Headers & Optimal Primary Length

[geoklass]

What is the optimal primary length, short, long, of somewhere in the middle. Like many things when building anything for a high performance engine, "it depends".

If you are building headers for a Top Fuel dragster, the primaries are not very long. Of course, the primaries are all there is to a Top Fuel header, there are no collectors. There is also a very small power band in a T/F dragster, it leaves the line at almost the same RPM as when it crosses the finish line.

The primaries on the same engine mounted in a Funny Car are a little longer. Not because they need to be for the power band, but because the wider body requires longer tubing. In a regular street/strip Mustang, the primary length of a Long Tube header is pretty much determined by the room inside the engine compartment (typically between 32" to 34"), and don't forget, these primaries empty into a "collector". We have found that the collector changes the dynamics of the header. Based on what we have learned on the dyno, the length of the primaries are not that important when the primaries go into a collector. A typical 1 3/4" Long Tube header has a 3" collector (8" to 12" in length), but the reality, if the customer is running a 3" exhaust system back to the mufflers, his collector is actually 4-feet long.

A well designed Mid Length header is very close in power to the same header in a Long Tube configuration, when running a full exhaust system. On typical dyno comparisons, the difference between the two (everything else being equal, as in 1 3/4" primairies and a 3" collector) is between 6 to 8 horsepower on an average, all through the horsepower band. The primary length is just not that significant. But when discussing Mid Length headers, the term "well designed" is significant.

 

[Noobz347]

Moved to Fox Tech

 

[geoklass]

Thank you, I'm in your debt.

 

[geoklass]

I have been a hot rodder since I was 15 years old, and started drag racing when I was 16 (1955) at Santa Ana Drag strip. When I started out, header design was pretty primitive. Most headers at the time were less restrictive because they had more gentle bends or radius, but not many used a "collector" to enhance scavenging. The terms "shorty" or "mid length" or "long tube" really did not exist when discussing the primary pipe length. "Smoother exhaust flow" was all that racers wanted. Of course, dragsters and out all-out race cars ran individual primary pipes, some short, some long, and some in the middle. The truth was that none seemed to be that much of an advantage over the other. A lot of the header design seemed to be being done around the Super Stock cars. In the late 1950's and very early 1960's, the "tri-y" type headers became popular, with four primaries merging into two slightly larger pipes and then into one pipe. Long tube headers started in the 1960's also, which is also when companies like Jardine, Hooker, Hedman, etc started to make them for racers and street machines. They worked fine, and then many Super Stock and F/X racers started experimenting with shorter length primaries, mainly to save weight on the front of the cars and to make it easier to work on the transmissions and clutches. Performance wise as far as horsepower, there was rarely much difference between the various primary pipe lengths, despite all the "theories". And ya know what? This is about where we are today, over 50 years later.

Just to show you how many of the greats in Super Stock racing realized that the Long Tube header was not the only way to go fast, and that the Mid Length header was just as fast and offered other advantages, here are a few photos of well known S/S racers running Mid Length headers:

 

[geoklass]

Here are a few more:

 

[geoklass]

It may seem like the Mid Length design mostly applies to the Dodge/Plymouth combos. Not really, it just has to do with the chassis design. The Fords and GM cars of that era had full frames, and they were a little wider than than the unit body chassis Mopars. The GM and Ford engine compartments had more room between the frames and the engines, and their headers usually went through those spaces. With the Mopars, pretty much all the headers went out the side panels. Here are some photos of Long Tube Headers on the Mopar S/S cars. It can give you a comparison of the primary length pipes between the Long Tubes and the Mid Length headers. Note the location of the collectors.