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LS SWAPS: Exhaust Systems Guide

The Gen III/IV engines have the same basic requirements for the exhaust system as other automotive engines, but these high-performance engines have specific EFI system requirements that must be met for the system to perform properly. All EFI systems require oxygen sensors to read the oxygen gas levels in the exhaust and tell the computer if the engine is running rich, lean, or just right. But that is not all. LS engines come with catalytic converters that burn up the exhaust gases to reduce emissions. This is a very important factor to consider and requires a bit of homework.

Most states have a classic car provision that exempts older vehicles from emissions standards and/or safety inspections, but it is absolutely imperative to research the current laws in your state before completing your swap. Some states require the vehicle emissions to match the standards for the year of the engine, not the vehicle, which requires the engine to retain all of the original smog equipment (including the EGR, AIR, and catalytic converters) in working order. Every state is different, and the laws are constantly being revised. It is up to you to make sure your LS swap is legal in your state.



 These LS7 flanges are very different from the classic three-bolt flange used for years. Get a fair section of the downpipes with the donor LS engine to ensure you have the new-style flange. (Photo Courtesy Street & Performance)


Exhaust Manifolds

Stock Gen III/IV exhaust manifolds are fairly efficient and can keep up with 400-plus hp for most engines. The trick is finding a pair that fits your vehicle’s specific chassis, as motor mounts and engine installation typically determine exhaust manifold compatibility. One set of manifolds may fit one installation and not another. Test fitting is the only way to verify clearances.


Flange Compatibility

Stock exhaust manifolds need a compatible flange that can be welded to the rest of the exhaust system. Several different flanges are used on the stock exhaust manifolds, but not all of them are reproduced. The trick here is to make sure to get some of the stock exhaust when the engine is first pulled from the donor vehicle. The section just below the flange is all that’s needed to match the exhaust manifolds being used. If your engine does not have the correct flange available, fabrication is your only option.

Speedway Motors reproduces exhaust flanges for LS1 and Vortec manifolds. Another source for flanges is used catalytic converters. Although you can’t buy a used converter, you can buy the flanges. Most salvage yards gladly cut them off and sell them.



 The stock manifolds may or may not fit your application; information on stock manifold fitment is sketchy at best. Truck manifolds are usually too big to clear the firewall in most muscle cars. (Photo Courtesy Street & Performance)


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stock downpipes

Stock downpipes also usually have catalytic converters. If they’re usable, it will save some cash down the road. (Photo Courtesy Street & Performance)


Steel Flanges

Using the exhaust gasket as a pattern, you can cut a flange using a plasma cutter. An acetylene torch certainly cuts the steel, but it’s not the correct tool for this job because it also warps the steel to the point where it isn’t usable as an exhaust flange. Depending on the installation, there are a couple of ways to cut new flanges.

The most common material for flanges is mild steel. For a good seal, 1/8-inch or thicker material should be used. This yields a high strength while being easier to cut than thicker steel. With a good basic outer dimension measured from the manifold or exhaust gasket, any local metal sup-ply shop should be able to cut a plate to fit. Depending on the shop, they may be able to cut out the entire part needed.


 truck manifold

 The truck manifolds fit most GM trucks, and clear the steering column. You can retain the heat shields or take them off. (Photo Courtesy Street & Performance)


Otherwise, using the basic plates, trace the inside of the gasket to the plate and then trim out the plate to the outside edge of the line with a plasma cutter. Any metal overhang can cause some turbulence, impede airflow, and reduce horsepower, but does not significantly affect performance if the overhang is small. If you do not have access to a plasma cutter, the following procedure also removes any metal overhang.

Using a drill bit, drill to the inside of the line marking the exhaust opening. If you are using Z06 manifolds, do this in each corner. Then place the flange in a vise and use a reciprocating saw, air body saw, or hack saw to cut out the center section. It may take awhile, but it does work. A plasma cutter, of course, certainly makes the quickest work of the job.


Aluminum Flanges

An aluminum flange looks great with a stainless, chrome, or polished exhaust. Cutting aluminum is easier than cutting steel, and you can cut a set of aluminum flanges from plate aluminum with a band saw and a scroll saw pretty easily. Aluminum is softer than steel, so aluminum flanges have to be thicker than their steel counterparts. Aluminum flanges should be made from at least 1/4-inch or 3/8-inch plate.


stock manifold

  Although factory manifolds fit a lot of applications, they don’t necessarily have the right look. These Hooker LS manifolds fit many applications and come with 2010 Camaro type flanges for a good seal without searching for parts or gaskets.



