Fresh Air: Testing the Benefits of a Cold Air Inlet System

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Ducting cool, outside air into your engine is hardly a new idea, but it can really pay off by improving part-throttle performance in ways you may not realize.

Words and Photos By Jeff Smith

Step outside on a cool morning, and take a deep breath. Feels good, doesn’t it? Your engine responds the same way. There are only three things you need to make power in an engine — a few pounds of air, some metered fuel, and a spark to ignite a compressed mixture — and we have combustion. Tweaking a few details along the way can easily improve upon that simple process and make a little more power at the same time.

We don’t have to tell you there hasn’t been a car built in the last 25 years that doesn’t pick up fresh, cool air from outside the engine compartment. Cold air packages started back in the 1960s with Mopar Air Grabbers, Ford shaker hoods, and cowl induction packages on early Camaros, to name a few famous examples.

Back then, it was all about making a little bit of extra horsepower, and that idea still holds true today. But frankly, at speeds above 40 mph, a dedicated cold air package under the hood of a muscle car will probably only see a minor improvement in inlet air temperature. There are so many air leak paths even on cars without a cowl hood that under-hood air temperatures at highway speeds aren’t all that warm. This means a cold air package really won’t accomplish much.

So about now, you’re probably wondering, “Then why am I reading this?” Because a cold air package can provide benefits beyond just high-speed peak horsepower. We’ll offer a few theoretical advantages and then show you how simple it was to install a Spectre system on a ’67 Chevy II, and what was gained as a result.

This is John Bianci’s ’67 Nova that will serve as our test car. It’s a nice street-driven small block with a 350, Muncie four-speed, and 3.08:1 rear gears.

This is John Bianci’s ’67 Nova that will serve as our test car. It’s a nice street-driven small block with a 350, Muncie four-speed, and 3.08:1 rear gears.

Cool inlet air is worth power. Conventional wisdom contends that for every 10-degree reduction in inlet air temperature, we can expect roughly a 1-percent power increase. This sounds good, but even on a mild 400 hp street engine, a reduction of inlet air temperature of 15 degrees is an inconsequential 6 hp. That’s hardly worth the effort. But, let’s take this cold air idea one step further.

Our good friend Tim Wusz at Rockett Brand Racing Gasoline recently informed us of a standard that’s been around the industry for decades, but gets very little attention. The standard (backed up by substantial OE testing) is that for every 25- degree reduction in inlet air temperature, an engine’s octane requirement will be reduced by one full point. This means by cooling the inlet air temperature by 25 degrees, it’s possible to use 90 instead of 91 octane gasoline. Plus, the cooler, denser air would be worth a slight power increase.

This demands an example. Let’s say you’ve built a healthy 10.0:1 compression, carbureted 5.0L small-block for an early Mustang. Even with a tight piston-to-head clearance to improve quench, the engine demands 91 octane fuel, especially on hot summer afternoons. You’ve tried the cheaper 89 octane, but on hot days, the engine rattles when you engage the clutch. Yet, at higher vehicle and engine speeds, the engine does not detonate, even at wide-open-throttle (WOT).

So, let’s assume  the inlet air temperature in this Mustang is around 140 degrees at lower vehicle speeds and lower rpm. Now, let’s install a cold air inlet air system that pulls air in from an area ahead of the radiator. If we reduce the inlet air temperature down to 90 degrees (with an outside temperature of 80), we’ve reduced the inlet air temperature to the engine by a solid 50 degrees.

The Spectre Muscle Car cold air system is pretty simple with a dual snorkel air cleaner with a 120-degree offset, a pair of ducting hoses, clamps, and the inlet flanges.  Spectre offers dozens of different kits and individual parts that can be combined to create your own custom application.

The Spectre Muscle Car cold air system is pretty simple with a dual snorkel air cleaner with a 120-degree offset, a pair of ducting hoses, clamps, and the inlet flanges. Spectre offers dozens of different kits and individual parts that can be combined to create your own custom application.

Using Rockett’s inlet air temperature factor, this theoretically should allow us to use an 89 octane fuel instead of 91. Or, we could retain the 91 octane fuel and perhaps add a few degrees of low-speed ignition timing, which will help throttle response.

