....

Numbers please? Anyone running a 5.0 or 5.7?

Thanks dudes!

Billpotter · #2 01-09-2014, 12:56 AM

My family had a 23 CC stern drive in the 70's that was delivered new with a 225 OMC (307 cid 5 liter) in 1975 that we ran for 600 hours till 1979, when the boat was then re powered with a new 260 OMC (350 cid 5.7 liter) that was then run for 200 hours. The boat went from a nice running boat with the 5 liter engine to a great running boat with the 5.7. This was before Loran C or GPS- no solid numbers: I'd go with the 5.7 unless the money thing is an issue- the 5.7 package, all other things being equal, delivered more of everything; something I have confirmed without exception in my marine surveying practice.

oldfielder · #3 01-09-2014, 06:51 AM

My 23 sf with carbed 5.7 alpha has always topped out right at 40 mph at around 4500 rpm. Max =4400-4800.
I like to,cruise around 28 at 3200 and right there she was burning just north of 10-10.5 gph when my fuel monitor was still working a while back.

FishStretcher · #4 01-09-2014, 09:13 AM

The local merc rep said that the EFI systems aren't picky about dirty/watery fuel, within reason, and based on my limited experience with a 2004 MAG MPI, I would agree. If it was me, I would go 5.0 injected over 5.7 carburated.

FLexpat · #5 01-09-2014, 12:33 PM

The sweet answer is obviously a 5.7 MPI. Just from rough numbers, replacement engine Mercruisers are running $35-$40 per horsepower. Marine Power replacement engines are running $30-$33 per horsepower. Very similar engines with different marketing, dealer networks, and warranties. A lot commercial guys run Marine Power. I had a Marine Power 290 HP 5.7 in my 20 Bertram; it worked really well and I liked it a lot. You may want to look into them.

bigeasy1 · #6 01-09-2014, 02:43 PM

Quote:

Originally Posted by oldfielder

My 23 sf with carbed 5.7 alpha has always topped out right at 40 mph at around 4500 rpm. Max =4400-4800.
I like to,cruise around 28 at 3200 and right there she was burning just north of 10-10.5 gph when my fuel monitor was still working a while back.

My figures are very very similar to this with a 5.7 mercruiser with an Edlebrock 600 cfm carburetor, on my 23' Tsunami.Fuel burn is also very close.I'm spinning a 15x17 Mirage ss prop.(1.65 I/O gears)

I bought another boat that was crashed,but had a very low hour 5.7 so i got a good deal,and used the engine,and junked the boat.
For my use,I prefer the carbed engine as there are very few dealers or marine mechanics that I trust to service MPI engines,where I boat.
At least with the carb,I can rube goldberg something good enough to get me home.

I also seem to remember that Meruiser had a take out program with their dealers,when repowers are done,you can pick up the original power plant for a decent price.I think they may have even offered a warranty with them.

Hooligan · #7 01-12-2014, 03:23 PM

Sounding like the 5.7 carb is the way to go: I am on the fence about buying a short block and building it up myself vs buying an already running takeout etc.

I like the MPI idea... only problem is the motor I was looking at went quicker than a school boy in a nudie booth. It was a sweet deal. This is the second 5.0 I screwed up on: last year I had an opportunity to buy a 2010 w 70 hrs on it (freshwater lake) for 2250.0.... woulda shoulda coulda.

Oldfielder, what prop are you turning and is it a bravo or alpha setup?

Ive an alpha one (Sterndrive engineering replacment) gen II and have a few props on hand to choose from.

Daysatsea · #8 01-14-2014, 10:25 PM

I've got an 07 300 hp/350 mag mpi in my 23 cc. Tops out around 40 kts with a light load turning around 4500. Not sure of the prop but I have a gen II alpha drive. Usually cruise around 26 kts at 2800.

Hooligan · #9 01-19-2014, 02:53 PM

Thanks Daysatsea- thats the kind of info Im looking for. Prop size would really help but if its inconvenient to look, no big deal.

FishStretcher · #10 01-19-2014, 04:23 PM

There is a 7.4 MPI with bravo near you on CL. Not sure if that's too much for a 23 footer?

And do you know why you have a dead hole? Because that might be easy if it is a valve.

It could be worth the price of a leak down tester if you don't have one.