2 ½" or 3" Exhaust? - Just For Our Muscle Car Folks

It's not uncommon to find a modified muscle car with an engine sized at 500 CI or larger. Most of these engines are also equipped with numerous high performance parts to increase HP, torque and improve driveability (def-the degree of smoothness and steadiness of acceleration of an automotive vehicle).

In most cases, a performance part is chosen based on an improvement in realizable performance based on real world tests. These 'premium" grade selections typically come with a significant increase in price too. Using an Edelbrock intake manifold as an example, the version tested to produce both excellent driveability along with the most torque and HP, costs over $150 more.
Edelbrock BBC Crate Engine
$15,770.32@ Summit Racing

But, the reasoning is rather simple. After spending $10,000+ on the engine, the choice of these necessary additional components could deliver anywhere between 450 HP and 600HP+, the higher for a few thousand dollars more. So why not? Right? And especially when they also deliver a similar level torque as broad and flat as Kansas; which is a very, very good thing. (See here for an example)

Time Honored Debate - Some Exhaust System Back Pressure IS or ISN'T Necessary
So the question most often debated is how to maintain all this extra performance you've already paid for when choosing the proper exhaust system. For decades, we've heard you need some back pressure in your exhaust system. 

But that "rumor" was started some 40 years ago, when numerous power improving advancements were yet to be developed and only engines revving higher than 6,500 made more than 400 HP.So what about in today's world. Does the "rumor" still hold true? Let's find out.

Now all of this info is good and applicable to large cubic inch, high horsepower cars, but if your Classic is is equipped with a smaller displacement engine in stock configuration, what should you do? Well, there are a few things to consider.

X Marks the Spot
Surprisingly, the above test systems did not include an X-pipe. While it remains untested conjecture with regard to the above video, on a typical 400-horsepower street car, adding an X-pipe to an existing dual exhaust system can add 7-12 hp. We wonder if the test HP/Torque spread would have remained intact.
Shows the two varieties. This company builds in both
Further it yields a smoother exhaust note and reduced interior resonance; added bonuses. Why is because it balances out the exhaust pulses while reducing the back pressure this causes. There is another option called the H-pipe. Similar in concept, the H-pipe will also reduce back pressure at low RPM, but after , say 3,000 RPM, loses nearly all effectiveness.

Why do These Work?
The firing order between the left and right bank of cylinders means that there is often a substantial difference in exhaust pressure between each bank. The result, at any given moment, is a traffic jam of exhaust in one bank of cylinders when minimal back pressure exists in the opposite bank. While this recurring scenario reduces engine efficiency and horsepower, the solution is as easy as integrating a balance pipe into the exhaust system.
L-Mandrel vs crinkle R-Mandrel vs. crush
Another point is to use a mandrel bent exhaust system instead of stock, crush or crinkle bent pipes. This method eliminates "kinking" or shrinking of the pipes as they are bent. The benefit is reduced back pressure. So the basic rules of thumb are:
  • The smaller the CID of and engine and the more stock it is, the less performance will be gained by using a "too-big" exhaust system. Most V8's up to 400CID can get along with a smaller pipe size.
  • Use low restriction mufflers. They can be quite or loud, just not restrictive.
  • Use an X-pipe and mandrel bent tubes
For readers looking for an even deeper read on the above, we suggest this article.

Until Next Time, Happy Motoring
The Garagistry Team