7.4 / 454 in 20ft bowrider

[93maxum4.3lx]

comparing a typical 20ft bowrider with a typical 305/350 alpha setup; vs. a 20ft bowrider with a 7.4/454 and bravo 1 drive..

other than the increase in fuel consumption, is there any negative to the big 454 in a 20ft boat? are there any ill handling effects on a 20ft from having such a large chunk of iron in the back? The owner claims 70mph verified speed which is impressive but I wonder what the tradeoffs are.

 

[180Fisherman]

Re: 7.4 / 454 in 20ft bowrider

I have two friends that each have 21' Mariah bowriders with FI 454's in them and they flat out fly. No downside that I can think of. They actually sip fuel especially the one with a BravoIII drive on it. They cruise at 45mph at something like 2800 rpms.

 

[bear_69cuda]

Re: 7.4 / 454 in 20ft bowrider

454 in a 20' boat sounds like a blast! You can never have to much HP!

 

[RobbyA]

Re: 7.4 / 454 in 20ft bowrider

The only downside I see is that it is not a 502 cu. in.!

 

[Fireman431]

Re: 7.4 / 454 in 20ft bowrider

The average 20' boat runs a 150HP engine.

The 7.4L (depending on year) is anywhere from 290-330HP. Probably closer to 310HP. That will show you the power difference.

Speed is also a question of tuning, hull design, and power-to-weight ratio. The average 150 O/B is (I'm guessing) 400+ lbs. The average I/O combo would probably be closer to 550-600 lbs. The 7.4L is probably 650-700 fully dressed. That concludes, all things being equal, twice the power for the same, roughly, weight.

The thing to remember though is that the 7.4L is going to work much less to attain speed as the other engines would. This means less fuel comsumption for the same performance. Win/win. Remember that big blocks don't like to be pushed too hard for too long. 4200-4500 RPM is their general WOT. You will probably like the performance and fuel economy at 3200-3500 RPM.

Believe it or not, my brother's 30' express cruiser has a single 8.1L and tops out at 52 MPH!

 

[180Fisherman]

Re: 7.4 / 454 in 20ft bowrider

The 7.4L in my buddy's Mariah is the 385hp version. Fun fun fun but is does get to chine walking above 70

 

[45Auto]

Re: 7.4 / 454 in 20ft bowrider

The last year for the 7.4 was 1999 I believe. Powerboat tested a 202 Shabah in 1997 with a 415HP 8.2L/502. It topped out at 60.1 MPH on radar. Tell him you'll give him his asking price for the boat if he'll give you back $1,000 for every MPH less than 70 it does on GPS. He'll have to give you back about $10,000.

The only disadvantage I've found to a big block in a smaller boat is that your buddies will want to drive it all the time .....

 

[H20Rat]

Re: 7.4 / 454 in 20ft bowrider

The average 20' boat runs a 150HP engine.

The 7.4L (depending on year) is anywhere from 290-330HP. Probably closer to 310HP. That will show you the power difference.

Speed is also a question of tuning, hull design, and power-to-weight ratio. The average 150 O/B is (I'm guessing) 400+ lbs. The average I/O combo would probably be closer to 550-600 lbs. The 7.4L is probably 650-700 fully dressed. That concludes, all things being equal, twice the power for the same, roughly, weight.

the 150hp outboard is right on, but the I/O's are heavier. Merc 2.5L v6 outboard at 150hp is 431 pounds. An alpha with a 350 bolted to it is 952 pounds, plus or minus a couple pounds for various accessories. (40 more pounds for a bravo I) The book weight on a 7.4 with bravo I is 1158. (those weights will vary some over the years and the construction used, but still going to be relatively close.)

long story short, 200 pounds difference, not that much at all... Don't assume the 7.4 is going to be quicker though, here is a mfg test report comparing a 350 vs a 7.4 in an identical boat. Because of less weight, the 350 was a little over 3 mph FASTER. The 7.4 had better acceleration though.

http://www.stingrayboats.com/products/reviews/tb2_99.html

 

[H20Rat]

Re: 7.4 / 454 in 20ft bowrider

The last year for the 7.4 was 1999 I believe. Powerboat tested a 202 Shabah in 1997 with a 415HP 8.2L/502. It topped out at 60.1 MPH on radar. Tell him you'll give him his asking price for the boat if he'll give you back $1,000 for every MPH less than 70 it does on GPS. He'll have to give you back about $10,000.

The only disadvantage I've found to a big block in a smaller boat is that your buddies will want to drive it all the time .....

What do you mean GPS? You mean that speedo on my dash isn't accurate?! Got into a similar discussion with a buddy lately, he claimed his boat could do 60 on a good day, 55 mph all day long. GPS told a slightly different story, 45mph with his hand on the throttle so hard the lever almost came off.

 

[45Auto]

Re: 7.4 / 454 in 20ft bowrider

What do you mean GPS? You mean that speedo on my dash isn't accurate?! Got into a similar discussion with a buddy lately, he claimed his boat could do 60 on a good day, 55 mph all day long. GPS told a slightly different story, 45mph with his hand on the throttle so hard the lever almost came off.

My first boat was an 18' Bayliner with a 120 Force. Used to be able to do over 50 on the speedo. Never had a GPS to check it with, and I never could figure out how all those people with 40 MPH boats were getting away from me so easily ......

..... a 17' Bayliner with a 120 Force was tested by Trailerboats magazine and topped out at a true 39 MPH. Imagine that!

 

[Fireman431]

Re: 7.4 / 454 in 20ft bowrider

The last year for the 7.4 was 1999 I believe. QUOTE]

My boat is a late 2001 Carver and came with twin 7.4L MPI's. I don't know when they actually switched engines, but I'd assume it would have to after that...

SmokingCrater, I was just guessing with the weights. I've moved O/B's, but never I/O's assembled. I figured if i could lift the outdrive assembly myself, than it couldn't weight too much! Maybe I'm a superhero or something!

 

[45Auto]

Re: 7.4 / 454 in 20ft bowrider

My boat is a late 2001 Carver and came with twin 7.4L MPI's

My bad, I was going off memory, knew the 8.1L came out in 2000 and figured that was when the older motors were discontinued. Looks like from the Mercruiser manuals that the 7.4L (454) and 8.2L (502) were offered through 2001 by some manufacturers.