Bolting on a performance exhaust system adds an exciting new tone to your car. But, that sweet song becomes annoying noise as corrosion eats away at your exhaust. If you want the music to last, start with Water is the natural enemy of an unprotected exhaust system. During proper combustion, water droplets form and drip from the tailpipe. Most of the water is forced out by the exhaust but a fraction of it stays behind—and, moisture and metal don't mix. This excess water gradually turns your shiny exhaust into ugly-rust from the inside out.
What Does Stainless Actually Mean?
Stainless steel exhaust components stop your pipes from corroding. But, not all steel is created equal. You see, some alloys corrode over time despite the name “stainless”. It doesn't necessarily mean “stain proof”. Corrosion resistance depends on the content of chromium and nickel found in the alloy.
What's Better, T409 or T304 Stainless Steel Exhaust Components?
The most frequently used stainless steel is T409. Found in many stainless mufflers, this mixture has enough chromium and nickel to resist corrosion. But, it lacks the strength to be used for fasteners. Next up in the ring for corrosion resistance and all-around toughness is the T304 steel used in Borla exhaust headers and Corsa exhaust. This is the highest quality stainless steel used in the automotive world. It is most often found in exhaust tips. Super tough and nearly impervious to rusting, T304 has around 20% chromium and 10% nickel content in the alloy. In fact, the ferrous metal content is so low that magnets won't stick to it, making identifying T409 from T304 very easy.
Making the Right Exhaust Choice
In short, when comparing T304 vs. T409, the best choice is T304. But, both alloys far surpass aluminized steel used in factory exhaust systems. There really is no wrong way to go as both a T409 and a T304 exhaust system.
Stainless Steel battle: Two Alloys Enter, Only One Remains
Many companies advertise their stainless steel products as just that - simply stainless steel. Some don't bother specifying the differences between 409 and 304 grade material in their items. But did you know that there are a few major differences in the metals that make one better for performance exhaust applications than the other?
Side-by-side the two look very similar, but corrosion resistance depends on the content of chromium and nickel found in the alloy. 409 stainless is a metal designed primarily for the automotive exhaust industry, although it has been used successfully in other industrial applications. It combines good elevated temperature corrosion resistance and medium strength, good formability and inexpensive overall cost. 304 stainless is one of the oldest variants of stainless steel and is typically used in household applications and is one of the most common metal alloys available today.
The Difference is in the Numbers
The most frequently used stainless steel in performance exhaust systems is T409. Usually found in Flowmaster, Invidia, and many other aftermarket stainless mufflers and catbacks, this mixture does not contain enough chromium to resist corrosion in cold-weather states where salted roads aid in corrosion development. T409 has a chromium content of 10.50% to 11.75% while its nickel content is only 0.50%. 304 Stainless is the highest quality stainless steel used in the automotive world, and it is most often found in exhaust tips, clamps, bolts and other small exhaust components where strength is very important. 304 grade stainless is tough and nearly impervious to rusting, because it has around 20% chromium and 10% nickel content in the alloy. However, because of its low ferrous content, it can develop a golden color when exposed to high heat over time. In fact, the ferrous metal content is so low that magnets won't stick to it, making the identification of T409 and T304 very easy.
What is the best option?
When considering an aftermarket exhaust for your Corvette your global location is an important factor. 409 Stainless systems will suffice for areas where road salt isn't an issue. Its temperature resistant alloy allows for superior strength, and rust generally isn't an immediate concern. 304 Stainless is great for rust prevention in northern/wintry areas, but susceptible to heat discoloring under extreme temperatures seen in most southern states. Below you can see the differences in corrosion after just one winter. The main 304sus piping is slightly discolored by resisting rust quite well, where the 409sus resonator section is showing signs of surface rust.
Automotive manufacturers have found that by saving a few extra bucks in production by using the cheaper 409 grade stainless, they can sell more units. While this is great for making money, do you as a consumer really trust the product? Do you know what grade steel your exhaust is made of? MAP uses only 304 stainless on all of our custom exhaust components and headers, so you can be sure that your performance parts will last for years to come without any rusting or corrosive threats!
When choosing an exhaust system for your vehicle, you may have trouble choosing between a stainless steel exhaust system and an aluminized steel exhaust system. In general, aluminized steel costs less, but both systems have advantages and disadvantages. It's important to consider your needs and especially your location before you make your decision.
Appearance
If the appearance of your exhaust system is important to you, stainless steel has a definite advantage. Stainless steel can be polished, so it will look newer longer compared to aluminized steel. Don't let the name fool you, though. Stainless steel is not actually stain proof. Over time, stainless steel may develop discoloration, especially because exhaust systems reach very high temperatures. Blueing is common, in fact, on stainless steel parts used on vehicle exhaust systems.
Of course, the location of the exhaust system can make it challenging to keep stainless steel polished. Unless you are dedicated to constant cleaning and polishing, stainless steel might not be worth the extra expense...at least as far as looks are concerned.
Corrosion Resistance
When it comes to resisting corrosion, however, stainless steel is the way to go. If you live in a dry climate that stays fairly warm in the winters, aluminized steel will be good enough for your car. However, if you live in an area near salt water or an area that receives a lot of snowfall and uses salt on the roadways, you should probably consider stainless steel. Because of stainless steel's high corrosion resistance, it's also easier to provide a warranty on a stainless steel exhaust system.
Stainless steel comes in different grades with different levels of resistance. If corrosion and pitting are a major concern, look at 304-grade stainless steel, also known as marine grade. An exhaust system made from 304-grade stainless steel can actually last about 20 years.
Lifespan
A quality stainless steel exhaust system might outlast your car (indeed, some exhaust manufacturers warranty stainless systems for 1,000,000 miles). Only you can determine how long you plan to own your vehicle. If you are looking for replacement parts on a vehicle you are likely to sell in the next five years, an aluminized steel part makes more sense. It costs less, and you don't need to worry about lengthy durability.
Stainless steel is sometimes more brittle than aluminized steel. This might lead to cracking, although this is not a common occurrence.
Density
Stainless steel is denser than aluminized steel, which could be seen as an advantage. However, the tubing in exhaust systems made from aluminized steel is typically thicker, which tends to offset any strength or weight advantage the denser stainless steel might have.
When it is time to consider a new exhaust taking all these factors into account will really help you make a wise decision.