In my boat project, I discovered that it probably has been repowered from a v8 to a Slant 6. Most likely staying within Mopar, so it may have had a 273, given it was a 1966 boat.

So does anyone know the specification differences between the 273 and the 225 in that of performance, power, torque, rpm range, and where in the curves does it peak?

Knowing that, is there a way for us to rebuild a 225 to outperform the 273, yet still keep the same curves, just improved?

yeah..... most of us here who have built up a slant 6 have probably exceeded the power of a stock unmodified 273 with our builds. There isn't a terrible lot of displacement difference there. and with the /6 having the long stroke that it does (especially the 225) torque is its strong point to begin with. What does your slant have for a manifold and carb?
They started with the 2 bbl on the slants for marine use, right around then// but if it was put in "later" who knows what they may have used? What makes you think they put a /6 in in place of a V8? the bellhousing between the 6 and the 8 don't match, the mounts are different. so they would have almost had to have a 2nd boat for a donor for all the parts that would have been different....
IF I had a boat with an inboard or I/O, and IF it had a Mopar V8 in it to begin with, and IF the engine was damaged beyond repair, I would have most likely have stuck another V8 in its place so that I wouldn't have all the "conversion" headaches. which makes me think it has always carried a /6.
I just bought a freshly rebuilt head from a guy that ran a charter off of Lake Michigan that had twin /6s on each of his 2 boats. looks no different from what I had laying here that came off of a late 70s car/truck.

If it were me, I would have stuck with a V8 back then as well. Bell housing would have to be changed, and because the slant front motor mount is actually just a plate on the front, with longer timing chain bolts (yep the front load is carried by 4 long timing chain bolts), that would have to be changed as well.

My reasoning for the V8 original is because of 2 clues. The Volvo Penta 270 outdrive is a D variant on its serial #. According to the book or internet, the 270 came with 3 different gear ratios. 1.61:1, 1.89:1 and 2.15:1. The 1.61 (D) was for 8s, the 1.89 for the 6s (C), and 2.15 (B)was for the 4 cyl. engines. And my tach, which I think may have been part of the original dash, has Chrysler Marine 8 on it. Those two clues give me that suspicion.

So my next step then, is while the engine is out, actually measure (count) the rotations then I will know for sure.

In the end, if I do have the 1.61 gears, then, if I understand things right, my boat will be a powerhouse of pulling power, but will be a bit slow in the speed department. And if that were the case, I could put a larger prop on it, with a more aggressive pitch, and in theory should shift some of that power over to speed.

I do have a 2 bbl carb. Carter Ball and Ball to be exact.

Or I could shift things with a different grind on the cam? Or increasing the compression ratio? So many variables. I am getting confused.

no, actually with the ratio as you say, I would think they used the 2.15 with the 4 cyl because it was underpowered and the lower gear (higher numerically) would help it not be such a slug. It would get there eventually with the V8 ratio but it would take longer, and teh v8 with the "4 cylinder" ratio would get up to speed faster but would be wound out tighter.
even cars and trucks with 4 bangers have to rev them higher to get what is wanted out of them while the bigger engines often just "loaf" along.
and that boat is over 50 years old. Are you sure the outdrive is original? and never had anything done to it (or changed out?) that tach could have also easily been swapped over the years. If that tach is set for a V8 and you are running a 6 off of it, then your revs shown will not be accurate.

Volaredon you are right. The 2.15 gears were for a 4 cyl. So it would seem that my outdrive was from a 4 cyl. engine. I confirmed the gear ratio by spinning it.

So If I wanted to keep the power band up to that of a 4 cyl, I would need to have a more aggressive cam?

By considering the mods to my /6 am I loosing out? By that I mean, with needing higher revs, am I throwing out fuel economy? Isn't the 225 best suited for low rpm torque and HP?

"It depends upon the build" is a terrible answer, yet it has some relevance.

