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Tech—Why the G-Meter is the Most Important Sensor for Going Fast


By Jason Reiss

Photography by Steve Baur, Kevin DiOssi, and Josh Ledford

In the sport of drag racing, acceleration is king. There is simply no other measurable metric that is as critical to performance as acceleration rate; not torque, nor RPM gain, power-adder boost, nor even horsepower. Of course, measuring data points from each of those items is important, as they provide a window into the performance of the vehicle, but the data point that must be elevated above all else is the rate of forward motion. Because, after all, we’re trying to get from point A to point B more quickly than the next guy—and the way to do that is to be faster than he is.

That’s where the g-meter comes into play. The g-meter, or accelerometer, has one task—to measure acceleration, or how hard the car is pulling when the hammer is down. Accelerometers are used in many different applications; from cell phones to laptop computers to airbag deployment systems, but it’s the accelerometer as used in drag racing that has helped racers to set records and document performances that improve year after year—and sometimes from race to race.

“We’re trying to accelerate from point A to point B as quickly as possible. The higher you can get the g-meter to read, the faster you’re accelerating,” says tuner Josh Ledford of Pro Line Racing.

“We’re using it in all classes where it’s permitted. It’s the most valuable tool we have in our arsenal,” says tuner Jason Lee of PTP Racing.

There are two graphs in this screenshot, displaying false g from a wheelie. This particular graph is from a small-tire radial car. The one on the bottom is the ride height sensor, and the top graph is that of the g meter, with two runs shown—a good one, in red, and a bad one, in green. The ride height sensor on the green run displays a huge spike, then dips below the graph from the run depicted in red, what would be considered a good run with no wheelie. We can see that as the car comes down from the wheelstand and the ride height dips below that of what would be considered a normal run, that the green accelerometer in the top graph has a corresponding dip before it recovers and basically follows the good run in red. Now, these look like huge swings, but this could depict just a small wheelstand of a foot or two. But every millisecond counts in racing, and the top tuners like Ledford and Lee are analyzing the data down to the smallest bit of information.

What is a g-meter?

Simply put, the g-meter is an electromechanical device that can be manufactured in a standalone configuration, or as part of a broader approach to datalogging. The goal of the meter is to measure the g-forces applied to it, then output those readings to a data collection system of some sort.

The forces it measures may be static, like the continuous force of gravity, or dynamic, as in the case of an accelerometer used in a drag racing application.

The simple definition of acceleration is: the measurement of a change in velocity (speed divided by time). This is not to be confused with the speed an object is moving. Although the object—a car in this instance—may be moving quite quickly, it is not necessarily accelerating. For example, if the tires are spinning, a car may not be accelerating even though it is moving at a rate of 80 mph. If a car accelerates from 0-60 in six seconds, it will have an acceleration rate of 10 mph/second.

In a drag racing application, the vehicle is most likely not accelerating at a constant rate; as boost ramps in, or more nitrous is added, a vehicle will accelerate more quickly at the beginning of a run, and as speed is attained, the rate of acceleration will drop off.

Accelerometers can be preinstalled in an engine management system like a FuelTech, Haltech, BigStuff3, or any of the other major racing engine management systems on the market. It can also be offered as part of a standalone datalogging system like those offered by Racepak and others, or even as part of a broader vehicle management system like the David Technologies Profiler.

Most of the data shown in this article is from a Racepak, but this screen shows some of the capabilities of the FuelTech datalogger system. In Group 1, you can see the basics of the engine tuneup, and in the second group there are several more items, and so on. This particular car was using an FT500 and had a Racepak installed, so there is no accelerometer on this particular image.

The unit is calibrated to read zero g, and at a constant velocity would continue to measure 0 g. It is only when the meter is subjected to acceleration or deceleration forces that it will measure positive or negative acceleration.

There are several different types of accelerometers: mechanical, capacitive, piezoelectric, and even semiconductor accelerometers, but all have one thing in common. They measure the rate of change of the acceleration of a mass of some sort, and transfer that measurement into numbers a savvy racer or tuner can see on a screen.

The FuelTech FT600 engine management system packs a ton of features into a small controller, which includes a datalogger, among other items.

What Information Does It Provide?

Accelerometers measure g-forces along more than one axis: horizontal (acceleration), vertical (wheelie) and side-to-side (pitch/yaw), but the only axis that really matters is the horizontal axis, as that is measuring how quickly the car is moving forward.

According to Ledford, this means that there’s also something called “false g”; this can be understood as a wheelie or side-to-side pitch of the car if traction is iffy, and a savvy tuner needs to be able to discern which is which.

“For example, if you stand the Racepak up on its end, it will show 1 g due to the earth’s gravity. Instead of reading 2 g of acceleration, it’s going to read 3 g (rounding the numbers for simplicity’s sake), because there’s one g of acceleration which is straight up and down,” says Ledford.

“All of our cars have ride height sensors on them; if the front end is higher in the air, you’re going to get a false g reading. When the front end sets down you can see the g-meter drop, and you need to know how to determine the difference between the forward acceleration and the vertical acceleration rate.”

