Autolite & Holley power valve 7.5HG 2100 4100 

This is a 7.5HG valve

As used in
Autolite 2100; 4100;
Motorcraft 2100; 4100 
Holley, many 2 & 4 barrel carburettors.

7.5 & 8.5 inch HG are the average size that are installed in the stock engine.

In competition engines and those with multiple carburettors fitted the vacuum will be reduced and will require a power valve that opens up quicker.

How to read the HG on our power valves.
Look at the face of the power valve (the shiny surface). Turn the valve so the number reads correctly and is on the right side. Example 8.
Read the number at the bottom. Example 5.
You have a power valve with 8.5HG.

The power valve stays closed until it reaches it's rated HG. In the example above the power valve won't open until the vacuum drops to 8.5HG. 

High performance and engines with multiple carburetors will have a lower vacuum reading thus requiring a lower power valve.

Determine your power valve size by connecting a vacuum gauge to the manifold and read the HG under power, divide by 2 and round to the next .5
For example a reading of 15 HG would require a power valve size of 7.5 HG.