On the 5.3, most LT's I've seen use 1 3/4" primary tubes. Why 1 3/4" ? Is that the optimum diameter? What about 1 5/8" ? I want low end torque, not top end performance. I would think that a LT would bolster low end torque over the stock cast iron exhaust manifold, if the primaries are of correct diameter. Theory explanation would be most appreciated on this topic.

Also, collector diameter is another concern. Theory explanation on that would be appreciated as well. I see mostly 3" collectors, but the dynatech has 2.5". I would think that a 2.5" collector will provide better bottom end torque over a 3", since exhaust gas velocity would be higher. Is that a good reason to go with the Supermaxx header?

 

good question. id like to know the same so i may choose wisely the first time or i guess i mean the second time since i have shorties lol

 

Here is an interesting article on different header configurations on the same motor. The type of header you buy depends on where you want the HP and TQ.

Article:
http://www.chevyhiperformance.com/te...aders_shootout

Summarized results:

 

I think you should optimize your primary tubes based on where your motor makes peak tq....make it compliment your cam/intake manifold, don't want the headers, intake manifold or CAM fighting each other, they are all tuned to a particular Tq peak have them match as much as possible.

Bigger is not always better.... 1 5/8'' and 2.5'' collectors all the way on a "stock" 5.3...It's a truck, takes strong bottom end to get a big boat moving. Plus think about when you are towing... I rather more overall power or power under curve on a heavy truck, especially a daily driver.

I think alot of it is marketing and most peeps just want to buy the bigger pipes even though they don't understand things like velocity, scavenging and power under curve vs peak hp. Bigger is better in most folks eyes and it sells like sex!

On a side note 1 3/4'' on a 5.3 will leave room for growth like big CAM's....

1 3/4'' is as big as I would ever go on my 6.2L and my 6.2L VVT actually pulls all the way to 6k.

 

Anyone know of any formulas for optimum primary diameter, collector diameter and length , and primary length? Anyone ever find the tradeoffs for these factors on LT's for the 5.3? Brands recommended? Is 1 1/2" primaries a consideration at all?

Also, the dyno data I've seen recently comparing different diameters of primaries usually starts @ 2500-3000 rpm.......with my truck, I'm more concerned with the off-idle to 2500 rpm performance. Any data in that range would be appreciated.

 

dynatechs actually measure 2.25 at the collectors [well atleastthe ones on a SSS]

the design of out motors dont produce the low rpm torque some of you are looking for. Thank the firing order for part of that.

but a 1 3/4 primary isnt going to hurt the torque. Our design for our motors produce the torque up top. thats just the way it is.

 

A stock 5.3 makes peak Tq around 4,400rpm... I think 1 5/8'' (1.625) would be the best match for a stock 5.3 intake manifold and CAM. 1.5'' would be killer off idle, part throttle, and for towing but would hurt a bit much up top if like to race any. 1.75'' moves tq higher up in the powerband on a 5.3L, probably not what you want if you are looking to optimize low end grunt....

http://media.gm.com/us/powertrain/en..._Avalanche.pdf

The peak torque RPM of 3,860 is achieved using primary header tubes of 1.50 inches in diameter with an engine displacement of 323.00 cubic inches.

The peak torque RPM of 4,531 is achieved using primary header tubes of 1.625 inches in diameter with an engine displacement of 323.00 cubic inches.

The peak torque RPM of 5,254 is achieved using primary header tubes of 1.75 inches in diameter with an engine displacement of 323.00 cubic inches.

Some nice reading from carcraft...
http://www.carcraft.com/techarticles...ics/index.html

http://www.carcraft.com/techarticles...que/index.html

from the last link..."One of the biggest mistakes made in exhaust-header application is the selection of primary tubes that are too large. Big primary tubes are only necessary to carry the gas volume generated at high engine speeds. Most headers with 1-1/2-inch primary tubes will carry an engine well into the 300hp range, while 1-5/8-inch headers can support up to 400 horsepower, and a little beyond in some cases"

 

optimum primary size/length and collector type/size/length go out the window for the most part these days when fitment and availability enter the equation. You basically get into into custom headers when those factors are at the top of your decision process. There are some good designs out there that will get you in the ballpark though

 

What good designs are in the ballpark?

 

1 5/8 is sweet on a fairly stock 5.3. Cammed, id jump it to a 1 3/4 tube.