Why Intake Temperatures Can’t Be Compared Between Positive Displacement and Centrifugal Superchargers
Abbreviations/Acronyms/Definitions in This Article:
- The words Blower and Supercharger are interchangeable.
- Centri = Centrifugal
- PD = Positive Displacement. This includes roots, roots improved, TVS and Twin Screw blowers.
- IAT1 = Intake Air Temperature before the blower.
- IAT2 = Intake Air Temperature after the blower and after the intercooler.
- IAT = Unless noted otherwise, I’ll be using IAT interchangeably with IAT2. It’s the IAT that really matters, and in a lot of cases the only one you know.
- DT = Blower discharge temp. This is what the temperature of the air is after it comes out of the blower and before it passes through the intercooler. This temp is rarely sampled in anything short of a full bore race car.
- IC = Intercooler.
- A2W = Air to Water. This is a type of intercooler system. All PD blowers come with these if they have an IC at all.
- A2A = Air to Air. This is the other type of intercooler system. Most Centri blowers come with an A2A. But a few come with A2W systems.
- Ambient or ambient temp = Outside air temp.
- Air charge = The compressed/heated air after the blower.
- Charge Air Cooler (CAC).
- Low Temp Radiator (LTR).
- Mass airflow sensor (MAF).
- WOT = Wide Open Throttle
- TB = Throttle Body
A few words on intercooler system/parts names/definitions/descriptions/and confusing stuff.
In different segments of the automotive world, some components are called different things but they’re exactly the same. The OEMs call one thing “A,” while the aftermarket calls the same part “B.” And the Ford crowd may call it “C.” And the tuner crowd can call it “D.” So, obviously, this can get confusing. I’ll clear that up here. Well, clear it up for this article. I won’t be fixing the internet!
In the “Mustang World” and This Article, You’ll See These Words Used for These Things:
-Intercooler. This is the cooler that sits in the intake manifold under the blower on PD blown cars.
-Intercooler. This is the cooler that sits behind the bumper cover and in front of the radiator/air conditioning condenser on A2A Centri blown cars.
-Intercooler. This is the cooler that sits in the “box,” usually in the engine compartment the air charge is run through on A2W Centri blown cars.
Confusing huh? Three different “intercoolers.”
-Heat Exchanger. This is the cooler that sits behind the bumper cover and in front of the radiator/air conditioning condenser on PD blown cars.
-Heat Exchanger. This is the cooler that sits behind the bumper cover and in front of the radiator/air conditioning condenser on A2W Centri blown cars.
In the World of OEM Manufacturers, You’ll See These Words Used for These Things:
-Charge Air Cooler (CAC). This is the cooler that sits in the intake manifold under the blower on PD blown cars.
-Charge Air Cooler (CAC). This is the cooler that sits behind the bumper cover and in front of the radiator/air conditioning condenser on A2A Centri blown cars. But there are no OEM blown Centri blown cars, so really you’ll only see this from the OEMs when describing turbo setups.
-Low Temp Radiator (LTR). This is the cooler that sits behind the bumper cover and in front of the radiator/air conditioning condenser on PD blown cars.
And One More:
Technically all of the intercoolers, heat exchangers, LTRs, CACs, etc. are……………..heat exchangers.
As you can see, it doesn’t take much for confusion to ensue. I’ll be using “Mustang World” terminology in this article.
“Centri Blowers Have Lower Intake Air Temps than PD Blowers Do!”
“Centri Blowers Run Cooler than PD Blowers Do!”
“Centri Blowers Don’t Heat Soak Like PD Blowers Do!”
I see the above quotes (and more like them) all the time. Someone will say something along the lines of “Centri blowers have lower IATs than PD blowers do.” That statement, in some cases, is true. But in other cases, it’s completely false. And there’s a 99.9% chance that people making that statement have no clue what they’re actually talking about. They’re probably repeating what others said. And that person didn’t know what they were talking about either. They’re making the blanket statement that Centri blowers always have lower IAT2s than PD blowers do, and that couldn’t be any more wrong. It’s a fairly complicated situation to understand, and most people, even people in the go fast parts industry (especially retailers and dyno operators/tuners), don’t possess the information to make statements like the above. So, to sum up, when you hear people making the above statements, you can be almost certain that they’re talking from a position of ignorance.
To fully understand why you can’t make blanket statements like “Centri Blowers Have Lower Intake Air Temps than PD Blowers Do!” you’re going to need a good understanding of a few things. Here they are:
Why You Have An Intercooler
Compressing air (boost) creates heat. Heat at a certain point causes detonation in the motor. Detonation is when the fuel’s octane can’t cope with the heat being generated. This results in cracked rings, cracked pistons, melted pistons, broken connecting rods and sometimes even cracked engine blocks. Detonation and temperature control is obviously a huge thing. You can turn a motor to scrap in a matter of a second if your IAT’s are too high.
