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  Conquest 23 IO Repower - Diesel to Petrol!

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Author Topic:   Conquest 23 IO Repower - Diesel to Petrol!
pgbj posted 09-20-2012 09:15 AM ET (US)   Profile for pgbj   Send Email to pgbj  
Hi, I have a Conquest 23 IO (2000) with Mercruiser 4.2 D-tronic diesel with Bravo 3. I have been thinking of upgrading the boat - but concluded it is perfect for me: beamy, spacious cockpit, dry and handles the Atlantic down here in Cornwall well (once you ease off the speed and use the trim tabs). I want to keep it another 12 years.

The core engine (an Italian VM) has been OK, but the "black" Merc ancillary stuff less so, and it is now something of an orphan having been passed back and forth from Merc to Cummins etc. So when the next big bill comes I'm thinking of re-powering. Replacing with diesel is no longer economical with the high engine price and Euro bureaucrats banning "red diesel". So I'm thinking petrol and a bit more power...

I know you guys are mostly outboard-powered but I recall a few IO owners on the forum. Anyone know what Merc petrol fits in the boat - was it supplied with the 5.x small-block V8? Would you think the 350 MAG MPI would fit and work well? How do you think a 300hp sterndrive would perform? Other options?

Any thoughts appreciated. Thanks

Jefecinco posted 09-20-2012 12:16 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
I would consult with Boston Whaler before making any decision on repowering. I would also consider a 300 HP Verado outboard or perhaps twin outboards for boating in the Atlantic.


David Pendleton posted 09-20-2012 03:27 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
My Conquest IO came with the small-block 5.7L EFI V-8. It is rated at 260 horsepower. I also have the Bravo III outdrive.

I know someone who replaced the same engine with a 350MAG MPI (although not in a Whaler), and it was bolt-compatible.

I have never seen the engine mounts for the D-Tronic, but I suspect they are not the same, but if you are re-powering, new engine mounts aren't a big deal.

The short answer is, yes, the small-block V-8's will fit, and the Bravo III is compatible with either engine. You'll pay a heck of a lot more for the 350, though.

I thought about the 350 once, too. I ended up re-propping and it was almost like getting a new engine.

tom976 posted 09-20-2012 04:12 PM ET (US)     Profile for tom976  Send Email to tom976     
A 5.7L motor is a 350. Small block chevy.
David Pendleton posted 09-20-2012 04:25 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Yes, I know that. I meant the 350MAG MPI in my last two references.
pgbj posted 09-21-2012 03:17 AM ET (US)     Profile for pgbj  Send Email to pgbj     
@Jefecinco. I'd love to put a couple of Yamaha 4-stroke outboards on the back - but there is no back! All the brackets I have seen are real ugly.

@David. Thanks. The 350 MAG MPI is indeed more expensive - the retail price including taxes and BIII is just shy of £20k in the UK. However, the rumoured price of the Merc QSD 4.2 350 and TDI V8 diesels is close to £45k!!

Out of interest what performance do you get with your EFI? My 225HP diesel does 36 mph at 3850(WOT) and 28mph cruise at 3200. Fuel consumption is so low I don't even think about it - fill up 3x a season tops.

David Pendleton posted 09-21-2012 11:22 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
My engine runs between 4400-4800 RPM at WOT. It depends on the props I have on.

At that RPM, the fastest I have ever run was 48MPH by my GPS. That was with about 20 gallons of fuel, no water and only me aboard. That was with my 26P props on. I only use those for big water trips now. I run 24P's on the river.

Normally I run with anywhere between 60-80 gallons of fuel, 15-20 gallons of water, with two people aboard and assorted stuff. Under these conditions 38-41MPH is more realistic.

I can cruise easily at 28-30MPH at about 3200 RPM.

I don't know my fuel burn numbers either, but at home I rarely fill up (120 gallons U.S.) more than once per season and top off when I get down to about 50 gallons.

This changes a bit when I'm travelling. I usually fill up before leaving and adjust from there.

Either way, I don't worry about it too much.

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