This state-of-the-art laser ride height sensor allows the tuner to measure the sensor’s distance from the ground, to determine whether the car is in a wheelstand, or the tire has too much or too little air, or how hard the tire is planted. For example, let’s say the sensor is mounted on the rearend housing 8.5-inches from the ground, but one second into the run, the sensor is measuring 6 inches from the ground. This shows the tuner the attitude of the car and gives them yet another weapon in the tuning arsenal to make accurate tuning changes. The use of these technology advances over the last decade or so has driven the amazing performance increases we’ve seen across all forms of drag racing where these items are permitted for use. 

As the ultimate goal of the racer is to accelerate as quickly as possible, knowing how and when and why the acceleration rate changes is critical to making tuning changes.

“Analyzing the data provided by the g-meter tells us whether the car is actually accelerating. In the case of a slick-tire car, the tire can be spinning or stuck to the track and the accelerometer is what tells us whether it’s going forward at the maximum acceleration rate. In a radial-tire car, it’s pretty evident when the tire is spinning or not. When I am naming files, I will always put into the filename if there is a wheelie, so that when we go back to look at data later, we know to discard that particular run when trying to compare to a non-wheelie run,” says Lee.

By decoding the information recorded by the accelerometer, and comparing it with the data from past runs, the tuner can make the determination as to whether there is more performance locked up in the car, or whether the engine can take more timing, or more boost, and where that can happen on the track.

“The g-meter gets rid of reaction time, it gets rid of deep staging, mile-per-hour clocks, it gets rid of all of that. We’re trying to make it to the finish line the quickest, and the g-meter is the elapsed time,” says Ledford.

This is a visual of the FuelTech fuel map in a grudge car. It shows the fuel and ignition tables, and if the engine were running, the section at the bottom would be displaying live data.

Which One To Use?

Each tuner may have a preference for the type of accelerometer they want their racers to use; for instance, Lee told us he prefers the system from Racepak. But most tuners will be perfectly comfortable using systems any of the reputable manufacturers, especially if a racer comes to them for tuning services after a car is already built, but they still have a preference and will likely direct you to their system of choice if you contact them before purchasing one.

Lee says the Racepak offers him everything he needs to work with, and likes the fact that the accelerometer—and datalogger itself—is separate from the rest of the engine management system. He explains that he prefers that the accelerometer only has that singular job to do, rather than being lumped in with the rest of the engine management software as it is with other systems.

Here is Racepak’s V300SD datalogger mounted in a vehicle. Although the perspective of the photo makes it look like it’s not sitting flat, rest assured that it is, the accelerometers inside are oriented properly for measuring g forces. Most chassis builders mount them in this position right next to the passenger door to make it easy to recover data after a run.

For instance, when a racer has completed a run, they are always instructed to pull the data from their engine management system up, while Lee pulls the Racepak data, so he can compare everything on two adjacent computers, and not have to flip back and forth between screens and applications. By having the logger’s information divorced from the engine management system, and being able to compare that data side-by-side with the EMS, rather than over top of it, he feels that isolating the data he’s looking for is more effective.

Ledford, on the other hand, typically uses the built-in logger that’s installed into the FuelTech system for nearly all of his customers. As he is one of the in-house tuners for Pro Line Racing—and PLR is the exclusive U.S. distributor of FuelTech’s products, he’s used to using the built-in accelerometer in FuelTech’s FT600 system for many of his upper-echelon customers. He also has extensive experience with many of the other systems on the market and uses Racepak’s accelerometer nearly as often as he does the unit built into the FuelTech system.

Ultimately, an accelerometer is an accelerometer, and which system it’s in doesn’t matter much, as the data it provides is identical.

How Analyzing Its Data Proves Beneficial

So if acceleration is king, as we mentioned in the first sentence of this article, how does the accelerometer help to improve it? In a number of ways, it turns out. That acceleration rate will show how a variety of changes affects the car’s performance on the track; how does a stator change, or a timing change, or a change in shift points improve or hinder the acceleration rate? The g-meter tells all.

This particular run is from a Pro Mod vehicle, and according to Ledford, has about the most comprehensive equipment available for datalogging. As we can see, there are nine separate columns of logging, each with six items stacked up. In this graph, the bottom green graph is driveshaft RPM, which jumps up at the hit and then settles down, rather than the straight line shown by the radial cars. The middle green line is the accelerometer, and the top red line is engine RPM. This particular car is one that has a laser ride height sensor on the rearend housing, denoted by Housing Height in the seventh column from the left.

“We tested one time with Tyler Crossnoe’s Ultra Street Mustang, and ended up moving the shift points around by 400-500 rpm or so to see where it maintained the g-meter. We found out that shifting it lower than max blower speed maintained the meter for a bunch of reasons. But we came out and were front-runners,” says Ledford. 

