Additional photos by Fred Menditto
In last month’s issue of Mopar Muscle, we completed the finishing touches on our ’10 Dodge Challenger bracket car and took the car to the dragstrip for some initial testing. While we were happy with the results, these preliminary test passes were primarily to make sure the car was safe, and went straight down the track. We felt there was a little more left in the Challenger, so we took the car back to the track to perform some tuning to make sure the car is not just safe, but also running the best it can.
If you’ve followed this project from its inception, you’re likely wondering why we would need to do any additional tuning since we already had the engine on the dyno at Indy Cylinder Head. The fact is, dyno tuning of the engine is only a part of the package. An engine dyno cell is a controlled environment, and can be very useful when tuning an engine for optimum power in the dyno cell. Conditions at the track, with the engine in the car, offer many more variables, and the engine should be tuned again at the track under racing conditions.
02 After a couple of burnouts and 60-foot passes, we were happy with the way the car perf
03 For our second test session, we took the car to Bradenton Motorsports Park to make adj
04 Our first change was to start dropping tire pressure from the 9 psi we had run so far.
In the dyno cell, an engine can be run under controlled conditions, with a consistent air supply throughout the rpm range where the engine operates. Mounted in the engine bay during a dragstrip pass, the engine operates in a narrower rpm band, and rpm drops and then increases again after every shift. Additionally, as the car accelerates, air from the hoodscoop feeds the carburetor at an ever increasing speed, providing fresh air that not only enters the engine, but travels past and around the engine and the carburetor’s air bleeds. Coolant temperature also changes with each pass, and as a car wins rounds (as we plan for this one to do) we may not have the luxury of completely cooling the engine between runs. For all of these reasons, final engine tuning should be accomplished at the track, where the car will run the most.
Another aspect of vehicle tuning that cannot be accomplished on a dyno is chassis tuning. We had the crew at Inline Performance Specialists weigh our car on four-wheel scales to set corner weight, and the car’s “instant center,” by adjusting the four-link bars. And while these settings in addition to the initial shock absorber settings will get the car close, final adjustments need to be made at the track. Tire pressures also need to be experimented with at the track, since each car and each combination is different. There is no “magic” tire pressure for a given combination, but trial and error will teach us a tire pressure range that this car likes. Once we figure out the proper tire pressure range, we can adjust the pressure within that range for conditions at any given track, and varying track temperatures.
Change one thing at a time, record the results, and develop a trend you can follow.
06 We made our jet changes in single-size increments, and our Hemi seemed to like more fu
07 Ignition timing and jetting go hand-in-hand, and adding fuel sometimes won’t make a di
08 Our final step of tuning our Challenger was to try different rpm settings leaving the
Our initial test of this vehicle was accomplished at Auto-Plus Raceway in Gainesville, Florida, but we decided to take our car to Bradenton Motorsports Park for our second session. We’ve raced and tuned cars at each of these facilities before, and have found both to have good track preparation and consistent timing systems. Bradenton is also closer to our Tampa, Florida, office, so testing on a Thursday night doesn’t require a long drive and overnight stay like Gainesville does. Wherever you track test your car, the procedure is the same: Change one thing at a time, record the results, and develop a trend you can follow. When tuning an engine, make one change, usually either to the air/fuel ratio through a jet or air bleed change, or to the ignition timing curve, and make a pass. The numbers on the time slip will quickly tell you if the change made a positive or negative difference. The elapsed time is a good indicator of if the chassis is working better or worse, and generally the mph the car runs will tell you if you’re making more or less power.
Most modern Hemi engines are fuel injected, but ours runs a carburetor, so tuning is accomplished with good ol’ fashioned jets, fuel pressure, and air-bleed changes. Since we already optimized our engine on the dyno, only minor adjustments should be necessary in terms of air/fuel mixture. Since there is no distributor (and no distributor provision) on the third generation Chrysler Hemi, we installed MSD’s Hemi 6 controller and coils, with Indy Cylinder Head’s coil relocation kit on our engine. This ignition controller really makes tuning easy, as ignition timing, rev limits, and even retard settings can be set easily using a laptop computer with the provided MSD software.
During our first test session, we were happy that the car went straight down the track, the transmission and converter performed properly, and the engine ran smoothly. We tested the brakes and steering on short passes, and then once we were confident the car was safe and there were no leaks or malfunctions, we made a couple of full passes resulting in elapsed times of ten seconds flat. Since our first test session was successful, we could concentrate more on optimizing the combination during our second test session. At Bradenton, we experimented with rear tire pressure, carburetor jetting, ignition timing, and both launch and shift rpm. Each pass, we recorded our results, and if a change resulted in a negative result, we’d go back to our baseline and try a change in the opposite direction.
When finished, it was apparent that we had our combination pretty close when we got to the track. After lowering rear tire pressure by some 3 psi, and experimenting with timing and carb jetting, we improved our car’s elapsed times eventually running several passes in the 9.8-second range. We’re happy with our results, and look forward to watching driver Rick Menditto compete with this Challenger. It was a lot of work, but our project car is ready to go some rounds. Be sure to look for this car at different tracks and events, and stop by and say hello, check the car out, and let us know what you think of our late-model Mopar race car.
...once we were confident the car was sage and there were no leaks of malfunctions, we made a couple of full passes resulting in elapsed times of 10 seconds flat.