Bigger carbs, manifolds, heads, and cams all are designed to let the engine take in as much air and fuel as possible, but the byproducts of combustion must be vacated quickly to make room for more of the good stuff (fresh air and fuel) on the next intake stroke. So why not just bolt on the biggest exhaust headers (in terms of primary tube diameter) with the largest collector you can find, and call it good? If 21⁄2-inch tailpipes are good and 3-inch pipes are better, then why not go to 4-inch pipes? Heck, let’s try 5-inch tailpipes. Import guys seem to think their four-banger engines need ’em, so why shouldn’t we? If you answered “to keep the exhaust flow velocity up,” that’s correct, but why does velocity matter?
Equal length four-into-one headers help to extract maximum power from a big-block.
Remember, the only event that actually produces power in your four-stroke engine is the power stroke, so how can the exhaust system create more cylinder pressure on the piston top? The key to this riddle is the effect that the exhaust gas flow has on the intake flow during valve overlap, the time when both exhaust and intake valves are open. If not for valve overlap, the bigger the exhaust pipe the better; just dump out all of that nasty combustion residue and be done with it. But because of valve overlap, a properly tuned exhaust system actually helps to activate the intake flow earlier in the cycle due to the scavenging effect of the exhaust gases flowing out the exhaust valve. With a header that is too large for the size of your engine and RPM range you need, all of the exhaust gases indeed get dumped out quickly; so quickly that there is little or nothing left to help pull in fresh air and fuel when the intake valve opens. As engine speed increases, even the larger header pipes have some amount of residual flow during valve overlap, which is why high-RPM racing engines are best served with larger header sizes.
OEM Exhaust Manifolds
Enough theory; let’s talk nuts and bolts. All production big-blocks use castiron exhaust manifolds, which are little more than sturdy chambers with four short legs attached to each exhaust port exit, dumping into a head pipe with a three-bolt flange. The primary virtue of cast-iron exhaust manifolds is that they are durable, and the heavy casting does a good job of dampening exhaust noise, a major consideration for your aunt Matilda.
Headers are frequently the first and most cost-effective performance upgrade that most gearheads make to their vehicle, and the more power your big-block is capable of, the greater the need for an exhaust system that matches the other performance parts in the engine. Most tubular exhaust headers are a four-intoone design, meaning that four individual primary tubes (one for each cylinder on that side of the engine) merge into a single collector. Street-driven vehicles also must have exhaust pipes that are typically 21⁄2 or 3 inches in diameter feeding two low-restriction mufflers. Most street headers have primary tube sizes of 13⁄4 inches or 2 inches dumping into a 3- or 31⁄2-inch collector, and typical competition headers start with 21⁄4-inch primaries feeding a 4-inch collector and go up from there, depending of the size of that monster mill under your hood.
Some competition headers feature a stepped design, where primary pipe diameter is increased before each pipe enters the collector, typically from 21⁄4 to 23⁄8 inches, or 23⁄8 to 21⁄2 inches. The stepped design is said to minimize reversion, which is a negative pressure wave that travels back up the header pipe after the initial exhaust pulse exits the primary tube. If the pipe length is just right, the timing of these pulses serve to help pull along the next exhaust pulse for a super scavenging affect; motorcycle racers refer to this as being “on the pipe.” Not all racing engine builders embrace the stepped header design, pointing out that the increasing diameter also slows the exhaust gas velocity.
The primary pipe diameter must be large enough to accommodate the volume of exhaust flow, so a larger displacement or an improvement in the volumetric efficiency of the engine calls for an increase in the diameter of the pipe, while primary pipe length is dictated by engine RPM; a short pipe becomes resonant at a higher frequency (thus a higher RPM) than a longer pipe.
A good set of headers is often the first and most cost-effective modification you can make to your big-block Chevy. This 304 stainless steel Hedman Hedder has 13⁄4-inch primary pipes dumping into 3-inch collectors. It fits first-generation Camaros and Novas, and is also available in painted or coated mild steel. (Photo Courtesy Hedman)
Many companies offer high-temperature ceramic-based coatings that offer complete protection from corrosion and act as a thermal barrier to reduce the heat radiated from the pipes. (Photo Courtesy Hedman)
Competition headers should have equal length primary tubes, usually from 28 to 30 inches long, so that all eight cylinders produce their peak power at the same RPM. Street headers may have a vast difference in primary pipe length due to cramped engine compartments, manufacturing considerations, and other variables. It’s really not that big a deal for most street applications, since the exhaust must be routed through tailpipes and mufflers anyway, which tends to negate the advantages of true equal length headers.
A variation of this theme was popular some time ago with the design of tri-Y headers, in which each pair of primary tubes are joined in a “Y” connection, and those two pipes are then joined in a similar “Y” connection at the collector. This design is excellent for producing good power at relatively low engine speeds, and could be just the ticket for tow trucks, RVs, or other low-speed, high-torque applications. A variation of the tri-Y design is the 4-2-1 header collector offered by some manufacturers. It joins each pair of primary tubes into a two-into-one connection; then those two pipes are merged into the final collector. While it is an interesting approach to exhaust tuning, it has not found favor with most professional-level racers.
