getting original 151 modded

[eagle.sx/4.wanted]

I have an original 1982 151 engine (iron duke).  What kind of mods internally can be done to this to get me in the 250 hp range, with new header, carb, etc. externally?  Will other Iron Duke parts fit this block?  Any info will help. It IS for a racing application. Would like maybe a little higher redline and power in the 4000/4500 to redline range.  Please let me know.  PM me if you want. 

Got me a 4cyl SX/4!!!  Just what I wanted.

Thanks, Josh

[eaglefreek]

What type of racing? Any rules regarding engine mods?

[eagle.sx/4.wanted]

it is supposed to be "stock appearing". Internally, you can do anything you want.  And the track we race at, as long as you are not beating the cars by a huge amount, they don't care.

[captspillane]

The weak part of the Iron Duke is the crank. Its designed for economy, which is right at 80 HP. The racing versions of the Iron Duke used a "Super Duty" crank that was much more robust. Years ago they sold quite a few of these to Pontiac Fiero guys, along with other Iron Duke upgrades. They are extremely rare and they were extremely expensive back then.

That same crank, however, is a dime a dozen today. Mercruiser purchased the rights to make the original Iron Duke casting, originally made by Chevy in only 1977 and 1978. For marine duty they also got the rights to make a super duty crank. Essentially every inboard boat between 16 and 20 feet that has been made in the last thirty years has that Mercrusier engine. I purchased three Mercruiser engines so far for $100 a piece. They're perfect for installation in an Eagle. The Merc Iron Dukes are still in production today.

The AMC Iron Duke, made in 1980 through early '83, is actually a 1979 Chevy engine. In 1980 Chevy changed the casting dramatically and abandoned the engine, and that's when AMC picked it up to use until their own 4 cylinder development was completed. There are several major differences between the 1978 and 1979 Iron Dukes, but the engine mounts and the bellhousing are the same. That's not true of the 1980 to 1992 Chevy Iron Dukes.

The early Merc Iron Duke 2.5L had the same size pistons as our AMC Iron Dukes. They were 120 HP motors. The later ones were factory overbored and stroked to 3.0L. Those were rated to 140 HP. The Merc 2.5L and 3.0L will bolt into an Eagle directly. There is one hole on the passenger side mount that needs to be elongated, but its minor. You also must take the Merc oil pan off and discard it because it is shaped dramatically different. The AMC Iron Duke oil pan and oil pickup will work.

There is a Mercruiser 3.7L four cylinder engine out there as well. That is often called an Iron Duke, but that is misleading. That engine was created by Mercruiser to be a directly interchangeable inboard engine with their 3.0L engine. They actually made an Aluminum block using the iron casting as a starting point. That means the bellhousing pattern is the same and the engine mounts should be the same. Instead of Chevy Iron Duke internals, however, they use Ford pistons, connecting rods, and a modified Ford V8 head. The Mercruiser aluminum block 3.7L engines were rated up to 190 hp. They would be a comparatively easy swap, indeed much easier than a Chevy Iron Duke would be. I have avoided this engine for one reason. The oil pan isn't an Iron Duke oil pan. It uses the Ford gaskets. That means you can't use the Eagle oil pan when converting it to car use. In order to install on in an Eagle you will need to make your own custom oil pan and oil pickup tube. 

The Merc Iron Dukes are non cross flow heads just like the '77 and '78 Chevy engines they are based on. That means that the carb is on the driver's side of the engine along with the exhaust. Our AMC engines, based on the '79 casting, is a cross flow head. The crossflow head from your Eagle might work, but you will not be able to use the intake manifold. The difference between a '78 casting and a '79 casting (that the AMC is based of) is that the distributor was moved further back to make room for the intake. You'll need to make a custom intake if you use your Eagle head on a Merc block. Alternatively you could use the Merc crank in your Eagle block, but then you can't take advantage of the 3.0L displacement and larger stroke that the Merc block has.

There is another head option. The racing Iron Dukes usually modify a Small block Chevy head a very small amount and use that. It dang near bolts up as is. If you look closely, the Iron Duke four cylinder actually uses a Chevy Small Block pattern for the valve cover. The Head Gasket pattern and water jackets are dang near identical as well. Those racing engines use the SBC heads because of the larger valves and performance upgrade possibilities. They then also use custom headers for both intake and exhaust.