These are tight-tuck block-hugger headers in Chromex. This coating helps dissipate heat for maximum efficiency and horse-power output. (Photo Courtesy Street & Performance)


The cutting process is the same. Plasma torches quickly cut aluminum, so be careful. If you hand-cut flanges, aluminum is softer, so it is a little easier. Also, aluminum cleans up nicely with carbide tips and a die grinder.


Custom Flanges

Local shops may use  CNC, plasma, or water jet equipment to cut the parts. The cutting process is usually not very expensive, but the program-writing time is typically around $50 per hour. Because flange design is really simple, it shouldn’t take a shop longer than an hour. Once the program has been written, a shop can make as many as needed. If you are doing multiple swaps, you can amortize the cost to several engines.

Bridging the gap between the factory manifold’s fitment and the performance of the headers are Hooker’s cast-iron LS manifolds. These manifolds provide tight fitment coupled with better flow performance, and they use 2010 Camaro-type flanges. The manifolds come with flanges and use OEM gaskets for a leak- free seal.



With all of the challenges of modifying stock manifolds, most swappers prefer aftermarket headers that are designed for engine swaps. Street & Performance offers several custom-fit headers for the most popular LS swap vehicles, as do several other manufacturers, including Hooker, Edelbrock, and Dynatech.

Choosing headers requires taking some measurements and doing some research, so be careful and do it right. Some headers might fit, but have the stock-style flange, and present the same mounting difficulties. Sure, the stock flange could be cut off and a new one welded on, but isn’t avoiding modification part of the reason for buying aftermarket headers? Some vehicle-specific headers fit late-model chassis and are designed for LS engines. These headers do not have the stock-style flange, making them excellent candidates for swap applications.

Vehicle-specific swap headers come with the universal three-bolt triangular flange that has been used for decades. This ensures the headers easily mate up to the rest of the exhaust. Some kits are available for specific vehicles that include the headers and exhaust, all in one package.


Street & Performance

Street & Performance makes headers for all the popular GM swap vehicles. They are designed for non-emissions-controlled vehicles. All their headers feature smooth-ground welds, optional Chromex coating inside and out, and matching flanges. Each header features stainless-steel head and exhaust flanges. These four-bolt flanges are a square design and have built-in oxygen bungs. There are separate applications for Gen III/IV engines and LS7 engines.


 custom headers

 If custom headers have to be made, this kit from Speedway Motors is a great way to go. It comes with LS flanges, primary tubes, and collectors. (Photo Courtesy Speedway Motors)


Applications include: 1954–1996 Corvette, 1978–1987 G-Body, 1967–1969 F-Body, 1964–1972 GM A-Body, 1962–1979 Chevy Nova, 1955–1957 Tri-Five Chevy, 1955-up GM trucks, street rod mid-length, and street rod block hugger.



The conversion kit from Edelbrock includes headers made of 409 stainless-steel tubing with 3/8-inch-thick port and collector flanges. The collectors use graphite donut gaskets instead of the leak-prone three-bolt collector and gasket. The kit fits 1964–1972 GM A-Body cars, including the Chevelle, Malibu, El Camino, Cutlass, 442, Sky-lark, Buick Special, GS350, GS455, GTO, LeMans, and Tempest. The Gen III/IV swap headers have 13⁄4-inch primary tubes stepped up to 17⁄8 inches for maximum flow and power.

Edelbrock recommends using its LS-series engine-mount kit (PN 6701) with Edelbrock swap headers. If you have Edelbrock E-Tec-series LS heads or Vortec Fast Burn heads, the Edelbrock swap headers (PN 65083) have the correct port flange configuration, so they are compatible with stock A-Body motor mounts. The Edelbrock LS swap exhaust system matches the headers and motor mounts, although the system also works well with other mounts and headers.

The exhaust system is constructed from 21⁄2-inch 409 stainless-steel tubing with an X-pipe assembly and includes a pair of Edelbrock SDT-series mufflers and a pair of polished stainless-steel tips.

This kit fits the same vehicles listed at left for the header conversion kit.


