Aftermarket Cold Air Intakes vs. Stock Intake:
Quantitative Testing of Change in Airflow and Performance
Quantitative Testing of Change in Airflow and Performance
By Sakib Elahi, Ph.D. (Sakib) and Travis Wingerson (Voltwings)
Copyright © 2015, Sakib Elahi
Aftermarket cold air intakes (CAIs) are one of the most common modifications that sports car owners make to their vehicles. The 2011-2014 V6 model Ford Mustang alone has at least 9 different brands of CAIs available. Drivers seek to increase the airflow to the engine with these intakes, thereby increasing horsepower and torque. These gains are said to come from a less restrictive design than the stock intake. Additional selling points of CAIs are louder engine noise and improved aesthetics under the hood.
However, the benefits of CAIs are frequently debated in the car enthusiast community. Some say that the larger diameter and straighter inlet tube of CAIs must certainly increase airflow over the clunkier stock intake, which has acoustic resonators that reduce engine noise and bends in the path to the throttle body. Others counter that the Mustang stock intake is already a cold air intake, with the duct located at the grille pulling in fresh air. When Jay Leno questioned Ford Mustang Chief Engineer Dave Peciak about aftermarket intakes, Peciak responded that the stock intake is engineered for maximum airflow, and that “this is the best intake you can get.” On the other hand, CAI manufacturers claim gains of up to 20 horsepower and 30 ft-lb torque in their product marketing. A caveat of these claims is that they are typically reported as “when combined with a tune.”
It is difficult to trust claims by either Ford engineers or aftermarket manufacturers, because each has obvious biases. There is a lack of objective, quantitative data on the effects of aftermarket CAIs on performance from an independent, unbiased source. Such data would advise consumers on purchasing decisions for maximizing their vehicle performance. We disclose that we neither work for nor have financial interests in aftermarket CAI manufacturers or Ford Motor Company.
In this study, we used a datalogger to record key vehicle and engine performance metrics with an aftermarket CAI versus a stock intake. We also measured air pressure at the front of the filter element to test ram air effects. To eliminate other factors, tests were performed on the same vehicle, same tank of gas, and on the same day. We also tested the effect of a tune, but using the stock intake instead of the CAI, to isolate how much of the claimed gains are from the tune alone. We hypothesized that the tune alone will lead to gains over the stock tune, and that the CAI combined with the tune will lead to gains over the tune alone. Our objective is to provide 2011-2014 V6 Mustang drivers quantitative, real-world data that will inform their purchasing decisions of aftermarket CAIs and tunes.
Tests were performed on a 2012 V6 Ford Mustang with an automatic transmission and 2.73:1 final drive ratio. The vehicle had ~52,000 miles on the odometer at the time of the experiment. The only modifications to the engine/exhaust are a JLT oil separator and Borla Touring mufflers, neither of which should have a significant effect on performance. The same tank of 93 octane gas was used for all runs.
We selected the Steeda Mustang CAI (Steeda Autosports, Pompano Beach, FL) for our experiment, because it has a largest diameter housing at the mass airflow (MAF) sensor of all aftermarket CAIs. Therefore, we assumed that the Steeda CAI will have the most gains in airflow, so if we do not measure an increase in performance when using the Steeda CAI, there is likely no increase in performance for other aftermarket CAIs either.
Table 1 lists the three tune/intake combinations that were tested. Test #1 is the setup straight from the factory: stock tune and stock intake. The purpose of test #2, the Steeda 93 octane tune with the stock intake, is to measure the change in performance due solely to the tune. Test #3 applies the Steeda CAI. The Steeda 93 octane CAI tune is used so that the effect of the tune is eliminated when comparing to test #2. Because of the larger MAF housing diameter, the Steeda CAI requires a tune for proper fueling, so we could not test the effect of the CAI combined with the stock tune.
Table 1. Tune/intake experimental groups and sample sizes. Each experimental group was tested four times, with the first run used as a pilot run and not included in the data analysis.
