<VV> RE: Ed's strange turbo behavior

[Jim Burkhard]

Sounds good Andy, but we were talking about somebody else's 
car, right (or am I confused?).  If the gauge just claims to 
read "psi" (rather than "psig" or "psia") as in your case, 
"0" with the engine off means the "psi" is really "psig" as 
it will be for most gauges. On *most* (not all) gauges that 
are intended to be automotive boost gauges, this will be the 
case. If it reads around 15 (14.7 more precisely), than it's 
reading in psia.

The original poster (Arjay) talked about his friend having a
"0-100 psi" gauge in place to measure boost. Clearly with a
psi range that big (regardless of it being psig or psia), it
is some sort of industrial gauge, and not normally something
you'd employ as a turbo boost gauge. Once you know that is
something peculiar is rigged up, you are immediately
wondering if it reads in gauge pressure or absolute
pressure.  If with the engine off, it shows around 15, it's
a psia gauge. You can use this, but you need to subtract off
the 15 when quoting numbers.  You are still at peak only
ready 1/4 range of the instrument, though, which is poor
engineering practice. Buy a different gauge if you want to
get more precise (and probably more accurate) readings.

I discounted other obvious things like it reads inches of
Mercury or kPa or some other units, because Arjay said it
was a "psi" gauge, and I trust he read the gauge face. The
values are reasonable for it being inches of Hg, though, so
that is something to really confirm. If it really says
"psi", than it's probably a "psia" gauge. Either that or it
is really broken. It's a simple matter to determine this. If
he's still in the dark, tee in another known good test gauge
back at the engine compartment and (temporarily) snake the
1/8" nylon tubing into the car through a window. It won't
take but 10 minutes and will reveal what is going on. The
car CAN'T be running 26 psig of boost if it's stock (you'll
be intake limited). If this was gotten around, the engine
wouldn't survive w/o extraordinary measures. In my work as
an automotive development engineer, I occasionally measure
something odd on a gauge or meter. If it doesn't pass a
sense check (and this doesn't from a couple avenues), you
test the measuring equipment.

Jim Burkhard

Andy Clark wrote:
> Hi, Jim. My boost gauge reads in psi and is at "0" with the engine off.
> Andy Clark
> Camano Island, WA.
> 66 140/4 Monza Sedan
> 66 140/4 Stinger Clone
> 66 180/4 Cord 8/10 #60 
> ----- Original Message ----- 
> From: "Jim Burkhard" <burkhard at rochester.rr.com>
> Sent: Friday, January 06, 2006 10:55 PM
> Subject: Re: <VV> RE: Ed's strange turbo behavior
> 
> 
> 
>>One big question ... look at the gauge face and tell us:
>>1) Does it read "psia" or "psig"?
>>2) When you shut the car off, what number does the boost 
>>gauge read?
>>
>>This is a (relatively) unmolested sorta stockish car, right? 
>>Boost should be limited by the restrictive Carter YH carb. 
>>If you switch to something else, you can make more boost, 
>>but at these pressures (if they are gauge pressures and not 
>>absolute pressures) you will be scattering stock cast 
>>pistons even if you somehow keep it out of spark knock (at 
>>those gauge pressures, you need water injection and/or 
>>intercooling and/or active knock sensing and spark retard, 3 
>>modifications it seems you are implying are unlikely.
>>
>>My vote is that the gauge reads psia.  Subtract off 14.7 and 
>>you get gauge boost pressures of between 5 and 11 psig ... 
>>right where they should be on a healthy stockish Vair.
>>
>>Jim Burkhard
> 
> snip..............
>