Forgive me in advance; I am a virgin on forums, only recently scouring them and never posting until now.
I have a 79 Spirit liftback (58k original miles) with an 81 Spirit 4 cyl driveline (91k miles) recently married and now on the road at an actual weight of 2515 lbs. My mission is light car, rwd, 4 cyl, manual trans, no power anything, no electronic anything, (because I am a Luddite) nice body and paint but not too nice, go anywhere reliability, 35-40 mpg, modest power level [my daily driver for the last 15 years is a supercharged 98 Prelude that makes 249 hp at 7800 rpm at the wheels using 132 cubes, so modest to me means about 1 hp per cube], and used as a daily driver.
While totally disassembling then re engineering, rebuilding, and then reassembling the 79, I acquired a spare crossflow head. In my 50 years of engine puttering, I have found that using a head with poor flow characteristics will render cam, intake, and carb changes worthless; headers always produce power gains. I hooked up the duke head to my flowbench and pulled baseline readings. I was horrified to see that intake flow peaked between .25 and .3" lift. Specs list the cam lift at the valve as .402". Zero flow increase after .3 means that .102 of lift is wasted and some power potential is lost. For comparison, an AMC V-8 stock head flow peaks north of .50" and valve lift is .424; the AMC 6 banger head flow peaks at .30 with a valve lift of .372; thus a 343 will respond to a higher lift cam (but no more duration) with more power, but a 258 will not.
An AMC V-8 bare head casting weighs 55 lbs and is 20 3/4" long. The duke head is 19 1/4" long and weighs 40 lbs. I matched the intake port to the gasket, straightened and widened the port all the way in to the pocket below the valve, and put the head back on the flowbench. No change in flow whatsoever. I then cleaned up the cone shaped area from just below the valve seat back to the area of my 1st step and reflowed it. Again, no change whatsover. Scratching my noggin in bewilderment, I examined, measured, made sketches, and then consulted my notes and sketches from porting 3 sets of V-8 heads and 2 heads for the 258. Now I notice that the along the floor of the intake port the flow tries to make a 110 degree turn on the short side radius (SSR), goes straight for 5/8" while the whole pocket narrows just below the valve seat. No way that the airflow stays attached here with that sharp of a corner, flow separation and turbulence result. The entire valve pocket region has walls only about .10" thick. Exactly where lots of metal needs to be removed to enlarge the SSR, there isn't any. With nothing to lose but a score of hours, I reshaped the SSR to a larger radius till I struck water and put it back on the flowbench. My 15 minutes with a grinder was rewarded with a 13% flow increase at .30" lift. Short of porting for max flow regardless of the holes that appear, welding those up, and reshaping again, I must stop there and reconsider the head issue.
Only then did I start researching the whole engine. From various sites, forums, and talking to engine builders I find that:
The block is thin and very weak. Overboring and honing without using a torque plate results in cylinder distortion of .003 to .004" when the head is torqued on; scored pistons soon follow. Numerous casting changes after the late 79 design that the 80-83 AMC dukes is based on finally produced a strong block, VIN codes E and A, found in Chevy S-10 pickups and Astro vans, 1985-1991. It has however, the 60 degree bell housing bolt pattern. The main webs have lots of meat and a nearly equal to the 4 bolt Super Duty block.
The crank is likewise light, weak, and insufficiently counterweighted. The wet noodle-like shaft always flexes but as rpms approach 5000 the flexing is so severe that the rods cannot resist the bending forces and snap. A vibration dampener is purposely designed to counteract (not eliminate) this flexing. The duke was designed without one. The S-10 engine has an SD4 crank, very strong with much added counterweighting.
Finding an S-10 engine and rebuilding the short block for my Spirit promises a durable unit of 150 hp or so but still leaves me short of a good flowing head. Pontiac had to redesign virtually every part of the duke to meet racing demands. They called the result the "Super Duty 4" 2 aluminum cylinder heads are available and 1 of cast iron. Pn 10049801 has intake ports moved closer together and raised, will accept 2.08 intake valves, and exhaust port moved and raised. However, stock intake and exhaust manifolds will not fit, custom fabrication required; expensive too! Pn 10045437 is also aluminum but less radical and will accept stock manifolds. "The cast iron performance head is designed to be used on the 151 or 153 engines pn 347056". Msrp for that casting is $427.14. My 1st direct question to post readers is: has anyone ever seen or used this head on their duke? I can find no reference to it on any forum or site except various GM parts books. I'd hate to rebuild an S-10 short block for my Spirit only to find that the head isn't the performance improvement GM claims it to be.
My other engine option for replacing the VIN code "1" engine in my Spirit is to go with the Mercruiser 3 liter 140 hp item. Forum member Captspillane has done this conversion and his post is informative, but ignores some important issues (for me) like the flywheel. I have a 3.0 available to me for cheap. I don't really need or want another 30 cubes and head flow is questionable. I found that the 3.0 head is a shortened 230 inch Chevy 6 head vintage 1962-1970. A good flowing head with an ancient design might be a hope and a dream.
OK readers, have at it, help me out.