Tests were performed on a straight, open road. The automatic transmission was locked in second gear, and the car was driven at wide-open throttle from a rolling start at ~10 mph until the engine redline (6800 rpm for stock tune, 7000 rpm for the two Steeda tunes). Each test was repeated 4 consecutive times. However, the first run of each group was used as a pilot run, and only the latter 3 runs were used for data analysis. Ambient temperature maintained at ~76 °F throughout the duration of the experiment, and density altitude ~850 feet.
Data was recorded using an SCT X4 Power Flash Programmer (SCT Performance, Sanford, FL), plugged directly into the OBD-II port under the steering wheel. Many parameters were recorded, but those relevant to the analysis in the report were: time (seconds), intake air temperature (°F), mass airflow rate (MAF, grams/second), engine speed (RPM), and vehicle speed (mph). The datalogger records these parameters at a sampling rate of ~65 Hz.
The air pressure at the front of the filter element was also measured using a magnehelic differential pressure gauge (Dwyer Instruments, Michigan City, IN). One end of the pressure gauge was connected to a hose that was positioned and secured through the grille and into the inlet of either the stock or the Steeda intake. The hose was routed across the hood and into the cabin. The other end of the pressure gauge was open to atmospheric pressure. Because the gauge is analog, we were unable to record the data. Changes in pressure were simply observed by eye during the runs.
All quantitative results below are reported as averages of the latter three trials of each tune/intake experimental group. Error bars are calculated as ±2 standard deviations of the three trials. Showing the data this way allows comparison of experimental groups with consideration of variations in the runs, i.e. experimental uncertainty. If two quantities have overlapping error bars, we can say with 95% confidence that the two quantities are equal. (The 95% confidence comes from the assumption that the data follows a Gaussian distribution. See here for a discussion on tolerance intervals in engineering if you are interested.)
Vehicle Speed vs. Time
The vehicle speed was measured for all trials. Since the runs began from a rolling start, the time axes of all runs were aligned at 20 mph for direct comparison. The vehicle speed vs. time starting at 20 mph and ending at engine redline is shown in Fig. 1.
The stock tune with the stock intake performs significantly slower than both setups that use the Steeda tune. However, the addition of the CAI with the tune makes no difference in vehicle speed over the tune alone, indicated by the overlapping blue and green lines in Fig. 1. For another quantitative comparison of this speed data, we determined the time to accelerate from 20-80 mph for each setup, shown in the bar graph inset of Fig. 1. Times were averaged for each trial, and error bars are ±2 standard deviations from the mean. The tune alone and the tune with the CAI both accelerate ~0.7 seconds faster from 20-80 mph than the stock setup, but the CAI has the same time, within error, as the stock intake with the tune.
Fig. 1. Vehicle Speed vs. Time. Steeda tune with the stock intake and the Steeda tune with the Steeda CAI both outperformed the stock tune with the stock intake, reaching significantly higher speeds by the end of the run. However, the vehicle speed is the same within experimental error between the tune with the CAI and the tune with the stock intake. Although it appears there are only two lines on the graph, the green and blue lines are almost perfectly overlapped. Inset) The time to accelerate from 20-80 mph was also equal within experimental error between the tune with stock intake (blue) and tune with the CAI (green), but both accelerated ~0.7 seconds faster than the stock setup.
Mass Air Flow vs. Engine Speed
Although the results above show that the CAI does not make any difference in vehicle speed over the tune alone, we recorded the readings of the engine’s mass air flow (MAF) sensor to assess whether or not the aftermarket CAI increases airflow to the engine. An engine’s MAF sensor measures true mass of the air per unit time (units grams/second), which accounts for variation in the density of the air. Therefore, variation in the intake air temperature does not affect this comparison--it is already accounted for in the MAF measurement.
