Mercruiser 454 distributer

BEERNUTS

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Hi All, I'm looking at a Mercruiser 454, its a 1990 model and was wondering why it dosn't have a HEI distributer, to my way of thinking this would be a better set up than the old points and coil setup, is there any reason not to fit a HEI or am i missing something?

TIA Rob
 

Don S

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Re: Mercruiser 454 distributer

Being a 90 motor, it should have a Thunderbolt IV ignition system. NOT points.

If it has points, then someone has been putting parts on that engine from whatever they could find. I would be very worried.
 

jaxnjil

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Re: Mercruiser 454 distributer

long shot here. if you were looking and expected to see a hei type car distributer and didn't. then you most likely looked at the tb 4. did you pull the cap and actually see points?


from nutcase's post below i find i didn't make mine clear enough.
if you looked at your distributer and didn't pull the cap, you may well think you have a points type distributer.
you might not suspect, that your T B 4 is a hei. if you are used to seeing a auto distributer with the large cap and married coil in the top of said cap.

with the divorced coil and small cap on the T B 4, i can see how you might think you have a point dis. if you do then other posts apply. if not your good to go.
 
Last edited:

zbnutcase

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Re: Mercruiser 454 distributer

Do NOT fit a GM HEI distrbutor, it is NOT USCG approved. And i'm with Don on that;in the rare event it does have points, you could well have somebodys collection of garbage can parts. Be afraid. 'nutcase
 

Bondo

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Re: Mercruiser 454 distributer

or am i missing something?

Ayuh,.... My Guess, like those above is,.. It ain't got Points now...
 

BEERNUTS

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Re: Mercruiser 454 distributer

Cheers for the replys guys, I must admit i didn't pull the cap, i just assumed it has points due to the divorced coil,I'll have a better look today.


The engine is a Thunderbolt 4 so what you guys are saying above does seem to be on the money.

The boat is a Formula 242 and is a full Americian import, trailer and all and does seem a very honest origional boat, its a 1990 too.

Thanks again for all the input and I'll let you know what i find today.
 

Don S

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Re: Mercruiser 454 distributer

The engine isn't a Thunderbolt 4, the ignition is. The engine is a 454.
If you look up in the Adults Only sticky at the top of this forum, you will find service manuals, for both your engine and outdrive.
#15 is the one you want for your engine.
 

BEERNUTS

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Re: Mercruiser 454 distributer

Thanks for clearing that up Don, and yes the dizzy is electronic, i took another look today to make sure.

Just out of curiosity what sort of revs should i get out of her?
At the moment 4-4.5k is tops, seems a little low in my book.
 

Don S

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Re: Mercruiser 454 distributer

Just out of curiosity what sort of revs should i get out of her?
At the moment 4-4.5k is tops, seems a little low in my book.

That depends on what you actually have. A 7.4MCM engine should have 4200 to 4600 rpm at wide open throttle.
You have to understand, this is not a car engine, top revs means nothing on boats. They have a max WOT (wide open throttle) rpm range that they need to run in to prevent overloading the engine, or over revving the engine.
This is determined by the manufacturer and is shown in the manual.
This RPM is adjusted by using different sized props in both pitch and diameter.

Have a look in the Prop forum here at iboats, I think you will get an idea of what is happening there.
 

BEERNUTS

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Re: Mercruiser 454 distributer

That depends on what you actually have. A 7.4MCM engine should have 4200 to 4600 rpm at wide open throttle.
You have to understand, this is not a car engine, top revs means nothing on boats. They have a max WOT (wide open throttle) rpm range that they need to run in to prevent overloading the engine, or over revving the engine.
This is determined by the manufacturer and is shown in the manual.
This RPM is adjusted by using different sized props in both pitch and diameter.

Have a look in the Prop forum here at iboats, I think you will get an idea of what is happening there.

Thats what it is Don, and its revs are in line with your reply so the propping should be in the ballpark.
I guess i just expected it to be faster than what it is, she does about 50MPH @WOT, does that seem about right?
 

Bondo

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Re: Mercruiser 454 distributer

At the moment 4-4.5k is tops, seems a little low in my book.
I guess i just expected it to be faster than what it is, she does about 50MPH @WOT, does that seem about right?

Ayuh,.... Sounds like the expectations of your book are alittle High, the boat sounds about Right...
 

45Auto

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Re: Mercruiser 454 distributer

50 MPH in a 24' boat with around 300 HP is right where it should be.
 

dakine

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Sep 2, 2009
Messages
71
Re: Mercruiser 454 distributer

That depends on what you actually have. A 7.4MCM engine should have 4200 to 4600 rpm at wide open throttle.
You have to understand, this is not a car engine, top revs means nothing on boats. They have a max WOT (wide open throttle) rpm range that they need to run in to prevent overloading the engine, or over revving the engine.
This is determined by the manufacturer and is shown in the manual.
This RPM is adjusted by using different sized props in both pitch and diameter.

Have a look in the Prop forum here at iboats, I think you will get an idea of what is happening there.

The motor is set up to operate near the torque peak with reasonable life.
Throw some heads that have ports big enough to stick your arm down on that puppy and wind it to 8000 rpm with the right prop.
It will last about 10 hours if it doesn't blow.
Just like a real race engine.
 

dakine

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Messages
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Re: Mercruiser 454 distributer

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