Hooker Headers

Hooker builds a couple of vehicle-specific LS swap headers (first-generation Camaro/Firebird and GM A-Body) in addition to its universal block-hugger LS headers.

1967–1969 F-Body, 1966–1972 GM A-Body: These headers are built with lightweight, 18-gauge tuned-length 13⁄4-inch primaries, with 3-inch smooth transition slip-fit-style collectors. This yields a leak-free fi t to the rest of the exhaust. The headers are designed to fit tight to the chassis, giving increased ground clearance, from 1 to 3 inches, which is especially helpful for lowered vehicles.

The cylinder head flanges are made from 5/16-inch machined steel for a good seal. To accommodate the oxygen sensors, the headers include 3- to 21⁄2-inch slip-fit reducers with oxygen sensor bungs welded in, plus a set of extra bungs for running a 3-inch exhaust system.

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Hooker LS swap headers fit most manual and automatic GM transmissions, including the Tremec 5- and 6-speed manual transmissions. Block Hugger: Hooker’s block-hugger LS swap header design is tuned for low- and mid-range street performance from idle to 3,500 rpm. The primaries are made from 18-gauge, 15⁄8-inch mild steel tubing that is mandrel-bent and tuned by length to reduce backpressure and increase exhaust velocity. This design delivers more power, torque, and crisp throttle response; the 21⁄2-inch collectors with a 3/8-inch-thick three-bolt flange make installation simple.

The head flange is machined to ensure a fl at surface for a tight seal. The tight-tuck tube design was used to fit within narrow street rod frames and provide a good fit in most vehicles that do not have a dedicated swap header. The collectors exit parallel with the oil pan for maximum ground clearance in lowered vehicles and are positioned to clear the starter and motor mounts.

Doug’s Headers: Designed for Tri-Five Chevy, Olds, and Buick cars, these shorty-style headers feature 17⁄8-inch primaries with a 3-inch collector made from 16-gauge steel. They are available in ceramic coated or natural finish.


1932 Ford Coupe

There is no hot rod platform more iconic than the 1932 Ford. From the very beginning of hot rodding, these cars have been at the forefront. Ever since 1955, people have been slapping small-block Chevys between the rails. What you don’t see very often is an LS under the hood. These rods are typically built with a lot of thought and emphasis on form rather than function.

Frankly, the Gen III/IV engines don’t have the same aesthetics as a classic small-block Chevy, but that doesn’t mean it can’t be done. Matt Diamond of Gearhead Garage in Gilbert, Arizona, planned to do just that. “The goal of the build was to give the customer a reliable daily driver with some modern amenities built on a vintage platform,” Matt told me. “The car has A/C, a killer stereo, and makes 400 hp on pump gas.” I think he nailed it.

The project began with a fiberglass 1932 Ford coupe body and frame from Atlanta Hot Rods. The front suspension consists of Pete & Jake’s components. The engine is an LQ4 salvage take-out from a 2002 Chevy Silverado, along with the matching 4L60E transmission. Mounting the engine to the frame is a set of motor mounts by Dirty Dingo Motorsports. Matt used an F-Body oil pan for ground clearance. The accessory drive features components from the Vortec system, an F-Body tensioner and water pump, mounted to Street Shop brackets. The water pump was machined so it could rotate 90 degrees, placing the outlet facing downward rather than out the side in order to provide clearance for the lower radiator hose. Gearhead Garage tuned the LQ4 ECM using HP Tuners software through a modified stock wiring harness.

One of the key issues for LS engines versus the typical small-block Chevy in a 1932 Ford is the look and fit of the exhaust. The owner of this car wanted the option to run hooded or with the hood sides removed, so the exhaust had to match. This is not an issue for a small-block Chevy, but the LQ4 proved to be tricky. None of the off-the-shelf headers or factory manifolds cleared the hood sides.

Matt built a custom set of headers to clear the frame and the hood sides while retaining the look. He added a set of electric cutouts on the headers with side dumps so the owner could choose when to get loud.

Once completed, the 1932 runs like a brand-new car with all the goodies expected on a high-end hot rod. It even features keyless push-to-start technology. Weighing just under 2,500 pounds, the 400-hp LQ4 makes this a beast on the drag strip too.


Written by Jefferson Bryant and posted with permission of CarTechBooks


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