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Replacing the 80 HP 2.5L Iron Duke with a 140 HP 3.0L Mercruiser Iron Duke

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captspillane:
I've decided to replace the Iron Duke in my Kammback with a boat engine. Turns out a 140 HP 3.0L Mercruiser engine is nearly a direct bolt in upgrade to replace the original 2.5L Iron Duke. It may be a better option for some people than a 4.2 swap. It almost sounds too good to be true. I'll post what I've learned so far and the progress I make in this thread. This project car won't be worked on until the end of the summer after I've finished the two SX4's I'm currently assembling.

I learned about this option when trying to figure out what engine was in my Kammback. As soon as I opened the hood I asked, "what's that? It's not an Iron Duke and it's not an AMC 150." The previous owner thought the car was all original. The owner before that had replaced the AMC Iron Duke with a rebuilt unit from a 1977 Chevy Monza. Thankfully the car came with a box full of reciepts that documented what work was done.

I might need to use the intake and exhaust manifold from the 1977 Iron Duke to install the Mercruiser 3.0. I'm going to try, but I'm not sure I can modify the marine version to work. If so that will make it harder for someone else to make the same swap. In my case I have everything I need for a bolt on installation. This is the current engine bay of my Kammback with the 1977 Iron Duke. The most obvious difference is that the carb is on the wrong side and the distributor is in the wrong spot.

rohnk:
Sounds like a neat project. I have to ask though, what is wrong with the current engine? I am assuming that it could not be rebuilt?

captspillane:
What's so great about the Mercruiser engine is the crank. Its the same crank as the "Super Duty" crank that Pontiac offered for race versions of the Fiero. It is obviously heavier and more robust. A Marine engine is designed to operate under full load and at the efficient RPM of the propellor, which is about 4K RPM, for a very long period of time. That's why a heavier crank was necessary. An automotive Iron Duke was designed for fuel efficiency only, so it was lightened and weakened on purpose to have just enough strength.

It’s also one of the most produced engines of all time. The first Iron Duke engines were made in 1977 using the dimensions and characteristics from another earlier Chevy engine. It was marketed as the "Iron Duke" because the Chevy Vega aluminum block engine was a disaster with lots of bad press. Those engines were prone to cylinder thermal expansion issues because of the iron pistons in an aluminum block. They were fixed by sleeving them, but that didn't fix the horrible reputation that engine had. The first engines had a Holley carb and non-cross flow head very similar to the older engine it was based off of. The valve cover gasket pattern, flywheel pattern, and bellhousing pattern was identical to a small block chevy. It even uses a SBC small diameter flywheel.

The 1977 and 1978 Iron Dukes got modified in 1979. The block casting was changed slightly. A new cross-flow head was added, which put the carb on the passenger side. A Rochester carb replaced the Holley. The Distributor was moved to a new location to make room for the intake manifold. The cross flow head is not compatible with the earlier blocks. In 1980 the block casting got changed again to replace the small block Chevy bellhousing pattern with the new 60 degree pattern.

Chevy continued to make the Iron Duke until 1993 with the new bellhousing pattern. They sold the rights and tooling to AMC for the 1979 casting. AMC continued to make the 1979 GM Iron Duke through mid-year 1983 when it got replaced by the AMC 150 four cylinder engine. The Chevy Iron Dukes are one of the most produced engines of all time. It got throttle body injection in 1982 which increased the power output from 80 hp to 90 hp. It jumped to 98 hp with changes to the TBI in later years.

Meanwhile Mercury marine engines decided to start making their own inboard engine. They started with the 1977 Iron Duke block casting. All the accessories bolted to the engine are different, including the oil pan, but there is no difference in the block or head themselves. Marine grade cranks, internal parts, salt resistant freeze plugs, and better gaskets are used throughout.

They initially made a 2.5L. They eventually came out with the Mercruiser 3.0L. This has a 4” bore and 3.6” stroke on a “Super Duty” crank. It’s essentially an Iron Duke stroker. It was offered with different carbs at 120 or 140 HP. Those numbers are painted right on the top of the motor to identify them. They should be identical internally. It is still identical crank flange to accept an automotive flywheel  and external characteristics.

