Rebuild recommendations and options

[brodie123]

Hi everyone;

Re: Mercruiser 250 I/O ~1988 L6 4.1L

I'm installing a new engine into one of our boats and will be the engine listed above. I will be re-building from scratch and wish to build it in a way as to get as much HP out of it as possible.

I already had the engine above in the boat with a 4 barrel carb that worked great but would appreciate any feedback here on what I should be looking at for max HP. Stainless prop is obvious but I'm seeking mostly just advice on the engine. I prefer to avoid a thru-hull exhaust - a little noisy for me.

It's going into a 19' fiberglass speed boat. I don't think I have enough room to install a larger engine but welcome any advice here.

thx!!!

 

[Bondo]

I will be re-building from scratch and wish to build it in a way as to get as much HP out of it as possible.

Ayuh,...... How many engines have you built from scratch,..??

If "Engine Builder" isn't in yer resume, I suggest you consult with someone who Does list that at the top of their resume,.....

No disrespect at all,.... just Honest advice from a guy that's been turnin' wrenches for over 5 decades,....
'n still does, daily,....

For the most usable horsepower in a long lived boat motor,.....
Google up GM HT383 long block,.....

 

[achris]

You're not going to get much more power from the 250.. Merc took it pretty much as far as it could go without compromising reliability. More power is going to need more engine. You say the space is limited, but a V8 is only 7" wider, and about 8" shorter. Same height and depth. A decent 350 with vortec heads and a standard cam will get you up into the 280hp area without much expense... Just my thoughts.

Oh, one more thing. That 250 can't be 1988. Merc stopped using them in 1982. And GM stopped making them in 1984.

Chris..........

 

[Bondo]

Wow, I misread that,.. I was thinkin' 350,......

 

[Scott Danforth]

If you want to stick with an inline

Go 292, you will need to fabricate a motor mount bracket
Go with a lump port head
also suggest a heat exchanger

The 250 can make some power, however the cam profiles youre limited to for marine use will limit the output to about 200hp, even with a lump port head. The 292 can make about 250 HP in marine trim

It wont be cheep. It will cost you more to build than a 383 stroker V8

Inliners.org will be a good resource

Agree with both Bondo and Chris....more motor is available for less money in a V8.

 

[Scott Danforth]

Also, unless you get into high RPM small blocks or big blocks, thru hull exhausts do not help unless you have a custom dry exhaust allowing higher cam durations

 

[brodie123]

Ayuh,...... How many engines have you built from scratch,..??

If "Engine Builder" isn't in yer resume, I suggest you consult with someone who Does list that at the top of their resume,.....

No disrespect at all,.... just Honest advice from a guy that's been turnin' wrenches for over 5 decades,....
'n still does, daily,....

For the most usable horsepower in a long lived boat motor,.....
Google up GM HT383 long block,.....

For sure. This will be #2 for me. It's part of my learning experience and I understand that mistakes may occur along the way but this site has helped me immensely.

 

[brodie123]

Thanks! I think I'll start shopping for a 350.

On the L6 in there now, the hull of the boat where the front motor mount sits only has a couple of inches of play - that's my biggest concern. If the V8 is shorter I'm not sure how to mount it other than build a wooden insert. I guess that's a better issue that being too long!

Once I find the engine, I will need a motor mount for the back that will fit the outdrive. Will I run into issues there? Any suggestions of what I should be looking for for engine mounts? (front and back) as I will need to purchase those separate from the engine unless I can find a trashed boat for sale with a 350.

Also, what about the exhaust?? What will I need to do there?

Would I be better to aim for the 290 given the above?? I assume that the 290 is a 4.3L? That at least is an upgrade - just have to find one.

 

[achris]

290 is the same as your 250, but with a higher block, therefore longer stroke. Still an inline 6.

Any V8 will not use the front mount, they use side mounts. You'll also need to be looking for a pre 1893 V8, as all the transom plates and dimensions changed in 1983. Any boat you buy with a pre 83 V8 will have all the exhaust etc. But then the drive ratio will be all wrong.

Probably going to be easiest to buy a boat with a blown/cracked late model V8. Get your create motor and then transfer the lot into the 19' speed demon.

Chris...

 

[brodie123]

Thanks. I'll take a look around for the "290" - is it actually 292??? (I googled and found 292 is why i ask)

I like the single exhaust etc. - unless someone here says "bad choice". I'm not looking for for a massive upgrade from the 250 - just a little.

 

[achris]

Yeah, the 292 was a good upgrade from the 250. Surprisingly Merc only ran it one year, 1969, and it was rated at 200hp. And being an undersquare engine, not so good for getting high revs, but bags of low end torque. :thumb: Also, the drive supplied with the 292 was a Mercruiser-II drive, with a 1.78:1 reduction. Your MC-1 at 1.65:1 should do just fine.

If you find one, then that is probably going to be the easiest path.

Chris.........

 

[Scott Danforth]

Chris, the 292 was used in Brazil for a long time, including as a marine motor

Brodie, depending where you are located, many municipal trucks from the late 70's and early 80's had 292's

you will need to source a core motor to build.

or you could simply buy it https://www.summitracing.com/search...e/4-8l-292/engine-family/gm-inline-6-cylinder

 

[brodie123]

Thanks guys - that gives me some goo options.

So as I understand it the 290 or 292 is a little more horsepower and would be able to use the same mounting that is already in the boat, just taller which I have room for.

The added bonus is that my cabin cruiser has twin 250's so I was hoping to stay with the engine type as I already have spare parts etc.

I'll start hunting. Thanks !!!

 

[Scott Danforth]

Thanks guys - that gives me some goo options.

So as I understand it the 290 or 292 is a little more horsepower and would be able to use the same mounting that is already in the boat, just taller which I have room for.

The added bonus is that my cabin cruiser has twin 250's so I was hoping to stay with the engine type as I already have spare parts etc.

I'll start hunting. Thanks !!!

you may need to make the engine mount as the difference between the 292 block and the 250 block is the starboard side bolt pattern besides the deck height - plenty of info on inliners (including a few marine builds)

if your running twins, I would rebuild both of them the same/ doesnt make sense in a twin installation to have one motor putting out 25-30% more power then the other.

 

[Scott Danforth]

Just noted your other post https://forums.iboats.com/forum/eng...drives/10657544-how-much-should-i-sell-it-for

the 3.7 and 4.1 are different animals. hope your not getting the two confused.

 

[brodie123]

Thanks. Long story Short:

I have a 19' that has a 250 in it and runs great. I want to beef that up with a tweaked 250 or 290.

My 30' cabin cruiser had twin 3.7L. Not a reliable engine. I already replaced one of them with a 250. I'm going to use the 250 from above to match the 2 engines in the cabin cruiser. So now I have twin 4.1L and get rid of the 3.7's.

End result: Very reliable, quiet and efficient cabin cruiser and a 19' with some beef.

 

[achris]

And spares that cover both boats.