Hi Ralph, when I first bought my Scimitar, I was anxious to get it in the water and I just done the work that had to be done to enjoy it a while. That boat had a 300hp factory longblock with about 20 hours in it when I got it, of course, I wanted to go faster, and thats when I started building one of the late model motors for it. The first motor was experimental for me because I had not had any experience with the roller cams, vortec heads and I didn't really know what to expect, but after a couple trials and errors, and starting with the wrong cam, it finally began to come alive. After I found out what was going to work for me I built the new motor, but I changed out the cam again in it.
Horsepower ? I don't really know, I asked the guy's at the machine shop about that, what kind of horsepower could I tell people it had, realistically, and they were pretty quick to come up with a conservative rating between 425/435 hp. I ran the motor about 40 hours before I changed the cam again, the new cam brought the speed up two miles per hour. The best speed it has run to date by the gps is 71.2 , it's turning 5200 rpm with the PowerTech prop, I can't really give you accurate prop spec's because the prop shop has worked on it so much, they have shaved it thin, ground off about 1/4" off the lower part of the leading edge of the blades, cupped the tips and actually curled the whole blades to hold water, it was a 14 1/4 X 23 to start with.
What can ya get out of a 260hp ? depending on how much work ya want to do on it. I have improved the performance on those motors quite a bit with just a little bit of effort, everybody don't agree with me about this, but, based on experience, I have made a sugnificant difference in the performance by simply changing out the timing chain and gear assy with a double roller Cloye set that the cam gear itself can be installed/reversed to advance the cam timing two degrees ahead of the crank. It is possible to do that with the motor in the boat, but it's better to do it with the motor out, to remove the pan to re-install the timing cover to be sure there will be no oil leaks, and if the motor is removed, it's not all that much more work to change out the cam.
I'm not looking at the spec's, but I think the initial timing on the 260hp is set at 6 or 8 degrees BTDC . The total timing at higher RPMs and type of distributor advance mechanism used will determine the initial timing that you set at the balancer with a timing gun. Initial timing (or static timing) plus the distributor timing equals the total timing advance. The total timing advance for a Mercruiser 260hp is 28 to 30 degrees, I Think, with the 22 degree module on the thunderbolt ignition. The 260hp has 9.5 to 1 compression ratio, because of the gas available. The 91 octane pump gas (the expensive stuff) is only good up to about 9.5:1 compression ratio. Anyway, by advancing the cam, and setting the total advance up to 34 degree's made a big difference. Anything over that, I think you need to be mixing the fuel blend to raise the octane level.
My motor is running a 10.25 to 1 compression ratio, maybe a little more, cause the heads were milled .030, and I'm running the total timing advance at 38 degree's, I feel good about that because they put Manley stainless steel valves in the heads when they built them, but I have melted some stock valve with that timing.
Wow, you're more lucky in finding them than me. I have a 1980 one but too much of a project for me and I'm also looking for one in a good condition too. Let us know which one it's gonna be (and which not ;))
I just finished going through all your rebuild posts, hats off. Red's Gold turned out beautiful. Your abilities are amazing.
I found a third boat all original in mint condition, just trying to decide which way will provide the most pleasure. Launching a water ready boat and enjoying it or rebuilding it the way I envision it which would take a couple of years because of the cost.
Question, any idea how much more can be gotten of the stock 260?
I was also curious what kind of RPM and HP you're producing with the new motor? I've learned something interesting but I want to verify it before I share it.
Thanks, that's funny I just hung up the phone with him. Seems like a good guy, I just need to pull the trigger, it's a little richer then I wanted for a project, there's no interior and the floor need doing.
