singledownloop New Reader
Dec. 22, 2014 2:51 p.m.

I've been reading that a 3.0 and 3.7 mercruiser engine bolts in place of the gm iron duke engine.From what I've read the 3.0 is 130 hp and the 3.7 is 190 hp?Have any of you ran a mercruiser engine in a car or been around one?Seems like a bolt in way to get a big increase in horsepower.

curtis73 UberDork
Dec. 22, 2014 3:07 p.m.

The marine 3.0L is a chevy engine (based on the old 153) not the Pontiac iron duke (based on the 151). Basically they are completely different critters, especially since the duke went through a ton of changes over the years - FWD, bellhousing changes, etc.

So to say that it will fit where a duke used to be is not accurate. Totally depends on which duke we're talking about.

Toyman01 MegaDork
Dec. 22, 2014 3:14 p.m.

I don't have any direct experience with the 4cyl MerCruisers, but a little digging turned up this on a boat forum.

The power ratings for both are not as far off as you may think, since the 470 was rated at the flywheel and the 3.0L is rated at the prop.

The 470 is the 3.7 engine. One is going to be gross HP (3.7) the other is net (3.0).

There was also some discussion of poor reliability on the 3.7 and overheating issues. Those may be a marine problems. There was also some discussion about not being a drop in conversion.

curtis73 UberDork
Dec. 22, 2014 3:16 p.m.

here is some swap info that might get you started:

Each number is its own entirely different engine casting, each often confused with the others. 1) 1977, 1978 Chevy Monza and all Mercruiser 2.5L and 3.0L engines are SBC with non crossflow heads. 2) 1979 only Chevy cars and 1980-1983.5 AMC cars are all SBC bellhousing pattern with crossflow heads. Distributor hole moved in the casting to allow room for intake on opposite side. 3) 1980 to 1993 Chevy cars and trucks are 60 degree GM pattern. Notice that a 1983 AMC engine is the same as a 1979 GM engine, but not the same as a 1983 GM engine. 4) Late 1983 and 1984 four cylinder Eagles are a different engine entirely. It's a TBI AMC150 four cylinders with 60 degree bellhousing. I've seen one once and would love to see another in this lifetime. 5) Mercruiser 3.7L engines, rated for 180 and 190 HP, are externally interchangeable with Merc 3.0L engines and often get called "Iron Dukes". Internally nothing is the same. They use Ford pistons and heads instead of Chevy V8 pistons and heads. The AMC oil pan will not match it. The SBC bellhousing and engine mounts should be identical, however.
Kenny_McCormic PowerDork
Dec. 22, 2014 3:24 p.m.

Sounds like this may be a misconception, as there is also an actual iron duke based (as in the 151/2.5 that went in everything in the 80s) 4 cylinder Pontiac racing engine called a "Super Duty" or SD4 that came in 2.7 and 3.2 liters.

singledownloop New Reader
Dec. 22, 2014 3:34 p.m.

For some clarification I have an 82 amc stock 4 cylinder car.It uses a sbc trans bolt pattern.It appears a lot of different variations are referred to an iron duke.To avoid confusion I have a 151 c.i. GM engine.The more info the better on the 3.0 and 3.7 in a car.

Kenny_McCormic PowerDork
Dec. 22, 2014 3:39 p.m.

Going to a small block chevy V8 will probably be the same amount of work, and cheaper.

Toyman01 MegaDork
Dec. 22, 2014 3:52 p.m.
Kenny_McCormic wrote: Going to a small block chevy V8 will probably be the same amount of work, and cheaper.

Quoted for truth.

curtis73 UberDork
Dec. 22, 2014 4:12 p.m.
singledownloop wrote: For some clarification I have an 82 amc stock 4 cylinder car.It uses a sbc trans bolt pattern.It appears a lot of different variations are referred to an iron duke.To avoid confusion I have a 151 c.i. GM engine.The more info the better on the 3.0 and 3.7 in a car.

Ah... then the marine engine should bolt in just fine. It has an SBC bellhousing pattern and the mounts are in the traditional chevy/GM position. Get a crossflow head so the turbo fits. hint hint.

Don't worry about reverse rotation unless the donor boat is 75 or older. By about 1982, all Merc, Volvo, and OMC applications were all forward rotation. Between 75 and 82, there were still a rare few reverse engines in Correct Craft ski boats, and rarely in some dual-engine applications. Since a dual-power 3.0L is highly unlikely, don't panic about it. Any reversing that happened with dual engine boats after that time was accomplished in the outdrive.

ID'ing it should be easy if the starter is on it. Reverse rotation engines used starters that bolted to the bellhousing and stuck out the back instead of being bolted to the block in the traditional orientation.

Don't use the marine water pump. It is just a circulator pump and isn't designed to really move water in and out of the block. That is the job of the pump in the outdrive. Make sure to use an automotive pump. Also re-use your distributor. Marine dizzys don't use vacuum advance. The marine carb will work with some tuning. Its not designed for the throttle to snap open and accelerator pump shots are pretty small. Even better, get an EFI version. The ECMs are designed for two-wire installation and there is no O2 sensor. Its all speed/density.

singledownloop New Reader
Dec. 22, 2014 4:19 p.m.

curtis73 You're kind of reading my mind.If everything is a bolt in for the 3.0-3.7 I see a turbo in it's future at some point.The smc is way easier to find but isn't a direct bolt in.Now curious if I can swap the 3.0 crank in my 2.5 engine.I've read,but have no personal knowledge,that the mercruiser engine has the same bore spacing as a sbc and splayed sbc heads have been used on them.

curtis73 UberDork
Dec. 22, 2014 4:23 p.m.

