I have a table of Holley jet #'s and their corresponding flow values in cc/min. My question is, I assume those flow ratings are measured under some vacuum reading, 28" H20 maybe? If so how are the flow rates affected by boost pressure. I'm thinking at 15psi boost you would probably double the flow rate since you are doubling atmospheric pressure. Just an academic question if anyone has any thoughts.......

Thanks,
Craig

 

That is a great question for Tom V.


Tom V.?

 

Yes, I was hoping Tom could answer this. I'm basically trying to get a theoretical carb jetting or fuel requirement based on a known HP, boost amount and BSFC value.

Craig

 

The test, to my understanding, was not under pressure as a normal carb is not under any pressure (other than atmospheric pressure of about 14.7psi).

I believe that the people used Stoddard's solvent (for safety reasons) (same Specific Gravity as gasoline and has been used for years by Holley when testing carbs on a bench).

Simple rig:

Stoddard's solvent in a vessel with a given carb jet in the drain line. Turn the flow valve
on for approximately one minute and collect the sample). Pour the sample into a 100 cc
burette and measure the number. If the flow volume is more than 100 cc/min then you
drain the previous measured amount, and measure the next amount in the same burette
until the test sample is gone.

The accuracy will be with-in a couple of ccs/min which is good enough for a magazine article.

You could do the same deal at home with a burette and some time and a simple jet holding
fixture. A Simple jet extension on one end of the jet (drain side) hooked to a rubber hose
and a second hose fitted over the round part of the jet which goes to the solvent vessel
would do the trick. Point the hoses straight down.

Tom Vaught

 

There was some pressure then, just not much. It was simply based on head pressure from the height of the solvent above the jet......I wonder how much though. Guess it would be easy to replicate that test with a sealed vessel of solvent and put air pressure on top the solvent. There's probably an easier way mathmatically though....need to dig out the fluid mechanics book.

Craig

 

I read that last night and did not think of it.
But, using a tall narrow burette and feeding the jet,
would that not slew the test due to the column?
As volume decreased the head pressure would also?

I have a headache.

 

flow would need pressure drop though and arent both sides of the jet the same pressure even under boost?

 

Robert, yes your right....you'd need to keep filling the burette so it stays at the same level and maintain a constant head pressure.

Trendsetter......yes, I think your right. Fuel is still 'pulled' thru the jet due to the boosters. Unless one is using vent tube extensions.....then it starts to be forced thru the jets it seems.

Oh well...dumb idea on my part.

Craig

 

I would have to disagree, the venturi causes a pressure drop, or fuel would not flow, since it would have to go upwards.

 

Correct there has to be a pressure differential or it doesn't work.

 

I think the head pressure issue could be avoided by using a good sized solvent vessel as a source. The minimal amount of solvent that was collected in one minute would not appreciably change the volume of the vessel or the amount of head pressure.


Troy

 

I think it would be interesting if you did a free atmosphere test as tom described, measure what flowed through and set your own reference point. Refill the source vessel to the same line, then seal and pressurize the vessel to 5psi and repeat the test. Repeat for 10,15,20,25 psi. Plot the data on a graph and let us all know if it's linear or not!

You could go one step further and try another jet size 20 sizes bigger or smaller and see if they produce the same curve of pressure vs flow.

Your right though, people well versed in fluid dynamics could probably just tell you.....

Remember though, technically it is never pulled through, it is always pushed through by the higher pressure!


Troy

 

Quote:

"I think it would be interesting if you did a free atmosphere test as tom described, measure what flowed through and set your own reference point. Refill the source vessel to the same line, then seal and pressurize the vessel to 5psi and repeat the test. Repeat for 10,15,20,25 psi. Plot the data on a graph and let us all know if it's linear or not!

You could go one step further and try another jet size 20 sizes bigger or smaller and see if they produce the same curve of pressure vs flow."

You are correct.

If you have a large volume vessel the amount of flow level drop in the vessel would be
small vs the flow through the jet. Agree also with the pressurizing of the vessel.

If you read my post again, the original flow testing is done to get the "sample" that you
then measure to determine how much flowed in one minute.

If you put one quart of any solvent in the vessel you fabricated you would have 946 ccs
of fluid. It would flow out at a given rate. You put the same 946 ccs and put a head pressure on the vessel of 5 psi, 10 psi, whatever, as was suggested the vessel would empty quicker in the measured minute but you would get a flow relationship vs pressure.

From what I remember the 110 jet flowed 2520 ccs in a minute.

Without going into a lot of math, I think your vessel would need pretty thick walls to
contain 25 psi of air pressure AND you would need a LARGE vessel assuming the 110
jet flows 2.66 quarts in one minute at normal atmospheric pressure and you would be
2.7 times that at absolute pressure. (14.7 plus 25 psi). Around 7 quarts in 1 minute.

