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Doug Thorley Headers for your 4WD vehicle


What are Tri-Y headers and how do they work? We frequently get calls asking us to explain the theories behind the Tri-Y headers, but it just can't be done properly over the phone. To make things easier, this information will give you a basic idea of the key elements of Tri-Y design theory.

Why are headers so important? It takes energy to push disorganized exhaust gases out through a vehicle's stock manifolds. Headers don't make horsepower they simply relieve the engine from the task of shoving exhaust out through stock manifolds. Installing properly designed headers frees up the energy previously wasted pushing out exhaust, and allows it to be used in propelling the vehicle. This increase in efficiency brought on by the installation of headers can be thought of as free horsepower.

What's the idea behind Tri-Y's? It all begins with firing order. For familiarity's sake, we'll use the Chrysler / GM firing order: 1, 8, 4, 3, 6, 5, 7, 2. (The same theory will apply on a Ford, though the firing order is different.) If you were to lay the cylinders out so that the first four cylinders in the firing order were on top of the second four, and then connect each cylinder on the upper row to its counterpart on the lower row, you'd have something that looks like the disgram to the right.

You've just connected, or paired, the cylinders that fire 180 distributor degrees apart from one another. Check this out on a distributor and you'll see that #1 is directly across from #6, #8 is directly across from #5, and so on. If you could pair your header tubes like this, you would have theoretically ideal headers. One pair of cylinders would fire every 180 degrees of distributor rotation. The high-pressure pulses from each paired set of tubes would be allowed one complete crankshaft revolution's worth of time to smooth out before its partner cylinder fired. The pulses would be organized so they could proceed out the tailpipe in an orderly fashion.

Best power for headers

So what does all this have to do with the Tri-Y design? Let's go back to our first four over second four firing order stack and see how close we can get to pairing the cylinders 180 degrees apart without having to cross over or under the engine. Cylinders 1, 3, 5, and 7 are on the driver's side of the engine. Cylinders 2, 4, 6, and 8 are on the passenger's side of the engine. Let's do the driver's side first by referencing the diagram to the right.

If we blend the gases from #1 with those from #5, and blend the gases from #3 with #7, we get the paired cylinders much closer to being 180 degrees opposite one another than if we just dumped all the exhaust gases together and let them fight their way out of the collector as a four-tube header does. Now check out the driver's side on the left.

Thats all there is to it. Relatively simple, very effective and applicable to V6, straight 6 and 4cyl engines as well.

So why aren't all headers Tri-Y's? Mainly because they're very labor intensive to manufacture and most companies are looking to produce a cheap header over a true performance header. This is also why Thorley takes the time to make every header they produce, 50-state SMOG legal. Each header comes with CARB certification papers and sticker. Very few headers on the market do this which makes their headers illegal in most places.

Why are most Doug Thorley designs Tri-Y's? Doug Thorley Headers firmly believes that where possible, the Tri-Y design is the way to Make It Go! We hope this article eliminates some of the mystery surrounding the Tri-Y designs, why they build them, and why we sell them!

Header construction. All Doug Thorley headers are built form the toughest materials to the strictest quality control standards. 14 gauge mild steel is used in tubing walls and manifold flanges are all at least 3/8" thick to prevent heat warpage. All headers also come with a nickel-chrome finish. Your header will have a shiny chrome finish at installation and Thorley's rust protection ensures it will look like a million bucks longer than your vehicle's body will last.



Street legal, CARB legal, SMOG legal. Most are not!

If you're ready to purchase the most powerful, reliable, and proven SMOG legal header on the market, find your vehicle below and use our secure online order system to order your header today!

Scroll down below our shopping cart items to see sketches of some more popular header designs.


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Ford 2WD
Ford 4WD
GMC and Chevy, 2WD
GMC and Chevy, 4WD
Suzuki and Tracker


Toyota Owners please call to order.

Too many variables in their design, O2 placement, and other factors to
safely list on our website. 888-801-7271



Order line: 888-801-7271



The Thorley 634Y is used in all 6 cyl Jeep applications.  







 The Thorley 550Y is used with fuel injected Toyota 20R and 22R engines. 





 The Thorley 364 series headers are used for a variety of Chevy and GMC vehicles from 73-81. 






 The Thorley 214 series headers are used on Ford 4WD F-250 and F-350 vehicles.