bopiela

Hey guys quick question because I'm at a complete loss to where I had this information. What is the vacuum rating of our 4.6 engines? My BOV has the wrong spring and I want to make sure I get the right one when I order.

Thanks

cliffyk

For a bit of background, the BOV spring holds the valve closed during periods of high manifold vaccum to prevent back flow of unmetered (and possibly un-flitered) air into the intake.

So, to select/adjust the spring you need to use a vacuum gauge to determine the peak manifold vacuum for your engine--it should be around 18 to 20 inHg at idle, however this depends on a number of factors including cam overlap and altitude--it could be anywhere from 15 to 22 inHg. Peak manifold vacuum will also be a bit higher under closed throttle deceleration, that should be determined as well.

Some manufacturers rate their BOV springs by psi which is somewhat confusing as it implies that the spring rating is linked to boost--it is not. The psi rating for BOV springs is the normal manifold vacuum (usually expressed in units of inches of mercury, inHg), plus a bit to allow for closed throttle deceleration, converted to psi; here is the conversion formula:

1 inHg = 0.491 psi

So if your manifold vacuum at idle is 20 inHg you would add a bit, let's say 1.5 inHg and convert 21.5 inHg to psi = 10.5 psi.

bopiela

Thanks cliffy. My tuner told me at one point and I believe the springs for my BOV come in a 20-22 in/hg setting so that's the one I needed. I just needed to confirm and wanted to double check before purchasing and correcting the problem.

Thank you much!!

cliffyk

I would check the actual manifold vacuum at idle, it does vary between engines, even dead stock engines--HF has a vacuum gauge kit for $13...

FrostByte

Or cal Vortech & tell them what u have. Im shure they will help you. Just a phone call.

bopiela

Thanks all. Will check. Appreciate the input!!

Kerns

When you are supercharged you don't go off vacuum rating... You just run a 3psi spring.

bopiela

my tuner told me to get the right spring because it was blowing off too early at times. and i can tell it will at weird times. which i assume is do to my blow through bov intercooled setup??

Kerns

manifold pressure will provide enough force to keep the bov closed under wot... Beyond that it should be open at all times unless you are 1/2 throttle or greater. If it is blowing off early its most likely a leak somewhere along the line.

bopiela

well it doesnt under wot. it does other times, so i guess that would be because its supposed to? so maybe i actually dont need to change it?