Don’t Ditch That AMC V-8!


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There is a lot of knowledge, history, and beauty held in the inventions of the past, and one of the more redeeming qualities of the Internet is that it has helped educate people on what is rare and what is valuable. We’re witnessing the economic aspect of it right now. Anything made before the ’80s is starting to skyrocket in price, and vehicles from the ’60s seem to be the toys of rich guys now. No doubt, you’ve noticed that fullsize Jeeps are the hot ticket for retro builds these days. Ten years ago, we’d never imagine seeing old fullsize Jeeps with more than $100,000 thrown at them, but lo and behold, it is clear that the world has started to appreciate the fullsize Jeep. And that’s part of what has us scratching our heads over why so many people are ditching their original AMC V-8s for some go-to LS swap.

We understand the power and fuel economy arguments. And the LS fuel injection is obviously nice. But how could one appreciate the fullsize Jeep and not also appreciate the venerable AMC V-8 that fullsize Jeeps (and lots of CJs) were outfitted with between 1965 to 1968 and again in 1970 and 1991? That got us to thinking. Maybe it’s time to explain why you shouldn’t ditch that AMC V-8.

We’ve taken a look at Jeep’s powerplant past and have compiled a bit of history—as well as some advice on how to build a great AMC V-8. We’ve also had the pleasure of interviewing Larry Ofria, the man who helped AMC design the dogleg heads in the late 1960s. He is an engine guru whose racing history goes back more than 50 years. Perhaps this will help you make the right decision when the next opportunity arises to build a custom AMC V-8.

The Original AMC V-8: 1956 to 1967

CEO George Romney ordered the original AMC V-8 to be built in 1954. Less than two years later, the AMC executive had his new V-8 ready to debut to the world. With a forged crankshaft and connecting rods, this overhead valve pushrod engine became the muscle car powerplant for Rambler, Nash, and Hudson vehicles—and the workhorse for the Jeep trucks and Wagoneer. The engine came in three sizes—the 250ci, the 287ci, and the 327ci—all with the same stroke but different bores. The 287ci replaced the 250ci after a couple years, and the 327ci was released a year after the original 250ci but with hydraulic lifters (that the 287ci also later got).

There is occasionally some confusion as to what powered the first V-8 J-trucks and Wagoneers, but it was not a Chevy 327 (which did not emerge until five years after the AMC 327). From 1965 until early 1968, Jeep offered the AMC 327 block as the Vigilante V-8.

Size Years Bore Stroke Special Notes
250ci 1956 to 1961 3.500 3.250 -The original AMC V-8 developed by Dave Potter

-Had solid lifters

287ci 1963 to 1965 3.750 3.250 -Replaced the 250ci engine

-Had 0.25 inch more bore (total of 3.75 inches bore)

327ci 1957 to 1967 4.000 3.250 -Known as the “Vigilante V-8” in Wagoneers and trucks (1965 to early 1968)

 

The Second Generation: 1966 to 1970

By the mid-’60s, AMC’s first-generation V-8s were old hat, and the company eagerly introduced its second generation of V-8s in 1965. The new V-8 shared the same basic design, but it was a physically smaller block and utilized a more modern thin-wall casting technology that allowed it to drop around 60 pounds over the first-gen V-8. The bore center remained the same, and different displacements were made using a combination of bores and strokes. These engines had rectangular port heads that flowed well, and AMC’s new V-8s were known for making great power with very little headwork. But AMC was not yet where it wanted to be in racing—and not yet satisfied.

In 1968, AMC released the AMX 390ci engine with a higher-nickel-content block, forged crankshaft, and forged connecting rods. It has been said that the reason for the forged parts was that AMC gave the engineers improper time to properly test cast parts with the increased power that the 390ci variant made, so they went for overkill to be safe. It would be a relatively low production number engine, so the higher parts cost (of forged parts and stronger block) was likely seen as a wash since they could forego the testing.

