Trying to fine tune my 383 with a wideband and have the Edlebrock power piston spring kit but have a question. The springs are listed as 4, 5, 6 or 8 inches vac. This appears to be the point at which the power system will be at full operation but what point do they start? I have an Edelbrock catalog and have searched the web but no luck. My car has about 12-13" vac at idle in gear and under moderate acceleration it goes lean when vac drops to around 11". This is why I need to know the cut in or starting point for these springs.

I don't know what Edelbrock is selling but the factory ones were rated at start and full enrichment. They made a 14-4 11-5 10-6 8-4 7-3 6-3. It is possible that Edelbrock is just selling the ones made for low manifold vacuum.I got some of the springs from the factory years ago but I think you haven't been able to get any for many years. I would try one out of a 70 or earlier Q-jet and work from there

Thanks. I know the factory used that type of designation and hoped that Edelbrock did as well.

I called Edelbrock tech and as usual, they were completely worthless. They could not give me any info about their own spring kit other than the full open rates.

So I called JET Performance in CA (jetchip.com or 800 736 8513) and they have impressed me again. According to them, the colored springs for Q jets all come from the same supplier and here is how they operate:

Red starts at 7" full opened at 3"
Orange 8" adn 4"
Black 10" and 6"
Yellow 6" and 3"

The guys at JET have been real helpful to me over the years and they really know their stuff. Hope this helps someone else.