“Let’s say the engine builder says to shift the engine at 8,800 rpm, and the driver is not experienced enough or just doesn’t have the feel to know that the engine is falling off at 8,400. Well, we can use the accelerometer to see that the acceleration rate drops off a few tenths, and that tells us we need to lower the shift point of the engine to try to keep the acceleration rate up. Or the builder says to shift at 8,500, but the engine is still pulling, so we need to adjust the shift point up to keep using the engine’s powerband,” says Lee.

Adds Ledford, “When we’re drag racing, I don’t care where it makes peak power; I care about the average power. If you can maintain a higher power level over a certain amount of time, then the car’s going to accelerate more quickly. The engine makes peak power at 8,000 rpm, but it keeps accelerating to 8,500. So if you shift it at 8,000 and the rpm falls to 7,000, but it makes less average power at 7,000 than it does if you shift it higher, your g-meter might fall off a little bit because you shifted lower. But if it recovers and your average g-meter is better, then the car is accelerating harder. You should base your shift point not on your engine RPM, but your g-meter. It all goes back to acceleration.”

Tailoring It To The Track

Whether it’s a part of a V300SD datalogging system from Racepak or the integrated gyroscope and accelerometer found inside the FuelTech FT600’s internal datalogger or another accelerometer system, the most important thing to note is consistency with installation. Most importantly, it needs to be flat and level, as the hardware requires this to achieve accurate readings. And the smallest change in the vehicle can affect the readings it puts out to the datalogger, so the racer needs to be conscious of this, especially during a race when things can get hectic and small steps of the tuning process could potentially be overlooked.

This graph depicts tire spin on a radial-tire car. The yellow line in the middle shows the lateral accelerometer data. From launch to approximately 1.5-seconds into the run, the attitude of the car is relatively tame, but then the car starts to pitch and yaw side-to-side. This corresponds with the data shown on the driveshaft graph (the red line which starts at the bottom and goes up). There are three bumps in the graph, starting just before three seconds into the run, which tells us that the tire is not stuck to the track like it should be. An interesting thing to note is that the bumps in driveshaft rpm correlate with small bumps in the engine RPM graph (top, in white) to show that the RPM is fluctuating due to the tire spin. “A radial-tire driveshaft graph should be dead pinned and almost straight up, like it is around one second on this graph. Downtrack, the car probably hit a couple of bumps, which upset the tire. The g meter shows that the driver is having to steer the car as the power comes in,” says Ledford. These graphs show the tuner that its time to make a shock adjustment, or pull timing, or whatever they determine is appropriate to correct the issue and maximize performance.

“If you change the ride height of the car, it might only be .01 of a difference, but it’s still a difference. If you raise or lower the front or rear of the car, it’s going to change the attitude of the sensor. Once you make a change, you have to manipulate the Racepak with the nudge function—when the car is first staged and sitting perfectly still, you’d move the line up or down to put it on the zero point, so then you get rid of the attitude of the car and account for changes you’ve made to ride height,” says Ledford.

Whether a racer is trying to analyze and compare two runs or 20 runs, Ledford says it’s extremely important to ensure that the zero data point lines up for all of those runs on the screen. Once this is done, then the racer can analyze what happens after the transbrake is released.

Believe it or not, it can also be used to help train the driver where to stage the car—a critical step when trying to maximize performance on the track.

Here is another graph showing the same tire spin on the radial car, except in this image we have the acceleration g meter instead of the lateral unit. We can see here with the three bumps that occur around three seconds that once the tire settles down around 3.5-seconds in, the g meter actually recovers and shows forward acceleration improving rather than continuing to drop off.

“We can use it to see whether the car is shallow or deep staged depending on the numbers. When we go back and zero the graph (overlay the runs with another) you can see if the car got a head start at the ET clocks and had a quicker 60-foot time, or was deep staged and had a slower 60-foot time and elapsed time—the accelerometer can tell you all of these things based on previous performance of the same car and driver,” says Lee.

How the data is interpreted and tuning decisions are made for on-track performance is where these guys really earn their keep.

“The accelerometer offers a basic measurement, but where the real talent comes in is being able to look at the track—which changes 100 times a day—and know what you can put into the tuneup to maximize performance with the given conditions. Can you make it hotter, or do you need to dial it back? Without the information the accelerometer provides, you can’t accurately make that call,” says Lee.

The bottom graph from this ProCharged car shows converter charge pressure, which shows where the transmission is actually shifting—right around 3 seconds in. The middle graph is the accelerometer, and the top graph is engine RPM. The large spikes in the converter charge pressure tell us that’s where the transmission is actually shifting, and there is a slight corresponding dip in acceleration, then we see the accelerometer’s readings pick back up. It’s interesting to note that in the run depicted in green, the shift point was pulled back enough that you can’t see an RPM spike at the top, yet you can still see where the g meter is measuring higher after the shift.



Pro Line Racing

PTP Racing


Mike Galimi
Mike Galimi
Mike Galimi is the Director of Content & Marketing at ProMedia Publishing and Events with nearly 20 years of experience in motorsport writing and photography.