23' Performance 5.0 vs 5.7 Mercruiser
 

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!

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.

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.

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.

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.

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.

awesome .02.
 

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.

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.

Thanks for the info, very valuable
 

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

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.

A friend of mine had one in a 23 Sceptre. Good rig if you like to burn lots of gas! On one trip with us from Palm Beach to Great Guana Cay, his 120 gallon tank was nearly empty by the time we got to Coopers Town on the main island of Great Abaco, a distance of about 120 miles!

So with a Bravo 3 on a 25, would that be as bad? Just thinking about the siamesed exhaust ports on a SBC. It still would weigh less than dual 6 cyinders.

I had a 5.7 efi with the bravo 3 in a 251 proline great power package would run all day at 3800 running about 35 top end about 48 at wot
Never checked but economy was great would guess about 3-4 mpg atleast

Fish, even though it weighs less than twin 6's, the 454/7.4L engine is still quite a bit heavier than the small block 350/5.7L, so it makes more sense in the much heavier 27 than it does in the 25, which is known to be sensitive to stern weight. CSC member 3rd Day has a 7.4L in a 27' Seamaster and was complaining about fuel consumption in it, so you might send him a PM to get more details. I think I'd pick a FWC 350 w/aluminum heads and a DuoProp or Bravo 3 drive over the big block engine. The small block MPI heads have lots of development work in them and make pretty good power, so if you can get enough speed at an acceptable vacuum level with that combo, why run anything heavier?

Water Rat (Chris) has a 7.4 in his 23 Sceptre with a Bravo drive (don't remember which model). If he sees this thread he'll provide performance/consumption #s. I do know that with a couple guys in the back of the boat, there's about an inch of water on deck due to the extra weight of the big block.

I've been on a 23 Sceptre with a carb'd 350 and Alpha drive which seems like a perfect combo for that boat.

Fish: I agree with Bushwacker:

" if you can get enough speed at an acceptable vacuum level with that combo"

The 5.7 with a duoprop(or bravo3) balances 0k in a 25, especially with a light load. Load it heavy and you need careful to put a lot of weight forward. The big block would definitely be worse for balance(important in the 25), but If you are looking for speed, you might consider a big block and moving the fuel tanks forward. Mine, with a 5.7 throttle body EFI, is a bit slower than I would like. Light load is fine, 24-26 knots cruise. Load it down, the vacuum drops fast and you have to slow down, 21 knots or there abouts. Fuel economy goes to hell as well. You might find a good solution in one of the higher horsepower small blocks with more modern EFI, not sure.

So I have some of the parts. I have a bravo 3 and 300 MAG MPI. I just need to burn the valves out of it at WOT so I can justify those aluminum vortec heads... Although I am not sure there is a lot more power with those heads. Although with good valves and seats, maybe you can run harder at cruise.

With the knock sensor, I can try higher toluene content race gas and see what a bit more power will get me. If I have the prop pitch to use it.

Hooligan, I have a Quicksilver 1916P19 prop on my alpha II. Think it is a 14.5" x 19" pitch.