Boost gauge bad? Or is it something else?

[Tennpenn83]

Hey ya'll got something for ya.

Running a 2.9" pulley, monitoring boost via mechanical Phantom II Autometer boost/vacuum gauge.

I have always observed around 14-15 psi (which I always thought was kinda low for a 2.9")

Saw the other day I was only boosting around 11-12 psi. (Gauge is only about 2 months old). Just being the paranoid mofo that I am, I took off the supercharger, intake manifold, inspected any related boost/vacuum lines and seals of everything. Gaskets for both the s/c to i/m and the i/m to head were inspected and turned out to be ok. Inspected the intercooler endplate seal and it's ok. Played with the boost bypass actuator (before and after teardown/re-install) and it's working fine. THERE IS ABSOLUTELY NO PROBLEM WITH THE INTAKE/BOOST SYSTEM.

For reference, when the engine is off and there is no boost/vacuum, the gauge has always read around 3-4 Hg on the vacuum side. Never has been exactly zero. Could this be the boost gauge itself being a factory defect? What are other people boosting on a 2.9" pulley?

Please keep in mind that I have a ZZP mid-length header/ 3" downpipe with 2.5" Hi-flow cat on so I assume that I will drop just a psi or so in boost because of better airflow, am I right?

[Mattman6]

boost gauge is messed up easy fix

[Tennpenn83]

Originally Posted by Mattman6

boost gauge is messed up easy fix

It's a mechanical gauge, shouldn't it be a little more reliable than that?

[HondaHater]

i have a 2.9 with that very same header/dp (as u know..lol) and many other mods.....i'm usually around 16.5lbs steadily

[Tennpenn83]

does ANYone know how to zero out a mechanical boost gauge?

[Tennpenn83]

Alright guys here's the dyno from earlier today:



As you can see I'm making decent power up to about 5500 rpm where the a/f ratio drops from 13 or so to 12 and then keeps dropping all the way to 6300 rpms, where it's so rich, the tailpipe wideband used can't even read it. At this point, the dyno was cut short at 6700 rpm (300 shy of redline) because the tech didn't want to damage my engine.

The solid slanted HP graph line would have kept going, probably all the way up to around 250 or higher is the a/f ratio had not dropped.

[Dainslaif]

Mine did that as well, nobody had a good answer as to why. Stage or custom tune?

[Tennpenn83]

custom tuned by a cobalt member. will be going back this weekend for a re-tune.

weird thing is, a month ago i was boosting 15lbs and lately it's been lucky to see 12. then the rear o2 sensor was acting up even though we turned it completely off with hpt.

[Dainslaif]

Mine's custom too, and when I got mine tuned it pulled fine all the way to 7000, but a few months later when I went to dyno I was dropping the AFR around 5500 on every pull. I need to save for a wideband and HPT to actually find out why.

[Tennpenn83]

wideband is a must, and I'll probably look into hpt for myself here soon. that and dual interceptors and we'll be set