Thankfully in the modern age the engine management systems (ECU/ECM/Computer/whatever you call it) are incredibly good and have all sorts of fail safes built in. In most cases if your IAT’s are too high you won’t turn your motor to scrap. The ECU will know that the IAT’s are too high and in turn will retard the ignition timing dramatically to keep everything in one piece. The penalty for that is a massive loss in horsepower though. It is not uncommon for a 500hp car to lose 100hp. I’ve seen 600-800hp cars loose 250hp when IAT’s aren’t kept in check. So, you may not blow motors up when IAT’s get out of hand like in the old days. But you do pay a huge HP penalty.
Most tuners (and OEM’s) want IAT’s below 135degF and set the ECU to pull timing at anything over that number. If you’re running e85 or good race fuel you can push that number up some. But that doesn’t apply to most people.
What’s the Difference between an Air to Air and Air to Water Intercooler System?
You’ll find two types of intercooler systems in most applications. Air to Air (A2A) and Air to Water (A2W). All PD blower systems use A2W IC systems. Most Centri blowers use A2A IC systems. But a few Centri blowers do use A2W.
Air to Air
A2A systems are fairly simple. The blower discharge air is run through an A2A IC that sits up behind the bumper cover and in front of the radiator. The hot air from the blower goes through the IC (inside) while the cooler ambient air is moved across/through it (outside). The temperature difference between the inside and outside of the IC cools the air charge. The air charge is then piped into the motor/TB/intake manifold—a very simple system with no moving parts.
Here is a picture of one. The red arrow is pointing to the intercooler. The green arrows are the piping that’s feeding the air from the blower to the intercooler and then the cooled air to the TB/intake/motor.
Air to Air Intercooler
Air to Water
A2W systems are quite a bit more complicated than A2A systems. Because PD blowers sit on top of the motor, it doesn’t leave enough room to run a 3”+ pipe out to an A2A IC and a 3”+ pipe back in. That would be a MESS even if you got it to work. So, PD blowers have the intercooler inside the intake manifold under the blower discharge port. Well, some are blowing “up” into an intercooler. But you get the point, the blower blows air directly through the intercooler.
Here’s a picture of an intercooler (blower removed)
Air to Water Intercooler
Water is pumped through the intercooler, which is then pumped out to the heat exchanger, which is located behind the bumper cover/in front of the radiator (pictured here).
Air To Water Heat Exchanger
When air is compressed, it heats up. In a perfect laboratory environment with a 100% thermal efficiency, compressing air to 1psi will raise its temperature 9.89deg (all temp measurements are in Fahrenheit). But, that’s in a perfect environment. Superchargers are not a perfect environment. They have some inefficiency when it comes to compressing air. Some more than others. And different blower speeds will result in an even greater variation. It can get confusing, so here are a few “data points” to consider/put in your memory banks:
- Twin Screw blowers are the most thermally efficient of the blowers. This is because they’re actual compressors where everything else is a “blower” to one degree or another. You’ll see about 12deg/psi of temp gain with a Twin Screw.
- TVS blowers are almost as good as a Twin Screw when it comes to thermal efficiency. Eaton has done a hell of a job making a Roots Improved blower (which is what a TVS really is) act like a Twin Screw. With a TVS you’ll see about 13deg/psi of temp gain.
- Centri blowers are not quite as thermally efficient as the Twin Screw and TVS blowers. They clock in at about 13.5deg/psi of temp gain.
- Roots Improved blowers are the least thermally efficient “popular” blowers out there at about 14deg/psi of temp gain.
- A true Roots blower (think old school blower with carbs on top) is the least thermally efficient at 16-20deg/psi of temp gain. You don’t see these much anymore though.
But Wait, There’s More.
- If you put a blower on that’s too small for the HP level that you’re trying to run at, you’ll have to spin it faster. Which will push it out of its thermal efficiency range. And the closer you get to the maximum RPM, the more heat they’ll make (it’s exponential). For example, a properly sized Roots Improved blower can have lower discharge temps than a Twin Screw that’s too small for the job.
- If the blower has an inlet restriction—like the elbow is too small (which you usually can’t improve), the throttle body is too small, or the cold air intake is too small—you’ll have to spin the blower faster than one that has properly sized inlet components. This is another case where a Roots Improved blower can have lower discharger temps than a Twin Screw.
