re-power CC Roamer Offshore (38ft)

When you get it running you are going to have to run it wide open for a short distance to make sure the props are the right size. If it won't turn the recommended rpm then you will need to remove pitch until it will. You should pick up about 200-300 rpm for each inch of pitch that you take out. If you have too much pitch it overloads the engines and causes similar problems that you have experienced.

If you saw 4200 rpm's and still had some throttle left, it doesn't sound like it's the props or being overpropped enough to destroy the v-drive. I have some doubts as to who is rebuilding the engines. What compression ratio do the motors have? I wouldn't want more than 9:1 compression in a gas big block (non computerized EFI). I know a lot of the chryslers installed in boats (as well as other gas engines in that era) ran a higher compression ratio like 10:1, BUT gas at the pump was almost 100 octane and leaded. Not 89 octane unleaded that you see at most marina's and with ethanol in it.

I'm not familiar with that particular boat but I put a pair of 3208 turbo cats in a 36' steel hull boat and it was 18-20 knt cruise boat. I think those motors were rated at 260 hp. I bought them out of a boat that had caught fire and they totaled it out. That is about the only way that you can justify a diesel change out, dollar wise.

Hi,

I sure seems as though you have been a victim of just plain bad luck with that situation as Marmot suggested in Post No 38.

A broken conrod is something that luckily does not happen all that often and often leads to catastrophic results.

When you rebuild it again make sure you either use new conrods or at least have the ones you are going to use crack tested and also insist on the use of new bolts for the rod caps

Engines

There is a nice set of 454's on ebay right now.

7.2 Does not sound right for a compression ratio of a gas big block. It sounds too low and should really be 8:1 or better. It does indeed sound like bad luck, I would use ARP bolts on good connecting rods. What condition are your exhaust manifolds and intake manifold in and how old are they? Water leaking into a cylinder would hydraulic a cylinder and bend/snap a connecting rod like that.

Chrysler V8

Factory specs call for a compression ratio of 8.0:1 for Chrysler M-413.

Yeah, and in #33 I suggested overpropped as well. Sometimes you make your own luck.

3500 rpm

3500 rpm should have got you to 26 mph.

"21 mph on the wheelhouse speedo"

Pitot type? How accurate is it?

A connecting rod failure, often follows immediately after a predet piston failure. Have You seen the piston in question?

Highly Suspicious. You have 2 modes of fail here, to Lean, or to much total ignition timing, creating to high of combustion pressure.

My question to the builder would be: "How come Mercury Marine can mass produce hundreds of thousands of 270hp rated 350ci's, with caveman points ignition, stock Quadrajets and non-forged pistons. . . . . With no piston or rod problems?"

Kiwi

the speedo is probably one of the last instruments I would rely on for troubleshooting a problem that is almost certainly related to overloading, ignition timing, or mixture related combustion issues.

If you can determine that none of those items could possibly be involved with the last failure then you have to chalk it up to bad luck or a material defect with the piston or rod that failed.

As Kiwipushrod suggested, the piston crown (and plug) might have a story to tell.

Hi,

Welcome back to YF.

I suggested a member here send you a message a while ago as your Gas V8 Knowledge is always welcome here and you had not posted anything for a while.

Offshore, The 454's would be fine but if Your tuner/installer does the same thing as they did with the smallblocks, You'll be back with another thread.

I don't think this problem is the fault of the builder.

I have a short strory that may be of some help to You.

A few years ago, I built a budget 350HP 350ci motor for a local kids 96' Camaro (Finish reading before You get all Marinized on Me). I set the total timing, advance wieghts full open, @38*BTDC which is MAX. I don't care what the idle timing is, as Total Timing is the key for longivity.

The rear diff ratio was a bit tall from the factory, 373 iirc. When You drove the car it would turn on like a switch @2800rpm's, and pull very hard from there on up to 5200rpm's. It was impressive to Me for what was into it.

The kid loved it for about 2 weeks, then 1 day He comes back and say's He's dissapointed that the car won't break the rear wheels loose from the starting line and asked what We had to do to remedy this. I told Him that a 2800-3000 rpm stall speed torque converter would fix it and make the car have a faster launch. The stall speed would allow the motor to rev higher before applying it's power to the transmission.

He asked about other cheaper options and I told Him that there were none, He then said that someone told Him to advance the timing and it would take care of low end power, I told Him "Don't touch it!" and that I sure wasn't gonna do it.

Low and behold, a few days later I get an angry call from this kid, cussing and screaming about how I built a ****** motor and that it had thrown a rod and I did'nt know what I was doing. I told him it was under warranty and to get it over to Me and I would take care of it.

I pulled the motor down and found a burned thru the ring lands piston and a busted connecting rod.

After a brief investigation I found out that a shop on the other side of town had givin Him the low end power, that He so desperately needed, by advancing the idle timing by 12 degrees, bring the total timing into the 50*btdc range.

He eventually apoligized and We put a stall speed converter into the car after repairing the engine for a nominal fee. 5 yrs later the motor still runs strong and breaks the tires loose at the line. . . . . 3 transmissions later and still @38* total timing

Kiwi