454 Big Block Engine Opinion

As others asked, what was the purpose of the rebuild?

Something is wrong.

Your engines should reach their maximum rated RPM, which will be somewhere in the 42-4600 range. This is important, and a boat should be run to its maximum RPM at least once a season. Doing so establishes the motors are running correctly and you are not overloading the vessel.

Those that think they are babying their engines because they never elevate the rpm's, might find they are in fact not babying them at all. Load on a motor is way more important than rpm (so long as you are not over-revving). A guy running big blocks all day at 2800 with a max WOT of 3400, is being way harder on them than a guy running 3600 with a max WOT of 4400.

As you correctly state, running at 3100 when the engines won't turn past 3400 is a recipe for disaster. It's a great way to burn holes in pistons, blow head gaskets, etc.

If the engines ran at or close to rated rpm range, and will not now, something is wrong. There are a lot of things that could be, but the old adage I've found very useful is, "If you worked on the motors, and a new problem comes up, it is almost certainly something you did."
 
It almost sounds like a set of open chamber heads were installed where closed chamber heads were original. The change of compression will be significantly downward and so will power. It would be very anemic and hard to start. But, Sheesh - any engine mechanic knows this.

Like said earlier - do a compression test and verify you are showing at least 110 psig on all of the cylinders. If you are down around 60 or 70 PSIG then you may know the issue is in the cylinder heads used. Also verify the casting numbers against what was original Mercruiser; the numbers are under the valve covers if Chevy or usually stamped on the front boss if aftermarket.
Another member had valve train issues where the heads were rebuilt and the OEM valve train didn't correctly adjust causing the valves to not fully close which will result in major performance issues and engine damage. This can be checked with a compression test but more accurately diagnosed with a Leak-Down test.
Or, ignition timing is incorrectly set.

BTW you should occasionally run those motors on the pins just like the diesel engines to exercise sensors, regulators, and linkages. Those bulwarks are good for 5500 RPM so fully loaded at 4800 RPM won't hurt them at all. The only thing hurt is your fuel budget....
 
As others have said, verify compression and even leak down to rule out the heads.

Second thing I'd look at is the timing... Timing had to be set because the distributor had to be removed. It may be slightly off. Also verify the advance system is working correctly (I don't know what type of ignition setup you have).

If all those things check out, start looking at fuel pressure under load.

It's likely something simple.
 
Agree with compression and possibly even a leak down to verify the mechanical side.
Older posts indicate your fuel was old last season. Have you tried running off of a secondary fuel source with known good fuel? You can't overlook the simple things.
 
I may have missed this in the replies, have you talked to the mechanic about this to ask them to do engine compression checks as well as a leak down test? Have you paid them in full yet?
 
km1125 said:
You should get the casting numbers off the current heads, then you can figure out what they are. You could also look up the stock heads using the serial numbers on the engines so you have something to compare the two (old vs new).

I'm really wondering what the motivator was to do ANYTHING to the engines at only 1000 hrs, especially since (it seems) you rarely run it at or near its rated output. If a lot of your cruising/hrs is closer to 2000RPM and some 3100, those engines should last well over 2000 hrs before needing attention. And I don't think 4100 is "redline" on these engines, but rather the rated RPM. "Redline" is probably closer to 4400-4600.
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I am guessing the original heads are long gone to wherever he got my current heads. Can always ask.
 
scoflaw said:
At the age of those engines there are more factors in play than just hours. Aftermarket heads won't have serial numbers but there should be some markings on the exterior to tell you what they are
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Got it
 
mrsrobinson said:
I may have missed this in the replies, have you talked to the mechanic about this to ask them to do engine compression checks as well as a leak down test? Have you paid them in full yet?
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Initial discussion with mechanic was loaded with excuses by him for rpm deficit.

The guy has been around awhile. A local marine supply merchant validated my guess that this guy was one of the bet around.

