As some of you know, I entered a 303Y block in the EMC Vintage Class, and finished 4th, a 375Y and finished 2nd. The rebuild of the 375Y after the EMC netted 613.3hp and 546 lb/ft torque. The 303Y after the EMC had a cam change and intake change and netted 466.4 hp and 399 lb/ft torque. The rules did not allow for tubes in the push rod holes, so I was limited in the head flow for the Challenge. I had a pair of iron heads that I wanted to use that flowed ~25cfm more than the heads entered in the challenge. Now I have installed those heads and spent a day on the dyno hoping to best my previous best with iron heads. There was a problem almost immediately, and the engine nose dived above 5800 rpm. It had been able to rev to 7200 rpm smoothly before and making peak power at 6700 rpm, still carrying 461.2hp at 7000 rpm. The issue with this new dyno session, what is the problem? The better heads were up 42.1 tq and 32 hp at 4000 rpm, up 32.4 tq and 30.9 hp at 5000, and 6100 were dead even in both torque and hp. After the nosedive it lost 52hp. The only real differences were the beehive valve springs from CC and using 11/32" valves. Same carb, same headers, same ignition, etc.. If it had been able to keep pulling to the 71-7200 rpm, it should have been up at least the 32 hp it was up all below the curve. We were stumped. Now on the day 2. I had another Y from my '55 Tbird, which turned out to be a 312 short block. I offset ground the crank to 3.580" with 2" journals, 6.250" Oliver rods, 3.580" Diamond pistons with metric ring package, windage tray, Mummert aluminum heads, Isky camshaft with 242-250* on 112 with .592" lift. Should have been a really strong engine. At 4000 rpm, it was making 81.5 more tq than the iron headed 303Y, and 62 hp more. At 5000 it was 56.3 hp better and at 6100 they were dead even, and this engine nosedived the same as the 303Y. Both had CC beehive springs and seat pressure of 135# and ~310# over the nose pressures. This 333Y made 463 tq and 453 hp before crashing. It should have been able to rev cleanly to 7000 and make close to 500 hp. They both had Harland Sharp roller rocker arms, Smith Brother push rods, MSD electronics/wires, and pump gas at 10.4:1 static compression. DCR 7.92 iron heads/8.23 with aluminum heads. Ideas? Joe-JDC