A fun project though.

Tom V.

 

I was thinking more about this last night and believe that testing under room conditions
would be fine. You could then apply the relationships (Ratio) for any given jet vs another jet
to boost pressure as far as actual CCs flow thru the jet but the ratio would still stay the same.

For most people, they would just take the data already provided by the magazine and use that.
Maybe I will do that calculation for the different ratios and post the ratios.

Tom V.

 

Keep in mind that increasing the boost pressure doesn't necessarily the differential across the jet. If the bowl and venturi inlets are at the same pressure increasing the pressure will not increase the jet flow. Well, that's not entirely true. The increase in pressure will increase air density. The increased density will increase the booster signal and of course the flow through the jet. You would probably never see more than a couple psi across the jet.

 

To me this would be correct. And I dont think we can change the density of the fuel ( atleast not from pressure).



Pontiac4ever, I dont think it is going to increase the flow but a very little bit because of the boost. But I am not sure myself. It may even make it flow less. If the air going through the venturi has more density, then it has more pressure (makes sense, boost) air with more density has slower velocity (I think this is right). What I dont know is, if the air has more density & slower velocity, would it create more vacume on the fuel curcuit than compaired to air with less density & more velocity. Some of these carberater guys can anwser that I bet.


But one thing I think you will find, is if you go testing flow of jets, the amount of differance between, say, 71 & 72 jet, will not be the same amount of diff as between a 76 & 77. And I bet you will be hard pressed to find two of the same size jets that will flow the same.


If you build a vessel to hold fluid to feed a jet, the placement of the feed line (going in the vessel ) will effect pressure that is placed on the jet. It may not be the bottom of the vessel that gives the most pressure if you tap into the side of it, with like a cylinder shaped vessel. I do not know how that would compair to taping into the bottom of a cylinder shaped vessel.


I am just wandering, do you have a exact reason for figuring this out. Not that it matters, but just curious.



Jess

 

No, otherwise no fuel would flow through the jet.

It is all about the pressure differential across the jet, which in turn is affected by the carb design. Venturi design and size, booster venturi design, velocity through the venturi, throttle blade opening, emulsion circuit, fuel level in the bowl, pressure seen in the bowl, etc are all variables that can change the differential pressure. I would surmise that the differential across the jet is probably under 1.0 psi on an average homebuilt carb, and perhaps a bit more on heavily modified units with a different emulsion package and modified annular boosters. Just a theory though.

 

Food for thought:

Holley carbs were calibrated (as were all carbs for many years) and tested for flow and function at 20.4" inches of water delta P across the carb. 20.4" H20 is 1.5" of mercury.

Even if you test at 28" of Water (H20) like some carb companies now do, this would work out to 2" of mercury (Hg) or basically ONE PSI of test pressure. The carb works fine in that range.

What this means is that on a normally aspirated carb the intake manifold might be at 13.7 psi (absolute pressure) and the air filter/ air horn might be at 14.7 psi. (Difference across the carb of ONE PSI).

Now we talk Boosted carb:

On a boosted carb the intake manifold might be at 23.7 psi (absolute pressure) with 10 psi boost showing on the boost gage, and the air filter/ air horn might be at 24.7 psi (absolute pressure). (Difference across the carb still of ONE PSI).

The only time you might start to have calibration issues is when the delta across the carb goes higher than ONE PSI. But Gordon did not see this until he was making over 1700 hp with his 509 chebby at 17psi boost with his 850 cfm style carb.

A comment on the vent tubes:

Flow across a jet is based on the delta P across it.

If the delta P is 1 psi you get a given flow for a given sized jet orifice diameter.

If you add a vent tube extension and now have 2 psi across the jet, you can reduce the orifice diameter to a size that would have given you the same flow at 1 psi. This allows you to go from say a 96 jet to say a mid 70s jet. The only real advantage to this deal is that Holley and others do not offer a lot of large jets past the 110 jet size. By using the vent tubes you can get the flow of the 110 (ot larger jets) with say a low 90s jet. You still have 15 more jets you could install in the carb before you run out of available jetting choices.

If you don't want to use the vent tubes, you can do the same added fuel strategy by now adding a second Power Valve Channel passage and restriction). The carb still sees the 1 psi from the bowl across the discharge of the booster but because you have more fuel flow area you get more flow for the same 1 psi delta p.

Different strategies for different folks. Adding a Vent tube is pretty easy for most, adding a
PVCR passage might not be too easy for some.

While I agree with Andrew that the flow testing across the jetting could be at atmospheric
pressure or at 1 psi pressure, some people would say "I see 20 psi in the bonnet so I have
to have 20 psi in the jet rig". I hope this helps people understand that that is not true.

Tom V.