So How Much Heat Are We Talking About Here?
- A correctly sized Twin Screw making 10psi will get you a temp rise of about 120deg. But then you have to add that to the outside air temp (ambient), which we’ll say is 80deg. That is a total discharge temp of 200deg.
- A correctly sized Centri making 10psi will get you a temp rise of about 135deg. Then add 80deg ambient, and you have 215deg. Not a massive difference. But when your goal is to have a maximum IAT2 of 135deg, the extra 15deg the centri is making represents are fairly large percentage.
Now let’s turn up the boost.
- A correctly sized Twin Screw making 15psi will get you a temp rise of about 180deg. Then add the ambient temp, which is 80deg. That’s a total discharge temp of 260deg.
- A correctly sized Centri making 15psi will get you a temp rise of about 202.5deg. Then add 80deg ambient, and you have a 282.5deg discharge temp.
At this point, the “out of the box” intercooler systems are struggling to keep IAT2s below 135deg (some won’t even be able to). That extra 22.5deg of discharge temp from the Centri is absolutely making a difference.
Now let’s turn the boost up one more time.
- A correctly sized Twin Screw making 20psi will get you a temp rise of about 240deg. Then add the ambient temp, which is 80deg. That’s a total discharge temp of 320deg.
- A correctly sized Centri making 20psi will get you a temp rise of about 270deg. Then add 80deg ambient, and you have a 350deg discharge temp.
At this point, we’re talking about some serious heat. There isn’t an “out of the box” intercooler system that will keep these sorts of temps down to a safe level (under 135deg). You would be hard pressed to build a custom A2W that can keep up. And you will never get an A2A that will get the job done.
Heat Transfer – Transfer Medium
Water transfers heat 24.17 times faster than air. Let that sink in. The water in an A2W system will transfer heat 24.17x faster than the air in a A2A system.
Water transfers heat 24.17x faster because it’s more molecularly dense. That means there’s more “stuff” to whisk the heat away. It also means that, for a given volume, let’s say a cubic foot, there’s more “storage” capacity to “store” that heat. Sounds like a clear winner, right? Give me an A2W setup any day! Well, just like most things in life, there are flip sides and “in this cases”…. Keep reading.
The Intake Air Temp Sensors Aren’t the Same
Right here is where the biggest confusion about IATs between PD and Centri blowers comes from. They don’t use the same kind of sensors. And they act wildly different, and they don’t give comparable data.
PD blowers have what I call a “bulb style” sensor. They react to temperature changes immediately. This is why you see the OEMs using them for their blower applications. They all use basically the same sensor. And the aftermarket uses OEM sensors in their kits. They’re instantaneous, accurate, inexpensive and reliable. Here’s a picture of one.
“Bulb” Style Sensor
Centri blowers “re-purpose” the car’s stock IAT sensor that’s built into the mass airflow sensor (MAF). They can do this because, unlike a PD blower, a Centri unit is blowing through the MAF with boost pressure (the MAF is before the blower on a PD setup so it “sucks” through the MAF). There’s a problem with using the IAT sensor built into the MAF though. It was never designed to deal with a blower application. The IAT sensor in the stock MAF was designed to measure air temps going into the motor that weren’t under boost. That means the sensor wasn’t designed to react quickly or to deal with really high spikes in temperature. It doesn’t need to when used as intended. In a naturally aspirated use, it will never see the extreme fluctuations that a boosted application will.
What’s the net result of using an IAT sensor that wasn’t designed for blower applications? Well, it’s problematic and not accurate at all.
The biggest issue is that the IAT sensor can’t “keep up” with radical temperature changes (going into boost). So, what you get is incorrect data. It takes the IAT sensors built into MAF sensors about 8sec to stabilize or “catch up.” Here’s an example of what happens:
You’re cruising down the road at normal speeds and not in boost. Your IAT temps are effectively ambient, let’s say 75deg. Your car makes 10lb of boost at WOT, which is an additional 135deg (roughly), which gets you a discharge temp of about 210deg.
Now you whack the throttle wide open. Unlike the bulb style IAT sensor used with PD blowers, the MAF IAT sensor can’t keep up with that temperature change. A tuner friend did a test to show how long it takes for the IAT sensor to stabilize and actually show you the correct IAT temperature. He did this for himself, but he posted the results to help educate the public. Here’s what he found out.
It took 7.88sec for the IAT sensor to read the actual intake air temp of 142deg. The rate at which the sensor “caught up” wasn’t linear (graph below). There’s a curve to it. That means, when you first go WOT, the sensor is WAY behind. Only as time elapses does it start to catch up faster. What does this mean in the real world?