The stock-length pushrods we thought would work fell far short, and we were puzzling until we did what some car crafters don't do-we read the directions. That's when we discovered these AFR heads come with 0.250-inch-longer intake valves and 0.100-inch-longer exhaust valves. This dictated custom-length pushrods to make the rocker-arm geometry work with the longer valves.
We used a pair of Comp Cams adjustable pushrods to determine the proper lengths. We started by adding 0.250 inch to the stock length of the intake pushrod and 0.100 inch to the length of the exhaust pushrod. Remember that Rat motors use different lengths for the intake and exhaust pushrods.
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.
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.
JGM resized our stock 31/48-inch rod bolt connecting rods to bring them into shape and the
The stock rods already had ARP rod bolts, which we reused, but we were careful to properly
The AFR heads we chose are the smallest rectangle-port heads AFR sells at 305 cc, which we
Rather than be conservative, we decided to step up to a more aggressive Comp Cams mechanic