Holley 670CFM or 770CFM - Impala Tech
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post #1 of 20 (permalink) Old 08-09-2011, 07:56 PM Thread Starter
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Holley 670CFM or 770CFM

SO Im thinking about swapping my carb for a Holley, Just dont know which one, The motor in the Impala is a 355, I ordered a 770cfm carb this morning, but if I can get some info and deside to go with the 670 Ill send it back..

Heres the build, Just a mild Street/Strip set up

Motor:
350 .30 Over
PnP Power pack heads
Hooker Headers
3in Exhaust (headers back)
Weiand Street Warrior Intake manifold
9.5 Comp Pistons
Comp 280H

Trans:
Th400 2200 Stall

Rear end:
GM 12 bolt, Moser axles, 4:11 Gears



Thanks
-Phil
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post #2 of 20 (permalink) Old 08-09-2011, 08:11 PM
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I'd personally go with the Holley 770 Street Avenger. Thats what I have on my Caprice. 355 with vortec heads, 3000 stall 700r4 trans, stock gears. Its great. Its the Aluminum one with 4 corner idle. I was going to get a Dp, but they suggested a Vs carb for better street use with great performance.
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post #3 of 20 (permalink) Old 08-09-2011, 10:32 PM
BA.
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we can be sure that the 670 has smaller venturii's than the 770 which translates to the 670 being more crisp in the sub-4000rpm range, and the 770 carb has a good chance of making a few more ponies at 6000rpm and up.

even if your engine doesn't "need" 770cfm of air-flow, small HP gains have been seen in magazine dyno testing at upper rpms due to the improved air flow.

I'm not familiar with the "PnP" Power Pack heads.....unless you're an IT guy and you're playing off of 'Plug n Play'. (yeah, I'm an IT guy)
Either way, if those are anything like old Power Pack heads....I'd do the 670 because the heads wouldn't flow very well at 5500 and up. Just one guys opinion.

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
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post #4 of 20 (permalink) Old 08-09-2011, 10:59 PM
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Power Pack heads where the second revission historically of the first design head Chevy cast for the small block. They came out in 1956 and have not changed since that time. (Chevy's next improvement to the head was the introduction of the fuelie head on the 1957 283 horse power 283 mechanical fuel injected motor, but it recieved several updates since that time).

Big Dave
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post #5 of 20 (permalink) Old 08-10-2011, 12:12 AM Thread Starter
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Thanks guys.. So I might try the 770.. If it is dumpping more fuel then I want then I have a 1405 Edelbrock I can use I guess.. Or just re jet the 770.. And when I say PnP I mean there ported and polished..


Thanks
-Phil
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post #6 of 20 (permalink) Old 08-10-2011, 12:18 AM Thread Starter
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P.s, I forgot to say that I was buying alum. Heads and a"higher rise" intake manifold with s hotter cam later this yr or the 1st part of nxt yr.. So a 770 might not be a bad choose..
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post #7 of 20 (permalink) Old 08-17-2011, 08:00 PM Thread Starter
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Heres how it turned out, Edlebrock RPM Air gap, Holley 770 carb...

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post #8 of 20 (permalink) Old 09-09-2011, 11:33 AM
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Quote:
Originally Posted by OldSkoolPala View Post
Heres how it turned out, Edlebrock RPM Air gap, Holley 770 carb...

I would think a 770 is more fuel than that engine needs. The street avenger is a great carburetor. The only thing an edelbrock is good for is a paperweight, IMO. You could probably get away with a 670 on that engine, or less, but once you get the heads, intake, and cam, your carb should be alright.
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post #9 of 20 (permalink) Old 09-09-2011, 04:27 PM
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Quote:
The only thing an edelbrock is good for is a paperweight, IMO
Damn, to think a paperweight has been flawlessly powering one of my cars for almost twenty years...

BTW, I vote 670.

Two doors, four doors, wagons, and ragtops.
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post #10 of 20 (permalink) Old 09-09-2011, 04:33 PM
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You can not change the jets to reduce the cfm of the carb. Reducing the jets only makes your 770 cfm carb run lean. The venturi determines the carb's cfm rating (It can be changed but it would cost more you than a new carb would unless you are going to amortize the $350 in specialized tools to change the ventures out over a lot of other carbs). The metering plate is drilled to meter fuel at the correct adiabatic ratio for the specified cfm (once again a good tuner can modify the air bleeds but it is best left alone). The cheapest way to change a cfm rating is to buy another carb (I have a shelf full of old Holley carbs that I have collected from flea markets and swap meets over the decades).

Finally the placement of the venturi in the carb has a measurable affect on fuel economy. A straight venturi will waste as much gas as it burns, or so it seams. A dog leg venturi will drop your main jets down two sizes and still maintain the same air fuel ratio as it will more effectively aerate the fuel, and it gets a stronger vacuum signal for better carburation from idle, to part throttle, to cruise. And the final offering is the Annular booster venturi. It is a venturi with in a venturi that magnifies the vacuum signal and drops the main jet size again another two numbers for the same cfm carb with a dog leg booster.

You shouldn't change a carb to affect how it drives (no amount of tuning will make a too big carb appear to the motor as a smaller carb). More unhappy owners swear their carb is no good because some expert helped them tune it away from the factory setting which is perfect to meet the needs of the cfm rating on the carb.

Big Dave
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post #11 of 20 (permalink) Old 09-10-2011, 09:45 PM
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I was just making the same decision a couple weeks ago for my 383 with flat tops .474 lift hydraulic cam, Edelbrock aluminum heads and an RPM Air Gap Performer intake. All the cfm calculators pointed to a cfm less than 670 unless I plugged in a super high rpm (like 7000) I bought the 670. Haven't driven it yet though. I dyno'd it with two different 750 cfm carbs and it starts to drop off after 6000 due to the cam anyway.

