Big-Block Chevy Engine Build - Dyno

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.
To ensure we had a solid induction system that could feed the fat Rat, we opted for an Ede
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.
Bolted to the Westech SuperFlow 901 dyno, the 496 delivered some impressive numbers, but o
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.
Lash caps fit between the valve-stem tip and the rocker arm and were originally designed f

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.
Tightening the lash from 0.016 inch to 0.013 inch increased the lobe duration at 0.050 inc