I have a pressure regulator I rigged up for testing actuators and wastegates. I was curious how accurate the stock gauge is, so I rigged it up this morning. The stock gauge is way off on the vacuum side (what is -10 boost anyway? Not 10"vacuum for sure!) It appears as though the stock MAP sensor used for the gauge is something like a 2.5-bar sensor (or so). It reads fairly linear, but not very accurate... The halfway mark b/t 0and +10 isn't 5psi, but about 4psi actual. The next mark should be 7.5psi but it is only 6psi. The +10psi mark is actually about 9psi. Maybe my gauge is wacked or the MAP was no good, I don't know,but that'* all I have to go by.

I had a 3-bar sensor in stock so I plugged it in and got these numbers:

0= still 0
first line= 5psi
second line= 7.5psi
third line= 11psi
+10= 15psi

The gauge pegs just past +10 at 16psi. The gauge readsmuch more accurately with the 3-bar sensor, just needs to have the numbers redone. The vacuum side still is inaccurate as far as the numbers go. The -10 should be 30" vacuum. The engine idles around the middle line, which is probably about 20".

There is a technique for scratching off the back of the silkscreen to allow light to pass thru. If the appropriate number were stenciled (backwards), one could use the 3-bar MAP and renumber the gauge. I just left the 3-bar on the car. I know the actual boost now, so numbners aren't necessary for me. I will try to renumber is later.

The only O.E. application for the 3-bar MAP sensor was the 89 Turbo Trans AM for the guage. In AC Delco, it'll cost about 60.00. Not too cheap, but cheaper and easier than installing an aftermarket gauge/pod.

FWIW....

FWIW, it varies from car to car. I have a good boost gauge in my Superchared SLE (which did not come with an OEM boost gauge).

Out of curiosity, I hooked it up to my 93 SSEi. It reads dead-nuts from 0-full boost. Dead on. But it does read manifold vacuum wrong.

-10 on the SSEi is actually -20 in reality, as the aftermarket gauge in the SLE reads.
0 is 0 on both cars.

Full boost on the SSEi'* gauge is accurate compared to the aftermarket boost gauge.


I'll move this to Forced Induction.

Mine was way off too that is why I went with an aftermarket gauge.

Let'* just mention...none of the gauges were ever intended to be "accurate"

We know that they are only rough reference at best.

I didn't buy a Bonneville to be cluttered up with aftermarket crap. That'* why I came up with a way to get "more boost from the stock gauge".

Anyway... I went to Walmart last night and bought some small white sticky numbers to "re-calibrate" the factory gauge. I'll take some pictures and post them when I get it done later this week.

Well, the only reason I added a gauge to my OTHER car is simply because it didn't have a boost gauge, and in my mind, that'* a critical gauge to have in an L67. That'* sometimes the first indication of a problem you'll see.

The point I'm trying to make here is that not ALL of them will be as inaccurate as some. There is a variance from car to car. On the positive side of the scale, my SSEi seems to be pretty accurate, as opposed to yours. I know what vac really is, so I'm not bothered by that.

I'm curious to see your 'fix', but I'd kinda also be curious to see what yet another gauge reads.

I used a Autometer boost gauge to calibrate both times. The stock MAP is not a 2-bar like I thought it would be. I plugged in a 2-bar and he gauge zeroed at the first line past 0. Funny, the stock MAP and the 3-bar MAP both zero at 0, but are scaled differently in boost. ???

What year do you have. I think Series II L67 PCMs used the boost signal...

It'* a '96. There are two MAP sensors, one for the pcm and one specifically for the gauge. The '89 TTA also used a MAP sensor just for the gauge (it was a MAF car). My 88 GA turbo split the signal from the ecm to the boost gauge. So does my 90 McLaren GP. The latter both used a 2-bar MAP sensor and boost gauges that read

-15/0/+15

They are dead-on accurate. I have tested both. My Bonne was not. It zeros a little below 0 anyway, so perhaps it lost/gained a little resistance somewhere along the way....