We've found that AL seems to be the sweet spot for IC pipes. The first pipes we made for diesels were in T304 stainless and they held heat for a duration of time about 46% longer than AL. The plastic holds heat longer than you may think. I'm digging through the archives to see if we kept the data.
Jesse, I assume you guys have some expertise in thermal conductivity of materials and, perhaps more importantly, the transfer of heat from the pipe to the air flowing through it (heat soak).
Perhaps you've seen this - it's by John Concialdi, Engineer and founder of AEM and you can find it online in a number of places:
Aluminum vs. Steel vs. Plastic Explained by John Concialdi, AEM Chief Engineer
5/28/2003
Aluminum vs. Steel vs. Plastic
The issue of heat absorption with an intake system has a degree of validity, however we have found that too much emphasis is placed on material selection, instead of the real issue of tuning the system. Our systems feature a unique shape and diameter because this is what we found to make the most useable torque and horsepower for each individual application in testing. However, for the purposes of this discussion, we will limit it to why we choose to make our systems from aluminum and the effects of heat absorption on all materials. If you do not wish to review all of this information right now, a quick synopsis of this discussion is outlined in the following bullet points, with complete topic discussions below:
We use aluminum to eliminate any chance of the system rusting, and it's lighter than steel
We limit our use of plastic because this material absorbs some of the sound energy we work to create in the inlet duct
Whether or not an inlet system is made from aluminum, steel or plastic, the thermal conductivity of the duct material has little effect on engine power
The rate at which air travels through the inlet path under open throttle, when one is asking the engine for maximum power, negates the effect of material heat soak, regardless of the material
We use aluminum or a combination of aluminum and plastic plenums for throttle-body-injected applications that require a special plenum for every intake we produce. This eliminates any chance of rust occurring on the inside of the inlet pipe. We have seen chrome-plated steel systems whose inner diameter became rusted over time, causing flakes of rust to travel along the inlet path. We also choose aluminum because of its lightweight properties. Heavier components place higher loads on the brackets they are attached toóor even worse, to the pipes they are attached to. We combine our lightweight aluminum design with a flexible coupling device we call a soft mount that connects the intake system to the body of the vehicle. In addition to the soft mount, we use doublers at the point where the mounting bracket is welded to the pipe for additional strength.
We limit our use of plastic because this material absorbs some of the sound energy we work to create in the inlet duct. Although we use the best plastic material for our plenums, it is still not as resilient and does not retain the visual appeal of aluminum over long-term use. Because we have to use plastic on throttle body applications, we take extra precautions to ensure that the aluminum retaining ring that attaches to the throttle body is anchored securely into the plastic plenum; this is done by making an interlocking mechanical link between the plastic and aluminum.
Whether or not an inlet system is made from aluminum, steel, or plastic, the thermal conductivity of the duct material has little effect on engine power. We have found that the tuning of the pipe, in addition to providing the coolest inlet air source, are the keys to making useable power. We perform engine inlet-air-temp studies when developing each application to determine the coolest location for sourcing inlet air. In addition to this, we determine the safest location for the inlet source to protect it from highly dusty conditions and water. To this end, we provide a stainless-steel heat shield to help minimize heat soak into the inlet area, as well as to provide protection from dust, dirt and mud.
At light throttle opening, air speed and airflow at the inlet system are relatively low. The high residence time of air in the inlet while at low-throttle settings will increase inlet charge temps when materials with high thermal conductivity are used. Typically, when someone is at light throttle they are not asking the engine to make power. Most likely, fuel economy is the issue.
When the throttle is fully opened however, air speed and airflow increase considerably. Typically, the inlet air speed of a 5.7L engine with a four-inch duct at full throttle is 34 feet-per-second, based on a volumetric efficiency of 70% and an engine speed of 3,000 rpm. Most inlet systems for every intake manufacturer for this engine are 30 inches or less. This means that the air in the duct of a 30-inch inlet length on this engine at the given rpm is 1/10th of a second hardly enough time to transfer an appreciable amount of heat into the air stream on any system.
Basically, the rate at which air travels through the inlet path under open throttle, when one is asking the engine for maximum power, negates the effect of material heat soak, regardless of the material. We hope that this helps to clear up the issues of material heat absorption in intake systems. Thank you for taking the time to read this, we welcome your comments and feedback!
