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

[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]

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?

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.

[A-A EAGLE]

Catspillane, what an interesting story and good research! I think it will bring some of us to a good idea, so a good egg for you!

Anjo
Holland

[captspillane]

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.

The non-cross flow head on the 3.0L is the same head as my 1977 Chevy Monza engine. I'll use that intake and exhaust manifold. I’m also planning on creating my own carb adaptor from scratch to attach a Howell TBI kit intended for a 4.2 where a Holley carb is now. I do know that the 3.0L intake and exhaust ports changed shape slightly around 1992, so I'm sticking with 80's boats.

Indeed it’s possible that the marine carb and manifolds will work just fine. I’ll look at that possibility and post my thoughts here. Even if they did work I wouldn’t use them because I want to retain my current exhaust pipe that I just made.

Unfortunately it’s hard to find 1977 and 1978 Iron Dukes. They're very rare. I know I sure didn't know what a Monza was until I started this research. I'm lucky to already have one in my Kammback. If I wanted to do this to a second Eagle I would try to make a custom intake and exhaust manifold from scratch. I think that the mounting surface and basic shape of the marine intake and exhaust would be a good starting point for someone skilled with a welder to make their own. It might even work as is.

I also have read of people using a small block Chevy cross flow head. Supposedly the bolt pattern is the same but one water jacket needs to be modified. It should be less difficult than installing a 4.0 head on a 4.2. That is supposed to allow for bigger valves for extreme racing engines. Of course the people who make modifications like that also make their own manifolds from scratch as well.   

I found one rebuilt 3.0L, engine only, for $700, one running complete boat for $500, and one 3.0L in need of rebuild for $300. I also found about a dozen other boats and 3.0L engines for sale within two hours of my home. Imagine finding a freshly rebuilt 140 HP Super Duty Pontiac Iron Duke race engine for $700. I can’t imagine finding one at all.

[captspillane]

Here is the comparison photo of a regular 151 crank and a Super Duty 181 crank  I got it from. I also included several photos of a 3.0L crate motor, of a Monza from the H-body forum, and my own Kammback.

http://myfiero.oceanmoon.com/how-to/engines/iron-duke-performance-faq

[bigdog56e]

 I remember some air compressor company was using the 3.0 engine on trailer mounted air compressors. Might look for a source of GM industrail engines. Chrysler made Air Raid sirens in the '50's with the 354 Industrial Hemi engine, and Ford had a industrial version of the 2.3L four cylinder.
                                                                                                  Eddie

[nwamcsx]

Any update/more info on this swap??


Thanks

[carnuck]

I was wondering if they used the SBC pattern 2.5L or the later 60 degree GM pattern in the SX4s.

[captspillane]

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.

[captspillane]

As for an update to my project status, I recently let the annual inspection expire on my Kammback. It needs a wheel alignment desperately and I'm not willing to align it until all the ball joints have been replaced.

If I need the car on the road in a hurry I'll simply fix and inspect it and keep running the current Monza 2.5L. It was professionally rebuilt in 1995 and should run strong for a long time once I replace the intake gasket.

If I don't need it quickly I'll pull the entire crossmember and engine and continue with this swap. I haven't decided if I'll use the TH400, 700R4, or T5 after the swap. I've got at least one of each already. I'm leaning toward the 700R4 but I'll probably stick with the T5.

I also used my Howell TBI kit in my 1980 Eagle Wagon. I'll need to source another kit or start making my own. I won't install my 3.0L Merc motor without a GM TBI already adapted to it. I'm going for full spark control since the GM HEI distributor is compatible.

I've got a large number of Louver kits to make and a SD33T turbodiesel swap to finish first. It will realistically be a year or two before I pick back up with this project.

[carnuck]

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.

I haven't seen many 4 cyl Eagles and none with the AMC 4 cyl, but that is the same motor used in '84-'86 Cherokees and a few '86 Comanches (then they went to TBI and later to MPFI)

The AMC 4 pattern is also the same as 2.8 Chev 6 and '80 up front wheel drive Caddy, Buick, etc (including the Northstar V8s)

[j2sax]

Althought the AMC 4 was not EFI'd in the Eagle. 

Jesse