Couple books say that your swanky turbo exhaust manifold should have equal length runners. But I saw a Saab Viggen mani the other day (divided outlet, with paired 1-4 and 2-3 runners) that had 1 & 4 @ equal-length, but 2 and 3 @ unequal length compared to each other, and both shorter than 1 or 4. This car runs 20 psi stock, so what's the deal here -- Saab's been in the turbo game since the '70s. Is there some equation to determine some percentage length the shorter runners should be vs the longer? It appears from looking at this manifold that the whole equal length deal just doesn't matter.

Thoughts, experience, and some tech is appreciated.

 

they should be, but not always.

unless your running some serious HP, it doesnt matter.

 

its more important that they flow well

 

 

Log manifold

 

THey certainly don't have to!
I had the chance to test quite a few manifold designs. Free flowing merges and wastegate placement are far more important.
here's an example: very unequal, yet surprizingly good working manifold!

 

a good OEM puts its focus on reliable, then cost effective, then its appeal to the masses, then finally performance.

a bad OEM goes cost effective, appeal to masses, reliable, performance.

 

equal length runners= over rated :P

 

Fixed it for you.

 

It seems like it wouldn't matter at all. How fast is all that exhaust gas coming out of there anyways? It seems to me like it's being expelled so quickly that it doesn't make any kind of difference unless the lengths were waaaaaay off.