Why use inches of wawter?

[Ol' CW]

Why use the 28 inches of water when flowing cylinder heads? I understand that this is where differences really show up between cylinder heads, but why use water instead of inches of mercury?

According to the conversion factor I found with a quick search, 28" water is 2.0594" mercury, is that correct? So then maybe it is because the water scale is bigger and reduces potential error???

[84Dave]

Ol' CW..... your conversion math is correct. And water, with a Specific Gravity of '1', is a convenient substance on which to reference pressures and depressions. It is more sensitive, in an observable fashion, in a manometer for instance, than mercury would be. Racing fuel, with a specific gravity near .720, would be slightly more sensitive than water, however......... we can see it now........ "There I was, reading my manometer with Sunoco 110 in it, and the darned thing blew up and caught on fire!!" Not a good thing! -Dave-

[Ol' CW]

Thank you for your reply!

[MorGo]

And anyone WOULD NOT WANT mercury nearby!Mercury is highly toxican so water is readily available and obiovisly not dangerous..

[volvobrynk]

Hello

A few questian from Denmark. Sorry for any incorrect spelling!

I have made a good deal on a set of industial vacuum meters, gauge type. Is it posible to use or will the reading be to slopy?

How importen are the 28 inches of water, can less be uset. For instens 20" ot 10"?

Best Regards
Rasmus Brynk

[Ol' CW]

I think at lower vacuum, your results between different heads or port modifications will not show up as well as at the 28"

[rookie]

Originally Posted by volvobrynk

Hello

A few questian from Denmark. Sorry for any incorrect spelling!

I have made a good deal on a set of industial vacuum meters, gauge type. Is it posible to use or will the reading be to slopy?

How importen are the 28 inches of water, can less be uset. For instens 20" ot 10"?

Best Regards
Rasmus Brynk

28” is an industry standard that is used to compare one head flow or one head porter’s claimed flow to another.

If you flow at a different setting like 10”, 20”or 25” you can convert any test pressure to 28” by dividing 28 by the test pressure you are flowing at then squared.
This is the equation the Superflow book gives for converting test pressures although their chart shows a little different #’s.

Forgive me if that is not the correct way to lay out mathematical equations but I’m not a math magician.

Example 28/25 x square root = 1.059, Superflow’s chart shows 1.060?
Example 28/20 x square root = 1.183, Superflow’s chart shows 1.180?
Example 28/10 x square root = 1.673, Superflow’s chart shows 1.670?
Example 28/7 x square root = 2.000, Superflows chart shows 2.000?

[Stan Weiss]

If all you are looking for is to see how changes you make effect flow you can use any units you want. I saw numbers that came off of Cummins Templeman flow bench that were measured at around 520 mm of water (around 20.5 inches of water) in Cubic meters per second as flow units and time. Not what I would call standard.

[rookie]

GM has started using grams per second?? I guess to confuse stupid people like me has to what these heads actually flow.
*- Camaro LS3

[Stan Weiss]

Since they are using 7kPa of depression this is 28.103 inches of water. Grams is a unit of weight. Since the weight of a cubic foot / air density changes with temperature that is another variable in the mix.