This is exactly what we decided to test on our friend John Bianci’s ’67 Chevy II. The car is currently equipped with a 350c.i. small-block with a Lunati hydraulic flat tappet cam (230/235 duration at 0.050 with 0.4489/0.507-inch lift), 9.0:1 compression, Edelbrock Performer RPM cylinder heads, and an Edelbrock Victor, Jr. intake fed by a FAST EZ-EFI 2.0 self-learning EFI throttle body system using an open element air cleaner pulling in hot, under-hood air.

The Nova is typical of a nice daily-driven early muscle car. John drives it nearly every day, including taking his kids to school. We started by locating where we wanted the inlet flanges on either side of the radiator.

The Nova is typical of a nice daily-driven early muscle car. John drives it nearly every day, including taking his kids to school. We started by locating where we wanted the inlet flanges on either side of the radiator.

After a recent tuning session, we noticed the inlet air temperature display indicated 131 degrees on a day when the ambient air temperature was closer to 80 degrees. We contacted Spectre, and after a discussion with Guy Smith, we decided on a dual inlet air cleaner assembly using a pair of plastic ducts fed by small inlet flanges that could be located on either side of the radiator.

After the system showed up on our doorstep, we broke out the four-inch hole saw and located a couple of suitable locations for the inlet flanges. After quick work with the hole saw and bolting the flanges to the core support, it only took a few minutes to mount the flanges using some stainless ¼-inch bolts we bought at the swap meet.

Test fitting the air cleaner housing meant using a short ½-inch Spectre spacer in order to clear the throttle linkage. This probably would not be necessary on a typical Holley carburetor, but the FAST EZ-EFI 2.0 throttle body is somewhat shorter and the linkage hit the underside of the air cleaner.

Test fitting the air cleaner housing meant using a short ½-inch Spectre spacer in order to clear the throttle linkage. This probably would not be necessary on a typical Holley carburetor, but the FAST EZ-EFI 2.0 throttle body is somewhat shorter and the linkage hit the underside of the air cleaner.

We also had to add a short ½-inch spacer between the air cleaner and the EZ-EFI throttle body to provide clearance for the throttle linkage and fuel lines. With the Spectre air cleaner in place and the hoses ducted to the air cleaner, we were ready for our test drive. The ambient air temperature on test day was pleasant at around 85 degrees. With the cold air ducting in place, a 20-minute cruise through typical Southern California in-town traffic produced some interesting results.

With the car at speeds above 20-25 mph, the inlet air temperature plummeted from our previous 130 to 100 degrees. Sitting at an extended stop light, the inlet air temperature would climb 10-15 degrees, but then quickly come back down once the Nova was rolling again. So, we can confidently state that even in sweltering August traffic, the Nova should be able to reduce the inlet air temperature by a solid 20 to as much as 35 degrees.

We used one of the four-inch flanges as a template and located the best positions for the holes in the radiator core support. There was just enough room to fit in our Milwaukee four-inch hole saw on our ½-inch drill motor.

We used one of the four-inch flanges as a template and located the best positions for the holes in the radiator core support. There was just enough room to fit in our Milwaukee four-inch hole saw on our ½-inch drill motor.

This data makes sense because when the car is sitting still, the engine is continuing to make heat and with no strong flow of cool air, that aluminum Spectre air cleaner assembly will begin to pull in heat. But because the housing is thin, once the car begins to move, the cool inlet air quickly reduces the inlet air temperature closer to ambient.

We also took the Nova out on the freeway, and as expected, the inlet air temperature was very close to ambient. A few days later on a cool evening freeway jaunt, the inlet air temperature appeared to be perhaps a few degrees cooler than ambient. On that evening, John saw the numbers hovering around 72 degrees, which is really good. Compared against the open element air cleaner, this was easily worth 20 to 25 degrees.

We attached the flanges with simple ¼-inch stainless bolts and nuts, but you could also use threaded inserts.

We attached the flanges with simple ¼-inch stainless bolts and nuts, but you could also use threaded inserts.

