dual GM TBIs [Fwd: CFM numbers]

[Raymond C Drouillard]

On Thu, 15 Oct 1998 19:40:36 -0600 bearbvd at sni.net (Greg Hermann) writes:
>
>>> Dear Sir:
>>>
>>> I am wondering if you know, or can point me toward a source for, cfm
>>> ratings for GM tbi units.  I haven't been able to find them anywhere.
>>> Thank you.
>>>
>>> Shannen Durphey
>>> shannen at mcn.netShannen,
>>
>>        My apologies for the delay. The GM tbi units are not rated by 
>cfm
>>numbers but by displacement size of engines. 4.3, 5.0, 5.7, and 7.4 
>etc.
>>        Although the 4.3 to 5.7 is approx 465-500cfm and the 7.4 is
>>approx. 550-670 cfm.
>>
>>                                REP. Troy Brown
>>                                Howell Engine Developments Inc.
>>                                6201 Industrial Way
>>                                Marine City, MI 48039
>>                                (810)765-5100
>
>Hey neat--now--are these (approximate) ratings based on 3 in Hg, 
>(pressure
>drop) like all two barrel carbs are, or on 1.5 in Hg like four barrels
>are??? I believe that Holley, for one, rates their two and four barrel
>Pro-Jection units on the two different standards. (All you gotta do is
>multiply one by .707 to get the other.) Just wonderin' and askin'!
>
>Regards, Greg

I believe that Holley rates its TB assemblies (injecters included) on the
amount of air you can run through it and still have a stoichiometric
mixture.  In other words, it's rated by the size of the injecters.  This
is based on the assertion by the sales tech that the smaller TB won't
restrict the air going into my engine, and by my experience with the
Pro-Jection system that I drive.  The TB is way oversized for my 360.  It
doesn't take much throttle to dump all of the vacuum out of my manifold
at low engine speeds, and there is no vaccuum in my manifold even at the
shift point of the automatic transmission.

It makes sense, fabrication wise, that they would use a standard throttle
body and mount different size injecters on it for the different
applications.

Ray Drouillard

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