Quote Originally Posted by Call_me_Doc
The 340 had specific heads from 1968-'70. '68-'69 and some '70 used the #2531894 "X" casting (so called for the large X cast near the spark-plug holes), which was, for a long time, considered the premier SBM cylinder head... until someone actually flow-tested the 3418915 heads, marked "U", "O" or "J" in the same spark-plug area and found that with the 2.02" intake valve used in '70-'71, flows identical to the "X" head on the intake side, and even better on the exhaust.

This 915 casting is also the same as used on the Challenger T/A and 'Cuda AAR 340 Six Pack engines, though they used revised pushrod-hole locations and therefore offset intake rockers so that the "pushrod humps" could be ground out of the intake ports. This very-same casting (with standard pushrod holes) was used in 1971-'72 on every 360 built, though on the 360 application it used a 1.88" intake valve but shared the 1.60" exhaust with the 340. All '71-'72 360s were two-barrels, and were C-body and truck only in those years, though a single '71 Dart mysteriously got built with one. Regardless, the 1.88"-valved "J" heads are identical castings to the 2.02" versions, needing only the seats enlarged and the bigger valves to be "340 spec" heads.

Starting in 1972, the 340 was phased down to the 1.88" valve used in the 360, but managed to hang on to its steel crankshaft. In '73, both the 340 and the 360 went to the far-inferior 3671587 (or simply "587") casting; it was a far cry from the 915 "J". It takes a lot of porting and an intake-valve increase to 2.02" just to get this head on par with the X or J heads. The '73 340 went to an iron crank and therefore uses a 1-year-only, 340-only dampener and flywheel (or converter, if your car doesn't have the right number of pedals )

The 340 went away at the end of the '73 model year, performance small-blocks then being based on the 587-headed 4v 360. Though the casting numbers changed over the years, the 360 head didn't change much; the minor revisions to the ports did worsen it as time passed but any 360 head can be made to perform pretty well if you pay someone, or have the patience, to port them.

Then, in 1989, we got a reprieve with the introduction of the best SBM head ever put on a production engine: the 308. This is a truck-only casting, used on 1989-'91 318s and 1989-'92 360s. Though it's only a 1.88" intake valve from the factory, opening it up to a 2.02" intake allows it to surpass the vaunted X and J castings, stock-for-stock. This was the head used on the early Mopar Performance 360 Commando crate engines (before the MP Magnum crates superseded it in the mid-'90s). Tons of these heads sleep quietly in a boneyard near you; I picked up a complete '91 B250 van 360 with a lifter tick for $150 from a nearby yard. One solid J casting will set you back that much these days (I've sold bare J castings for as much as $400 per pair).

A couple of caveats:

1. There are later 318 heads with an X or J cast into them, which I've only found on trucks. These are not the good 340/360 heads. Check the numbers, but unless the X or J is over 1" tall, keep looking.

2. The current rage among head shops is to try and sell you the "swirl port" 302 cylinder heads. First off, they're not swirl port, they're swirl chamber. Second, they're 318 heads, originally equipped with a 1.78"/1.50" valve combo. Yes, they will garner you nearly a point of compression due to their heart-shaped, closed combustion chambers... but opened up to 2.02"/1.60" valve and fully ported (and I mean, like $1200 worth of work here), they flow less than a stock 2.02" X/J/308 head. Oh, and the upsized valves destroy the swirl effect--this straight from a Chrysler engineer (not me) While compression is nice, it's fairly useless when there's nothing to compress: horsepower is all about airflow. So why the big push? Virtually every non-cop 318 from 1985-1989 (except '89 trucks) had these heads--there are a million out there. So, the shops push the compression aspect and folks eat it up. Magazines further embellish the myth, because these shops pay to advertise with them (the real money in magazines). Unless you're building a stump-pulling, trailer-towing truck motor, don't fall for it. If you don't believe me, then look under the hood of any '80s Mopar cop car with a 318: they all had later ("945"-casting) 360 heads, as well as 360 intake manifolds and carbs.

3. The Edelbrock Performer intake, while lighter, makes less power than any factory SBM iron four-barrel intake. It has smaller ports, so you'll feel an improvement off-idle in the seat of your pants, but in the middle and on the top end, you'll lose. The lighter weight doesn't account for the HP loss. I've seen the dyno runs and flow-tested them; numbers don't lie. Also, avoid the '70s-era Streetmaster and SP2P intakes like the plague; you can clog the ports with two fingers. Both were designed with towing and fuel mileage in mind and have no place on a performance engine.

That's all I can think of at the moment... I'm pooped. Anyone got a beer?
now thats what im talking about , you know your stuff about small block mopars