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7.4I magnum

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Old 08-18-2007, 07:56 PM
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Default 7.4I magnum

Can anyone tell me the difference between the 7.4 and 7.4I magnum? Would the 7.4I magnum be enough to power a 32 Envision? I'm looking at this boat hard, but don't want to get it if it is underpowered from the get go.

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Old 08-18-2007, 09:36 PM
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Can anyone tell me the difference between the 7.4 and 7.4I magnum? Would the 7.4I magnum be enough to power a 32 Envision? I'm looking at this boat hard, but don't want to get it if it is underpowered from the get go.

thanx
There is a 7.4, and a 454 Magnum. No such thing as a 7.4 Magnum.

Assuming that they are EFI engines, the 7.4 is 330 HP, at a max RPM of 4600. The 454 Mag is 385 HP at a max RPM of 5000.

The 454 Mag has all the good forged parts in it, while the 7.4 was not really intended to be a high performance engine.
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Old 08-19-2007, 05:02 PM
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Like Duke said, the 454MAG is a much better package. Especially if you plan on any performance upgrades.



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Old 08-19-2007, 06:58 PM
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Either way, if its a single in a 32 Envision, its way underpowered. That boat needs twins.
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Old 08-19-2007, 08:20 PM
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Depends on what yr the motor is.... if its a mark IV mid to late 80's then MERC labeled 330 hp oval port head 454's with the MAGNUM cuz it was pushin an alpha I 1.32 ratio.

A non mag 454 can be built for performance use. Your building all your HP at a relatively low RPM. 4800 to 5500.
No problem for a pro built BBC.

Would be nice if it did have the big rods, forged crank and forged pistions BUT

Rectangular Port heads are not any better building torque/hp at boating RPM's


I would look at how well the boat was maintained.

HP is like Hamburger.. You can buy as much as you want.

good luck with the purchase
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Old 08-19-2007, 10:14 PM
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The boat is a 1995 Envision Intruder combo, see attached pic of motor.
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Old 08-19-2007, 11:02 PM
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As it says its a 454 mag and 385hp. Still way underpowered for a 32' boat. It would probably run 50mph on its best day.
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Old 08-20-2007, 10:50 PM
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Is there a formula to use to establish how much more HP an engine will need to get me to a certain speed? Mid 60s is where I want to be.
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Old 08-20-2007, 11:17 PM
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Is there a formula to use to establish how much more HP an engine will need to get me to a certain speed? Mid 60s is where I want to be.
There are a ton of variables, size weight etc. Plus as you get faster, it takes more HP per mph to gain. It might take 100 HP to gain 5 mph, but then another 150 HP to gain the next 5 mph. For a pretty good guess, figure 15-20 HP per mph, 100 HP= 5 mph and so on.
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Old 08-20-2007, 11:33 PM
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I've been looking for the formula for that insurance brokers use. I used to find easily and now I'm drawing a blank on how I searched for last time.
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