4-2-1 exhaust (Tri-Y)

[Farmer]

OK, I have now done quite a bit of reading, and been "under the hood" of about a dozen different modern cars that use 4-2-1 exhaust manifolds.
I have pretty much found out how to get close on calculating the primary tube length and size. But I cannot find any material dealing with length and size of the secondaries. Only than it, naturally, has to be larger than the primaries.

It seems that most newer cars have long secondaries. Up to about 3 feet long. Some even more than that. In most cases the secondarys are LONGER than the primarys.

But if I take a look at f.i. the T4 Tangerine manifold that Racer Chris made for Raby, the secondarys are shorter than the primaries. Also the Tri-y for a Porsche 356 race enginine has short secondaries. (Hondas, Yes I know, too)

Is there someone that could tell me the general rule on, how and what dimension and length on the secondaries make influence on the engine characteristics ?

Thankyou.
Torben