Headers - How Important is Primary Size?

[BubbaGreen]

How much of a difference will it make if one installs
1-5/8" vs 1-3/4" primary full lenght headers. Let me
add the part numbers that I am questioning is the
hedman elite series HED-69890 vs HED-69088.
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This would be for a streetable driver, not a race truck.
TIA...

[cst70rocket]

primary size is very important, too big, can kill low end torque, too small can kill upper rpm hp. With a tbi 350, go with the 1 5/8".
Kirk

[BubbaGreen]

Guess I should have stated, this is for use with a GM Crate motor RAMJET-350 TPI.
The included manual specifies 1-3/4" Primary dual exhaust w/ H-Pipe.
The price difference between header models is approx 40-50 bucks.

[Zeke]

Try this link lots of info on building headers and what different size primaries will get you.
http://www.headersbyed.com/index.htm

[LUV2XCLR8]

I had very limited choices on this for my rig, on a 4wd everything reasonable is
5/8 unless you want custom or very expensive headers, to rich for my blood

[Sweet72]

Man Zeke, what a informative site. I didn't know and I'm sure many others don't know how much potential power can be lost (read that as not gained) by bolting on a set of their favorite headers.

[Dual quad]

Bubba, the header pipe size dictates max flow rate, velocity, and to a certain degree scavenging. Normally SBC header pipe of 1 5/8 is ideal. 1 3/4 is appropriate with a radiacal SBC making major HP #s. The Ramjet stock cam is relatively mild and as CST said, the larger pipe size will kill your low end torque.

The included manual specifies 1-3/4" Primary dual exhaust w/ H-Pipe

I think what the manual is refering to here is the actual exhaust pipe diameter, not the header tube diameter. Personally, I'd go for at least 2" (dual) on the exhaust pipe and 1 5/8 on the header tube.

[Stroker]

http://www.headerdesign.com/index.asp is a great site for header and exhaust information. I think you'll find that the 1 5/8 is a better choice than the 1 3/4 for your application. To bad they don't make headers with longer primarys.

[BubbaGreen]

According to the ship along ramjet-350 manual.....it makes reference to all testing and tuning using 1-3/4 primary into 3" collector using 2-1/2" exhausting tubing w/ an H-Crossover pipe.
But, I am thinking if I install the 1-3/4 inch primary header, that I will suffer the low end torque. I do plan on going with 2-1/2 inch exhaust.
Havent had time to check out the posted link, but will do so later.

[ChevLoRay]

I haven't seen it mentioned, yet, but remember that an internal combustion engine is very much an air compressor. With that in mind, you have to concern yourself with the engines' ability to process the incoming air and that means also exhausting it. Starving it for air, will limit its' efficiency. To use a too-restrictive exhaust will also limit the efficiency. So, it is with those ideas in mind that the engineers and test folks test what works and what doesn't. While their tests were done with the larger of the two sizes of tubing, it doesn't mean that the engine won't function with the lesser size. Going for the larger diameter will allow more air to exit the engine, specifically at higher rpm's. However, like stated earlier, the smaller diameter tubing will aid in low-end torque, which is what you feel from the "git-go".

The H-pipe balance tube, is designed to even out the pulses and help create a scavenging effect, causing the exhaust system to effectively create a vacuum effect from one side, to the other. The only problem for my truck, is that the optimal place for the balance tube, is directly beneath the pan on my transmission. So, I am running without one.

The balance of the exhaust system also will affect performance, but there has to be some back pressure in it, for optimal street efficiency. How much? Good question. I've got a 2.5-inch system, from the headers, to the 40-series Flowmasters, then 2.25-inch from the mufflers.

[Sweet72]

Originally Posted by ChevLoRay

...there has to be some back pressure in it...

I made a similar statement on here a couple years ago and it went over like a fart in church. Bigger isn't always better, or... "it's not the size of the boat, but the motion of the ocean".