The MAF curve versus engine speed in RPM is shown in Fig. 2. The Steeda CAI begins outperforming the stock intake in terms of MAF at an engine speed of ~5000 RPM. The increased MAF using the CAI continues to grow over the stock intake until the engine redlines. The peak MAF of the Steeda CAI with tune, stock intake with Steeda tune, and stock intake with stock tune are 24,470 ± 90 g/s, 22,510 ± 30 g/s, and 22,400 ± 50 g/s, respectively. All of these peak increases occur at the engine redline. This equates to an increase in MAF of ~9% for the Steeda CAI over the stock intake. The MAF is equal within experimental uncertainty for the two groups that use the stock intake, both with the stock tune and the Steeda tune, for the entire power range up to 6800 RPM. The slight increase in peak MAF with the tune is due to the increased redline of the Steeda tune from 6800 RPM to 7000 RPM.
Fig. 2. Mass Air Flow (MAF) vs. Engine Speed. The Steeda CAI drew more air into the engine than both setups that use the stock intake. All increases occurred at engine speeds greater than ~5000 RPM; below 5000 RPM the two intakes perform approximately the same. The Steeda tune makes no difference in MAF over the stock tune when using the stock intake, with both curves overlapping within experimental uncertainty throughout the powerband.
Ram Air Effect
To assess any potential “ram air effect” gained by the aftermarket CAI, we measured the air pressure in front of the filter element during all runs. The Magnahelic pressure gauge that we used only had an analog readout, so we were unable to digitally record this data, the way we did the other results presented in this paper. The results reported here are approximations that were observed by eye (by the passenger) during the runs.
The pressure gauge measures the pressure differential across the front of the filter and atmosphere. A positive pressure means that the ram air effect is occurring, and pressure in front of the filter is higher than atmospheric pressure. A negative pressure means that a vacuum is forming in front of the filter, and that the engine is pulling in air faster than the intake can feed it.
During our pulls with the stock airbox we observed a steady decrease in pressure (increases in vacuum) as RPM increased, up to a maximum vacuum of ~1.75” of water at redline. On the contrary, after switching to the Steeda intake, we observed steadily increasing positive pressure in front of the intake, up to a maximum of roughly 1” to 1.5” of water at redline. While these results are of not as high precision as the other quantitative results reported in this study, it is significant that the Steeda intake reaches a positive pressure (ram air effect) while the stock intake creates a vacuum (engine starved for air). For reference, 27.7” of water is equal to 1 psi.
Idle Air Temperature
A common concern of an open element filter, like the aftermarket CAIs, is that it is exposed to engine bay heat when idling, whereas an enclosed stock airbox is not. To test the effects of heat soak at idle on both the enclosed stock airbox and the open Steeda CAI, we recorded intake air temperature while stationary for several seconds before conducting each wide open throttle pull.
The intake air temperatures at idle showed large variation over all trials. The average and 2 standard deviation errors for the three groups are shown in Fig. 3. The Steeda CAI has a higher average intake air temperature at idle of 94°F ± 7°F than the stock intake, which averages 89°F ± 4°F with the stock tune and 86°F ± 1°F with the Steeda tune. The temperatures with the stock intake are the same, within experimental uncertainty, no matter what tune is used. Although the uncertainty of ± 7°F with the CAI overlaps with the uncertainty of the stock intake measurements, the average temperature for the CAI is still greater by a large margin than the average temperatures for the stock intake. Furthermore, of the individual trials that were averaged, the minimum temperature measured on the CAI was 90°F, but the maximum temperature measured on either of the stock intake trials was also 90°F. Therefore, it can be concluded with reasonable confidence (but not 95% confidence) that the Steeda CAI pulls significantly warmer air at idle than the stock intake.
Fig. 3. Intake Air Temperature at Idle. Average intake air temperature at idle for the Steeda CAI is greater than that of the stock intake, both with the Steeda tune and the stock tune. Variation in the trials was large, especially for the CAI, so error bars overlap. However, even the minimum temperature measured for the Steeda CAI was equal to the maximum temperature for the stock intake (90°F).
