Step primarys
The Step design headder extends the useful horsepower band higher than the band will be with a single diameter primary. This step magnifies the exhaust pulse going back to the exhaust valve from the collector to enhance the scavenging effect at higher RPM's. However like everything in physics there are side effects this causes at lower RPM,s. I use this style on engines that spend most of there time above the peak horsepower RPM. Drag racing engines often and circle track on longer tracks will benefit by extending the peak horsepower RPM 3 to 4 hundred higher. The bottom end will often be sour below sometimes 4 thousand RPM under full load condition. Where folks often go wrong is the step is too close to the head, which causes the sour rpm to be higher than your engine likes or can deal with. The step or steps need to be arrived at by the predicted exhaust velosity for the engine and cam profile and vehicle weight. The heavy car will not like a step unless the engine can rev freely to the tuned point. A light dragster with a big powerful engine will like a step closer relative to the head than a heavier car will. The converter or load will need to be higher in the rpm band to take advantage of this set up. It is not as simple as just stepping the primary one size larger, X dimension from the exhaust valve. Lots of factors must be taken in to account for every application. Not every headder builder will bother to educate there customers on the physics that are involved in a great set of working pipes and some don't even know them selves. They are just fabricators and could care less how there product works or doesn't. Most buyers are pleased if the pipes fit there chassis and if they look cool on there engine. The performance of the pipes are seldom checked and mostly presumed by the less knowledgeable customer's of the big well known builders. The cam spec's, rocker ratio, converter stall or trans gear ratio, weight of race car, type of racing, engine asperation, and fuel type are just some of the major factors that MUST be evaluated to estimate where a step might work best in any given race engine. This is why step pipes are a hand bone for second hand users. What are the odds that the step is in the best location for your engine? NOT GOOD I assure you. I have fabricated headders after the better known builders were paid to on some customers race cars and have fixed tuning problems that were built into the pipes from malicious step locations. Plagiarism is the ultimate sin in expensive racing engines and headders. If you don't know how to calculate the appropriate step location for your engine, don't do it, you WILL most likely loose torque, kill bottom end horsepower and tune-ability over a single diameter primary.
Good luck in 09