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Discussion Starter #1
may change from my edlebrock 600 4 barrel to a holly 2d tbi that is 670cfm, but the holly is rated up to 275 hp so it won't work on my new 355 around 325hp?? but the 600 will?

what im asking is is CFM the same across the board if so then the tbi should work right?

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John Krenn
1970 malibu SS look-a-like well kinda
307/200-4r (soon to be 355 or 383)

Ft.Worth, Tx
 

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thats wierd ,, 670cfm is up there as far as small blocks go ,, seemd it should be rated at a higher HP ,, whats the next step in cfm in the holly ,, unless you have really trick heads that breath alot you should stay under 700cfm as far as I know your small block should be able to flow 670 cubic feet per minute <cfm> just dont under stand why they rated the max HP so low ,, 670 is at the max end of what a small block can flow with out lots help

[This message has been edited by catman (edited 11-30-99).]
 

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John,
The one thing to remember is that CFM does not neccessarily equate to hp capabilities, especially when you get into fuel injection. Look at the Nascar motors, they are getting over 700 hp using an 850 cfm carb. I would bet that the reason for the low rating on the Holley is due to the fuel flow capabilities of the injectors themselves. Remember that a motor is just a big air pump, a given motor lets say a 350 at a particular rpm, lets say 6500, will pump the same amount of air no matter if it is making 50 HP or making 650 hp. This is a bit of an extreme example but is the truth.
Back to your original question though, I think that injector capacity and possibly computer capability is what limits the Holley TBI unit, not air flow.
Hope this helps,

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Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100



[This message has been edited by BillK (edited 11-30-99).]
 

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John, this may be off the point, but I know for a fact that 2 bbl are rated differently ( at a different vaccum I think) than 4 bbls. For example a 500 cfm 2 bbl Holley only flows about 375 cfm in relation to a 4 bbl. What the 2 bbl/4bbl vaccum readings are, I don;t know. But I was told this by Holley. It may indeed apply to a 2 bbl TBI. If the relationship holds as in standard 2bbl v 4 bbl, the 670 would actually flow 500 cfm in "4 bbl terms".

Sorry for the confusion, but I don;t know the actual difference in how 2 bbls and 4bbls are calculated. They are not comparable however.
 

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A one or two barrel is rated with 3" Hg of vacuum. A four barrel is rated with 1.5" Hg of vacuum. Because there is less vacuum, there is less air flow, so a two barrel carb that is rated at 500 cfm (at 3") flows only 354 cfm (at 1.5") rated as a 4-barrel.

Take the two barrel rating, and divide by 1.414 to convert to four barrel rating. Source: Holley Carbs and Manifolds, Urich and Fisher. H.P. Books

The Corvette Tri-Power, for example, using a center carb of 350 cfm, and a pair of outboard carbs at 446 cfm, totalling 1242 cfm, actually flows only 878 cfm rated as a four barrel.

I don't know, but I suspect that the two barrel throttle body injectors are rated the same as two barrel carbs, so a 670 cfm throttle body would flow only 473 cfm. Yeah, that'd support about 275 HP!
 

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With fuel injection don't worry about too large of a cfm rating on the throttle body. You will not experience bogs if it is tuned right. This is because their is no accelerator pump on a fuel injection system and it doesn't rely on vacuum to operate. i.e. a 900CFM throttle-body will work just fine on a 350cid engine.
 

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Discussion Starter #7
well i found a fella that has a like new holley 2d fi set up with everything and o2 sensor option also, he had it on a diplomat (didn't ask) and didn't like the performance so he said he'd take $300 but he thinks the o2 sensor that came with it is bad. but he has everything even the original box. now this will work on my 307 but im very close to finishing my 355 and am worried if im shooting myself in the foot>>>

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John Krenn
1970 malibu SS look-a-like well kinda
307/200-4r (soon to be 355 or 383)

Ft.Worth, Tx
 

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Shurkey, thnx much for the clarification. I was told my 3 500 cfm 2 bbls would flow "About 1100". And now I know exactly why and that the real number is 1062 cfm.

[This message has been edited by Gene Chas (edited 12-03-99).]
 
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