Hey all. I'm new to this board (usually on the PROM/trans/swap boards) so I'm not too sure if this has ever been asked or answered before. I'm told that everything between the 4.3L V6 and 7.4L V8 all use the same tbi units (model 220 I believe). Does anybody happen to know the actual cfm's of these or even a close estimate? I called a few speed shops and all I got was, "well what the hell do you care, tear all that crap off and put a carb on it". Thanx for any help

 

the 7.4L TBI is bigger. The Holley 670 is about the same size. I used to know the CFMs off the top of my head but I forgot.

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-Tas
'89 Formula WS-6

305, TBI, auto, 14x3 chrome flat based open element with K&N, Milodon 160* thermo, functional Formula hood, cross-flow Flowmaster, '99z28 rear pipes and tips....

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It's been posted before as being in the 450-500 CFM range for all the SB ones. Even the 350s had the same TBI unit as our 305s, just the injectors were bigger. The 7.4L big block has a 2" bore instead of our 1 11/16". Sorry, but I don't know the flow ratings on it. If the Holley 2" TBI is flowing about 670, I'd say it's a good bet that the GM one is in that neighborhood.

Jason

[This message has been edited by 89BlwnRs (edited September 07, 2001).]

 

holley designed the gm one in the first place. gm contracted holley to design and build it, then took it and contracted rochester to make it instead. Holley filed suit but i dont think it ever went anywere. this was all told to me by somebody a while ago while who knew somebody who worked for holley when it all happened. so, i dont know if its true, but it seems it could be very likely. there are not enough differences in the holley 670 unit and the rochester cast 454 unit to have any significant flow differences.

if you want bigger than that you will have to go with either affordable efi's setup that controls a holley 4bbl with a gm computer. nobody else has figured out how to do that effectively. or you could help fund my 2bbl 2.25 inch billet unit, that still hasnt been built.

another thing to consider is how they were flowed ( at what vacuum). as well as that they are throttle bodies, not carbs, and work differently and are tough to compare. i think the consensus is that the sb unit can support ~230hp on a 305, before becoming a restriction, and ~330 on a 350.

 

I have seen reports, i.e. ads, from Turbo City (they modify GM t-bodies for those who don't know) saying that the stock TB for both 305's and 350's flow 535cfm @3"Hg. and their modified TBs run 585cfm @ 3"Hg.

I vagely remember reading somewhere else that the 454 GM TB flows like ~620-640cfm @ 3"Hg.

These are both 2v tests (again for those who don't know). When converted to 4v flow numbers @ 1.5"Hg by using this formula ("Known Flow" @ 3"Hg x 0.707), the 535cfm @3"Hg becomes equivalent to ~378 cfm @1.5"Hg.

That's an unfair comparison really. It's sorta comparing apples to oranges. But it gives you some idea of what type of 4v carb flow #s they run similar to.

Hope this helps,
AJ

 

Only 230 HP? Uh...what should I do? Invest in a "performance" TB unit? I didn't want to..I was just going to get a new fuel pump...

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1992 Camaro RS (Auto) 5.0L TBI
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those are before they become a restriction. though you can get around it. like NOS< supercharger, etc.. increase velocity in the manifold/ you can go around it. thats with a base engine i would guess.....you dont need a bigger one, but hey, never hurts to have a bigger one.