Choosing The Budget Iron Eagle HeadsSo why did we chose to run a pair of Dart's Iron Eagles? The reason was twofold. First, we already had one, which we did a basic porting exercise on and saw good results that came pretty easy. Just a good clean up and bowl blend resulted in some healthy flow numbers, as is evident from the graph. Secondly, the heads are a good value for the money and they avoid a ton of hassles trying to recondition a set of 25-year-old stock castings, which frankly are not even in the same ballpark when it comes to quality. The castings we used were the small-port ones (nominally 308 cc). These are normally equipped with 2.250/1.88-inch valves, but we opted to step up the intake valve to 2.3 inches. On the flow bench, this resulted in a small but worthwhile increase in flow in the 0 to .500-inch lift range, although top-end flow remained virtually unchanged. Overall, our head porter, Nathan Bornitz, was able to coax between 8 and 22 cfm more out of the head's intake ports in the lift range from .100- to .700-inch lift. All this was achieved with only about a 5cc increase in port volume, and most of that in the bowl area. The result was an increase in flow and port velocity. On the exhaust side, flow was increased by 3 cfm at 100 thousandths lift and 23 cfm at .700-inch lift. Again, this was done with minimal increase in port volume.
After porting, the heads were milled at T&L by 70 thousandths, which dropped the combustion chamber volume from 121 cc to 107 cc. The last move was to assemble the heads using COMP Cams' new big-block Beehive springs. These delivered 150 lbs on the seat (410 lbs over the nose) with our street roller cam and 1.7:1 ratio rockers. For those of you not familiar with this high-tech spring, those numbers might look a little on the low side. But to understand how effective they can be, you need to bare in mind that a swap to these low-mass springs (a steel beehive retainer is so small it weighs less than a conventional titanium retainer) has more effect on increasing rpm than making a change to titanium valves. A ballpark figure is that with 10 percent less spring, the engine will turn about 12 percent more rpm and show better control all the way up the rpm range.
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 Putting our Standard Abrasives...  Putting our Standard Abrasives porting supplies to work on our Iron Eagles showed that even simple porting procedures pay off. End results were 380 cfm on the intake and 296 cfm on the exhaust with fat curves on the way up. |
 We re-cut the intake seats...  We re-cut the intake seats of our Dart Iron Eagle heads to accommodate larger 2.3-inch valves. The Dart's standard size on the small-port head is 2.25 inches. |