JGM resized our stock 31/48-inch rod bolt connecting rods to bring them into shape and the
The technique is very simple: Establish a pushrod length that will place the roller tip of the rocker arm on the inboard third of the valve tip. This can be done by marking the top of a valve with a felt-tip pen and turning the engine so the lifter for that valve is on the base circle of the lobe. Install the rocker arm over the adjustable pushrod and wiggle the rocker to make a witness mark on the valve tip. Adjust the pushrod length until the witness mark is on the inboard third of the valve tip. This allows the rocker tip room to travel across the top of the valve tip without running off the edge. A too-long pushrod will place the pushrod too far outboard at the beginning of its travel. A too-short pushrod runs the risk of running off the inboard edge of the valve tip. We ended up with intake pushrods that were 8.500 inches long and exhaust pushrods that were a long 9.400.
Next, we discovered the proper intake pushrod length placed the inside radius of the rocker arm very close to the retainer. If we lengthened the pushrod, it only made the placement of the rocker tip on the valve-stem tip less desirable by moving it outboard, which we didn't want. The solution came from Westech's Steve Brul, who suggested running valve-lash caps on the intakes. This increased our clearance between the rocker arm and the retainer and also increased the contact area between the rocker-arm roller tip and the valve stem.
Next, we carefully assembled the Rat with new Federal-Mogul main and rod bearings. We need
On The Dyno
After we had carefully assembled the Rat and double-checked all our clearances, we loaded up the rotund rodent and bolted it up to Westech's SuperFlow dyno. To finalize the combination, dyno guru Steve Brul suggested a set of 211/44-inch Hedman dyno headers that connected to a pair of massive 4-inch exhaust pipes and a pair of Hedman 4-inch mufflers. To tell the whole story, Westech uses an electric water pump just to make testing simpler. But this means there are a couple of free horsepower here by not driving a mechanical water pump.
As for ignition, our budget was so bent, stapled, and mutilated there was only enough money left for a basic, used HEI and PerTronix 8mm budget plug wires, so that's what we went with, along with a set of cold-heat-range Bosch plugs to light the fire. As you can see from the test, this was more than enough to get the job done.
Brul always makes the very first pull on the engine after the oil and water temps are up to normal, and then it's a quick yank at a mid-rpm point to see if the timing at 36 degrees and the stock jetting are where they should be. The first pull showed us 599 hp at 5,500 rpm. That's when we knew this was going to be a good day.
The stock rods already had ARP rod bolts, which we reused, but we were careful to properly
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
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.