Performance Exhaust Install - Exit Velocity

For any given displacement engine, higher power output means greater average cylinder pressure during the power (expansion) stroke. For a given compression ratio and exhaust valve timing, any increase in the average cylinder pressure will also lead to an increased cylinder pressure at the moment the exhaust valve opens. Opening the exhaust valve against the higher pressure results in a louder exhaust note. Thus, higher performance engines are typically louder.

There’s more to it, however. Higher performance engines usually alter the exhaust valve timing to open earlier during the last part of the power stroke. This creates a longer exhaust event, which aids in exhaust flow at higher rpm, thus producing more power. Opening the exhaust valve earlier results in even higher cylinder pressure being initially sent out the exhaust, again creating a louder exhaust note.

At the other end of the exhaust-valve timing, high performance engines typically delay the closing of the exhaust valve (while simultaneously getting a head start by opening the intake valve earlier). This results in a period when both valves are open at the same time, called overlap. If the exhaust system is restrictive, pressure will build up. The result is not only less efficient scavenging of the exhaust gases from the cylinder, but also the reversion of exhaust gases backing up into the cylinder during overlap, and if the restriction is really severe, exhaust gases backing up all the way into the intake ports. The end result, as you can imagine, is a poor running engine and loss of power.

Stock exhaust systems, which are designed to keep the noise low, can therefore be quite restrictive to exhaust flow, due to all the torturous paths they are designed to squeeze through in hopes of stripping off the pressure wave energy that results in noise. Low performance engines don’t have a great deal of valve overlap, so they can get away with a somewhat restrictive exhaust system. But with the increased overlap in our high performance engines, a restrictive exhaust system will quickly undo a bunch of effort at the engine end to make more power.

Pontiac engineers knew this and equipped high performance models with freer flowing dual exhaust, and in the case of H.O.’s, Ram Airs, and SDs, low-restriction manifolds. There was even a short-lived vacuum-operated muffler bypass system known as V.O.E.

Increased power output gained through larger displacement and/or more aggressive valve timing will be compromised with a restrictive stock exhaust system. Such was the case with our ’66 GTO in which we bumped up the engine displacement by more than a liter, replaced the bumpstick with a Melling Ram Air IV unit, and have future plans for large quantities of hidden nitrous. Thus we needed an exhaust system upgrade.

Our plan was to maximize the flow potential (and minimize any restriction which could create reversion problems with our additional valve overlap), while at the same time stay well within any old folk’s noise tolerance. We also didn’t want to go too big, since overkill on the exhaust system can reduce gas velocity, which will again reduce scavenging efficiency. Somewhere in between too big and too small is just right.

We started at the engine with a set of ceramic-coated, four-tube Doug’s headers, which then dump into a 3.00-inch, mandrel-bent dual exhaust system kit from Pypes. For a bit more power potential, and less low-end boom, we went with an X-type pipe option after the headers.

Our muffler selection was based on the extensive work by Jim and Tom Hand, who compared several mufflers for restriction, power output, and noise levels. We chose the 18-inch Goerlich Xlerators, since they represented a good compromise between noise level and performance potential.