Why are Most Headers 1-3/4"?
 

Why are most LS engine Headers 1-3/4" primary? When I was really into the 4th gens all the LS headers were 1-7/8" and people had proof they gained up top and lost nothing down low compared to 1-3/4" yet it seems most the of the LS conversion headers are the smaller primary?

Is it back to where there is no noticeable gain?

1.75 works well for a wide range of motors being used in the swaps would be my guess. Don't forget, the 5.3 is used a lot in these swaps and that motor does not need 1.875 headers. 1.75 headers also work very well in built 6.0s. You may give some up over the 1.875 headers but not much. Combine that with the fact that the 1.875 headers are probably harder to package in the engine bay even thought it's only 1/8th more. Just a guess on my part really. I wouldn't worry about using 1.75 headers on anything smaller than a serious LS3 if that's all that was available.

There are gains to be had with the 1.875" primaries on bigger engines & higher hp (500-800hp).. However, a lot of the swaps have limited room to run a larger header. We try and go with the 1.875 most often when we can though.
A few years ago we used an off the shelf Stainless Works header on a 67 Nova we built with an LS7 built to 650hp. Ran mid 11's at 125+ all day long, using 1.75" primaries.
A call into stainless works or other header companies may give you some more info, that's all I got :)

-Dale

I remember LG Motorsports had a youtube video about this at one point where they basically explained that 1 3/4 is more than enough for the vast majority of applications, and that they exclusively ran 1 3/4 on their actual RACE cars due to marginally better low end torque.

If you want a larger primary, then it is a custom header. I had a set made by Lemons headers 2" Primary for my LS 418 with a TVS 2300. This set was made to fit a DSE subframe, and the fit was fine. Actually, this set will go up for sale, because I had sled alley change the engine mounts. Same as Camaro XV, so it required new headers.

Off the shelf mass production headers or Low Rate production are always a smaller diameter primary. I remember my first set of Aluminacoat Blackjack headers for my 67 Camaro on SBC was 1 5/8" in 1984. Manufacturers develop a process to keep the cost low, so it based on material, process and tooling costs.

Majority of the header companies offer larger sizes, but they will just sell the mandrel bends. So you can build the size for your application.

Jeff