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Old 07 March 2014, 15:45   #1
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Country: UK - England
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Educate me about Mercruiser outdrives

Cut a long story short, I have a BMW M57 (530d) engine that I plan to marinise, obviously I'll be needing an outdrive or maybe a jetdrive... also need a boat to fit it into but thats another story all together.


However Mercruiser outdrives seem plentiful I am very genned up on Mercury outboards & gearboxes etc but I know very little when it comes to outdrives & their workings.

It would appear the lower units are the same as the outboard units, is that so? I'm assuming the 160bhp M57 engine would have a similar output & torque to the V8 units so this is the area I would be interested in looking into.

Pics & info would be great, also links that explain their workings & differences would be brill.
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Old 07 March 2014, 16:11   #2
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Quote:
Originally Posted by cptsideways View Post
I'm assuming the 160bhp M57 engine would have a similar output & torque to the V8 units
Ahh. no.

A Merc (GM) 5.7ltr V8 puts out 260hp and more torque than the Queen Mary as it's in a very low state of tune.

A 3ltr TDI BMW lump with virtually half the cubes, would be a total wet fart by comparison.

Small capacity, diesel turbo/intercooled engines don't marinise well due to their narrow powerbands. No gears on a boat, it's all about grunt right through the rev range.
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Old 07 March 2014, 17:21   #3
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Quote:
Originally Posted by Mollers View Post
Ahh. no.

A Merc (GM) 5.7ltr V8 puts out 260hp and more torque than the Queen Mary as it's in a very low state of tune.

A 3ltr TDI BMW lump with virtually half the cubes, would be a total wet fart by comparison.

Small capacity, diesel turbo/intercooled engines don't marinise well due to their narrow powerbands. No gears on a boat, it's all about grunt right through the rev range.
Hmmm, well my knowledge of petrol v turbo diesel is quite vast, tuned & standard stuff. So I pulled up some dyno plots as reference.

530d lump, grey lines are stock, red lines are tweaked



And the Mercruiser lump think its the pokier one listed here obviously

So the TD lump makes 500nm or 350ft/lb ish at 2k and peak hp is at available for 2/3 of the rev range. The V8 graph starts at 2500 rpm with 360ft/lb and only makes its power in the top 1/4 of it rev range.

So according to those graphs the TD is pokier up till it runs of puff at 4500rpm, a tweaked one makes 150% of the V8 torque & will use 1/3rd of the fuel to do it


So going by that I think the little 3.0 smoker would need a V8 spec outdrive at least
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Old 07 March 2014, 17:46   #4
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I think yanmar were/are using the 3.0BMW block on their 260 engines and highly regarded. This will be very torquey (I know I have 3.0l twin turbo version in an X5) so you will want a duo prop leg to avoid torque steering/prop walking problems at low speed or offset a single prop leg to account for it. Mercruiser XR (heavier gears to cope) in either bravo 1 or 3 but if money is no object go for IMCO who effectively build there drives around coping with high output V8s in the states.

Hamilton jet drive is worth considering but it will rob you of some top end speed but there is something appealing about a jet - probably because i have scratched that itch!
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Old 07 March 2014, 18:12   #5
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You will need one of the bravo range so you get the cone clutch rather than the dog clutch of the alpha range

If its in a rib you could use any of the 3 available bravo 1 uses the same props as the alpha & large outboards bravo 2 uses large dia props & large dia gearcase usually specified for larger cruisers etc but often used on commercial boats as its a bit less troublesome than the bravo 3 which is the twin prop version which is supposed to give better straight line tracking & other benefits but in reality the gains are fairly small
With regards to the engine it is the basis of the yanmar 6by engine which got a bad reputation in its early days & has never found the popularity of the 6lp big brother it cost yanmar an arm & leg in recalls & warranty
Also bear in mind a marine engine is on load all the time & it doesnt have any hills to go down
an ex vehicle engine which has done a life already & is amateur marinised will suffer reliability problems & they usually end up being a money pit & sold on as unfinished projects
Id be interested to see how you get on but please excuse me scepticism
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