EXHAUST TYPES

[Corzette]

The first thing to consider is the size of the primary tubes. Bigger tubes will give better breathing for top-end power, but the low velocity at lower rpm will make for more reversion to contaminate the next charge, so larger tubes will usually cost some low-end power. It is a common mistake to port the heads to match the headers primary pipes, but this does more harm than good. The primary tubes should be larger than the exhaust ports, this makes a reversion dam to limit reverse flow. This is most important on the port floor where velocity is the lowest. If your looking to enhance power in the 1500-3500 rpm range, you'll want 1 1/2"- 1 5/8" primaries on a small-block and 1 3/4"-1 7/8" on bigger cubic inch big blocks. Big tube race headers can help power above 5500 rpm, but may cost some low-end power. If you don't intend to rev the motor much past 5500 rpm, go with a smaller tube header. On a small block Chevy, going bigger than 1 3/4" will require an adapter plate that will need to be ported to the head and you must use a header with a special bolt pattern to bolt up to it.

 

Primary Tube Length

 

The shorter the primary tubes are the higher in the rpm range they will help power. Each exhaust pulse causes a high pressure wave to travel toward the collector. When it reaches the collector it is inverted and travels back toward the cylinder as a low pressure wave. It is this low pressure area that helps scavenge exhaust out of the cylinder during the overlap period. This happens when the low pressure area reaches the exhaust valve during the overlap period. The low pressure area helps draw out the exhaust and draw in more fresh intake charge. All the pressure waves travel at the speed of sound (1200-1300 ft/sec in the hot exhaust). With all the pressure waves traveling at a constant speed, you can see that the header can be only be tuned to a narrow rpm range. On a street car that needs low-end, the tubes should be longer, in a high rpm drag car they will need to be much shorter. To get you close to the optimum primary tube length, use this formula:

 

Length (in inches) = (CID x 1900) ÷ (rpm x pri.OD2)

 

Equal Length Primaries

 

Equal length headers can be an advantage on a street car with a full exhaust system, but even more so an open header engine race engine. Whether they are worth it on a street car really depends on price, they are usually much more expensive. That money may get more bang for the buck elsewhere in the engine. As long as the primaries differ no more than 2 inches, they will be fine for a full exhaust system street car. Equal length headers will work the best when the banks of an engine are even firing (and most V8's are not), 180° crankshafts even out the cylinder firing and work the best with equal length headers, but the costs and the fact that they cannot be perfectly balanced makes them impractical for the street.

 

H-Pipes

 

Adding an simple connection between the pipes can boost power in a certain rpm range. Most header primaries are tuned to operate on the second set of pressure waves, to tune the crossover to the same rpm range it will need to operate off the 1st set of waves. If your primary tube ends 30 inches. From the back of the valve and is using the second set of pressure waves, putting the crossover 60 inches from the valves will help power at the same rpm range using the 1st set of pressure waves. In order to be effective, the crossover should be at least 90% of the diameter of the pipes.

 

X-Pipes

 

This is a more expensive crossover, but works a little better. The X design allows the pressure to go form one pipe to another much easier by eliminating the 90° corner. The gasses have smooth bend to follow. X-pipes take up much more room which makes it harder to place them in the best location when ground clearance is a problem. When figuring the placement of the X pipe, measure the point where the pipes meet.

 

Thermal Coatings

 

The biggest performance benefit of coated headers is that they help keep the heat in the exhaust system. As the exhaust gases cool, they also lose velocity. By keeping the exhaust hot, the velocity will be higher and you will get more of a scavenging effect. At low rpm, reverse flow will be reduced also. By keeping the heat in the exhaust system, you also reduce under hood temperatures, which can really be a plus if your carb is breathing under hood air. Thermal coatings (like Jot Hot) also look nice and resist rust to help the headers live longer.

 

Thermal Wraps

 

Thermal wrap does a better job at insulating heat and reducing under hood temperatures, but they also tend to hold in moisture and cause rust. If you use wrap on a street car, you should use a coated header to help it last. Headers will not last as long if they are run hotter, and wraps make the header run much hotter. The metal will fatigue much quicker. Gaskets will also fail sooner if the system is hotter, especially the collector gaskets.