My opinion is it shouldn't vary by much if leaving at the same RPM. The only real difference being suspension loaded (FB) or not loaded (TB) and how your car reacts to that difference.
Just my personal experience, but leaving off the foot, I always try and leave at a low enough rpm to where the suspension isn’t preloaded up.
I leave at 1500-2000, usually 2000. It’s “ relaxed” at that rpm.
If you have a 5 k converter, the instant you mat it, it’s going right to that point pretty much instantly.
If you’re using a brake and have the chip set at say 3600, when you let go of the button it’s still instantly going to 5k where the vert flashes at.
The biggest difference I have seen either way is reaction time, off the foot your reaction time is obviously going to be a bit slower, but that isn’t going to effect the ET, just the reaction time.
I posted this on here before, I used to have non minitubbed 70 Duster, 002/003 leafs, ran well into the 9’s at 3350 pounds.
Many on here who know me remember the car.
For about 4 or 5 seasons I was in a 10 flat index race series with it. 400 pro tree. Had a Griner brake in it ran 1.34-1.35 consistently, always wheels up.
The same nights they had the index race, they also had a footbrake bracket race. I always entered both classes.
5400 vert, launched it at 2k off the foot, around 3800-3900 off the brake. The time slips were always virtually identical, more air off the brake, less off the foot, but same ET slip.
Know plenty of guys going high 8’s low 9’s off the foot in 3200-3300 pound cars..way wheels up 1.2xx 60 foots( NSS cars)
Not saying there can’t be a difference, but its extremely unusual if it’s more than a couple numbers one way or the other. Some cars are actually faster off the foot. Less line pressure= less parasitic loss
