boost guage install

[mickster8888]

i have an 1983 930 row and want to install a boost guage i purchased from north hollywood speedo. it is the one that replaces the clock. i bought their install kit to hook to brake booster and after doing so got no reading. the check valve works but i found that i'm only getting vacuum at the hose that connects to the brake booster. i don't know why this is. i had the small vdo boost guage in the car which is tap off the intercooler and is connected with a tiny plastic line. when i connect this line to the nhs boost guage it wasn't very sensitive and only registered when i was on full boost. do i need to run a bigger line to the nhs boost guage? the smaller guage was very responsive with the small line. the tap for this is next to the fitting on the intercooler for the factory electric boost gauge in the tach. any help would be greatly appreciated.

[lucittm]

I would use the line connected to the intercooler since you already have it installed. No reason why the NHS unit should not work as well or better than the small VDO boost gauge you had before. I would call NHS and see what they say. They treated me well.

Mark

[mickster8888]

yeah i talked to them already about the problem and they said a larger diameter hose might make a difference. i was wondering if anyone else has had this problem. thanks for your response.

[billjam]

Mick,
I had a similar problem recently with a newly-installed mechanical boost gauge which was not reading as it should.

I ditched my clock and installed a gauge in the clock hole. It was connected via a 4mm (ID) hose to a fitting I drilled and tapped into the LH end of the manifold (where the smog pump used to be). From this fitting, the hose runs through the car to the gauge via an Innovate LMA-3 under the RH seat.
I considered tapping into the brake booster line at the booster, but decided to run a separate line to get what I thought would be accurate boost to the LMA-3.
The gauge was reading vacuum OK, but as boost came on, the needle moved up to zero then moved back into the vacuum range until boost got towards 0.5 bar (on the electric tacho gauge). As boost continued to rise to about 1 bar on the electric gauge, the needle would move eventually up into the boost range, but never to more than about 0.5 bar.
The electric gauge in the tacho "feels" correct and I tested the mechanical gauge out of the car with vacuum and boost using a similar length of hose and it works just fine.

The only thing I can think of is that as the air is flowing past the hole that I drilled in the manifold, some kind of venturi effect is affecting the pressure at this port.
I have since re-positioned my connection point to a tee fitting in the 12mm hose to the booster. There is a plastic connector in this hose about 8" out from the engine, so I replaced this with a home-made copper tee and connected my 4mm hose here (just visible right in the fat hose at the LH edge of the photo).

Unfortunately, I can't tell you if it fixed the problem just yet because my fuel distributor is out of town and my suspension parts are in transit from PP, but it seems that there is more to connecting up a boost gauge than meets the eye.

I'd also be interested to know the "ideal" tapping point for vacuum and boost.

Bill

[Helmsy]

My clock-replacement boost/vac gauge is tapped into the line at the brake booster. Seems to work fine and accurate.

Maybe hose diameter does have an effect?

[356-930]

Originally Posted by billjam

. . .The gauge was reading vacuum OK, but as boost came on, the needle moved up to zero then moved back into the vacuum range until boost got towards 0.5 bar (on the electric tacho gauge). As boost continued to rise to about 1 bar on the electric gauge, the needle would move eventually up into the boost range, but never to more than about 0.5 bar.

Bill

Because the vac/boost pressure sense tap point is fundamentally looking directly into #2 cylinder's intake "plenum," it is subjected to pressure pulses in the 'plenum' that occur with cylinder intake/exhaust valve overlap. And, the higher the rpm, the faster the pulses.
Classic vac/boost/ mechanical “C-type” pressure gages (like NHW's boost gage) cannot keep up with some and can't even "see" most of the pulses but they raise hell with the C-tube – you saw it.
The brake booster line should be a good source to tap. At the engine, a good pick off point is a T in the line to the WUR’s boost enrichment port.
One should use a line/hose from the engine to the gage with a minimum ID of 5/16" (8mm).
The hose is a mechanical filter that pneumatically damps engine pressure change to a frequency the gage can handle.

[billjam]

Originally Posted by 356-930

Because the vac/boost pressure sense tap point is fundamentally looking directly into #2 cylinder's intake "plenum," it is subjected to pressure pulses in the 'plenum' that occur with cylinder intake/exhaust valve overlap. And, the higher the rpm, the faster the pulses.
Classic vac/boost/ mechanical “C-type” pressure gages (like NHW's boost gage) cannot keep up with some and can't even "see" most of the pulses but they raise hell with the C-tube – you saw it.
The brake booster line should be a good source to tap. At the engine, a good pick off point is a T in the line to the WUR’s boost enrichment port.
One should use a line/hose from the engine to the gage with a minimum ID of 5/16" (8mm).
The hose is a mechanical filter that pneumatically damps engine pressure change to a frequency the gage can handle.

Thanks - most of this makes sense.
Although I don't see why the larger vacuum connection for the booster (which is in a very similar location - close to inlet for #1) wouldn't also suffer a similar problem with pressure pulses. Maybe size has some effect resulting in pressure fluctuations getting damped.

[356-930]

Originally Posted by billjam

. . . Maybe size has some effect resulting in pressure fluctuations getting damped.

Absolutely; the greater the line's volume, the greater the damping. Moving the pressure tap away from the source of the pulses and into an open "plenum" damps the pressure pulse signal a bunch.

[mickster8888]

is the tap in my intercooler a good pickup point.?it's already tapped so it would be the easiest to use.

[billjam]

Originally Posted by mickster8888

is the tap in my intercooler a good pickup point.?it's already tapped so it would be the easiest to use.

Tapping into the IC will allow you to measure boost pressure only .
If your gauge is for vacuum and boost, you will never see vacuum at the gauge.

[fredmeister]

To see vacuum you need to be after the throttle plate.
Also tapping into the intake after the throttle plate sees what the engine sees with regards boost level. Tapping before the throttle plate should read higher because of the pressure drop across the throttle plate which I guess may be as high as 1psi or so.
Anyone know this value at 1 bar boost from the turbo?

[mickster8888]

thanks guys, i'm just reading boost. i'm gonna run a larger line back this weekend. thanks again for clearing this up for me.

[wanabe911 owner]

Does anyone have a picture they could post of where exactly you tap into at the brake booster to instal a boost gauge? Thanks in advance.

[spuggy]

Originally Posted by mickster8888

thanks guys, i'm just reading boost. i'm gonna run a larger line back this weekend. thanks again for clearing this up for me.

My gauge is into the throttle body under the throttle plate. PO ran a dedicated vacuum line back to the dash down the tunnel.

The diameter of the line is tiny. I see no fluctuations or pulses in the reading. Just another data point.

If I were doing it myself, I think I'd be very inclined to just tee off the brake booster line.

[wanabe911 owner]

So, yesterday I tapped into the brake booster line and connected my boost gauge. But there is something wrong, or I connected it wrong. The gauge seems to work fine at first, you start the car and it starts to read vaccum like between, the it stays there and continues reading vaccum, even when you shut the car down! It stays like this until I pull the line from the back of the gauge. Any thoughts? Did I install it wrong? The gauge might be bad? I tapped into the 15/32 line that is a U shape that comes from the booster to a fitting on the car.

[lucittm]

I'm sure there is a check valve in the line to keep the vacuum in the system. You might have tapped into the line after the check valve. Sorry, I don't know where it is in your car.