We successfully conducted all experiments that we designed to test the performance of the aftermarket Steeda CAI versus the Ford stock intake. Most importantly, we found that the Steeda CAI does not increase the vehicle speed, at all, over the stock air intake that is running a Steeda tune (Fig. 1). The increased acceleration that results in faster 20-80mph time of ~0.7 sec over the stock intake results entirely from the Steeda tune. Adding the CAI on top of the tune does not make the vehicle faster.
However, we also found the interesting result that the air flow to the engine significantly increases with the Steeda CAI over the stock intake--even when running the Steeda tune with the stock intake (Fig. 2). This result is further evidenced by the observation that the CAI forms a ram air effect of higher pressure in front of the intake at high speed, whereas the stock intake forms a vacuum, indicating that the engine is starved for air. This is a surprising result, given the previous finding that the vehicle speed increases with the Steeda tune on a stock intake, but does not further increase when adding the CAI.
With this evidence we can conclude that the engine and the tune are unable to utilize the increased air flow that is provided by the Steeda CAI. Since an in-depth analysis of the tune was beyond the scope of this study, we do not have the evidence we need to answer the question why the engine is unable to utilize the extra air. However, it is a reasonable conjecture that combining the CAI with other engine modifications that improve air flow (e.g. larger throttle body, ported intake manifold, more free-flowing exhaust) could allow the engine and tune to take advantage of the increased intake MAF. Further testing would be valuable to test the hypothesis that a Steeda CAI offers advantages over the stock intake when combined with other engine modifications.
The air flow from the Steeda CAI did not surpass that of the stock intake until ~5000 RPM. Below 5000 RPM, the MAF of both the CAI and the stock intake were nearly identical; in fact, the CAI briefly has a slightly lower MAF than the stock intake at ~3000 RPM, which is likely due to the higher air temperature at low speeds. This result is important to “daily drivers” that are on the market for a CAI. Increased MAF at engine speeds >5000 RPM will likely only be achieved under hard acceleration. The MAF at normal driving speeds will be the same as for a stock intake.
We found that the air temperature at idle is modestly higher for the CAI than the stock intake--an ironic result considering the name “cold air intake” (Fig. 3). Although the temperature is higher, we also found that the MAF is nearly identical for both intakes at moderate engine speeds and greater for the CAI at high engine speeds (Fig. 2). Since MAF is a measure of air mass, the effects of intake temperature are accounted for in that measurement. Warmer air is less dense and therefore has less mass than cooler air, but an increased volume of warmer air can have the same or greater mass than a smaller volume of cooler air. The MAF sensor measures mass, which is the relevant parameter for engine performance. Our results show that although the intake air temperature of the CAI is greater than the stock intake, the MAF of the CAI is still the same or higher than for stock. This means that the larger intake volume is compensating for the warmer air, and that the intake temperature is decreasing to ambient at high engine speeds. That said, the MAF for the CAI may be lower than for the stock intake when cruising in the city at low speeds, when underhood temperatures cannot decrease as fast as they did during our wide-open-throttle runs.
Our results prove that for a 2011-2014 V6 Mustang, adding a Steeda CAI with a Steeda tune does not increase vehicle acceleration any more than the Steeda tune does by itself with a stock intake. Both the CAI and the tune with the stock intake offer acceleration gains of ~0.7 seconds from 20-80mph over the stock tune with the stock intake. Our results also prove that the Steeda CAI does indeed increase air flow to the engine, and even creates a ram air effect in front of the filter element. However, the stock engine/exhaust is unable to utilize this extra air flow to create more power. We found that the intake air temperature at idle is moderately higher when using the open element Steeda CAI than the sealed stock intake, but that this increased temperature is offset by the increased air volume that the CAI allows versus the stock intake, especially at high engine speeds. These results suggest that V6 Mustang owners who want to improve the performance of their vehicles should initially purchase only a tune, and keep using the stock air intake. The CAI may lead to increases in performance, but not when keeping the rest of the engine/exhaust stock. Further testing is needed to prove that the CAI offers gains when combined with other engine modifications.