Sadly, I paid him as we went..... because I trusted him to do a good job. I am into him about 400 - 600.00 (currently owed), a fraction of what I've paid him so far. Nevertheless, this remaining balance ain't going to him uintil my engines are right.......or never.
 
mrsrobinson said:
I may have missed this in the replies, have you talked to the mechanic about this to ask them to do engine compression checks as well as a leak down test? Have you paid them in full yet?
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Good thought. A number of people have suggested compression test. I am thinking I will get somebody else to do it, for if it reveals a problem this guy is not going to want to deal with it. Not hard to fudge a comp test to avoid responsibility. Given the excuses he was spraying at me (in my original post), I am now way leery of his integrity.
 
techmitch said:
Best question asked so far. X10!
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sbw1 said:
Why did you rebuild engines that were performing somewhat close WOT spec?
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Agreed. Good question. I had an overheating problem on both engines. He sent me pictures of the cooling ports ( ?) in the heads or somewhere and scared me into the rebuild I got. I know overheating is a much simpler issue, but the pictures he sent were unnerving...and he does know his inboards (on the whole), so I went for the rebuild.
 
PlayDate said:
So….just to recap… your “mechanic” decides to replace your heads with a rebuilt set and now the engines flatten out at 3400.

So that definitely is in the category that he got the wrong heads for your engines or they were poorly rebuilt. Additionally he may have used a thicker head gasket that reduced compression.

The next step is a compression test to see if anything is addressable without replacing the heads. We had another thread where the rebuild “mechanic” screwed up the valve job so they didn’t seal.

Sorry you are going through this but there are a lot of incompetent people out there who think they are mechanics.

Do the compression test and post the results. Also shoot a picture of the spark plugs.
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Will do.

Thanks for the offer to look the results over.
 
Rivertender said:
I am guessing the original heads are long gone to wherever he got my current heads. Can always ask.
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Re-read my post. You don't need the original heads, just look those parts up based on the serial numbers of your engines. Then get the casting numbers off the heads you CURRENTLY have to determine what they are so you can compare the two.
 
Rivertender said:
Agreed. Good question. I had an overheating problem on both engines. He sent me pictures of the cooling ports ( ?) in the heads or somewhere and scared me into the rebuild I got. I know overheating is a much simpler issue, but the pictures he sent were unnerving...and he does know his inboards (on the whole), so I went for the rebuild.
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When were the impellers changed last?
 
It should still rev out above 3500 rpm even with open chamber heads . Street raced for years and any big block would rev to 5000 easily, possibly timing or fuel
 
Scott215 said:
It should still rev out above 3500 rpm even with open chamber heads . Street raced for years and any big block would rev to 5000 easily, possibly timing or fuel
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Even pulling a trailer up a hill? I don't think an auto with a transmission is a fair comparison to a heavy boat trying to get on step.
@Rivertender, I read some of your earlier posts on this and another forum. It sounds like you replaced the distributors with EST Voyagers before the head swap. Is that the case?
Your symptoms as others have mentioned could easily just be a timing problem.
The Voyager distributors work great in jet boats (my experience with them), but I have my doubts with an i/o application. They do not hit their full advance until 3600 or something like that. Jets don't care as it is just a pump in front of a nozzle (restriction). The original Thunderbolt distributors can achieve full advance as low 2000 rpm.
Have you checked the timing with your engines loaded?
We are all trying to help.
 
keokie said:
As others asked, what was the purpose of the rebuild?

Something is wrong.

Your engines should reach their maximum rated RPM, which will be somewhere in the 42-4600 range. This is important, and a boat should be run to its maximum RPM at least once a season. Doing so establishes the motors are running correctly and you are not overloading the vessel.

Those that think they are babying their engines because they never elevate the rpm's, might find they are in fact not babying them at all. Load on a motor is way more important than rpm (so long as you are not over-revving). A guy running big blocks all day at 2800 with a max WOT of 3400, is being way harder on them than a guy running 3600 with a max WOT of 4400.

As you correctly state, running at 3100 when the engines won't turn past 3400 is a recipe for disaster. It's a great way to burn holes in pistons, blow head gaskets, etc.

If the engines ran at or close to rated rpm range, and will not now, something is wrong. There are a lot of things that could be, but the old adage I've found very useful is, "If you worked on the motors, and a new problem comes up, it is almost certainly something you did."
Click to expand...









A good adage and an easy one to agree with............
 
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