Keith, at some points in the rev range, no matter how long you stay there you will NEVER come on boost.


Take your T4 cossie at 2500rpm up a hill where its steep enough to keep you at 2500rpm in the gear you are in and floor it, you will NEVER come on boost.

Thats being outside the boost threshold, you are at a point in the rev range where there just isnt enough pressure in the exhaust housing of the turbo to spin it up enough to generate boost.



Do the same test on a hill suitable for holding you at 5000rpm off boost and you will find that when you floor it, nothing happens inititally, then after a second or two you get a load of boost, THAT is lag.


so the boost threshold is where in the rev range you can spool the turbo, and lag is how long it takes to do so within that threshold when you go from a closed throttle to an open one.