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Good evening, I need to make a fully headers for a stock LS3 engine, but I've never done one and would like to know the external diameter of the tube. Thank you very much.
if this was for all out your would want the hearder where it bolts up to the head to be exactly the same shape as the exhaust port. On my L92 heads the widest spot I measured was around 1 5/8". This what the inside diameter of a 1 3/4" tube is with .0625" wall.
You would also want to use the largest radius bend that will still fit in the car as the first one out from the port. After that I would step it up to 1 7/8" and possibly 2" further down towards the collector. This is all going to depend on how much room you have. After all the header has to fit. This is only if you are looking for every last horsepower.
As to the collector the merge style if done correctly will help reduce reversion. There is a science to header design.
Edit: I almost forgot the cone. This device goes on where the 4 primary pipes come together at the collector.
1989GTA thanks for the recommendations, but I would make a headers based on technical data that you provide me. I have to import the material (elbow) of your country and my country here have many problems with imports and now I have 17/8 pipe 2 inches in diameter.
The space in the car is spacious, it is a 80 malibu.
Your comment cone'll take it into account when manufacturing the merge collectors.
Thanks again.
Not sure of what your are trying to do with these headers, or how hard it would be to get shipping taken care of, but someone should make a set of headers for the ls3 in your car...
Buddy in my country (Venezuela) is very complicated to import from the United States. Here we do not manufacture the elbows to make custom headers. For this reason ask that diameters could be used in the manufacture of headers for an LS3. I have read and if I want to use in the primary tube 1 7/8 I need to improve with porting the cylinder head. Here a picture of a work in a 243 cylinder heads I'm working.
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Recommendations for building a headers to LS3 
