7.4l with 300hp vs. 454 with 365hp

J

jeff 220

New Member
Apr 18, 2007
2
I have bought a 95 220 Select. I have never drove it yet, got it i the winter. The boat has a 7.4l 300hp engine 21 pitch prop.
Any idea how fast this boat is?
Second question: I have seen you can get this boat with a 454 with 365 hp, what is the difference in performance?
Third question: Technically what is the difference in the two motors, is it simple like a cam shaft or is it a total rebuild?
Forth question: If you were to enhance the performance of this boat, what would be the logical steps to do it?

Looking forward to some feed back.
 
Jeff - First off welcome to the board. Congratulations on the boat. :grin:

Need a few more specifics to help answer a few of your questions. Is the motor Carbed, EFI, MPI?

What drive do you have? Alpha / Bravo 1 / Bravo 3?

The more details you provide the better the members can help you out.

:grin:
 
deterine HP increase percentage, take the square root of that number, and that is a good estimate of speed increase percentage. Most of the typical HP increase steps would get you about 5 mph increases.


I have a 4500 lb Baja with 310 HP version of that engine, and get about 56 mph. I think your boat may be a little lighter, but I may guess that the hull might not be as quick. My wild arse guess would be 55-60

But, get on it and take a GPS,
 
If it makes you feel any better. . .most 22' bowriders come with 220hp engines -> and are pretty quick.

Your top speed is also a function of water conditions. You won't see 40+mph with 3' waves. On a flat lake. . .I would bet your in the upper 40's.
 
Just so you know, a 7.4 IS a 454.

And, so you have something to compare to, my 99 190 gets up to about 48 MPH with a 240 HP EFI Merc 5.0 litre(305 CI). That's with a High Five prop. With a 3-blade SS prop, I'd be willing to bet I could get to around 52 MPH.

As STIHLBOLTS has already suggested, you'll need to give us a little more info so we can help you explain the difference. If I was to guess, I would say that the higher HP motor is fuel injected.

Paul
 
Thanks everyone for your comments, I appreciate the feed back.

To give everyone a little more detail. The motor is carburated and has a Brovo 1 out drive with a SS 21 pitch prop.

I realize the 7.4L and 454 block are the same block, but in the owners manual, the boat has an optional motor called a 454 365hp, what I was getting at was, what is the difference in the motors as far as carb, cam,etc. I was thinking that if there wasn't much difference in the motors, I would upgrade mine to the 365 hp.

My boat does not have Thru hall exhaust, can anyone tell me what hp difference that would make?

Jeff
 
My boat does not have Thru hall exhaust, can anyone tell me what hp difference that would make?
Click to expand...

The short answer, the same difference a set of headers and open exhaust would to a car. It would allow the motor to breath better. Translates into more useable horse power. How much would require some technical equipment to measure the difference. But if for nothing else, it would sound a whole lot louder. :grin:

We have a member who placed a 500hp motor in a 240 sundeck. That is one screamer! :thumbsup:
 
the 7.4 and the 454 mag are basically the same block. But the mag has upgraded camshafts, and some other goodies. This allows a higher max RPM and higher HP. Also, the mag requires thru hull exhaust. Also, adding additional HP increases - boost - is not recommended for the lower HP engine. Mine is a 1999 and is a 7.4mpi with 310 HP, the 454 mag with injection produced 385 HP.

Now, the 496 mag replaces the 454 mag, and the 496 HO replaces the 502 mag for near equivalent HP.
 
The biggest difference in the motors is that the 454 Mag has forged rods and a forged crank. This is makes the Mag much more suitable for power mods like a blower. Adding thru-hull exhaust will make a bigger difference than headers, but headers will help a little, too. If you do both, don't expect an increase of more than 2 or at most 3 MPH.

I have repowered a couple early 80's 260 Dancers. Starting with a 350 and Alpha I got very high 30's in terms of top speed. 454 and Bravo III with open exhaust got 44-46MPH. Going to a 502 Bravo III and EMI headers (with exhaust open) gets 53-54MPH with full fuel, several passengers and in the rev limiter. I am surprised how much more power the 502 has than the 454. Also, it gets better mileage in the 260 than either the 454 or even 350. In fairness, the other two were carb motors.
 
My opinion here, but using manufacturers HP ratings can be misleading, and this is one of those circumstances. The "normal" HP from a 454 has been about 330-350 HP - for decades now. So a 300 HP version - if there was one - had to either be very de-tuned or understated in the literature.

Likewise a 385HP version would have been either a specialty engine or overstated. These days Mercury sells a 350 ci (5.7L) engine with a stated 300HP output. This size engine has historically been 250-260HP. In the same operating rpm ranges these new engines produce the same HP as they always did. The difference is the manufacturers have upped the maximum rpms, which is the only place you get that increased HP, since speed is part of the formula. This is true for most of the Mercury engine product line at this point, including the mentioned 8.1L HO engine. You only get these HP outputs at 5000 rpm or more. Otherwise you get the HP people have always gotten from these engine sizes.

Since you are interested in max speed, maybe this helps. But most of us don't run our engines at 5000 rpm. And, you won't for long either. And, of course, custom modifications are a whole 'nother ballgame.
 
In the case of the old 454 versus 454 Mag, there are differences. I mentioned the forged components in an earlier post, but also the cams are different, valve springs are different, and compression ratios are different among other things. This does make these motors perform differently. The 8.1's have differences between each other, too. The HP ratings were not arbitrary.
 
I didn't mean that the HP ratings were arbitrary or incorrect. It's just a question of where that HP is. They know that you can increase HP if you can increase RPM. So all those things you mention are intended to achieve more RPM and hold things together at those speeds. But, if the original engines 454 or 8.1, have the same power as the HO models up to 4800 rpms, and the HO models only give more power up to 5200 rpm, well, in my view, users will make very little use of that power for the money they paid.
 
But, if the more powerfull engine allows a larger pitch prop, the the engine can be run at lower RPMs for the same speed. Probably, this is not more fuel efficient, but less wear and tear on the engine.
 
Cap10pat's point about how HP is achieved is correct in some circumstances. If you look at the torque curve, it will tell what the engine's real capabilities are. With the 454's, the Mag has a higher torque curve through most of the RPM range. Some other motors (350 and 350 mag as well as the 8.1 and 8.1 HO) primarily have a difference of power at higher RPM's. But, it is not the higher RPM that creates the power, it is the amount of power created at the higher RPM.

That is, if you look at the HP and Torque curves for the 8.1 Mag and 8.1 HO, you'll see the HO hits 400 HP at 4200 RPM. The other 8.1 never gets to 400 HP. Both motors have the same peak torque (500lb ft), but the 8.1 Mag hits it about 1000 rpm lower than the 8.1 HO.
 
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