I know exactly what you mean and had a good look at the file, the transition from steady state zero forward G (which was not as "flat" as you would imagine since keeping the thing stable in 3rd at 40mph require constant feathering of the throttle) to a "little bit" of G -ie when the throttle is mashed was at 0.4s - I am confident that this is when the hammer was dropped.
If you look at the pic you can see that it climbs relatively slowly from the 0.4s point. This engine doesn't actually have any lag between hitting WOT and attaining "some" positive Gs but I guess doing it in a higher gear (like 5th) will give amuch gentler G increase slope which may be easier to compare car for car