454 Big Block Engine Opinion

Rivertender

Rivertender

Member
Nov 7, 2006
123
Tampa Bay
Boat Info
370 aft cabin S Ray 1997
Engines
Twin 454
Inboard
Had a mechanic rebuild my '97 engines.
.......1000 hrs on them.
........4000- 4100 rpm WOT - 24 mph WOT with the boat at rebuild time. This is a capability, but never operated above 31 to 32k rpm.

Repair sea trial revealed the WOThrottle rpms to be 3400 on BOTH engines.
........I know he used aftermarket heads.
.........I also learned from our conversation about the inadequate rpm-s...... that he used something other than the OEM spec on the head because he, in an attempt to justify his part choice, explained and simultaneously revealed that the heads he used INCREASE the compression which, he explained, should deliver even more rpms. (Why, for the life of me, a mechanic would deviate from OEM spec parts is beyond me - anything other than mftgr spec is a subjective guess as to how it will perform).
.........seems to me that higher compression heads are the exact thing to NOT do to older engines where high performance is not an issue

I knew I was dead in the water with this guy when he was justifying the low rpm for WOT with:
....how much water is in your water tank?
....look at all these tools and stuff on your back deck (rags, a bottle of anti freeze, and cardboard boxes)
....these are old and tired engines - you can't expect more than this of them
,,,,3400 is good, right where it should be
....we need to adjust the props
,,,,bottom not clean (I explained it is cleaned monthly by a diver and then he said "when this month was it cleaned?"
....all these things (above listed) I am mentioning influence rpms
...."you can't run your engines higher than 3400, not these old engines-you don't want to be up there in rpms" . This of course ignores the fact that my colossal bill from him needs to deliver engines that perform to spec and provide enough power to climb onto a plane so that the engines can be backed down to about 3100 rpm to stay on top. No one operating these engines with any care will cruise/run the engines above 32 or 33k or anywhere near the 4100 rpm redline. Too, there are emergency situations that will require the engines to make full rpm-s and power.

My aft cabin is now functionally a trawler because putting it onto a plane and keeping it there at 3100 rpm will be destructively close to the new redline of 3400 rpm. By default then, the boat will need to be operating around 2,000 rpm or 6 knots. Too, the engines seem to be struggling and roaring at WOT trying deperately to get the boat onto a plane (which it just barely achieves if you keep the power on).

So, the opinion I am asking for is....................would you accept an engine rebuild that was almost 1,000 off on available rpm-s?

Many thanks for your thoughts.
 
Did he "rebuild the engines" or just replace the heads?

A slight increase in compression should not be an issue, but that all depends on what "slight" means. If the original engine was designed/certified to operate at 86 or 87 octane and you're now operating at 89 or above, then a "slight increase" would likely be inconsequential to the operation. Should actually result in slightly better performance or economy, but you might never be able to actually measure it. Definitely shouldn't result in a dramatic LOSS in performance unless a lot of detonation is occurring, which will destroy the engine in short order.

Is this carb or FI? Has the ignition timing been validated or verified to be correct?

To answer your final question, "NO!" Definitely not acceptable.
 
I don’t know, seems 1000 hours is not really a lot of hours on a big block imo. Maybe halfway through their life span? 4100 RPM sounds right for a stock motor. Before the rebuild did you try changing the props to a different pitch?
 
What casting number on original heads and what did he replace them with? Peanut, oval and rectangular are the 3 styles
 
How old is the fuel? Is it possible it is degraded and causing the timing to retard under load?
 
km1125 said:
Did he "rebuild the engines" or just replace the heads?


.........................................heads only

A slight increase in compression should not be an issue, but that all depends on what "slight" means. If the original engine was designed/certified to operate at 86 or 87 octane and you're now operating at 89 or above, then a "slight increase" would likely be inconsequential to the operation. Should actually result in slightly better performance or economy, but you might never be able to actually measure it.


..............................I hear you on slight,but I would never deviate from OEM



Definitely shouldn't result in a dramatic LOSS in performance unless a lot of detonation is occurring, which will destroy the engine in short order.


.........................my thought similar. Seems to me that a higher compression or reduced detonation chamber area/volume would stress the other components.


Is this carb or FI?

...................FI (sorry, meant to include that)



Has the ignition timing been validated or verified to be correct?

......................yes. Book is 8 to 10 degrees and he has it at 10

To answer your final question, "NO!" Definitely not acceptable.
Click to expand...
km1125 said:
Did he "rebuild the engines" or just replace the heads?

...............................................thanks.

A slight increase in compression should not be an issue, but that all depends on what "slight" means. If the original engine was designed/certified to operate at 86 or 87 octane and you're now operating at 89 or above, then a "slight increase" would likely be inconsequential to the operation. Should actually result in slightly better performance or economy, but you might never be able to actually measure it. Definitely shouldn't result in a dramatic LOSS in performance unless a lot of detonation is occurring, which will destroy the engine in short order.

Is this carb or FI? Has the ignition timing been validated or verified to be correct?

