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Old July 6th, 2006, 11:39 AM   #1
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Default AMC V8's (and why PavementPounder bitches so much about CJ Chevy V8 swaps).

Read this article to understand a few things about the misunderstood AMC V8.

AMC V8 Engines

The first V8 AMC used was the Packard 320. Packard's "Ultramatic" automatic transmission was the only trans available with this engine. It was used only in 1955 and some 1956 models. AMC had talked with Packard in the late 40's about possible merger. AMC officials still hoped this would happen and felt a cooperative venture would strengthen bonds between the two companies and pave the way. AMC and Packard agreed to use each other as parts suppliers -- or so AMC thought. In reality Packard president James Nance felt he was doing AMC a favor by selling them engines and felt no obligations to purchase from them (Packard was still a healthy company in 1955) . A few bids were sent over just for show, but were rejected as being too high. This took place early in the hand-shake agreement.

This incensed AMC president George Mason so much that he ordered his engineers to develop a V8 engine as soon as possible (the hand-shake agreement was meant to foment trust between the two companies -- we see how that worked!). In order to do this AMC hired former Kaiser engineer Dave Potter. He had already worked on V8 designs at Kaiser and was able to have an engine ready for installation in a vehicle in less than 18 months!

In order to meet the short development time the engine couldn't be cutting edge technology. Instead it used all proven design and build techniques. It was on a par with other V8s of the time, but not the then-new, cutting-edge Chevrolet small block (also introduced in 1955). In other words the engine was relatively bulky and heavy for its displacement, but very strong. There wasn't time to build and test cast rods and crankshafts so forged parts were used. With the bulk of the block and forged crank and rods, this proved to be an exceptionally strong engine. Today a few racers have discovered this and are using them for high boost turbocharging. The only thing required is custom forged pistons -- the crank and rods are as strong or stronger than aftermarket performance parts after a little preparation.

The only drawback to the design was the heads. They used conventional vertical overhead valves. With the valve going straight down into the head there was limited room -- the valves could only be so big before shrouding affected flow. The block was capable of supporting over 400 cubic inches, but the heads would need to be redesigned to support large enough valves. Only slightly larger valves can be installed in these heads, and it may not be worth the cost. The turbo racers mentioned earlier overcome valve size somewhat with boost -- up to 23 psi -- but limit displacement to near stock, only boring to take out wear.

Note that Chevrolet didn't make their small block 327 until 1962. The early AMC V8 more closely resembles the Chevrolet 396 big block, though there are very notable differences. This is as close to a "big block" as AMC ever made.

AMC engineers knew that the heavy V8 couldn't compete with more modern small blocks introduced by the competition (Chevy in 55, Chrysler 55, Ford 63). The heavy engines weren't in line with AMC's economy image either, though they were very much responsible for the Rambler's reliability and smoothness reputations.

AMC started development on a new V8 shortly after the first was in full production. The new engine, introduced in mid 1966, shared some features of modern small blocks and others more common with big block of the era. It had wide bore centers comparable to a big block. This made the block a little longer but provided plenty of room for future growth and increased stability. For this reason some publications call it a "mid block". It had a Buick style oil pump made into the timing cover. The distributor was driven off the camshaft via a gear that bolts to the front of the camshaft. The oil pump drive shaft was slotted to fit a tang on the end of the distributor shaft.

A 390 cubic inch performance model was introduced for the 1968 AMX. This used the same dimensions as the 290 and 343 but had thicker main bearing webs for added stiffness in that area. AMC never produced a factory four bolt main bearing engine as they felt the two bolt cap was adequate, but they did cast the webs thick enough to be drilled for aftermarket four bolt main caps for racing purposes. To keep high reliability with the longer stroke all 390 and larger AMC engines used forged crankshafts and rods. According to AMC engineers forgings were originally used due to inadequate time to test cast parts. AMC decided to keep the forgings, either due to low numbers of the engines or to retain high reliability. In either case an AMC 390/401 is much stronger than comparable small block 400 engines. No aftermarket cranks or rods are required for racing, just careful preparation of the stock parts.

