Vacuum during a 1/4 mile run

I realize engine vacuum drops off at the hit of the throttle but does it return in any amount during the run. Just curious. Thanks.

Depends on the size of the carb or throttle body relative to the engine's displacement.

I used to have 1" of vac going down the track with a 408 and a 850. I believe the theory is 1.5"@full throttle equals correct carb size.

493", 2-750 eddy carbs, 12.4 cr, 275-283 roller .650, 3710 lbs.

Are you using a evac on the engine or is this
just the intake/vac number your looking for

No evac. Just wondering if metering rod springs that were too light could cause the carbs to go lean on top by the vacuum possibly sucking the rods back down even slightly.

Not with dual 750 carbs. Some NHRA stock class guys have problems with high vacuum since they are forced to run small carbs. Same thing happens with the 350 cfm carb circle track guys. But with dual 750 carbs you probably are running less than 1.5 inches of vacuum at WOT.

I also run a pair of 750s.
What jets and rods are in yours?

I run a pair of 1407 Eddy 750. 572, 285/292 cam, 15-1, 6 psi fuel pressure. Summer tune is orange springs 113 primary, 110 secondary, .042" rod. Car has been 9.09@147 in summer air. Up one step (116/113) in cool air 9.04@148.
Doug

Carb setup on mine is .113 pri. .107 sec, .071x.047 rods. orange springs .043 shooters middle or top hole on the lever. Floats set by the book 7/16 drop, .110 n&s. 6lb.fp. Sometimes increase the sec. jet to .110
Runs pretty nice this way. Plan on trying smaller shooters .035 this season and also would like to try different secondary boosters from a set of older afb's (3721 afb from 327 chevy 300hp)just to see how it will work. Anyone been down this road?
Thanks for the replys.
Steve

I prefer boost