Effect of inlet air pressure on A/F

[oldjohnno]

Assuming an effective air scoop or ducting, what could be reasonably expected with regard to a/f change with the small increase in pressure resulting from a speed of say 150mph? What I'm trying to ascertain is what sort of jetting adjustment would be required from what the engine wants on a (stationary, obviously) chassis dyno to around 150 - 160 on salt? Bowl vents are in the air horn, but I'm guessing the increased air density would require some amount of enrichment. Any thoughts or ballpark starting points?

[MadBill]

Assuming an effective air scoop or ducting, what could be reasonably expected with regard to a/f change with the small increase in pressure resulting from a speed of say 150mph? What I'm trying to ascertain is what sort of jetting adjustment would be required from what the engine wants on a (stationary, obviously) chassis dyno to around 150 - 160 on salt? Bowl vents are in the air horn, but I'm guessing the increased air density would require some amount of enrichment. Any thoughts or ballpark starting points?

As long as the bowl vents are seeing the same pressure as the air horn, the difference is too small to matter. For example, at sea level, 150 MPH produces a theoretical ram pressure of 0.397 psi or 2.70%. Holley suggests a one jet size decrease per 2000' increased altitude. That increase results in about a 4 psi reduction in barometric pressure, Therefore 150 MPH = 1/10th of a jet.

[F-BIRD'88]

At the drag strip your mileage may vary.

Try 4 jets bigger from dyno jet and tune back down watching mph and for too lean AFR.
I find many cars respond to bigger jets with a proper hood scoop at the track.

[SupStk]

I have found there is more power to be had from the cooler air than any benefit from ram air. It don't take much air pressure over the bleeds to screw up the mixture. Recall one experiment whereas ram-air was ducted into the carbs and completely sealed. That instance the car ran best with three 1" holes in the back of the scoop to vent pressure. Came on that "fix" after building lot of fins and baffles.

[randy331]

At the drag strip your mileage may vary.

Try 4 jets bigger from dyno jet and tune back down watching mph and for too lean AFR.
I find many cars respond to bigger jets with a proper hood scoop at the track.

I never see that kinda difference from dyno to track.
Seriously,...Why bother to dyno if you have to start the tune up all over at the track ???

You must have flawed testing procedures when you dyno.

Randy

[Rick360]

If your scoop is converting all of the velocity into pressure, it would be .89"hg more and would result in HPCF increase ~ 1.5%. The jet required for the extra pressure would be .6 more jet in the Holley jet 80-90 range.

Rick