Do some poking around on the 3.7L. Marine 3.7s were problematic - overheating, head gasket failures, etc. Not sure if that was a design issue (like thin castings, too much bore, etc) or if it was a simple thing like not enough water pump flow or a bad round of thermostats.

Kenny_McCormic PowerDork
Dec. 22, 2014 4:27 p.m.

In reply to singledownloop:

I believe later 2.5s used a SBC head pattern, I wanna say I read someplace the Mercruiser motors used ford 460 heads but don't bank on it.

singledownloop New Reader
Dec. 22, 2014 4:29 p.m.

Kenny_McCormic from what I'm finding the 3.7 uses big block ford heads.

alstevens New Reader
Dec. 22, 2014 5:26 p.m.

The 3.7 aka 170 or 470 is an aluminum block with cast iron sleeves and was designed by Mercury Marine that uses a Ford 460 head. The motor has plenty of power. It was also a closed cooling system - meaning that it had a heat exchanger and used anti freeze circulating throughout the engine for cooling. Some of the poor dependability was due to the aluminum block and the cast iron head that when overheated would expand and contract at different rates and would cause head gasket problems. I could go on but. The 3.0 ltr. is a non rev happy 4 cyl OHV engine but would be far better in a car but not as cool or as powerful

singledownloop New Reader
Dec. 22, 2014 6:56 p.m.

alstevens I'm curious why a 3.0 would be better then a 3.7 in a car.I know nothing about either so hoping you could discuss further,thanks.

alstevens New Reader
Dec. 22, 2014 9:38 p.m.

Mainly because there are so many more of them available for parts

tuna55 UltimaDork
Dec. 23, 2014 7:05 a.m.

I've wanted to get my hands on a 3.7 for a long time. It's a 460 with a small block chevy bolt pattern for the bellhousing. It's an aluminum block and iron head, so switching to an aftermarket aluminum BBF head will save tons of weight, and I imagine that the rest of the issues can be overcome. Internet lore suggests a guy went landspeed racing with fancy parts and learned some things, but his stuff isn't really well documented on the net other than notes about cautioning folks not to overlighten the crankshaft and that camshaft selection was weird since you're pretty much making an intake out of half of a 460 intake.

stuart in mn PowerDork
Dec. 23, 2014 8:09 a.m.

I've seen a few Mercruiser engine builds over on the HAMB board, you may be able to find them with a search.

iadr HalfDork
Dec. 23, 2014 11:47 a.m.

I'm building a 3.7. Generally boat people are extremely hard on stuff- if they can blow up Volvo red-blocks and complain about how unreliable they are- and they do do both- then their opinions can be discounted 110%. Mostly the problems can be traced to detonation. (same as cars- I did teardowns for a semi-volume or at least mainstream machineshop, and accross all makes failures were split 50% detonation, 25% driven hard when cold, 20% other failures-known design defects, or driven with lack of oil or coolant, or dirty oil- less than 5% worn out)

SBC bellhousing ...kinda, but 5/8" deeper to the crankshaft flange...and Ford Flywheel. International V8 starter.

In my case I'm using an aluminum head that saves no weight... because it's huge. Kaase Boss9 x 1 pce. Cam is reground by Lunati. 220/226 @ .050, ~.560 lift, clatter free hydraulic. This is not a small project. You need to completely rethink induction and cooling systems and build from scratch. With some help from the Locost board, I'm set my cooling system plan. Fiat "deadhead-T" thermostat, fabrication. Intake is 4 x Chrysler 3.5 V6 (they use two/engine, so two cars worth) throttle bodies, on their sides, and fabrication.
There are no brackets for A/c or P/S, and only really hokey ones for Alt. So...fabrication.

If you wanted to do carb and cast exhaust manifold, it would be easier.

iadr HalfDork
Dec. 23, 2014 12:06 p.m.

The trick to doing a T5 trans with these is to use the unloved 95-98 Mustang long input shaft, which is the exact extra length needed.

Then you use an aftermarket SBC-T5 bell or an Astro 4.3 manual bell, or in my case an 80-81 Jeep 2.5GM bell. The Camaro SBC to T5 puts the trans on an awkward slant of about 13-17 degrees, but may work.

You may not realize how big these are. Not much at all shorter than a BMW small six. 4 x 4.90 bore spacing=19.6" vs 3.6 x 6=21.6" This pic is a Hyundai Accent 1.6L gasket set on the core block, prior to it going out for machining.

Lots and lots of work, thinking, researching, learning and fabrication.
You have to want this and nothing else. Not a budget solution for anything. Even doing a lot on my own, I'll be into mine for ~11K CDN, including the head ($3700US right there for mine) F.I. soft and hardware, machine work, and subletting the TIG welds, and a custom header.
Not many people are as anti-v8 and as anti-turbo as I am.

I have tons of ideas to share if someone is serious- eg. Weber IDA have bore that are within .040" of exact alignment. You could use the IDA-a-like-throttle bodies to save a few steps in putting together an induction system. Or you could buy a Ford V8 manifold and cut it in half and put say a GM TPI thottle body on it. I photoshopped that up, and could look and work well.

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