You’ll notice there is a slight production overlap in engine generations; the Vigilante 327 V-8 made it into 1968 in the Wagoneers, while the new second-gen 290ci V-8 was released in 1966. Interestingly, Jeep never used the second-gen AMC V-8 and instead sourced a Buick-derived 350ci with a very flat torque curve for 1968 until 1971. We have not yet found a definite answer as to why.

Size Years Bore Stroke Special Notes
290ci 1966 to 1969 3.750 3.280 -Used for Trans-Am racing
343ci 1967 to 1969 4.080 3.280
390ci 1968 and 1969 4.165 3.574 -High-nickel-content block

-Forged cranks and connecting rods

-Heavier main bearing support webbing

There were only three years of AMC 390-branded blocks: 1968, 1969, and 1970. The ’68 and ’69 were second-gen engines, while the one-year-only 1970 390ci was a third-gen with a special rod-and-piston combo to achieve 11 ci less than the 401. The 390s were never offered in Jeeps.

The Third Generation (aka Second-Gen Mark II): 1970 to 1991

Eager to do even better in Trans-Am racing, the second-generation V-8 was short lived. In 1970, AMC released a very similar but improved version with a taller deck height and improved dogleg heads. This was the final and best version of the AMC V-8. While the second-gen version was good, the taller deck height allowed for a larger displacement and the dogleg heads flowed even better than the already impressive rectangular port heads. And perhaps that is the answer as to why the second-gen engine never made it into Jeeps; maybe AMC knew it would soon be making a “Mark II” version of the engine and decided to wait with the Jeeps, sourcing Buick engines in the interim.

How good was that dogleg head? It’s said it was so good that, in stock form, it could outflow even the best custom-ported and -polished Chevy, Ford, or Dodge heads of its day. This was the engine AMC had been waiting for to give it the edge in Trans-Am racing. The 290ci turned into the 304ci, the 343ci became the 360ci, and the AMX 390 became the mighty 401—the ultimate factory powerplant to ever outfit a Jeep. It should be noted that there was a one-year-only 1970 390ci third-generation engine prior to the 401. AMC accomplished this with a special rod-and-piston combination for a smaller stroke to keep the displacement at 390 ci. It is said the reason this was done was due to the popularity of the AMX 390 name, and to kill that success after only a two-year run would be silly. A year later, the 401 was born.

Considering when these engines were designed, have you ever thought to yourself, Why the 304ci? It was a time just prior to any fuel crunch or emissions restrictions—when bigger was always better. The 304ci still sported the same stroke as a 360ci but with a smaller bore and was barely (laughably) better on fuel. So why bother making the smaller-displacement 304? A good reason could be found within the rule books of the Trans-Am racing. The engine size limit of a Trans-Am car: 305 ci.

Unfortunately, all good things come to an end. Starting in 1980, the 401 was dropped after being practically suffocated by ill-working emissions equipment in its final years. The 304 was dropped by the end of 1981—likely due to the fact that AMC’s six-cylinder was making more horsepower than it, and its emissions were no better than the 360’s. The 360 stuck around until 1991, powering the wood-paneled Grand Wagoneers that so many of us have fond memories of riding in. It should be noted that the Wagoneer had the longest production run of any Jeep, consequently giving the AMC 360 V-8 an extremely long life as well.

The 401 made it into the fullsize Jeeps last in 1979, but the 360ci variant was used until 1991—and is one of the last carbureted engines in the U.S.

Size Years Bore Stroke Special Notes
304ci 1970 to 1981 3.750 3.440 -Used in various Jeep products as well as Trans-Am Racing (AMC’s last championship would be in 1976 with a Javelin)

-Same stroke as 360ci with smaller bore

360ci 1970 to 1991 4.080 3.440 -Longest production run of any AMC engine

-Most common size of AMC blocks

390ci 1970 4.165 3.574 -One year only: special rod and piston for smaller stroke to stay at 390 ci

-High-nickel-content block

-Forged crankshaft and rods

401ci 1971 to 1978 4.165 3.680 -Widely considered the ultimate AMC block

-High-nickel-content block

-Forged crankshaft and pistons

-Heavier main bearing support webbing

-Used in 1971 to 1978 Jeep J-trucks, Wagoneers, and Cherokees

SR block Beginning in 1970 4.080 -Service Replacement block with same high nickel casting as 401 but bore of 360

-Also sold as a heavy-duty racing block

-Did not have displacement size cast into the side

Isn’t this freshly built AMC 401 sitting at Valley Head Service just so much sexier than some powdered-metal LS?

