Reading the Boost Gauge

J

John Valerioti

Registered User
Being new to SC's...I read both the owners manual and the shop manual to find out how to read the factory boost gauge in the dash and neither gives an explanation. At what point does the Super charger kick in what do the numbers really mean?
 
First, this is both a vaccuum gauge and a boost gauge because it is essentially measuring air pressure relative to atmospheric air pressure. The left side is a normal vaccuum gauge that measures the lower-than-atmospheric pressure in the intake manifold duirng normal running, non boost situations. The units are inches of mercury from 0 straight up to 30 far left. It is same gauge we all used years ago on normally aspirated engines. About 30 inches of mercury vaccuum would mean that there was a perfect vaccuum in the intake manifold. Similarly, the right side of the gauge is a boost gauge that could be used for turbos or blowers. Again, it is measuring air pressure but in different units, pounds per square inch ABOVE atmosperic pressure. It shows 0 straight up to about 20 psi far right. 14 psi on the right side would mean that the blower has raised the pressure in the intake manifold to about double atmospheric pressure. Stock SCs run up to about 12 psi max.

So, 0 striaght up means no vaccuum and no boost or that the intake mainfold is sitting at about atmospheric pressure. You are in boost when the gauge is on the right side. You are out of boost when it is straight up or on the left side. You should idle at about 20 inches of mercury on the left side.

Note that most of these gauges don't rest at exactly zero with the engine off (like mine). In this case you have to do a mental correction to get the right value. My needle sits a little left of 0 when off. So when I am in boost and my gauge says about 10 I'm really pushing about 12 psi.

Jon
 
Last edited:
I think I'm right on this, anyone please correct me if I'm wrong. But there's a butterfly valve on the intake that puts air directly into the intake manifold below 2 inches of vacuum. from 2 in-hg up into boost the butterfly valve directs air flow through the supercharger.
 
oxygen822 said:
I think I'm right on this, anyone please correct me if I'm wrong. But there's a butterfly valve on the intake that puts air directly into the intake manifold below 2 inches of vacuum. from 2 in-hg up into boost the butterfly valve directs air flow through the supercharger.
Click to expand...

That is almost correct. Since the supercharger is a positive displacement type, it is always moving air thru the supercharger, IC tubes and IC. When there is negative manifold pressure (vacuum) the bypass valve remains open and any excess air from the supercharger is routed back into the intake side of the supercharger. Hit the throttle hard and vacuum drops towards "O" and at about 1 or 2 inches of vacuum the spring loaded bypass valve closes, forcing all the air moved by the supercharger into the engine.

There may be some situations where air flows the opposite direction (like at idle), but generally the air flowing thru the bypass will flow from the lower IC tube side of the valve to the supercharger inlet side.

David
 
Thanks for the more detailed explanation Dave & Patrick.

I was just trying to give the basic explanation of the use of the boost gauge.

Jon
 
Those are good explanations on how to read it...

But make sure you know how to use it to your advantage.

Like previously stated a idle vacuum of 19-20" Hg is normal. Anything less would idicate some sort of vacuum leak issue. Or your running a big cam.

If you somehow got more than 20" Hg vacuum during idle, then you've got a really good running SC engine.

When driving (not sure if you have a 5-speed or auto) make sure that the needle doesn't swing past "0" on the boost guage in order to get the best economy. This way the computer won't dump fuel into the engine.

Also, things to note, Supercharger "pressure" is also a measure of "backpressure". If you open the exhaust up with larger, freer flowing components then you will see less "boost", yet the engine is flowing more air under less pressure. Putting a Overdrive pulley on the supercharger will increase the pressure.
 
jonroe said:
First, this is both a vaccuum gauge and a boost gauge because it is essentially measuring air pressure relative to atmospheric air pressure. The left side is a normal vaccuum gauge that measures the lower-than-atmospheric pressure in the intake manifold duirng normal running, non boost situations. The units are inches of mercury from 0 straight up to 30 far left. It is same gauge we all used years ago on normally aspirated engines. About 30 inches of mercury vaccuum would mean that there was a perfect vaccuum in the intake manifold. Similarly, the right side of the gauge is a boost gauge that could be used for turbos or blowers. Again, it is measuring air pressure but in different units, pounds per square inch ABOVE atmosperic pressure. It shows 0 straight up to about 20 psi far right. 14 psi on the right side would mean that the blower has raised the pressure in the intake manifold to about double atmospheric pressure. Stock SCs run up to about 12 psi max.

So, 0 striaght up means no vaccuum and no boost or that the intake mainfold is sitting at about atmospheric pressure. You are in boost when the gauge is on the right side. You are out of boost when it is straight up or on the left side. You should idle at about 20 inches of mercury on the left side.

Note that most of these gauges don't rest at exactly zero with the engine off (like mine). In this case you have to do a mental correction to get the right value. My needle sits a little left of 0 when off. So when I am in boost and my gauge says about 10 I'm really pushing about 12 psi.

Jon
Click to expand...
Thanks jon....I feel like you just gave me a lession in Physicis !
 
John,

You're welcome! Glad to help.

The one interesting point I missed was that the two scales work in the same gauge because of the units. The vaccuum gauge goes 0 to 30 inches of mercury on the left and the boost gauge goes 0 to 15 psi on the right because 30 inches of mercury and 15 psi are about the same absolute value, a little over one atmosphere of pressure. So when the gauge goes 20 inches of mercury on the left that's about the same difference in pressure as 10 psi on the right.

Jon
 
You must log in or register to reply here.
Back
Top