Durg
1962 Impala SS
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post #12 of 20 (permalink) Old 09-11-2011, 12:07 AM
 
 
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Phil, Is it vacuum secondary Carb ?
Here is all I know and please re-educate if I am wrong. If this is going in a big heavy car the smaller carb is most likely the better choice. Simple physics the more mass is harder to move and responce vs fuel air supply is slower given more mass to move. So reguardless the engine will be a little slower to get up in RPM. This is why lighter cars can handle a bigger carb much better then a heavier one. The CFM you need for a build and desired RPM (feeding the air pump) can be calulated here.
http://www.carburetion.com/calc.asp
If a carb is a 750 that does not necessarily mean it flows 750 CFM it may be more or it may be less. Do not know what cabs you mentioned will flow that is somethign for you to find out.

I think if running a vacuum secondary carb over size is not that big of a deal. It will self meter and only give what your particular engine demands at a given vacuum. Although they may also be slower to react vs a properly setup mechanical. If this is primarily a street driven car the smaller carb may work better and actually have better in the seat feel accerleration.

I am not a carb expert but if they are vacuum seconday the 770 should be fine as long as it is not under carburating your car. The only example I hae with the vacuum secondary this is the late 60s Vettes (which is really a mechanical and Vacuum). They put 3x2 Holleys on which total 1250CFM seems like a lot for a 427 BBC. The end carbs do not kick in till 4000RPM then hold on : )
The deeper gears, air gap, and even a carb spacer may help.
By the way your SBC looks BAs I like it.

Last edited by bowtieman427; 09-11-2011 at 12:09 AM. Reason: add
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post #13 of 20 (permalink) Old 11-19-2011, 06:14 PM Thread Starter
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I havent been on here in sometime... But I sent the 770 back and got a 650 duel feed... Thanks for all the help guys.... After my car is done I'm thinking about doing a huge build... Something that myself hasn't seen in a Impala let alone a 70'... Once income tax comes I'll be doing the swap... Wish me luck... The old motor is going to be a doner to my nxt project.. Something like a Nova or Camaro.. Maybe a Monte
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post #14 of 20 (permalink) Old 11-19-2011, 06:21 PM Thread Starter
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P.s the Motor is the same but now with Vortec heads, Edlebrock Performer RPM Air Gap with a Holley 650, Swapped the Th400 out for a 700R4 still have the 12 Bolt with 4:11s

-1970 Impala: LSx/th350/12bolt/4:10
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post #15 of 20 (permalink) Old 11-19-2011, 09:52 PM
 
 
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What ET are you looking for whit your setup ? 411 is pretty low

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post #16 of 20 (permalink) Old 11-19-2011, 10:23 PM
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What ET are you looking for whit your setup ? 411 is pretty low
Not with a 700R4 transmission it isn't. He leaves the line with a 12.54:1 torque multiplication thanks to a 3.06:1 first gear, yet can cruise all day at 70 mph while turning the engine no more than 2,600 RPM because the tranny makes the final drive ratio look like he has a 2.87 rear gear with his 4.10 ring and pinion installed.

Big Dave
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post #17 of 20 (permalink) Old 03-12-2012, 05:38 AM
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A 350 small block can only pull a little over 600 cfm at 6 grand... Do the math...
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post #18 of 20 (permalink) Old 03-12-2012, 08:04 AM
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A 350 small block can only pull a little over 600 cfm at 6 grand... Do the math...
This is a year old post so I assume he is happy with his combination.

Further the "math" is based upon a false assumption of 100% pumping efficiency which doesn't ever happen in a stock motor (on a good day you might hit maybe 70%). It only occurs in a race built and modified motor in a fairly narrow RPM band at or slightly above your motor's peak torque.

With a tuned equal length set of tubular exhaust headers, open to the atmosphere, and independent runners on the intake manifold (tunnel ram or long runner fuel injection) you can broaden that sweet spot out a bit more to get above 100% volumetric efficiency for nearly 10 to 15 percent of the full RPM band (but it isn't easy and the sweet spot changes with the barometer and changing humidity, aka the weather).

A larger carburetor often will make more power than the "correct" cfm carburetor will but at the expense of throttle response and mileage. A smaller than needed carb offers a torque increase and snappy throttle response but limits your total available RPM range (which is what GM did when it introduced the TBI form of EFI). This allowed a small 305 to move a heavy vehicle but severely limited the effective RPM range which in turn limits your overall power band.

I personally would have gone for the 670 on top of the built 355 based upon a lot of experience in building motors. I also know that a customer wants a bigger carb (and heads and cam) but isn't aware of how it will affect their driving style. Without the overdrive transmission and tall gears I wouldn't recommend it, but because he does it worked out for him.

Big Dave
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post #19 of 20 (permalink) Old 03-12-2012, 09:41 AM
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Had a 350 built last spring. 4 bolt block, forged crank, forged pistons, Edelbrock Perf RPM alum heads (9.6:1 comp), roller rockers, Howard's hyd roller cam (#110245-10) & lifters. My engine builder suggested Howards. Said he uses a lot of them - good power & can't be beat for the money. Runs great and on his dyno got 450HP & 450 Torque!

It has a 670 CFM Holley & it runs great. You don't need a 770.
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post #20 of 20 (permalink) Old 03-19-2012, 02:18 AM
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670 on a small block, 770 for a big block...
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