Dark Art of Exhaust System Tuning

by James Prunty

It was not unusual for car manufactures to conventionally use whatever was inexpensive and relatively efficient for exhaust systems.  They were more concerned with installing things that wouldn’t wear out within the warranty period than extracting the maximum performance from their engines.  However, electronic technology, environmental concerns and performance issues have changed that viewpoint in the last decade.  The Corvette exhaust system, especially the ZO6, does a good job of flowing exhaust.

Building a quiet exhaust system that allows your LS engine to develop more power is entirely practical.  Be aware that knowing what it takes in this department can easily deliver a 35+-plus hp advantage over your less-informed competition.

Before we delve into the dark art of exhaust tuning theory, let’s talk about California, smog tests, AQMD etc.    Smog checks are a $300+ million industry in California.  If you have a license to do smog checks, you don’t want to lose it.  There are several components to a smog test, and the one I would like to mention here is the visual inspection.  “The smog technician must locate and verify that all emissions components are present and properly connected.  Along with emissions components, the smog technician will also be looking for any defective or disconnected electrical connections, vacuum hoses and/or any pipe or plumbing changes which would affect engine performance and ultimately increase harmful smog emissions.”

That being said, headers and high flow cats will flunk the smog test immediately, and in some cases so will an “X” pipe (you can appeal an “X” pipe claiming it’s a “balance pipe replacement,” there’s a process for that, and with every State of California process there’s a fee)!  So, it decision time!  Here’s a suggestion.  Go to whomever you have business a relationship with that does smog tests and ask if they’ll pass or flunk your car based on the modifications you’re going to do to your exhaust system.  If you do make changes, KEEP YOUR OLD PARTS!

A performance exhaust system is all about flow!  We select intake and exhaust components based on flow capacity rather than size because engines are flow sensitive, not size sensitive.  A choke point on the LS engine is the exhaust manifold.  Headers, in effect, create an exhaust manifold for each cylinder and can add considerable rear wheel horsepower, and increase fuel economy.

I replaced my entire exhaust system from the heads to the mufflers with a 3” ZO6 exhaust system.  If you’re looking in the engine compartment, it’s difficult to notice unless you’re very familiar with Corvettes.  From underneath, it takes a keen eye to spot the difference between 2.5” and 3” exhaust pipes.  And, all the parts have GM part numbers.  My investment was $400.00 and a day in the pit.  Doing aftermarket headers and a cat back can set you back upwards of $2,000.00 or more for the parts alone.

Exhaust system design is a balancing act between complex events and their timing. Even with the best compromise of exhaust pipe diameter and length, the collector outlet sizing can make or break the best design. The bottom line on any exhaust system design is to create the best, most useful power curve.  The exhaust system on a well-tuned engine can exert a partial vacuum as high as 6-7 psi at the exhaust valve at and around TDC. Since this occurs during the overlap period (on naturally aspirated engines), as much as 4-5 psi of this partial vacuum is communicated via the open intake valve to the intake port runner. Given these numbers you can see the exhaust system draws on the intake port as much as 500 percent harder than the piston going down the bore. The conclusion I draw from this is that the exhaust becomes a principal means of induction, not the piston moving down the bore. The result of these exhaust-induced pressure differences are that the intake port velocity can be as much as 100 ft./sec. (almost 70 mph) even though the piston is parked at TDC! In practice then, you can see the exhaust phenomenon makes a high performance engine a five-cycle unit with two consecutive induction events.  All theory aside, the final judgment is how the engine likes the exhaust tuning on the dyno and on the track.

To achieve a zero-loss muffled high-performance exhaust system you need to work with the two key exhaust system factors, 1) the pressure wave tuning from length/diameter selection, and 2) minimizing backpressure by selecting mufflers of suitable flow capacity for your car. If you do this then a quiet (street-legal noise levels) zero-loss system on your car is totally achievable without a great deal of effort.  Ultimately, it boils down to nothing more than knowledgeable component selection and installation, so let’s look at what it takes in detail.

All the necessary components to build a highly effective, no-loss system are at hand, and not necessarily that much money either.

 Header Basics

The two main components of a header are the primary tubes and the collector.  The first consideration is the proper tube diameter.  Many people think “Bigger is Better”, but this is not the case.  The smallest diameter that will flow enough air to handle the engine’s displacement. at your desired Red Line R.P.M. should be used.

The second consideration is the proper tube length.  The length directly controls the power band in the RPM range.  Longer tube lengths pull the torque down to a lower R.P.M. range.  Shorter tubes move the power band up into a higher R.P.M. range.  Engines that Red Line at 10,000 R.P.M. would need short tube lengths about 26″ long.  Engines that are torquers and Red Line at 5,500+ R.P.M.s would need a tube length of 36″.  This is what is meant by the term “Tuned Length”.  The tube length is tuned to make the engine operate at a desired R.P.M. range.

Collector size, simple, two sizes larger than primaries.  If you have 1 5/8 diameter primaries, the collector should be 3.25”.

Muffler Flow Basics

An engine’s output is influenced minimally by size but dramatically by flow capability. Buying a muffler based on pipe diameter has no performance merit. The only reason you need to know the muffler pipe size is for fitment purposes. The engine cares little what size the muffler pipe diameters are but it certainly does care what the muffler flows and muffler flow is largely dictated by the design of the internals. What this means is that you should select mufflers based on flow, not pipe size

It seems safe to assume that the more a muffler flows, the better.  Increasing muffler flow unlocks potential engine power. However, once all the potential power is unlocked, further increases in exhaust system flow will not produce any further benefits in terms of power. But what may be good for power may not be good for noise as any excess flow capability can lead to a noisier system.

Virtually all V-8 exhaust systems can be refined by the addition of a balance or X-pipe. They have two potential attributes: increased power and reduced noise. Extensive testing on both of these factors has indicated balance and X-pipes are 100 percent successful at reducing noise.  As far as power is concerned, things are a little less certain.  I’ve seen some really outrageous horsepower claims, but 5-8 being the most common. Based on my research, I concluded that a balance or X-pipe is always a positive asset and never a negative.