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Old 10-11-2009, 07:33 AM
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How big is too for a carb

Hello, I'm tired of my 600 cfm Edelbrock 1405. No matter how much I try my motor doesn't seem to like it. A buddy of mine has an unused Holley 82751 750cfm carb http://www.race-mart.com/items/produ...751-detail.htm . I am wondering if it may be too big for my 355.
The specs are
3970010 block bored .30 5.7 rods 3.48 stroke
Speed pro Flat tops 4 valve relief (10.2:1 @ 64cc heads)
Brodix IK 200
Edelbrock Performer Intake
mild porting on heads and intake
lunati Voodoo 268/276 60103
(http://www.lunatipower.com/Product.aspx?id=1984)
Mallory HEI Dizzy
Backed by a 5 speed

Is this 750 cfm carb to big for this motor or should I look for a 670 or 700 area carb?

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Old 10-11-2009, 08:28 AM
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The Holley 750 is perfect for your combo, buy it. I can see why you would have a problem with a Edel 1405 on this engine, it would require a lot of changes.
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Old 10-11-2009, 09:28 AM
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I would go with the 670cfm. Great, right out of the box! wouldn't even need to touch it other than mounting it on the engine. Good streetable carb and track.
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Old 10-11-2009, 09:48 AM
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I'd modify the 750 vs 650 carb with this thought.
650/670 with a vac sec is hard to beat for drivability. Probably bolt right on. The 750 may required some fine tuning to get it just right. The shooter may need need replacing up or down (unlikely), may have to change jets a little, i wouldn't mess with the air bleeds unless you have an AFR and really understan what they do. The vac sec will need to be a yellow or short yellow spring, and the pump cam may need some tuning.

All this after you get the timing right. 34-36 total at 2500 or so.

Personally I have a 750 3310-3 on my hot rod with an Air gap manifold. It's a little difficult to live with when it is 35 deg out side but once it is warmed up it is fine. I'm still fine tuning it but it pulls hard right up to red line.
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Old 10-11-2009, 10:15 AM
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carb

Go with the 750 for sure. That Edel is way to small for your set up. That series of Edel carb was designed for stock engines.


I would venture a guess if you took a vac reading at WOT you would have 3 or 4 HG of vacuum. The ideal carb would produce approx 1 HG of vaccum at WOT.


Keith
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Old 10-11-2009, 02:33 PM
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Yeah, i'm going to get the 750, has anybody have used the 4 corner idle. I have heard they are a PITA to set/tune. Is that true?
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Old 10-11-2009, 03:18 PM
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Quote:
Originally Posted by jonahb
Yeah, i'm going to get the 750, has anybody have used the 4 corner idle. I have heard they are a PITA to set/tune. Is that true?
Nope, it makes it even easier to tune. You have more control over adjustments.
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Old 10-11-2009, 04:49 PM
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keep in mind that q jets are 750 and 800cfm

how would a 650 be too big???

jetting is what to worry about..

out of the box aftermarket carbs always tend to be rich and dump alot of fuel.. unless youre sticking it on a huge engine maybe

good luck!
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Old 10-11-2009, 06:04 PM
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I'm running the same 82751 on my 355. So far I've been really happy with it. It was a bit lean at idle out of the box, but up top it runs great and pulls nice right up to my redline. I'm running a rebuilt old set of brodix -8 heads, 10:2:1 comp and a 292 cam.
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Old 10-11-2009, 08:49 PM
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Quote:
Originally Posted by fast68
keep in mind that q jets are 750 and 800cfm

how would a 650 be too big???
Quadrajets, like Thermoquads, have air valve controlled secondaries. A 1000 cfm Thermoquad will not over fuel a 350, but a 950 squarebore will.
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Old 10-12-2009, 11:04 AM
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BTW: That Holley in the picture is not a 750. It's a 780. 750 holley uses the striate booster leg and the 780 uses down booster leg. Throttle reponse is going to be a little sluggish at low end when using a 780 vs 750.
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Old 10-12-2009, 01:28 PM
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Quote:
Originally Posted by lg1969
BTW: That Holley in the picture is not a 750. It's a 780. 750 holley uses the striate booster leg and the 780 uses down booster leg. Throttle reponse is going to be a little sluggish at low end when using a 780 vs 750.
This throttle response quote is total B.S.
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Old 10-12-2009, 01:45 PM
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Be nice ericnova. It's only an a opinion
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Old 10-12-2009, 02:31 PM
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Carb Choice

Hey Jonah, There's some real good advise here. But, if you do the calculations, your cam only makes power to 6200, at 355 CI with 100% VE you only need 636 CFM. At 110%-700 CFM, at 120%-764 CFM. I'm a firm believer in not over carbureting. I have found that matching CFM to the engine makes the carb easier to tune and greatly improves the throttle response. Mechanical secondaries with stick shift, vacuum secondaries with automatics, my preference. If it were mine, I'd put a 650 cfm Holley double pumper w/ mechanical secondaries because of the stick shift. It'll pull like a locomotive. olnolan
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Old 10-12-2009, 02:45 PM
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Continuing with the OLNOLAN line of thinking, it's gonna be damned tough to exceed 105% VE on a street/strip build and all the parts have to be very friendly with each other to reach 100% VE. Automatic with less than 3000 stall converter, vacuum secondaries. Automatic with more than 3000 stall converter or manual transmission, double pumper.
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