To answer your final question, "NO!" Definitely not acceptable.
Click to expand...
km1125 said:
Did he "rebuild the engines" or just replace the heads?

A slight increase in compression should not be an issue, but that all depends on what "slight" means. If the original engine was designed/certified to operate at 86 or 87 octane and you're now operating at 89 or above, then a "slight increase" would likely be inconsequential to the operation. Should actually result in slightly better performance or economy, but you might never be able to actually measure it. Definitely shouldn't result in a dramatic LOSS in performance unless a lot of detonation is occurring, which will destroy the engine in short order.

Is this carb or FI? Has the ignition timing been validated or verified to be correct?

To answer your final question, "NO!" Definitely not acceptable.
Click to expand...
 
Angus said:
I don’t know, seems 1000 hours is not really a lot of hours on a big block imo.

...........................my previous small blocks had 1500 and ran ok



Maybe halfway through their life span?


.........................maybe. I asked around yers ago and many said 2k lifespan, but some said 3 to 5k can happen.


4100 RPM sounds right for a stock motor.

....................right on the motor on a sticker



Before the rebuild did you try changing the props to a different pitch?
Click to expand...

.........................nope. Never a thought, given that it ran well
 
scoflaw said:
What casting number on original heads and what did he replace them with? Peanut, oval and rectangular are the 3 styles
Click to expand...


...........I am sure he gave my original heads to the place he got my newheads from....a core? Have no idea to answer your question. As I am thinking about this travesty, it occurred to me that having the heads would be an ideal situation.

Let me ask you. Is there a Mercruiser part number for my heads that my engine serial number will lead me too?

Thanks.
 
Golfman25 said:
Do a compression check. Any bets he got something wrong.
Click to expand...


.......................ok. Good advice.

Is it possible that the compression can be......too high? I am familiar with loss of compression, but never heard of the other way around.
 
Rivertender said:
........................old. Possible, but I never had this as an issue before with older fuel.
Click to expand...
Was just a thought. Would like to think a competent mechanic would not have changed compression enough, without discussing it, for you to have to increase your fuel grade from 87.
Old/degraded fuel can be a contributor to detonation.
Knock control on your engine will retard the timing to protect itself if it is sensed.
 
Golfman25 said:
Do a compression check. Any bets he got something wrong.
Click to expand...



Apolgies, Golfman. I tried to reply to Pirate Lady but was blocked by the site. Am putting my reply here ....because it works.



Riverman,
I can’t speak to your mechanic opinion. i have a 30yo 454 in a 5600# 250. It had a valve job right before I bought it. The heads were off and at shop as I was looking to buy it so ya new job. It runs great,

...................in the boating world of ...hmmmm....uneven repair people...........God bless and good for you.



That said, it pops on plane at 3200 to never more than 3400, then back down to 2800 to 3100 for cruise. In 3 years with it I have never pegged throttle.

.......................my sweet 1980, 318 powered, F26 bridge equipped Trojan did the exact same thing, though 2800 was a challenge to stay on top without a slight breeze under the nose to help keep it there............hah ha. She liked 29 or 2950.


What my point? Why push it on older motor if don’t need to. 2800 to 3400 range works fine, i dont need to see what it can do.

............I fully agree, but failed to articulate clearly that my 24k pounds needs a bit more to get it up on top. It is put onto a plane and run (32k) for 10 or 15 mins........rarely, perhaps once or twice a year because my mechanic friends say that it is hard to make a case against "blowing out" the motors every once in awhile.

When going for a cruising speed, the throttles go up near or at peg and the hull lifts -----as the rpms are increasing. The response is decently quick given my 370's lack of much of a vee. I am guessing the rpms are probably at 36k to 38, maybe 39k literally for only a moment becz as soon as that hull reaches plane, I am, like you, pulling the throttles back and eager to get back down to 31-ish k. Remember, this is only a very few times a year.

Typically, my motoring is 6 knots at 1800 to 2200 rpm..yes, trawler-ish as i mentioned in my original post. Sounds like ......what is the fuss about? For the money I spent and as I mentioned earlier, the engines are designed to deliver 4100 and that's a reserve power that is comforting to know is there and that's what I need to know is in reserve should I need it in an emergency. Cruising at high speeds/rpm-s does one thing - sends out cosmic message for a marine mechanic to come and start the clock at 125.00 per hour [here in FL].



My car speedo say 160, don’t feel the need what it can do.

...............with an aft cabin, there is no interest in seeing what it can do. Kind of like rider lawn mowers, no need to see what they can do.


What situation can you find yourself to need 4600rpm?

............................none. My redline is 4100 and as described, it never gets there other than once when the boat was surveyed as I made the purchase.
 
Probably not aftermarket heads if they took the old ones as cores. The new and old casting numbers relatively important to rule that out as your problem
 
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.
 
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
 
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.
 
Have you paid him?

Imagine trying to explain this when you go to sell the boat. No way should you accept and pay for a hack rebuild...
 
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