The graph reveals that the 30-degree back cut (test 2) offers the best flow curve under 0.500-inch valve lift. Changes that improve the mid-lift flow numbers often result in excellent torque gains. The graph reveals that the 30-degree back cut (test 2) offers the best flow curve under 0.500-inch valve lift. Changes that improve the mid-lift flow numbers often result in excellent torque gains. Cam Specs Comp Cams SX290S Advertised Duration Duration at 0.050 Lift in inches (net lift minus lash) Lobe Separation Angle Intake 290 252 0.598 (0.582) 110 Exhaust 298 260 0.598 (0.582) --- Intake Flow Valve Lift Test 1 Test 2 Test 3 Test 4 Test 5 0.100 68 70 (+2) 69 (+1) 71 (+3) 69 (+1) 0.200 129 144 (+15) 145 (+16) 138 (+9) 140 (+11) 0.300 193 205 (+12) 202 (+9) 195 (+2) 203 (+10) 0.400 240 251 (+11) 247 (+7) 245 (+5) 249 (+9) 0.500 264 277 (+13) 272 (+8) 270 (+6) 276 (+12) 0.600 274 275 (+1) 281 (+7) 280 (+6) 281 (+7) 0.700 271 266 (-5) 266 (-5) 269 (-2) 265 (-6) Avg. 205.6 212.5 211.7 209.9 211.8 We used the same Comp springs but changed to Comp’s Ultra Tech rockers with the stock 1.7:1 ratio. We set the lash at 0.016 inch for both the intake and exhaust. We used the same Comp springs but changed to Comp’s Ultra Tech rockers with the stock 1.7: Flow Test Results Test 1: Stock 049 iron heads with 2.18-inch intake—on "good” port Test 2: Mild pocket blend, new Manley valves, five-angle valve job, 30-degree back cut Test 3: Same as above with 32-degree back cut Test 4: Same as test 2 with 35-degree back cut Test 5: Same as test 2 with 28-degree back cut We used a different carburetor for this series of tests, adding a Pro Systems 950-cfm, 4150-style carburetor. We literally set the idle mixture and speed and never touched it again. The jetting and fuel curve were nearly spot on. We used a different carburetor for this series of tests, adding a Pro Systems 950-cfm, 415 Power Testing Dyno testing is a lot like bracket night. You only have one shot to get it right. With the previous dyno test performed with JMS as our baseline, the pressure was on to see if all our head work would pay off. Yes, we used a different intake manifold—the Summit single plane—and a larger 950-cfm Pro Systems Holley E85 carburetor (the previous carb was a 750 cfm from Quick Fuel), so this isn't a true evaluation of the head work alone. Despite the differences, we forged ahead. We didn't need to tune the Pro Systems carburetor at all, and after a quick timing session, we learned that 37 degrees was what the Rat liked best. This resulted in a peak horsepower gain of nearly 40 hp. Torque also improved at the peak by 12 lb-ft—from 538 to 550 lb-ft. But the real story is the torque gains down low. Looking at the power numbers, our larger carburetor might have helped with a better fuel curve at the lower speeds and certainly our improved flow numbers helped, but we gained more than 100 lb-ft at 3,900 and 52 lb-ft at 4,100 rpm! This is all fantastic news and reveals what can be done with only simple head work, a good carburetor, and good tuning. Of course, there's always more to be done. The iron-head flow numbers indicate that more valve lift would be helpful. Using what we learned on the flow bench, with a 0.700 inch lift (or more) mechanical roller cam, we'd change the back cut on the valve from the 30-degree to the 28-degree angle in search of a more stable flow curve through the higher valve-lift portion of the curve. We probably wouldn't change the duration, so with an Extreme Energy mechanical roller, we could expect to push the lift up to 0.660 on both the intake and exhaust. This change would also demand better valvesprings and roller lifters, which will add roughly $600 to the cost of the engine, but this might be worth another 20 hp and push BC up into the 600hp area. But let's not skip over the fact that we simulated this Rat using Quarter, Pro in a 3,500-pound Chevelle with 3.55 gears and a 3,000-rpm-stall speed converter, and the simulation reported 11-teens at 121 mph. Not bad for an iron-head big-block that looks like a 396. Blue collar, indeed! « | 1 | 2 | 3 | » | View Full Article By Jeff Smith Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!