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What limits the RPM and what is safe to Spin to? 7.4L

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Old 05-21-2009, 10:35 PM
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Default What limits the RPM and what is safe to Spin to? 7.4L

I have the 330HP Merc 7.4L/Bravo in my boat. On the lid it says WOT rpm is 4200-4600. I looked at the carb, so I am assuming the RPM is limited by ignition?

If I were to change the prop, and setup the ignition differently, would I be able to gain some RPM, and a little more top end?

What is a safe spin speed for a stock cast motor? I would think low 5000s would be ok.

Sorry for the ignorance, as I am trying to relate automotive to marine.

Thanks for any input and replys!
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Old 05-21-2009, 10:44 PM
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First off you really shouldn't make too many comparisons to auto/car set ups.

A marine engine is always under a huge load, kinda like a car constantly going up a 30% grade.

I would say that max rpm varies a bit depending on the boat that the motor is in. If you have a 19 ft boat, I would say 5000 rpm max. If you have a 24 ft boat, I would say 4800 rpm.

In any boat, you prop it so that under perfect conditions it will pull up to max rpm. If you prop it too small, then you will always be on the limited which is bad. It is a trial and error kinda thing as is also dependant on what you use the boat for.
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Old 05-21-2009, 10:44 PM
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Hey!

I have the 7.4's as well in my boat. There's a lot of differnt opinions when it comes to building them up. The big problem with them is cast internals rather than forged. The rotating assembly won't stand up to spinning more than about 4800rpm for any amount of time before bad things start to happen. However there are some cool old threads around that get into some relatively cost effienct mods (intake, heads, cam). If you want to go serious power though, you probably better off selling them and using the cash towards some blue motors.
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Old 05-21-2009, 10:55 PM
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That motor is 15 years old, if you want to make it to 16,17,18yr and on DON'T CHANGE the rev limiter. If you want more performance, change the prop. Anything more than that you might as well just buy a used "built motor" and swap them out.

Smurf, I think you just got that boat, and for a hell of a deal?? (if my memory is working) trust me when I say enjoy what you got for a little while, price out a replacement motor and labor, that should change your mind.

BTW a merc 500 Efi racing motor rev limiter is set at 5200rpm, and they are built way better than the 330's to give you an idea.

Best of luck either way

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Old 05-21-2009, 11:28 PM
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Excellent info, and I totally understand where you are all coming from.

I have no intentions of changing anything with the motor right now... I was just more curious than anything... Like I said before, its my ignorance of auto to marine differences.

I am going to hunt for a 502 this winter and swap that in. I know a guy locally had a few for sale not too long ago (on here), maybe he will have something come winter, if not I am sure i will find somebody with something.
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Old 05-22-2009, 12:08 AM
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Quote:
Originally Posted by BAJA WILL View Post
That motor is 15 years old, if you want to make it to 16,17,18yr and on DON'T CHANGE the rev limiter. If you want more performance, change the prop. Anything more than that you might as well just buy a used "built motor" and swap them out.

Smurf, I think you just got that boat, and for a hell of a deal?? (if my memory is working) trust me when I say enjoy what you got for a little while, price out a replacement motor and labor, that should change your mind.

BTW a merc 500 Efi racing motor rev limiter is set at 5200rpm, and they are built way better than the 330's to give you an idea.

Best of luck either way

WILL
my 330's have the stock ignition are you saying there is a rev limiter in the merc thundervolt? mine are 86'
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Old 05-22-2009, 01:10 AM
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Its Thunderbolt !!! and yes there is !!!
I stand corrected. its thunderbolt IV to be exact. where's the limit? what RPM, this thing launched like a rocket but was done at 4300 or so. I agree with the prop for performance. just want to know where its limited. just rebuilt them with intake cam and exhaust upgrades and plan on new props
thanks for the knowlege
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Old 05-22-2009, 02:44 AM
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Originally Posted by bobbowobbo View Post
I stand corrected. its thunderbolt IV to be exact. where's the limit? what RPM, this thing launched like a rocket but was done at 4300 or so. I agree with the prop for performance. just want to know where its limited. just rebuilt them with intake cam and exhaust upgrades and plan on new props
thanks for the knowlege
I could be wrong here but I do not think merc was doing rev limits yet in 1986, Do you have a old manual of any kind laying around or do you still have the OEM flame arester on the carb or any old stickers on the engine that would say what max RPM you should be running at when it was still stock that will give you a starting point, To me 4300 just sounds a little to low of a RPM.
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Old 05-22-2009, 10:26 AM
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Quote:
Originally Posted by bobbowobbo View Post
I stand corrected. its thunderbolt IV to be exact. where's the limit? what RPM, this thing launched like a rocket but was done at 4300 or so. I agree with the prop for performance. just want to know where its limited. just rebuilt them with intake cam and exhaust upgrades and plan on new props
thanks for the knowlege

The only TBIV ignitions that have a rev limiter are
the HP series engines.

They are an external module ( item # 43 in this pic )

http://www.mercruiserparts.com/Show_...2%2DF213244%29

Your engines ( 330's) don't have a rev limiter.
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Old 05-22-2009, 11:11 AM
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Last year my stock 454 330hp would get up to about 4800rpm's. I did a edelbrock performer intake, and lighting headers for performance mods. The boat now gets up to 5,000rpm's but Iam not ever going to hold it that high for any length of time since I would like for this motor to live this year.
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