Displacements were increased in 1970 by lengthening the deck height of the block by 0.16" for that much longer stroke. The 390 had built such a performance reputation that a new rod was made to keep a 390 for 1970. The stroke was changed only 0.11" for the big engine to bring displacement to 401 cubic inches for 1971. It was felt that more than 400 inches would be larger than needed. (EDIT: But possible.) Even then the block had to be notched at the bottom of the bores to clear the crankshaft counterweights. The higher deck height meant a slightly wider intake was necessary.

The heads were also changed in 1970. 1966-1969 heads have rectangular exhaust ports. 1970 and later heads have a "dog leg" or "pork chop" shaped exhaust port. The larger port increased exhaust flow by around 50%, making AMC heads the best flowing production heads available. For this reason the Chrysler "Magnum" V8 head was based on the AMC design. The new ports also required new exhaust manifolds.

AMC V8 engines are generally classified as GEN-1, GEN-2, and GEN-3 (GEN for generation). The GEN-1 engine is the large 1955-66 250-327 block, GEN-2 the smaller 1966-69 290/343/390, and GEN-3 the taller 304/360/401 (and 1970 390) model. Although the GEN-2 and GEN-3 share essentially the same block except for the 0.16" deck height increase, the head, intake manifold, and exhaust manifold changes justify the separate designation. GEN-3 engines also use 1/2" head bolts, GEN-2 uses 7/16" head bolts. Heads will interchange between the two as long as the bolt size is accounted for. Step dowels are made to fit the better flowing GEN-3 heads on GEN-2 blocks, but for racing purposes it is better to drill and tap the older block for 1/2" head bolts. To put GEN-2 heads on a GEN-3 block the bolt holes must be reamed to fit 1/2" bolts.

Technically AMC didn't build a small block or big block, they just made one V8 engine with the exception of the short overlap in 1966. In reality the engines are compared with the competition. With this in mind the GEN-1 can be considered a "big block" because of its external dimensions and weight, and the GEN-2 and GEN-3 small blocks for the same reasons. Some publications have mistakenly called the 390 and 401 "big blocks" because of the displacements. Externally all GEN-2 and GEN-3 engines are the same size with the exception of height and width. All bolt patterns and external bolt on parts (except intake and exhaust manifolds) are identical. All internal parts interchange, though crankshaft and rod swaps may require custom pistons. GEN-1 parts are unique to that engine.

All 1970 and later AMC heads use the same port design. All 360, 390, and 401 heads are identical. These use 2.025" intake and 1.680" exhaust valves (early 70 used a 1.625" exhaust valve) and have the high flow "dog leg" exhaust ports. If building an AMC race engine simply order pistons for the desired compression ratio and forget the smaller chamber heads -- it won't cost any more (maybe less!) if the pistons need replacing anyway. 1970-early 71 304 heads use a different casting (3199517) but are essentially the same as the 360/390/401 heads. Ports may be slightly smaller and castings a bit thinner, but according to all AMC technical data 2.02"/1.62" valves can be installed. Combustion chamber volume for the 304 head is 52.20cc and produced a compression ratio of 9.0:1 with stock pistons. Later 304 heads have a 58.92cc chamber and produce 8.4:1 compression.

The bore size of GEN-1 engines is cast into the left rear of the block. This is covered by the bell housing when engine is in a vehicle with transmission installed. The cubic inch size of all GEN-2 and GEN-3 engines is cast into each side of the block just behind the engine mount plates in the center of the engine. An exception is the 1970 thick cast 360 (used in Donohue Javelins to homologate the thick wall 360 block for Trans-Am racing) and some service replacement engines. Since the Engine Day Build Code or serial number is located on a removable tag this is the only reliable way to identify engine size. This does not apply to the Packard V8 engine.
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