I have seen that my 1977 Monza engine has one bolt slightly off on the passenger side engine mount. Its very minor and the rest of the bolts are in the exact same spot as an AMC Iron Duke. Mine has the hole enlarged by the person who made the swap in 1995. The Mercruiser engine will bolt up to the crossmember engine mounts and transmission of an Iron Duke AMC Eagle with little or no modification whatsoever.

Mercruiser eventually decided to make their own 3.7L midgrade engine. They took their Iron Duke tooling and applied it to an Aluminum block. The head and pistons are identical to a Ford V8. The bellhousing pattern, mounting pattern and outside of the block looks like an Iron Duke and it’s commonly called an Iron Duke. Those had different carbs to make a 160, 170, and 190 HP motor. I initially wanted one of those because they should bolt up just as easily as the Mercruiser 3.0 but I found out that the oil pan is different. I looked up the gasket patterns to confirm. If you use a Mercruiser 3.7 you will have to take the oil pan and cut it to make your own. On the 3.0L engines you simply bolt your Eagle Iron Duke oil pump pickup and oil pan on to replace the funky Marine oil pan. There is also no sign of anyone ever installing a Merc 3.7 in a car before while the 3.0 has been many times, so I’m going to avoid that engine for now.

The 3.0 has had EFI added to it recently. You can buy fuel injected crate engines. Those are to be avoided, however, because they don’t work for automotive applications. The EFI is non-feedback, which means that it works fine for a limited RPM range and that’s it. It doesn’t adapt the fuel mixture to account for different RPM’s and engine load that you see driving. The timing advance is also very limited in comparison to a car distributor.

You might have good results with the Marine carbs IF AND ONLY IF you change the ignition system to have the proper timing advance. Supposedly a Chevy distributor works fine since the Iron Duke cap is a V8 cap with four blank spots where wires would go.

I’m personally going to try to adapt a Howell TBI kit intended for a Jeep 4.2. It should have the right size injectors for this engine since a 4.2 with TBI gets right around 140 HP just as this thing will.

The Mercruiser 3.0 did have changes in production that affect the rear main seal. There are three year ranges. I believe it was 1992 before they changed the seal. All ‘80s boats should have an engine that I can rely on being compatible with my Eagle oil pans.

The Mercruiser 3.0 is in nearly every small inboard boat from 16 to 19 feet long. They are extremely common. They’ve been produced by three companies including GM and are still produced today. That engine and its compatible marine grade Chevy V8s are the industry standard now, so as near as I can tell all the new diesels made by “Merc-Cummins” actually uses the SBC pattern and similar, if not identical, engine mounts as the 3.0 and in turn our AMC Eagles. I’m hoping to find a Marine diesel engine to install in an Eagle one day too.  

captspillane:

--- Quote from: rohnk on March 25, 2012, 04:04:53 PM ---Sounds like a neat project. I have to ask though, what is wrong with the current engine? I am assuming that it could not be rebuilt?

--- End quote ---

I've driven most of my life with an Iron Duke SR4. They're great and very practical, but it always left me desperate for more power when I had to slow down every time I went up a hill. 80 HP is not enough for me. Its just not. The 115 hp in a stock 258 is barely enough to appease my appetite. 140 HP is pretty much the least amount I will accept in a car I'm going to keep long term.

The Iron Duke that is in the car was overbored and professionally rebuilt in 1995. The car has 208K miles, so that engine probably has 60K or less miles. The Holley carb is set ok and idles very well, but it doesn't accelerate smoothly or run well under load. Its probably a timing issue. I'm going to pull it out and stick it in a parts car that  I want to move under its own power. Long term I'll sell or give it away to someone with the patience to use it. I have three other running Iron Dukes with the same fate. I'm planning on driving this Kammback as one of my maiin cars, so it needs more power, a TBI, and a paint job.

eaglebeek:
Catspillane...about your exhaust/intake manifolds...inboard and stern drive marine engines have water-cooled/jacketed exhaust manifolds. I'm not sure how they would work in an automotive application. My concern would be if this exhaust manifold would burn out if water wasn't circulating through it.

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