Boost/vac gauge readings question.

[v10kingsnake]

In anticipation of my car getting the ugr paxton treatment, I installed a boost/vac gauge in the car tonight. It is a mechanical gauge which I tapped into the MAP sensor line at the rear of the block of the passenger side of the intake manifold. Tator advised me that it was as good a place as any to tap into the motor. When I started the car the gauge moved to about -12 or -13 on the vac side which I expected to see but when I hit the gas the pointer moved up towards 0 but fell sharply to -20 or so when I held the rpms up at 3k or so steady. I figured it would stay close to 0 while the car was being rev'd but it didn't. Never expected that. I suppose that's normal but I really never had a boost gauge installed in a car before the blower was done so because the car is running perfectly I am thinking it is ok. Does anyone know how it should react pressure wise in a vehicle not running a blower (yet). Josh

 

[Joseph Dell]

WOT = 0. part throttle = variable. idle is 10-13 deg of vac at start and 7-9" of vac after it has warmed up.

 

[v10kingsnake]

I guess it was fine then. Never went full thrpttle and never really had ample time to warm up but cold pressure was in check to those figures. The 3 k hold which was only for a couple seconds brought the gauge closer to 0 but sharply back to -20 or so after a solid second/ I know theres no leaks so I guess it's fine. Guess I gotta roll it full throttle tomorrow to make sure it reads 0 to be positve. Oh snap, tomorrow is mischief night so the car wont be leaving the garage until after halloween. Dont need a ****** charge because some teenager thought it was funny to toss eggs at a viper.

 

[SYNFULL]

I installed an autometer vacuum / boost gauge and I am a little puzzled over it's readings as well. I am running an 8lb pulley on the roe but at full throttle the vacuum goes to 0- never positive. I also have it installed on the map sensor line. Anyone have any insight on this?

Thanks
Gary

In anticipation of my car getting the ugr paxton treatment, I installed a boost/vac gauge in the car tonight. It is a mechanical gauge which I tapped into the MAP sensor line at the rear of the block of the passenger side of the intake manifold. Tator advised me that it was as good a place as any to tap into the motor. When I started the car the gauge moved to about -12 or -13 on the vac side which I expected to see but when I hit the gas the pointer moved up towards 0 but fell sharply to -20 or so when I held the rpms up at 3k or so steady. I figured it would stay close to 0 while the car was being rev'd but it didn't. Never expected that. I suppose that's normal but I really never had a boost gauge installed in a car before the blower was done so because the car is running perfectly I am thinking it is ok. Does anyone know how it should react pressure wise in a vehicle not running a blower (yet). Josh

 

[mjorgensen Woodhouse]

Gary,

Are you talking about WOT on the road or just in neutral? Have you tried another point or accessing the manifold pressure?

Generally the gauge will not show real boost in neutral, a little momentarily but not sustained. If it is doing this on the road I would check for faulty boost relief valve or a hose that has blown off (not always easy to find visually).

Good luck

I installed an autometer vacuum / boost gauge and I am a little puzzled over it's readings as well. I am running an 8lb pulley on the roe but at full throttle the vacuum goes to 0- never positive. I also have it installed on the map sensor line. Anyone have any insight on this?

Thanks
Gary

 

[SYNFULL]

I'm talking about wot throttle on the road. I think I will try the vacuum line coming out of the manifold going to the vec.

 

[mjorgensen Woodhouse]

Sounds like a good place to start

 

[SYNFULL]

That worked! Tee'd into the vec line coming from the blower intake.
Thanks!

Sounds like a good place to start

 

[dave6666]

A good test would be to move your vac/boost port connection upstream of the TB's, closer to the blower. If you do have positive pressure there and under the same conditions have negative pressure at the intake, but want positive pressure at both, those are some good clues that TB size and intake plenum size are inadequate.

Also, your distribution from front to rear in the plenum will be better the higher the pressure is in the plenum. You would like the pressure drop to be right as each individual runner goes to each cylinder in the head. Of course if you do ever go to even slight positive pressure in the intake, everything else has to be able to respond to and accept that. Like bolts, sensors etc. When you start needing Kevlar straps across the top of your intake turn the boost down 10%. You are there.