Tubular manifold runner size


Active Member
If you are using standard cams you need the following

exhaust valve opening = 4 degrees
exhaust valve duration = 248 degrees
exhaust valve centreline = 120 degrees
Peak RPM = you choose (say 7,200)
Volumetric efficiency = 0.60 (Brake specific something - forgot - used for turbo engines, NA is 0.50)

Hope this helps

The Doc

Staff member
Measure your exhaust port and calculate the cross sectional area, now make sure whatever tube you use has the same or larger csa, as you need to keep gas velocity up as well.


Well-Known Member
Thanks everyone.

Reading up on manifolds a bit more earlier i want to try and keep the runners as small as i can get away with to spool the turbo as fast as possible. I'm not too worried if i lose a little bit of top end power as spool is more important for what i use my car for.

My aim is to have more than double the bhp of a standard car with stock spool or better!


Well-Known Member
Not yet ;-) Just waiting until i've got enough spare cash to buy everything in one go, turbo/manifold flanges/elbows/v bands etc etc