No matter if you are racing a four-cylinder in a Mini Stock class or a no-holds-barred, alcohol-burning V-8 Super Late Model, all stock car racing engines work on the same principles. And one of the most critical is that it's up to your carburetor to provide the proper mixture of air and atomized fuel to maximize power.
Automotive Specialists’ Jeff...
Automotive Specialists’ Jeff Dorton makes a carburetor tuning change to this Pro Cup car during a practice session while a crewmember also works on the suspension.
When your engine builder handed over your race engine, it was optimized to the absolute best performance under a given set of conditions. Of course, those conditions are always what is found in the dyno room that day. But at the racetrack there's no way to control temperature, humidity, altitude, barometric pressure, and a host of other things. If those conditions don't match those on the day your engine was tuned on the dyno, you may be down on power.
This is why being able to properly tune your race engine at the track is so critical. NASCAR Sprint Cup teams keep a person on staff to handle the tuning duties, but most of us simply can't afford that luxury. Dedicated engine builders will often spend their weekends visiting different racetracks to support their customers, but what happens if your builder isn't at the track that day? Then it's up to you to make sure your engine is at least close to peak power.
To find out more, we spent a Saturday shadowing Keith and Jeff Dorton of race engine builder Automotive Specialists at a CARS Pro Cup race. As the owner of Automotive Specialists, Keith Dorton has built the engines powering multiple champions in the Pro Cup Series, and he regularly follows the series to support his customers at the racetrack. The track-tuning practices he's developed are certainly proven to work, and he was willing to share them with us. And as long as you're racing a carbureted engine, the same principles should work for you.
No matter if you call it practice or hot laps, before you hit the track for the first time there are a few things that should always be done. Some can be taken care of before you leave your shop if you prefer, but a few should always be done just before you leave the pits.
"The first thing to be done is just to make a visual inspection of the car," Dorton says. "Make sure nothing looks wrong. Check the belts to make sure the tension is good and there isn't excessive wear. You should also look over the plug wires to make sure they haven't been touching the headers and have spots that are melted. Plug wires can take a lot of heat, but if you see spots where the insulation is melted or turned black from touching the headers, you should probably go ahead and consider changing them."
Next, fire the engine up and check the timing. Check both the timing at idle speed and at race rpm. If you're running a mechanical advance, make sure the mechanism advances the timing consistently when you rev the motor. If you have the advance locked out, make sure the timing doesn't change as you rev the motor.
Keith Dorton, owner of Automotive Specialists, reminds racers not to get too caught up in spending practice time chasing small carburetor changes. As long as the engine isn’t down on power, the best way to whittle down your lap times is by perfecting your suspension setup.
During practice, Dorton says he generally tries to stay out of the way. And he cautions car owners not to waste too much of their practice time experimenting with different carburetor settings. Instead, concentrate on the areas that have the potential to give you the biggest speed gains. "You're going to get a lot more out of a sway bar change than you will a jet change," he explains.
High-g turns can sometimes...
High-g turns can sometimes cause the fuel in the float bowls to slosh away from the secondaries. You can use extensions, like you see here, to move the pickups toward the rear of the bowl.
In other words, chassis adjustments can make a major difference in lap times when you're trying to find the optimum setup during practice, while an engine-tuning change--unless something is really wrong--generally won't result in a major difference in lap times. The biggest reason to make something like a carb tuning change during practice is if the carburetor isn't functioning properly, or there's a driveability issue. Since practice time is valuable, concentrate on getting the best-handling race car possible and don't allow the little things to distract your focus.
Of course, if your race car is significantly off the lap times the leaders are producing, you're going to have to figure out the source of the trouble. Is it a chassis problem, or is the engine down on power? That's a debate between engine builders and chassis specialists that's raged for decades, and it likely won't be going away any time soon.
You should always keep in...
You should always keep in good communication with your driver because he or she is the only one in the driver seat. Make sure to have your driver monitor the water and oil temps and pressures during practice. He or she should also provide feedback on how the engine feels, which not only includes a seat-of-the-pants perspective, but also what is heard as well as if there are any unusual vibrations.
One of the best ways to determine if your car's slower times are a result of a problem with the engine or chassis is with the stopwatch. Start comparing times between your driver and the fastest cars on the track. But don't time complete laps. Instead, start clocking segments. First, compare your driver's times to the fastest cars just through Turns 1 and 2. Then do the same thing through Turns 3 and 4. Finally, compare your driver to the leaders through the straights. Pick a point to begin timing where most cars have straightened out and the drivers are just getting to full throttle, and stop at the point where most drivers are just getting out of the throttle.
If the biggest difference between your driver and the fastest cars' times is in the turns, then you should begin looking for a chassis problem. After all, every engine produces the same amount of power when the carburetor's throttle blades are closed. There's probably some problem hindering your driver from getting through the turns cleanly. But if the biggest loss of time came on the straights, then you may have an engine that's down on power. Just make sure to always look at the big picture. If handling is so poor on turn exit that your driver is picking up the throttle later than the competition, then his straightaway speeds are obviously going to be lower too.