1970 Hemi Challenger 250 Chassis Dyno Test - Heavy Breathing Hemi Part II

Ed. Note: Last month, with the help of Dave Dudek of Shelby Township, Michigan, we highlighted the buildup of a 484ci Hemi that looks like it just rolled off the assembly line in 1970. Keep in mind the old adage, "beauty is only skin deep." Underneath its stock appearance lurks enough power to make any motorhead happy. With the engine installed in his '70 Challenger, Dave spent some time behind the wheel making sure the big lung was ready to go racing. For our dyno test, we used a Mustang 250 chassis dyno at Performance Automotive in Clinton Township, Michigan. A chassis dyno was chosen because the car can be tuned for the type of driving it would do with a complete exhaust system and everything else installed.

Spinnin' The Big Wheel
We showed up at the dyno a tad early and started our day with a Katech whistler. "What's that?" you ask. It's a tool that measures the exact compression of any engine, unaffected by cam overlap and other variables. People racing in circles at dirt tracks may know about this machine because there are many classes with compression-ratio limitations. If you want to know the exact compression ratio, perhaps to pick out a better cam, this is but one of the many services Performance Automotive offers. On a normal engine, this is a five-minute task, but on the mighty Hemi, we had to pull a valve cover because the "thing-a-ma-jig" that screws into the spark-plug hole wouldn't reach. With the whistler finally installed, we turned the engine over by hand and found a 12.8:1 compression.

With the Hemi buttoned up, we backed it onto the dyno and installed the wide-band oxygen sensor to keep track of the air/fuel mixture. Once everything was hooked up, we warmed up the big Hemi to operating temperature and dropped the hammer. While we watched the air/fuel monitor on the computer as the Hemi revved through the rpm range, everything looked great-from 4,000-6,500 rpm, the air/fuel held between 12.8 and 13.0. With that, we realized no tuning was required. Not normally so lucky, we knew we'd better make a backup pull.

After the second pull, we found the results mirrored the first pass. Since the dyno indicated that everything was dead-on, we called it a day and made plans for drag testing. It was an easy day at the dyno, and, more importantly, we posted baseline horsepower and torque numbers to use in the future. So, if you followed the buildup last month, consider yourself "dyno-tuned."

Carb Jetting
Front Carb
Driver-side front-.096
Driver-side rear-.078
Passenger-side front-.094
Passenger-side rear-.078

Rear Carb
Driver-side front-.080
Driver-side rear-.073
Passenger-side front-.094
Passenger-side rear-.077

Notes:
All four metering rods measure .064 and .059.

Fuel jets and rods were measured with calipers.

Track Day
With the dyno time behind us, we were off to the track. Mother Nature blessed us with clear skies and a high temperature of about 59 degrees-the weather was definitely working in our favor. We unloaded the car, got the engine up to operating temperature, and set the valve lash at .010 inch.

The game plan called for a shakedown pass to get a baseline. After a lengthy burnout to get the Goodyear Polyglas GTs ready, the car was staged, and after the lights dropped, a gentle launch led to firm shifts of the four-speed. The first pass, with a .699 reaction time, netted a 60-foot time of 2.09 and an 11.80 at 122.9 mph-far better than we expected. Still smiling from the results of the first pass, we immediately staged the Hemi again.

This time, the launching and shifting were a little more than firm, and the second pass had a .811 reaction time and a 60-foot time of 1.94. The e.t. was an 11.63 at 124.8 mph. We were happy with the way the car launched, but the shift to Second gear was a little unsteady. All shifts were executed at 6,200 rpm.

For a third time-without shutting the Hemi down-it was pulled back around, and the reaction time of .581 and a 60-foot time of 1.96 netted us an 11.61 at 124.6 mph. It was almost a carbon copy of the run before, but this time, we headed for the pits to let the Hemi cool down a bit.

The rear tire pressure, which we should have checked before the first pass, showed 24 pounds on the passenger side and 26 pounds on the driver side. In previous testing, 34 pounds was the magic number in the Challenger, so that's what we used.

A check of the ignition timing showed we were running 34 degrees total advance, and we left it there for the time being. To see how the Hemi would react, we loosened the valve lash to .016, at operating temp, and after a 40-minute cool-down, we hit the track again.

This time, we were rewarded with an 11.45 at 124.92-the car's best ever quarter-mile pass. It had a 1.89 60-foot time and minimal tire spin, which, for the most part, occurred in Second gear. The car seemed to like the changes we made, and a quick, hot lap back to the staging lanes rewarded us with a 60-foot time of 1.90 and a final e.t. of 11.41 at 124.89 mph.

During this second hot lap, the Hemi was shifted at 6,300 rpm. The next 18 or so passes involved three more valve-lash adjustments. We started by setting the lash at .012 intake and .020 exhaust, and then .020 intake and .012 exhaust, followed by .020 on both intake and exhaust. All e.t.'s, except for a few passes where we lost traction, were in the 11.40-11.50-e.t. range. During this time, our 60-foots ranged from 2.04-1.90. It appeared the lash setting did not affect the engine. Maybe the dyno would have been a better place to test valve lash.

On what was starting to become a long day, we made one final lash adjustment to .012 intake and .016 exhaust. We contrived this setting based on the data we had, thinking it would be a good place to settle. We cranked up the timing to 36 degrees and took a late lunch. After the longest cool-down of the day-almost an hour-it was getting late, and we only had time for a couple more passes.

The temperature had dropped closer to 50 degrees when we staged the Hemi. Launching the car with a .601 reaction time put out a 60-foot time at 1.87. The car launched hard, and Second gear had minimal tire spin. At the 330-foot mark, the Hemi posted an impressive 5.000 e.t. Everything came together, and the Challenger posted what we believe is the fastest e.t. ever from a Stock Appearing class car. When the lights lit up at the big end, the Challenger posted an 11.37 e.t. at an astounding 125.88 mph. All testing was done as the car raced, including the use of repop Polyglas tires.

Just imagine what this Hemi would be capable of with a free-flowing exhaust and aftermarket intake and carb! But that's the fun of racing in the Stock Appearing class.