Repowering with twin Mercruiser 4.5L's

HI, Has anybody had installed new Mercruiser 4.5L Sterndrives?
I am seriously thinking of re-powering from 2 aging straight 4 Volvo’s, but I am confused regarding the specified dimensions.
The old Volvos are fairly compact but they are a tight fit height-wise. I have around 800mm height in the engine locker at the front – tailing off slightly towards the transom. Now.. the overall specification of the Mercruiser says that it is 559mm high, but – when you look at the installation drawing it shows a height at the centre of the flywheel as 567mm with a 2” riser then about 242mm below the flywheel. This gives a height of 809mm and that is not counting the installation position.
So I am just at the planning stage now and need to remove the old drives and engines etc, before preparing the locker, beds etc
I am doubtful two 4.5L’s will fit and looking for basic guidance before the expense of engaging the engine suppliers. I could go to a single 6.2L 350 hp which I know fits in a similar boat (a 27 foot Targa Cruiser), but like the idea of Twins due to the waters I sail in.
Thank you.

Ayuh,..... Regardless which way you go, the transom will need to be filled in, 'n recut, atleast wider,....

'n you might need to build a different motorbox cover as well,....

V-motors are considerably wider than in-line motors,....

Hi - many thanks for that. I did appreciate that the transom will have to be modified, but I am stuck with the height I am afraid as there isn't a motorbox cover as such because it forms part of the rear seating moulding etc. I am leaning towards a single engine installation at the moment as it seems the most economical option both in work and costs. The boat did have the option of a large single diesel when new, but again - now the modern diesels are quite tall.

You will have to check the X height of the Mercruiser at the transom position you expect them to be installed. As you move the engines outboard that dimension becomes more critical.
X height is the crankshaft centerline vs the prop shaft centerline.

It may be cheaper and less dependent on critical skills subcontractors ( but more work for you) to leave the existing Volvo drives in place and replace the existing Volvo engines with 3L (150HP) GM engines. If drives are tired, replace them with 280 or 290 series drives ( with correct ratio)... They will bolt right up to your existing transom shields... assuming they are post circa 1968...

Hi, absolutely - that is my main concern I believe it will get very tight at best. The current position of the drives will certainly need to be moved outboard so major transom work will be needed. I am in the process of getting installation drawings which will certainly help.
Just need to get over my fear of a single engine installation!!

Looking at Targa 27 pics on the web, it would appear that your best option is to stick with what you have and start shopping for another boat. The repower that you are contemplating will cost more than the boat will be worth after you have finished. That, and if you have to remodel the existing weeny cockpit area to fit these new lumps, there won't be any room left. Unless this is some super amazing thing with such sentimental value that you can't possibly let it go, then yeh. The world is chock full of 30-something-foot express cruisers. I am certain that there is another out there with twin 6 cylinder turbo diesels that will fulfill your need for speed.

Apologies Sandkicker I missed this post.
This would be the ideal situation as one of my engines has had a catastrophic failure and parts are non existent. Unfortunately here in the UK we are limited by engine suppliers and those who do sell the GM engines are reluctant to give dimensions or offer backup. The drives are good 290 SP-A's well looked after with new props and regularly serviced. It may mean in the long run (cheaper and easier) to import a couple of crate Engines from the USA.
the boat itself is perfect for me and would be even better with reliable engines. The engines were originally 167hp each but due to age probably no where near that now so 2 x 150HP's wouldn't be much different.
Quick question if I may.. if I go this route, do I keep the old flywheel?
No hurry yet to make a decision so will keep looking at alternatives.

moresparks said:

Apologies Sandkicker I missed this post.
This would be the ideal situation as one of my engines has had a catastrophic failure and parts are non existent. Unfortunately here in the UK we are limited by engine suppliers and those who do sell the GM engines are reluctant to give dimensions or offer backup. The drives are good 290 SP-A's well looked after with new props and regularly serviced. It may mean in the long run (cheaper and easier) to import a couple of crate Engines from the USA.
the boat itself is perfect for me and would be even better with reliable engines. The engines were originally 167hp each but due to age probably no where near that now so 2 x 150HP's wouldn't be much different.
Quick question if I may.. if I go this route, do I keep the old flywheel?
No hurry yet to make a decision so will keep looking at alternatives.

Click to expand...

Ayuh,.... Flywheels are motor specific, not part of the drives,....

Realistic expectations:
Keeping same drives with new engines +horsepower: Your boat will perform more or less exactly the same as it does currently. Adding horsepower will assist only with top end, even wide open speed by a few knots but that's all you'll get with the current props and drive ratio. New may or may not equal reliability.
New drives and new engines: Possible increase in top end and WOT speed of over 10 knots. With Bravo III drives you get better tracking, backing, and close-in maneuverability. Optional joystick controls. Will require professional propeller testing and fitment, which should come with the purchase.
Petrol v Diesel: Modern turbodiesels are cleaner, quieter and put out a lot of power in the lower RPM range which is where you want it on a boat. Petrol engines need to be at high RPM to get the horsepower, which may or may not be suited to your style of boating. Marine turbodiesels tend to be 4 and 6 cylinder with the 6 being far more docile and powerful. Since they don't require elaborate intakes like petrol engines, they tend to be dimensionally lower. You might wish to seriously consider the Mercruiser 3.0 270 hp diesel with Bravo III drives. These can situate closer together as all the servicing is done at the front. Mercruiser offers financing so you can repower and not go broke doing it.
No matter how you slice it this is going to be major money.

moresparks said:

Apologies Sandkicker I missed this post.
This would be the ideal situation as one of my engines has had a catastrophic failure and parts are non existent. Unfortunately here in the UK we are limited by engine suppliers and those who do sell the GM engines are reluctant to give dimensions or offer backup. The drives are good 290 SP-A's well looked after with new props and regularly serviced. It may mean in the long run (cheaper and easier) to import a couple of crate Engines from the USA.
the boat itself is perfect for me and would be even better with reliable engines. The engines were originally 167hp each but due to age probably no where near that now so 2 x 150HP's wouldn't be much different.
Quick question if I may.. if I go this route, do I keep the old flywheel?
No hurry yet to make a decision so will keep looking at alternatives.

Click to expand...

Volvo used that block in front of some of their later drives....stick with the 290s... If you want the advantages of a BRAVO 3, just swap out the lower 1/3 single prop drive section for Volvo 290 duo prop lowers.. or find 290 duoprop drives. Just make very sure they are for a 4 cylinder engine (gear reduction issues). No expensive ( and highly skill dependent) transom surgery. As posted... more work for you ( sourcing)....less yard mechanic Quid..
Personally, not a big fan of major surgery on the transom.

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