This is the dogleg head that replaced the rectangular ported heads and gave AMC the winning edge they needed.

The Inventor of the Dogleg Heads

To get better insight into the last and best of the AMC V-8 blocks, we took a trip over to Northridge, California, to meet with Larry Ofria at Valley Head Service. The name of the shop—rather humble—does not begin to tell the stories of what has come from inside. The Thunderpower 32-valve heads are a Valley Head product, and Ofria’s original Dry Sump Pump external oiling system hasn’t changed much since 1966 and is still a good seller. Over the years, Smokey Yunick, Bill Stroppe, Holman & Moody, Carroll Shelby, Dan Gurney, and Bud Moore all came to Ofria for his expertise. AMC did, too.

Today, Ofria is one of the last of the original, old-school builders. Most of the machines in his shop were made long before computers were ever controlling them, and yet they’re incredibly precise. He’s still in the shop almost every day (he goes racing on weekends sometimes), and at 83 years old he’s been modifying and rebuilding engines for seven decades. Opening the doors of Valley Head Service on May 1, 1965, Ofria quickly made a name for himself as a premier head flow guy. In 1968, he got a call from AMC, and in 1970 the dogleg head would become the only head available for the new larger-displacement (with a taller deck height) AMC V-8.

Larry tells us, “The AMC was the ugliest motor at the track, but don’t ever mess with it, because if it was built right it’d eat you alive! And it was a good competitor. What killed AMCs in racing was having a two-bolt main instead of a four-bolt. The AMCs would lead the first half of the race, and either burn up the rear or drop the crank out during the second half.

Larry continues, “I got the call from AMC in ’68 because of all the work I was doing for Holman & Moody, Shelby, Stroppe, Bud Moore, and Dan Gurney. Back then, we were doing just about everyone’s head work except for Penske and the Chrysler Black Shadow Group guys. In 1968, a gentleman by the name of Ronnie Kaplan, outta Chicago, contacted me, and after we hung up, they started to make the arrangements. Their head flow guy had retired and moved to Mexico, so they found him, pulled him out of retirement, and sent him to spend three weeks with me on the flow bench. Back then, there were only about five real good flow benches across the country. We started testing and we simply found out where the air wanted to be—so we moved the port offset. The old AMC heads would just flow air straight in, not swirl it, and it would bounce back out. We moved the port to make a more drastic turn to swirl the air.”

The dogleg port would be AMC’s edge they were looking for in racing, and it brought even more notoriety to Ofria’s genius. He continued to be a head specialist until around 1974, when he expanded and started building complete engines. Today, Ofria continues to man the helm at Valley Head Service in Northridge, California, still pumping out some of the best custom-built engines in the automotive world.

The AMC 401

If there is one AMC V-8 block to have, it’s the 401. Well, technically there are two blocks to have. We have to include the one-year-only 1970 390ci that was made before they increased the stroke by 0.11 inch. The AMC 401 is probably a better choice, but the cool factor of the one-year-only variant is too much to pass up. Why a 401 block over a 360 or 304? The 401 was a higher-nickel-content block that used a forged crankshaft and connecting rods with larger 2.248-inch rod journal diameters. It has been stated that the factory parts will survive 7,000 rpms in built-up engines. The thicker webbing around the main bearings gives enough room to drill and tap two additional bolts to accommodate aftermarket four-bolt main bearing caps if you desire to do so (AMC V-8s were never four-bolt—and always two-bolt—mains from the factory). Hot rodders loved them because they were not compression sensitive and they held together under extreme force. AMC never had a big-block, but many Chevy, Dodge, Ford, and Pontiac fans still complained about the large-displacement small-block AMC 401.