Most tubular exhaust headers are made from mild steel, though more expensive sets are often manufactured from stainless steel for its better strength, corrosion resistance, lower coefficient of thermal conductivity, and appearance. Many companies offer high-temperature coatings, usually a ceramic-based chemical, for their mild steel tubes. When properly applied, these coatings offer complete protection from corrosion and act as a thermal barrier to reduce the heat radiated from the pipes. Also, header pipe diameters are measured on the outside, so the internal dimensions are smaller by the wall thickness. Most bargain-priced headers use 18-gauge steel, while more expensive sets may go up to 14 gauge.
Competition headers lead a pampered life compared to their street-driven compadres; so many racing headers are also made from 18-gauge tubing to reduce weight. The thickness of the header flange where it bolts to the cylinder heads can be as thin as 1/4 inch or as heavy as 3/8 inch for the more pricey headers. Thicker flanges are always better since they are less likely to bend and distort while tightening, and they offer a better gasket seal and longer life. You can improve the chances of gasket survival by regularly re-tightening the header bolts, since high heat and vibration tend to loosen them.
Mufflers and Exhaust Pipes
All street cars and some competition cars have to be equipped with noisereducing mufflers, and the aftermarket performance industry has responded with an impressive selection of highflow mufflers and exhaust systems. Today’s selection of performance mufflers have evolved into scientifically designed sound-reducing chambers with baffles, sound wave deflection chambers, unequal length passages to reduce or cancel harmonic resonance, and other advanced engineering features. Some still incorporate perforated steel passages with a packing material to absorb sound waves, while others rely on solid steel construction with no packing material that might blow out.
This 572-cube Chevelle uses two 3-inch Flowmaster mufflers and 3-inch mandrel-bent pipes to evacuate the exhaust from 620 hp.
X-pipes between the headers and the mufflers improve the flow capacity of the exhaust system and tame-down annoying harmonic vibrations and booming. This Hedman cross-pipe is offered in 21⁄2 and 3-inch sizes. (Photo Courtesy Hedman)
Typical performance mufflers for your big-block feature 21⁄2- or 3-inch inlets and outlets; racing mufflers are available up to 5 inches in inlet/outlet sizes. For best performance, the entire exhaust system should match the diameter of the mufflers’ inlet and outlet. The minimum size for any big-block is 21⁄2 inches, and that is on the small side. Serious street performance big-blocks need 3-inch pipes and mufflers.
A crossover tube between the left and right banks of your exhaust system is beneficial for both more power and quieter operation. Traditionally, the cross-over tube is placed behind the header collectors and in front of the mufflers in an “H” pipe configuration, but many more modern systems use an “X” configuration where each head pipe is curved inward to meet its mate from the other side, with a substantial opening where the two are welded together. When a single pulse from one side of the engine exits the header collector and heads toward the mufflers, it now has two low-pressure passages available: one from the muffler straight ahead, and one from the crossover pipe leading to the other bank of the exhaust system. Because the flow path through the crossover is a little longer than the flow path to the muffler on the same side as the header, the split pulse exits the two tailpipes at different times, and you hear a higher frequency with less booming and harmonic resonance than an isolated dual exhaust produces.
An Exhausting Summary
The majority of street performance big-blocks are miles ahead with even a modest set of tubular exhaust headers dumping into 3-inch pipes and mufflers, again with a crossover pipe. Twoinch primary tubes provide excellent power for most high-performance street engines and support more than 550 hp, if that’s what your Rat motor is capable of producing. Larger diameter exhaust pipes not only flow more, they are also louder, so if your objective is to build a “stealth” street machine, don’t go too large on the exhaust pipe size; 21⁄2-inch pipes may be best.
Zoomies are the simplest type of competition headers; four individual primary pipes with no collector. They are usually found on the most powerful supercharged engines, and most folks think that they don’t have collectors because those engines are so powerful that they don’t need the additional mid-range torque that collectors help to generate. That’s not true. Of course, tubing diameter and length must still be matched to the engine’s flow volume and RPM range for best power.
Pan-evacuation kit from Moroso includes the exhaust probes, anti-backfire valves, valve cover breathers, and grommets. (Photo Courtesy Moroso)
Pan evacuators take advantage of exhaust flow to create a low-pressure signal to the crankcase breathers in the valve covers, which can be good for as much as 10 more horsepower. Although they have largely fallen out of favor with today’s professional racers since belt-driven vacuum pumps are far more efficient, they still offer a good bang for the buck for many Sportsman and Bracket racers. Pan evacuators are only for open exhaust headers and should not be used on street-driven vehicles with mufflers.
Serious engines need serious pipes. This all-aluminum Rat motor has spread port heads, sheet-metal intake manifold, and two stages of direct port nitrous. Fabshop 21⁄2-inch headers feed into a merge collector where an O2 sensor gathers data for tuning.
These Borla stainless steel race headers have a muffler built into the collector for racing organizations that have maximum noise restrictions. They are also useful in bracket-style drag racing, allowing you to “sneak up” on your competitor if you have the faster car.
Competition machines usually are allowed to run open headers, and primary tube size starts at 21⁄4 inches and goes up from there. Consider step headers for large-displacement and/or high-RPM engines, and consult the manufacturer for specific recommendations.
So-called street car racing organizations and classes are really designed for full-on racing vehicles with mufflers, but the good news is that there are competition mufflers and exhaust systems big enough to handle 1,000-plus-hp engines. Several manufacturers offer header collector mufflers that simply replace the existing collector on a set of racing headers.