Quote:
Originally Posted by codydubb
the bigger the tubes the better for performance anyways, longtube headers all the way, you get very little advantage running shorties over stock manifolds
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None of those things are true.
Making power is all about velocity. Bigger is NOT always better. If you want to make power at 10K RPM then you need big headers to prevent restriction and keep the exhaust velocity high. If you're mostly driving around town and are using an engine built to make power under 6K RPM (which Vortecs are perfect for, and most trucks are used in this way), then you don't need large tubes and are better off with smaller tubes. That's right, *smaller* headers make more power in these circumstances.
Longtube headers often make more power than shorties, but not always. And the installation and clearance issues often make them not worth the hassle. That's part of the reason that Tri-Y headers became so popular. They offered most of the performance of long tubes but were easier to package than longtubes or equal length shorties.
Shorty headers can be a huge advantage over stock headers. They are more uniform in size and shape so the exhaust gases don't have to constantly speed up and slow down in the tube. Look at most any SBC exhaust manifold and you'll see that the exhaust gas has to take a sharp 90 degree turn as soon as it exits the port. Headers smooth out this turn with a larger radius which aids velocity. Additionally, because they are made of formed steel and not cast iron they are smoother and less turbulent inside. Also, many of the headers from the 60's to 80's are "log style" which cram gasses from multiple cylinders a single small tube. The different pulses hit the collector at irregular intervals and cause issues with flow. Even a crappy shorty header is going to be more efficient than most older stock cast iron manifolds.
Here's an excerpt from a
Chevy High Performance header test that was conducted a few years ago:
"As for the smallest of the headers tested, we meant to provide a baseline number but instead netted the best overall power curve for a street/strip vehicle. We feel comfortable concluding that the shortest and smallest tube design held true to our initial theories by promoting as much upper-end horsepower as possible with a short tube while building an impressive amount of torque with a small diameter."
Keep in mind that the headers they're talking about are Flowtechs with only 1.5" primaries. And this is on a fairly stout 383 making over 430 hp! They'd certainly be large enough for a milder 350 or 305.