If you craft your engine build to make peak torque (which usually coincides with the best volumetric efficiency) at your cruise RPM then you have just maximized Fuel economy potential. Further tuning required to get the highest vacuum at that RPM required...after the engine is built and installed. (back in the 80's it is my understanding that camshaft companies would recommend choosing a cam duration based upon the cruse RPM of the build. I suspect that was their way of helping people choose a cam that they would be satisfied with)

So if it were a stock rebuild, stock cam and and and, then plan on a fairly low cruise RPM for max fuel efficiency.

IF you put a cam in that moves the torque curve farther up the RPM range, and build the rest of the engine to support-complement that shift, then that particular engine will get better fuel efficiency than the stock engine being asked to spin at a higher cruise RPM...Because the built up engine is more able to completely expel all of the exhaust residue at that higher RPM.. so each combustion cycle contains more "space" within the combustion chamber for a fresh charge of fuel-air and assuming ignition timing is optimum it burns completely. Leftover exhaust in the combustion space does not lead to great fuel efficiency. I believe part of the reason the slant 6 can be fuel efficient, is because the exhaust side of the cylinder head flows comparatively better than the intake side.

What I do not know for sure, is if a "built up" engine, that is one that is crafted to optimize fuel efficiency at 3000RPM, be as efficient at 1700RPM as a "stock" engine? My guess is that it may not be quite as efficient, but It would certainly be more efficient than a stock engine being asked to cruise at 3000RPM.

Thanks Jase, That explains things quite well, even I can understand. So If I look at a Cam card, or spec sheet, they talk about RPM range. Some lower, some Higher, but pretty wide range either way. If I divide the range in half lets say, and use that as a target rpm?

It is my understanding that those "RPM range" figures on a cam card or advertisement, are NOT specifically tailored to a Slant 6 even though you are looking at a slant 6 cam....

Best to consider them general reference, and (based upon what I have read) should be treated as entertaining reading, rather than evidence-based scientific "fact". Others here have lots more experience with selecting camshafts... IF you decide what RPM where you want to emphasize fuel economy, then the experienced here will be able to contribute from their knowledge base.

It is my thinking that deciding what RPM range the engine will operate at most is important.

Also what fuel you want to use. An engine optimized for 93 octane will be a different animal than one built-optimized for 87 octane.

Also if you are willing to do some head work, then the camshaft choice may be effected....Depending upon the RPM desired.

Still if you are looking for a speed boat, it will be easier to accomplish with a 318, than a 225 and be cheaper (in terms of engine performance for every dollar spent) .. But engine swaps are or can be an involved process, and it would seem to me cheaper to rebuild what you have, optimize it, and get out on the water and have fun with it.

You hit the nail on the head. I think at this point, I want to do a little bit of tweaking and improvements, and enjoy being on the water. Then consider where things are and where they could be and through the current process, know what things can be done to change or improve any situation. Then consider the changes. In this case, for my boat engine, a Cam swap can be done without removing the block. I would have to remove the front mount which bolts directly onto the timing chain cover, but I can suspend the block to do that. Replacing a cam, would be a 1-2 hour job with 2 people. Probably a lot less.

As much as I like the Slant, I'd stay with a SB Mopar for a boat. Boats, for whatever reason, seem to use power inefficiently....I've been in some pretty big-engined boats that still were pretty slow.

I had a friend with a 34' cabin cruiser on Lake Superior. He bought it so he and his wife could visit the various ports on the weekends for mini-vacations. It had twin 502 Chevy big blocks. Get this...it would not plane on only one engine..it needed both.

He eventually sold it...said it cost so much to run it was far cheaper to fly in, rent a car, and stay in a fancy hotel.

Has alot to do with type of boat..... Ski boat, tug boat, charter boat, bass boat.... Hull design. Weight of boat. Prop pitch. And more that I am not even thinking about right now.
I recently bought a ready to run rebuild /6 head from a guy on CL. Back in the day he ran 2 charter boats/fishing guide type thing on lake Michigan, each boat ran twin slant 6s. We didn't really get into talking "performance" while I was there, he just complained about burning exhaust valves on them after they took lead out of the fuel. And how he always had a head on the shelf ready to go to keep down time to the minimum. He made it sound like this was his biggest (most likely) mechanical issue with them over the years that he ran them
But he was on the water "every day, all day" from April 1 til Halloween. So lots of hours on each one.