How unequal can "equal length" headers be?
 

Should "equal length" headers be equal to within 1/4"? 1"?, 2"?
How does one measure the length? Longest path? Center of the tube?
My headers vary by 2" along the centerline. Is that within the range of acceptability? Engine for now is stock 3.2 with PMOs. Track use only.

High quality headers have primaries with lengths equal to 1/100 of an inch. The guys who made the Buckley headers (which meet this spec, if I remember the spec correctly) measured a lot of headers, and found there was quite a lot of variance.

I don't know just how they measured. If you were fabricating headers, you'd know by the lengths of tube involved. I don't know how you could measure the cross sectional center of a tube, but if you apply the same method to all the tubes, that ought at least to tell you how equal or unequal they are. An exterior centerline defined as a horizontal through the cross section center, marked on both sides of the tube, and averaged, ought to come close for doing the fluid dynamics or whatnot in trying to model the effects of the chosen primary lengths.

I really hope someone steps up with some good info, but I do not think any normal header has ever been made to the 1/100th of an inch. Also you have to look at where the valve is in the head not just the header tubes.

An old method for measurement is to fill the individual tubes with water.
When you empty the tubes into separate containers...you can see visually if the lengths and diameters are equal.
When the tubes are bent...the diameter can change...so the total volume is the key.
The collector should be the same for all...so that hopefully is a constant.
Bob

Wouldn't the diameter and length be variables in this case? Shorter and fatter would hold the same amount of water as longer and thinner.
Bill K

Headers are relatively insensitive to differences in the the lengths of the pipes, you can have an inch or more difference w/o catastrophic results, the cams affect this too, modern smog cams have so little overlap that exhaust tuning has little effect.