The big improvement is clearly around town in slow speed traffic. Our experience has shown these higher inlet air temperatures force either a less aggressive timing curve or higher octane fuel to prevent part-throttle detonation. By reducing the inlet air temperature at these low speeds by as much as 30 to 35 degrees, we could now go back and add a little more timing.

This is the main information screen on the EZ-EFI 2.0 system. Note the inlet air temperature in this example was 131 degrees. This was taken on a very mild Southern California day with the ambient air temperature at around 78 degrees using a typical open element air cleaner drawing in hot under-hood air.

This is the main information screen on the EZ-EFI 2.0 system. Note the inlet air temperature in this example was 131 degrees. This was taken on a very mild Southern California day with the ambient air temperature at around 78 degrees using a typical open element air cleaner drawing in hot under-hood air.

Our next step will be to add one or perhaps two more degrees of initial timing. With the hot inlet air, this attempt was met with off-idle detonation on hot days. We also plan to add a degree of timing to the vacuum advance curve, which will certainly improve the overall throttle response. We will have to wait for the really hot days of summer to see if the engine will accept these changes without rattling, but the cooler inlet air is certainly a positive move.

If you are running a carburetor, an easy way to measure inlet air temperature is with a hand-held volt-ohm meter. Our Klein meter temp lead was just long enough to set the probe on the outside of the air filter element while running through the firewall to the gauge in the interior. Our small-block El Camino recorded a peak of 143 degrees in slow speed traffic on an 80-degree day. The engine temperature was 190 degrees, while average inlet air numbers were in the 125- to 130-degree range.

If you are running a carburetor, an easy way to measure inlet air temperature is with a hand-held volt-ohm meter. Our Klein meter temp lead was just long enough to set the probe on the outside of the air filter element while running through the firewall to the gauge in the interior. Our small-block El Camino recorded a peak of 143 degrees in slow speed traffic on an 80-degree day. The engine temperature was 190 degrees, while average inlet air numbers were in the 125- to 130-degree range.

Another aspect of engine performance many enthusiasts don’t appreciate is that adding a degree or two of initial timing also can contribute to lower low-speed engine operating temperature. Starting the timing sooner means more of the combustion process will have been completed earlier with the piston farther up the bore. This means more heat will be converted into pressure to push the piston down, rather than heat transferred to the cooling system.

We shot this cold air package on our friend Dick Eytchison’s ’65 Chevelle autocrosser. His system employs a single inlet and dedicated the passenger side high-beam headlight as a ram air inlet.

We shot this cold air package on our friend Dick Eytchison’s ’65 Chevelle autocrosser. His system employs a single inlet and dedicated the passenger side high-beam headlight as a ram air inlet.

On a carbureted engine, it’s possible this much cooler inlet air temp will slightly lean the air-fuel mixture, but it won’t result in much of a change. With the EZ-EFI 2.0, the fuel injection instantly compensates. But with a carburetor, you might see a very minor change in air-fuel ratio.

Overall, installing the Spectre system on this Nova was not only very easy, but completely successful in achieving our goal of reducing the inlet air temperature. If we have a criticism of the Spectre kit, it would only be that the large screw clamps were a little weak and easily stripped. We’d replace them with better clamps.

It only required about three hours of unhurried effort to install this system. The only custom tools we needed was a four-inch hole saw purchased from the hardware store.

It only required about three hours of unhurried effort to install this system. The only custom tools we needed was a four-inch hole saw purchased from the hardware store.

While a cold air system won’t add 50 hp or 10 mpg, we have improved the car’s drivability in traffic, and it’s much more responsive to the throttle now than with the open element air cleaner. It’s always good when you can replace hot air with a cool breeze!

Parts List

 

Description PN Source Price
Spectre Muscle Car cold air intake 733 Summit Racing $163.64
Spectre air cleaner spacer 4954 Summit Racing $8.97
Sprectre Cowl air cleaner w/ tray, 4” tall filter 98564 Summit Racing $354.97
Spectre headlight air funnel 9789 Summit Racing $24.65
Klein multi-meter with temperature probe CL800 Amazon $104.99

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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