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We feel for the most part, the majority of street performance oriented users sporting mostly bolt on mods are better off either with a Tri-Y or a 4-1 using long runners, small runner diameters or both, and a merged collector.
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| Underneath all of those shrouds the stock FR-S manifold is actually a pretty decent Tri-Y . The Nameless header next to it is also a Tri-Y and is good for around 10 whp over the stock part. Note that the Nameless part has a close coupled cat of its own incorporated in the design. Just because it has a cat, doesn't mean it's legal in Kalifornia…. |
For street cars it is essential that the headers you purchase have provisions for all of the vehicle's stock O2 sensors, EGR fittings and any other emission controls that the vehicle originally had fitted to the exhaust manifold. Most modern emission controls do not rob any wide open throttle horsepower. The common EGR valve which reduces toxic oxides of nitrogen, closes and has no effect at wide open throttle. Most air injection devices usually operate either on cold start or under closed throttle deceleration on typical modern cars. Removing these controls does not help power and pollutes the air. This is not good for a street car as we must all do our part to help keep our planet clean.
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| Look at the super crappy log exhaust manifolds on a Toyota Tacoma compared to the Doug Thorley headers. Even though the Thorley headers have all the fittings for smog doesn't mean they are legal. Check you local laws. In California, just because it passes the smog test, it doesn't mean it's legal. |
Just because your headers have provisions for all of your smog equipment, don’t assume that it is street legal. Due to the intelligence of some of our local government agencies, unless an aftermarket part is CARB approved with a CARB EO number, it is not legal in some states no matter how clean the gasses coming from the tail pipe are! So if avoiding smog certification hassles is important to you, either check your local laws before installing or make sure that the part you buy has a CARB EO number. Your parts dealer should be able to answer that question.
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| Doug Thorely's short tube Tundra headers are CARB approved and thus California legal. They use the stock cats. Even though the short tube design is not optimal for power production, but rather for cat light off instead, it is said that they produce over 15 whp. |
You can generally expect a power gain of about 5-25 hp at the wheels from a well designed header on most cars depending on how bad the factory exhaust manifold was. If you drive the same, not exploiting your new found power too frequently, you can expect better mileage with a header due to the improved pumping efficiency it produces. In buying a header look for thick wall mild steel tubing, at least 16 gauge, and preferably 14 gauge. If you can afford it, buy a stainless steel header, stick to 304 or 321 stainless. The increase in life is well worth the extra cost. Stainless also has an advantage of usually producing more power. This is because it has about half the thermo conductivity of mild steel. A stainless header keeps more of its heat in the tubes so pulse energy is conserved to help scavenge the cylinders. Ceramic coating can help in this way as well. Rust and heat resistant ceramic coated or stainless steel primary pipes are preferred for longer life as well. Look for thick flanges as these will resist exhaust leaks and last much longer.
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| Skunk 2's off the shelf headers are really nice looking parts. We will be testing some on Project Civic soon. |
So this time around we explained how a header works. In our next edition we will delve into some formulas to determine a close approximation of what header will work the best for your engine and how to design a custom header for your particular application.
More on Mike's continuing series on how engine stuff works.
