By experimenting with timing finalized at 34 degrees and jetting with two sizes out of the primary and secondary, our fat Rat leaned out to the tune of 685 hp. After a short skull session, we decided on a tighter lash on the intake lobes to increase the effective duration. This kicked the power up to the curve you see in Test 1 at 701 hp at 6,400 and 603 lb-ft of torque at 5,400.
Brul then suggested a simple little plastic 1-inch spacer under the carburetor because we were seeing as much as 2 inches of manifold vacuum at peak power, which indicated (as we expected) that the carb was a little too small for this power level. The spacer took the power to an amazing 707 hp at 6,400 rpm. Finally, we even tried a 14-inch-diameter K&N air cleaner that pushed the peak horsepower up to 708. We liked the 707 number so well, we decided to stick with that. Not bad for a cast-crank, pump-gas 496
Dyno Flog
Test 1 was run with two steps leaner jets than stock in the Demon 850 carburetor with timing set at 34 degrees total. We also tightened the lash on the intake valves from 0.016 inch to 0.012 inch to increase the intake duration.
Test 2 used all the conditions of Test 1 along with a simple 1-inch-tall, plastic, open-plenum spacer under the carburetor to improve the top-end power. In a final test (not shown here), we added a 14-inch, K&N, 3-inch-tall, air-cleaner assembly and peak power climbed to 708 hp. Also note the brake-specific fuel consumption (BSFC) numbers that dip into the 0.390s below peak torque. These are excellent numbers indicating very efficient use of the fuel. BSFC is expressed as pounds of fuel used per horsepower-hour (lb/hp-hr). Generally, any number under 0.500 is considered good; sub-0.400 is excellent.
| RPM | TEST 1 | TEST 2 | |
| | TQ | HP | TQ | HP | BSFC |
| 3,000 | 514 | 294 | 518 | 296 | 0.463 |
| 3,200 | 528 | 322 | 527 | 321 | 0.442 |
| 3,400 | 544 | 352 | 549 | 355 | 0.422 |
| 3,600 | 547 | 375 | 552 | 378 | 0.414 |
| 3,800 | 545 | 395 | 550 | 398 | 0.412 |
| 4,000 | 552 | 421 | 554 | 422 | 0.407 |
| 4,200 | 564 | 451 | 562 | 450 | 0.395 |
| 4,400 | 576 | 482 | 574 | 481 | 0.387 |
| 4,600 | 586 | 513 | 585 | 512 | 0.389 |
| 4,800 | 590 | 539 | 591 | 540 | 0.393 |
| 5,000 | 593 | 565 | 596 | 567 | 0.403 |
| 5,200 | 600 | 594 | 603 | 597 | 0.409 |
| 5,400 | 603 | 621 | 603 | 620 | 0.419 |
| 5,600 | 601 | 640 | 602 | 642 | 0.422 |
| 5,800 | 598 | 660 | 601 | 664 | 0.415 |
| 6,000 | 595 | 680 | 599 | 684 | 0.418 |
| 6,200 | 588 | 695 | 593 | 700 | 0.432 |
| 6,400 | 575 | 701 | 580 | 707 | 0.449 |
| 6,600 | 557 | 700 | 561 | 705 | 0.446 |
| Avg. | 572 | 527 | 575 | 529 | 0.417 |
| Power/ci | 1.21 | 1.41 | 1.21 | 1.42 |
To ensure we had a solid induction system that could feed the fat Rat, we opted for an Edelbrock Victor Jr. single-plane intake and a Barry Grant 850-cfm Mighty Demon carb. Everything we've read claims that even a mild street motor like this could benefit from a larger-cfm carburetor, but our budget demanded a smaller carburetor.
Bolted to the Westech SuperFlow 901 dyno, the 496 delivered some impressive numbers, but only after we ran through the requisite timing, jetting, and valve-lash tuning exercises. On pump gas, our motor was happiest at 34 degrees of total lead and made 701 hp with a lash change.
Lash caps fit between the valve-stem tip and the rocker arm and were originally designed for titanium valves to increase their durability. Using lash caps requires special valve locks that create a locating groove for the lash cap. Our AFR heads came with lash-cap-ready valve locks.
Tightening the lash from 0.016 inch to 0.013 inch increased the lobe duration at 0.050 inch roughly 3 degrees. This cost a little power below peak torque but bumped peak horsepower by 5 and got us over the 700hp threshold.