kz1000j 1981 exhaust size

i recently bought my first kz1000j

i want to know what is the best size exhaust headers to use on my bike according to the previous owner the headers that is on the bike is to small id(inner diameter ) so i want to upgrade the exhaust system while i am busy restoring the bike.

the bike currently has a cowley exhaust with headers not sure about the current size have not yet removed the exhaust to see what the id on the headers ,i first want to know what size wil be the best for performance so that i know if i take it off i am going to replace the system or not photos of the bike is uploaded with my post

There was a long discussion of this some years ago. The thread may be archived by now. It may have been started by user, Old Man Rock.

The diameter of the head pipes is only part of the equation. The primary pipe diameter and length, plus the design, diameter and length of the collector, plus the diameter and design of the muffler all need to work in harmony.
As I recall, the smaller diameter tubes provide a stronger scavaging signal, to a point. Beyond that point the smaller size will restrict flow.
Generally, the shorter primary pipes emphasize high rpm power, while longer ones favor a lower power peak.
The collector may be a 4-2-1, a true 4-1, a 4-2, or a 4-4 like the original Z1. Some collectors flow very smoothly, while others require the gasses to make really abrupt turns. Smoother is better. Also, the diameter and length of the collector will help determine whether the system performs best at low, medium or high rpm.
If you add in engine modifications like big bore pistons, ported head, racier cams and bigger carbs, this gets quite a bit more complicated.
Honestly, though, unless you plan to build a custom system, there are relatively few choices to be had.
If you currently have a race system on a stock engine, then you are not getting the best of what either the engine or exhaust system have to offer. Several guys with stock or nearly stock engines have reported good results with the Delkevic 4-1 system.
Are you experiencing any problems that could be exhaust related?

Based on what I can see in the picture, if it were mine, I would restore that system and love it.

TexasKZ wrote: Based on what I can see in the picture, if it were mine, I would restore that system and love it.

I agree. Sandblasting followed by ceramic coating would be sweet. Hopefully it's not all bashed up on the bottom.

Our KZ's are 40 years old these days, trying to squeeze out every last bit of horsepower from them is a poor value proposition.