In my case I have a '77 Monza Iron Duke already installed in my Kammback from a previous owner, so I have that intake and exhaust ready to bolt on to the Merc motor. I doubt you'll be able to find one but that is another possibility for a bolt on swap.

[captspillane]

When using a "land-fish" motor, don't forget to discard the Marine distributor. Marine engines are designed to come up to the efficient point of the propellor and stay at exactly that range for a long time. That is why the Crank must be so heavy, the Engine operates at a high RPM and a high duty cycle.

Marine distributors are not the same as automotive ones. They have a much smaller range of advance to correspond with the smaller service range. The good news is that any SBC V8 distributor is compatible with our Iron Dukes. My '77 Monza engine has a cap that is identical to the SBC one, except that every other post is deleted.

The fuel injection on the newer Mercruiser Iron Dukes is also very limited and unreliable. They have a feedback loop, but it is not able to keep up with the broad RPM range of an automotive motor.

[carnuck]

S10 trucks and Cavaliers came with TBI on the 2.5L but not sure what you'd need to make it fit.

[eagle.sx/4.wanted]

the car would be carbureted.  not fuel injected.  I know that you can get 350 hp out of a GM 2.5L (151).  But the more modern (84-92) blocks and parts are not the same, right?  Or can I swap in one, will it bolt to the trans.?

[thereverendbill]

the car would be carbureted.  not fuel injected.  I know that you can get 350 hp out of a GM 2.5L (151).  But the more modern (84-92) blocks and parts are not the same, right?  Or can I swap in one, will it bolt to the trans.?

not to sound mean or anything but i am curious to where you got a 350hp number out of an iron duke?

[captspillane]

The modern Iron dukes have the same crank, connecting rods, and pistons as our Iron Dukes. They have slightly better power outputs because of the fuel injection and the different camshaft that goes along with it. Its in the order of 100 HP instead of 80. They will explode long before 350 HP.

A 350 HP Iron Duke is plausible. Its been done. It is accomplished by using a super duty crank, overboring and stroking the engine, putting a high flow head on it, roller cam with obnoxious specifications and the custom rockers, and then jacking the compression ratio up to absurd heights with custom cast pistons and the like. Then you have to run jet fuel or kerosene. Then you have to run it at 9,000 RPMs. That will get you 350 HP for a duty cycle of about a minute before it explodes. I've personally met someone who built a 232 (He chose a 232 instead of a 258 because the square stroke equal to bore is supposed to assist in high RPM operation) that reached over 400 HP using similar methods. It cost an obnoxious amount of money and wore out in about a dozen races.

A 350 HP Iron Duke is totally custom, high end parts with an expected lifetime measured in hours. Such an engine will realistically take 10 grand to build (My 401 cost 13K to build) and you'll need at least four of them to make it through a race season. If you could afford to do something like that or had the knowledge to pull it off you wouldn't be on this forum asking the questions you're asking. I know I sure can't afford to do that sort of stuff and I sure don't have the skills or knowledge to pull it off either.  

The AMC Iron Duke is a 1979 GM Iron Duke. In 1980 they changed the casting to 60 degree GM pattern, but otherwise left the crank and internals exactly the same. Everything that is possible with a later Iron Duke is possible with the AMC version. Pontiac Fiero guys in the early 80's played with the engine consistantly by buying "Super Duty" cranks, overboring blocks, and adapting SBC heads. There is a limit you can overbore the engine before the sidewall is compromised. The Merc motors had the casting changed, so it already has a larger overbore and stroke than is even feasible with a S10 motor. It already has the Super Duty crank. The later GM Iron Dukes require a great deal of expense spent at the transmission because that bolt pattern is meant for light duty cars. Our engines and the Merc engines have the SBC pattern that allows you easily and cheaply to source a heavy duty transmission from a full size Chevy truck or racing application.

The guys who dump crazy amounts of time and money into overboring their later GM Iron Dukes usually achieve about 120 HP. The really good ones make it up to 140 HP. The Merc motor comes from the factory at 140 HP. It could reach more through better camshaft selection, custom forged pistons, and other standard high performance upgrades. I would expect 180 HP to be a reasonable limit for the Merc 3.0L.  

Remember that the Merc 3.7L came from the factory at 190 HP. It looks like an Iron Duke and is often called an Iron Duke (even though it has Ford internals and an Aluminum block). That is your best starting point if you're serious about getting the most out of a four cylinder and don't mind welding up a custom oil pan.