Originally Posted by AU N EGL

(from CORVETTEFORUM)

This has been posted a few times before. SOme things to think about are what are you going to use you car for? also dyno numbers show differences, but how often is you car running from 5500 to 7000 rpms?

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Headers

The LONGER primaries of the LG's & Dynatech tend to LOSE their scavenging effect above 3500 rpms.. They are GREAT for LOW END TORQUE at LOWER RPMS. Perfect for street driving & road racers..

However for DRAG racing, the brands with shorter primaries KOOKS, AR, TPIS, will OUT PERFORM from 4000-7000 rpms, exactly where the power band is @ WOT....

The dyno results may only offer minimal if any gains, but on the drag strip, a .05-.1 quicker ET can be had.

Dyno tests with headers having primary lengths adjustable in three-inch increments show that lengths between 24 and 36 inches have only a minor effect on the power curve of V-8s that you and I can typically afford, although the longer pipes do marginally favor the low end.

PRIMARY PIPE LENGTH
"The overall length of the primary header pipe is governed almost exclusively by the target engine's rpm range, which is dependent upon wave tuning. Typically, a lower engine rpm range likes a longer primary pipe, while a high rpm engine prefers a shorter primary."

The length of the primary pipes also affects torque. Where diameter affects the torque peak relative to rpm, the length affects the shape of the torque curve. Longer primaries provide more torque below the peak and reduce it past the peak. Shorter primaries provide more torque above the peak at the expense of below-peak torque. More torque in the low- to mid-rpm range is important to drivers who want that feeling of seat-of-your-pants performance on the street. Longer primaries also reduce the chance of escaping exhaust's being drawn back up another pipe.

In order to keep the torque curve the same for all eight cylinders, it is important that primary pipes be equal in length. Exactly how equal they have to be is more critical on uncorked race cars than for the vast majority of mild-engined street cars running through mufflers. In most applications, pipe length deviation of 2 to 3 inches on a set of full-length headers is not a problem.

Backpressure

From a certain perspective, you will always have backpressure. The only Zero-Backpressure system out there is in outer space. As long as you are running an engine in atmospheric air, there will be "backpressure" from the static pressure at the end of the tailpipe.

I've always found it odd that people don't give more thought to where their pipes terminate, as some placements are better than others. If you dump the pipe into an area of smooth, hi-speed flow, then Bournoulli tells us that you have lower pressure there, and the exhaust flow will be enhanced because it is essentially being "sucked" out of the pipe.

This is why going to big on the pipe diameter can really kill power, because the exhaust flow has more atmosphere to fight against. Pipe diameter is always a compromise-- the ideal only exists for ONE combination of throttle position, timing, load, IAT, etc etc-- everything else is compromised to some degree.


Source: https://ls1tech.com/forums/showpost.p...80&postcount=4

Pipe Size

For exhaust, pipe size mismatch will show up in the power curve, with oversized pipe losing at low RPM torque and undersized pipe suffering upper RPM hp.

1 7/8” headers for low to mid range tq and hp 2” for high end HP or big cube motors