By David Reher, Reher-Morrison Racing Engines
“Some racers wear out the threads on their carburetors trying to adjust for weather conditions.”
As I write these words, we’re loading our Speedco Pontiac Grand Am into the transporter for the annual trek to Denver. Preparing to race at a mile above sea level brings home the reality of racing under adverse conditions. While we make many adjustments for Bandimere Speedway, one of the items that is definitely not on our to-do list is to change carburetor jets.
Some racers just about wear out the threads on their carburetors trying to adjust for altitude and weather conditions. The truth is that you really can’t compensate for bad air by changing jets. When it comes to carburetor-equipped racing engines, you can’t fight Mother Nature.
It is a misconception that you must lean out a carburetor at high altitude. The fact is that a properly tuned engine will use the same jets in Denver as it does at sea level.
So why do cars run so much slower in bad air than they do in good air? The obvious answer is that the engine is making less power. When an engine is tested on a dyno, a correction factor is applied to the raw numbers to adjust the observed power to standard conditions. This allows us to compare the dyno test results that are made at different times of the year and under very different conditions. But when you are running a car down a race track, the correction factor is irrelevant. The only power that is available to accelerate the car is the engine’s actual output at that particular moment in time. If the engine is producing more or less power than it would at standard conditions, that’s what you’ve got to work with.
In this age of digital everything, carburetors have an undeserved reputation as low-tech devices. In fact, a racing carburetor is a very ingenious system. A carburetor responds to differential pressure, and therefore it self-compensates for changes in barometric pressure. The gas in the float bowl is always subject to the prevailing atmospheric pressure; the jets deliver fuel in proportion to the differential between the pressure in the float bowl and the pressure in the induction system. So when the barometric pressure falls, as it does so dramatically in Denver, there is less pressure differential and therefore fuel flow is reduced accordingly.
You don’t have to go to Bandimere to experience the effects of thin air. Even if you don’t travel, the changes in your race car’s performance at your local track from February to August will be substantial. On a typical summer day with 90-degree heat, the relative altitude can easily approach 4,000 feet. The unfortunate fact is that there is very little you can do to regain the missing horsepower by tuning the engine. While you might have a zero correction factor in January, it’s common to see a correction factor approaching eight percent in the summertime – and in Denver, we see 22 percent!
The harsh truth is that you’ve got a car with less horsepower in the summer, so you must figure out how to race it. What can you work on? You can work on the car – the torque convertor or clutch, the transmission ratios, the rearend gears, the tires, and the chassis – to work around the power deficiency. That’s really what we do in Pro Stock, and that’s why you seldom see Pro racers working on engines at the track aside from routine maintenance. We simply take the power we’ve got and try to make our cars use it as efficiently as possible.
In general, drag racers tend to be more engine-oriented than racers in other forms of motorsports. Perhaps that is because we spend relatively little time on the track compared to oval-track and road racing drivers. In my infrequent visits to NASCAR events, I find that most teams regard the engine as a small variable at the track because their engines are developed and tested at the shop. They spend the majority of their track time adjusting the chassis and working on suspension setups. I’ve seen competent teams qualify their Busch Series cars faster than a Nextel Cup car – despite the fact that the Busch Series engines have about 110 horsepower less than the Cup engines!
That simply shows how important the chassis setup is in circle track racing – and points out that drag racers could benefit from spending more time on chassis adjustments and less time on carburetor jets when the weather and altitude conditions are bad. The best place to work on an engine is on a dyno; the best place to work on a car is at a race track. As a professional engine builder, that’s a difficult statement for me to make, but I think it’s the truth.
In reality, the first 1/8th mile pretty well determines a drag race car’s elapsed time. If you’re competing in “go-fast” class such as a Quick 32 or Top Sportsman eliminator, the setup that made you fast in February isn’t going to make you a winner in August. When the relative altitude has changed 3,000 or 4,000 feet, you’re not going to be successful using the same convertor and the same gear ratios that you used in winter.
My recommendation for running in bad air is to target the engine’s rpm range and then make the necessary changes that will allow the engine to achieve that range. A racing engine’s peak power and torque is fixed by the intake manifold’s runner length, the airflow capacity of the ports, the camshaft timing and other factors. The engine is going to perform at its best when it runs at the speed that it was intended to run. Therefore if you go to a high-altitude track, or if you encounter high relative altitude conditions, you have to gear the car to allow the engine to reach its optimum rpm. When you’re missing 300 rpm at the top end due to a change in weather, you must work on regaining the engine speed to maximize performance. Maybe it’s a set of shorter rear tires (if the available traction permits), or perhaps a numerically higher rearend ratio. You still won’t run as fast as you did under ideal conditions, but you will close the gap.
Most of us race to go fast. We love to see a good number on the scoreboard. When the conditions are bad, the e.t.s aren’t as satisfying to our egos, but we can use those times to work on the race car and to learn how it responds. Challenge yourself to regain as much performance as the conditions will allow. The payoff will come when the “good air” returns because your race car will be faster and you will be smarter. Gaining an understanding of how a car responds to different conditions will make you a more formidable racer.