Why You Should NOT Use Exhaust Wrap on Your Cast Iron Exhaust Manifold

What kind of thermal barrier/insulation do you use on cast iron exhaust manifolds? It’s one of the most frequent questions we get at Heatshield Products. The majority of customers ask about if they should use or what kind of exhaust wrap to use on their vehicle’s cast iron exhaust manifolds. Our answer: do NOT use exhaust wrap on a cast iron exhaust manifold.

One more time. Do NOT use exhaust wrap on a cast iron exhaust manifold.

The main thing behind why you don’t use exhaust wrap on cast iron manifolds is the cast iron needs to breathe so it doesn’t exceed its maximum rated temperature. Similar to installing Heatshield Armor/Muffler Armor exhaust insulation, where you leave a section of pipe uncovered to allow the pipe to breathe. Before diving in further, a few points about cast iron that are worth knowing:

• Cast iron, in general, is an alloy with more than 2% carbon content
• Has a relatively low melting temperature
• Great resistance to deformation and wear and oxidation damage
• Automotive cast iron manifolds are made from “grey” cast iron
• Very high volumetric heat capacity (once hot, stays hot for a long time)
• High emissivity (radiates a lot of heat from its surface)

Cast iron is one of the oldest alloys known to man, like “5th century B.C.”-old. It’s a versatile material for use in a lot of things. Most notably for our purposes, for casting engine blocks, cylinder heads, intake manifolds (though uncommon now), and exhaust manifolds. Because of the carbon and silicone content of cast iron, it’s ideal for creating cast parts. This makes it a favorite of the automotive industry even today.

ABOVE: Cast iron is the perfect material for making castings in the automotive world. It has the perfect combination of durability, ease of production, and low-cost auto manufacturers have loved. But it also radiates a lot of heat due to its metallurgical properties, which contribute to high underhood temperatures. Insulating cast iron manifolds can drastically reduce underhood temperatures, but you never want to use exhaust wrap to do so.

Do You Need A Heat Shield for an Exhaust Manifold?

When it comes to exhaust manifolds, because of the cast iron construction, they radiate a lot of heat under the underhood, and they stay hot for a long while even after the vehicle has been shot off. This makes exhaust manifolds a major source of radiated heat underhood. And since the manifolds are usually in close proximity to a lot of underhood components, they subject those things to radiated heat exposure and wear. Anything you can do to cut the heat coming from the exhaust manifold(s) will provide a major reduction in underhood heat and heat exposure for underhood components. Recognizing the problem of heat radiating from the manifolds, most manufacturers started installing thin sheet metal heat shields on their exhaust manifolds from the factory. These factory shields end up being mostly ineffectual, as they usually just become heat-saturated and don't offer much in the way of protection for nearby components.

Because people are used to employing exhaust wrap on headers, it’s assumed exhaust wrap is what to use on cast iron exhaust manifolds. But the reality is that’s the last thing you want to do. Because of the cast iron’s properties, the manifolds need to be able to breathe so the material doesn’t trap too much heat and exceed its melting point. It’s the same situation as using Heatshield Armor to insulate exhaust pipes. A small section of pipe must be left uninsulated so the pipe can breathe just enough not to exceed its maximum rated temperature. Also, when exhaust wrap is used on cast iron manifolds, it creates a situation where the manifold doesn't cool off evenly. What happens is the flanges on the manifold will cool off quicker than the body. The flanges are typically where exhaust manifolds will crack or break, and uneven cooling will exacerbate this problem.

ABOVE: Another benefit of insulating cast iron exhaust manifolds is reducing the amount of heat exposure components neat the exhaust manifolds are subjected to. Constant high heat exposure adds wear and tear to components and leads to failures and problems that’ll ruin your day and cost you money. Very common with later-model vehicles that have more enclosed and component-filled engine bays. Later model vehicles had heat shields on the manifolds from the factory (as seen in the photo), but these shields were mostly ineffective. In most cases, they became saturated with heat and ceased to be heat shields but just another conduit for the manifolds to radiate heat.

Exhaust Manifold Heat Shield: Necessary or Not?

Installing any exhaust wrap on a cast iron exhaust manifold totally encapsulates the manifold and doesn’t allow for any heat release. This leads to the manifold heating up past its rated temperature, leading to manifold degradation and failure. The proper way to go for decreasing exhaust manifold radiated heat is to use Header Armor®.Made from the same BioCoolTM-infused technology as our Heatshield Armor exhaust insulation, Header Armor insulates the outward (non-engine side) of the exhaust manifold to drastically cut the amount of heat radiated outward towards components near the manifolds and underhood overall. The engine side of the manifold remains uncovered, providing enough surface area to release some heat from the manifold so it doesn’t exceed the maximum temperature. This also allows for the exhaust manifold to cool off evenly, reducing the risk of the flanges warping and/or cracking and requiring repair.

ABOVE: Header Armor is the proper exhaust manifold insulation: it doesn’t cause damage. Made with Heatshield Products specially-formulated BioCoolTM insulation, Header Armor handles up to 1800 degrees F of constant heat without degrading. Kits come with everything needed for installation and can often be installed without having to remove the exhaust manifold. Heatshield Armor insulates the important parts of the manifold while leaving just enough cast iron exposed so the manifold can still breathe and not exceed its maximum rated temperature.

Header Armor® exhaust manifold insulation is easier to install than exhaust wrap (added bonus), is removable, and can be reinstalled if necessary. Precut kits are available for standard manifolds along with inline-six engines (like the Jeep 4.0L) and V6 manifolds. Kits come with high-temperature stainless-steel Inconel wire to secure the insulation to the manifold, or you can use one of our HP Power Anchor kits. Either way, you end up with a properly insulated exhaust manifold, reduced underhood temperatures, and a better-performing exhaust system thanks to increased exhaust gas temperatures.