>> Header Design
>> Tailpipe Design
>> Carburetor Sizing

  Header Design for a 454Chevy Powered Dragster
You have decided to build a dragster out of a late 60’s Camaro. Your design goal is to make a quarter-mile dragster that is not overly difficult to drive, but is much faster than any street performance car. You are going to race against your dial-in time. You select a 454Chevy engine to satisfy your horsepower requirement. The motor must last the entire racing season to fit your budget, so it can’t be too radical. You also select a three-speed automatic transmission.

The Camaro complete with driver and tubular frame weighs 3200Lbs. You have tubbed the rear to accommodate 33 by 13 drag slicks. The engine starts with a hood scoop, a 1000CFM carburetor, a single plane intake manifold, rectangular port heads, 2.25” intake valves, 1.90” exhaust valves, and 12.5 to 1 compression ratio. This V8 engine has 4.250” cylinder bores and a 4.000” stroke.

You want a broad horsepower band to match radical shift points, but don’t want to compromise valvetrain life by using radical lifts. Your cylinder head intake and exhaust ports have a well balanced flow bias. Your camshaft design is as follows: Intake lobe has 260 degrees duration, 0.700” lift using 1.7 to 1 rockers, and ILCA of 107degrees. Exhaust lobe has 260degrees duration, 0.700” lift using 1.7 to 1 rockers, and ELCA of 117degrees.

With open headers, the engine produces 685horsepower at 6750RPM at the flywheel, and 623foot-pounds of torque at 4750RPM. Horsepower is 651 at your shift point of 7500RPM. A torque converter stall speed of 4500RPM is needed with the three-speed transmission. A 4.86 rear-end gear should give best quarter-mile times with the 7500RPM shift point and the 33inch tires.

To design your 4-into-1 open headers, you must evaluate the engine’s operating RPM requirement. The torque converter stall speed of 4500RPM is at an RPM somewhat lower than the peak torque RPM. In this case a header designed with a Performance Factor of 5 would maximize torque at this launch RPM. But since the engine will be operated well past the peak horsepower RPM, a Performance Factor of 4 will be too conservative. A header designed with a Performance Factor of 7 will maximize horsepower right in the middle of the engine’s operating RPM range, and is the best choice. In fact a Performance Factor of 7 is the most conservative setting recommended for all-out drag racing, and should be selected for three-speed transmissions. If the engine had more conservative valve springs, and therefore lower shift points and torque converter stall speed, then a header designed at a Performance Factor of 5 or 6 would probably give better elapsed times.

The Header Design Program recommends primary header pipes having 1.84” inside diameter, and 31.1” long measured off the back of the exhaust valve. The collector is calculated to have an inside diameter of 3.14” and a length of 12.8”. The actual header will use 2” outside diameter by 29” long primaries, and 3.25” outside diameter by 13” long collectors. The collector length can be increased a little at a time when testing at the drag strip until lowest times are realized. The Camaro is expected to run a 10.36 at 128MPH in the quarter. The car should also run a 6.57 at 106MPH in the eighth-mile. This car is much faster than any stock performance car. [ top ]

 Tailpipe Design for a 350Chevy Powered Street Machine
You have completed the assembly of your engine, and have made an excellent set of properly designed headers. You drop the engine in and bolt on the headers. You break-in the engine, adjust the carburetor and ignition, then scream off through the neighborhood toward the custom exhaust shop. What will you ask for when you get there? The Header Design Program told you where to put the H-pipe, but what size tailpipes should you use?

To size the tailpipes you must first evaluate your driving style and the importance of part-throttle operation. You built your car for spirited street performance, but also need it for a not-so-exciting daily commute. A monthly trip to the dragstrip is not out of the question, but you don’t want to compromise overall drivability and fuel economy for lowest possible ET.

Your engine is a 350Chevy with 4.030” bore and 3.480” stroke. With a compression ratio of 9.2, your engine produces 335flywheel horsepower at 4750RPM with open headers. To size the tailpipes for your dual exhaust system you will need to know the size of the primary header pipes calculated by the Header Design Program. The program says you need primary header pipes with a 1.42” inside diameter for your operating RPM range. Next you need to further consider your performance requirements.

Given your requirements, HeaderDesign.com recommends you use a tailpipe area that is 50% of your total primary header pipe area. You need a total tailpipe area of about 6.3in². The pipes on your dual exhaust system should have an inside diameter of about 2.01in. The closest size pipe is 2.125”, with an inside diameter of 2.00”. This gives a 50% area ratio, which will give good daily driving performance.

Use the 2.125” tailpipes for your dual exhaust system, and a pair of free-flowing mufflers and converters if necessary. A single 3” diameter tailpipe would also work well (51%), but the construction of the headers with Y-pipe collectors is much more complicated than the dual exhaust system with H-pipe. A dual exhaust system also looks great if done tastefully. If the tailpipe length were quite long, as is the case for trucks and RV’s, the single 3” diameter tailpipe would probably be the better choice.

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  Carburetor Selection for the 454 Camaro Dragster
To select the proper size carburetor for the 454 cubic inch powered Camaro dragster in the example above, you will need to estimate the volumetric efficiency of the engine at launch RPM, as well as at the RPM levels just before and after a shift. Using your engine simulation program, you find that the engine’s volumetric efficiency at launch (4500RPM) is 94%. The minimum RPM after shifting into the second gear of your 3-speed automatic transmission is also 4500. At your shift RPM of 7500, volumetric efficiency is 83%.

To ensure that the carburetor will function properly immediately after shifting into second gear, the average pressure drop through the four wide-open throttle-bores should be kept above about 0.5inches of Mercury at 4500RPM. This will ensure good fuel atomization and maximize fuel evaporation. The engine will noticeably “lay-down” after the automatic transmission shifts gears if the air/fuel mixture isn’t accurate and well atomized. If you were using a tunnel-ram intake manifold, with the intake runners pointed directly at the throttle bores, you could probably go slightly lower than 0.5inches of Mercury pressure drop.

You decide to try a four-barrel carburetor that has a flow rating of 1000CFM at 1.5inches of Mercury pressure drop. After inputting the proper values into the CFM Calculator, you find that at your launch RPM of 4500 the actual pressure drop is only 0.46inches of Mercury. You will need to get the accelerator pump shot tuned correctly to get the car to launch off the starting line. Also, the engine might hesitate slightly after you shift into 2nd gear. Near the shift RPM, the actual pressure drop is about 1.00inches of Mercury, which is optimal. This carburetor might be too large for this engine to operate at 4500RPM. A 950CFM carburetor might be a better choice.