The other day we got my friends flowbench going. Does anyone know what the most accurate way to test the flow? Does it need to be tested at 1.5hg or 3.0hg? I really need to find son information on this cause we're getting ready to bore and port my tbi and we really want to test it so that we get an accurate measurement. Anyon with any information on flow testing carbs or tbis please let me know. Thanks,

For carb's (which we all know TBI is not a carb), 2-bbl carb's are flowed and rated at 3" Hg.

4-bbl carb's are flowed and rated at 1.5" Hg.

When it comes to the TBI units. They are flowed and rated at the 1.5" Hg drop. Even the 2-bbl units such as the stock ones.

The stock 1-11/16" bore TBI units flow about 490 CFM at 1.5" Hg.

RBob.

RBob is a solid source of info. And not only that, but 1.5" is what I would like to hear. This is an issue that has much confusion for many people.

But...I am still not sure because that these TBI units are really rated at 1.5" though (no offense). I have read in the Holley projection instructions that their 670 cfm rating is at 3.0" pressure drop.

See here from Holley, page 4, second paragraph under Introduction.

http://holley.com/data/Products/Tech...7-6%20scan.pdf

Unless that is a typo, Holley rates their 670 cfm unit measued at 3.0 inches. That is mathematically equal to about 474 cfm measured at 1.5 inches. Logic says that the holley with 2.0" bores must flow more than the stock TBI with 1-11/16" bores. Therefore, it doesn't add up for the stock TBI to be 490 cfm @ 1.5" while the bigger Holley is only 474 cfm @ 1.5".

That implies that either Holley has a typo in their instructions about the 3.0" or maybe the stock TBI 490 cfm rating is not at @ 1.5", but a higher number, maybe 3.0".

Since you are about to flowtest your TBI, could you please post back to this thread with your results? It would be quite helpful to put this issue to bed if you could post the flow ratings at either (or preferably both) 1.5" and 3.0" for the stock TBI unit.

Sorry to rain on everyone's parade, and I will be glad to be proven wrong.

Well, when it comes to TBI Holley has done more of a dis-service then any other company that I know of. From their original TBI units using the worst injectors of all time to making false statements such as the one you noted.

Starting with the sentence from the install manual, "The 2-barrel throttle body used in this system flows up to 670 cfm at 3.0 inches of manifold vacuum, placing it in the same category as most performance 4-barrel carburetors."

Now, we know that 4-bbl carb's are rated at 1.5" of pressure drop (Hg), so lets convert 670 cfm at 3.0" to a 1.5" drop rating:

New CFM = SqRt(1.5 / 3.0) * 670 = 474 CFM

Do performance 4-bbl carb's generally run about 474 CFM? Not really, so the Holley tech writer is clueless about the 2-bbl TBI unit they are writing about. All the while disseminating false information.

There has been other flow testing done on TBI units. And they all line up with the ratings being made at 1.5" Hg.

Here are links to some of this data:

http://www.thirdgen.org/techboard/al...ctor-info.html (Throttle body & injector info)
http://www.fullsizechevy.com/forums/...=145234&page=4

Note that 28" of water is 2.06" of Hg

RBob.

RBob is right.

Looking at his first link, it states "Stock 4.3/5.0/5.7 2bbl TBI complete -- 574.1 cfm (dry)" @ 28 inches of water based on actual testing.

Doing the math, that IS equal to the 490 cfm measured @ 1.5" mercury.

I had previously looked for a thread with flow testing but didn't find that one. It is now going into my favorites.

I am VERY happy to be wrong about this one. Assuming then that the holley 670 cfm rating should have been at 1.5" mercury (not 3.0" as they stated), my engine is going to pick up some power on the dyno software tonight when I adjust the flowrate. Cool.

Is the flow you cite without inj pod and injectors? I think so. Did not yet read the links you provided.

I just read first link! flowed both ways!

second link shows the 7.4L info.