Fuel Injection Options

The AMC V-8 never was fuel injected from the factory in a Jeep product. In fact, it was one of the last carbureted engines built in North America—up until 1991. But there are some electronic fuel injection options out there. One of our favorites: Howell’s plug-and-play throttle-body kit. It’s 50-state legal, gets rid of a lot of terrible emissions equipment and vacuum hoses, and runs absolutely beautifully, giving smooth power response and increasing fuel economy. There are two- or four-barrel systems, but Howell tunes them specifically for your engine alterations and sends you a ready-to-go EFI system that bolts on top of any standard intake manifold for a pretty reasonable price.

An Awesome AMC V-8 Recipe

Looking to build yourself an awesome AMC for your Jeep? We asked some of the most knowledgeable guys we know for tips and combined that with our own engine-building experience. This won’t be the one that will make the most horsepower on the track, but it’s the one that will endure grueling conditions and make great power at the same time. In other words, we’re not sacrificing strength or longevity for horsepower with this recipe.

Block—It has to be the 401 block or an SR (service replacement block). Both are thicker around the mains and can be modified to a four-bolt main. The 304 and 360 blocks are fine, but if you’re looking for the best, go with the 401. Don’t bore more than 0.060 over.

Heads—Don’t mess too much with a good thing. The original dogleg heads were hard to beat. Maybe give them a polish. Factory AMC heads can be modified to put in Chevy valves. If you insist on new heads, there are a couple aluminum aftermarket options that will drop weight off the total package.

Crankshaft—If you’ve got a forged 401 crankshaft, don’t do a thing. If you’ve got one from a 304 or 360, you should probably still do nothing; these are externally balanced cranks and will be fine for any decent 450hp build. If you’re going to get crazy, there are aftermarket steel options.

Pistons—KB Pistons has a decently priced hypereutectic piston for the AMC. Your block should be okay with 0.060 over (added bore) if you need to go that much.

Connecting Rods—If you’re not using stock 401 rods, a Chevy rod might be an option for your custom build. Some machine down the rod journal to use a Chevy rod.

Camshaft—Don’t get too crazy with the lobes. A little lope sounds good, but you don’t want too much of that while you’re rockcrawling. Get the right grind for your application. Comp Cams has a nice option that we’ve tried. A roller cam is a better choice, but it’s a more expensive option.

Valves—We used SI Valves one-piece forged valves with fusion-welded tips in our last couple of builds. They haven’t let us down yet. A four-angle valve job (35-, 45-, 55-, and finally 75 degrees) on the heads will help smooth flow out. Stock valve size for all third-gen AMCs: 2.025 intake and 1.680 exhaust.

Rockers— Roller rockers are nice to add and can cut down on friction (power loss) as well, but stock rocker arms can work just fine.

Intake Manifold—Get rid of that heavy iron thing on your engine; there are other options. Edelbrock even has a 50-state-legal aluminum intake manifold.

Timing Cover—These can wear out and cause issues for the AMCs. Proform actually came up with a reproduction piece if you can’t find one that’s not worn through.

Water Pump—AMC V-8s don’t have big overheating issues unless water pumps wear out or radiators clog. If you could use a new water pump, FlowKooler has a high-flow unit with an increased vane count.

Oiling—There can be inadequate lubrication if the aluminum housing of the pump wears out. A Milodon single or dual pickup bypasses the stock restriction and provides superior oiling. You can also run an external line to the back of the block in the lifter valley to provide better oiling the back of the block.

 

If you’ve got an AMC engine and you want a shop with decades of experience, you won’t find many with much more history than Valley Head Service.

If your AMC block suddenly achieves some “speed” holes like this, you’ll likely be shopping for a whole new engine, but we implore you to look for an AMC replacement instead of a cheap-and-easy, go-to swap.



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