Carb options - help me decide

[JD08 42]

The engine is a basically stock 351C - 4V. I have a Holley 770 Street Avenger that is in questionable shape that came with it. I was thinking that may be a little too much carb and was debating getting an Edelbrock 600 Performer. So I decided to pose the question here and get input. I should also point out that my carb experience is pretty limited. I haven't dealt with them since the eighties and I really hated the one I had back then.

 

Should I rebuild the 770 SA or buy the Edlebrock 600?

[970fizz 19]

I have a 351w In my 70, first I had a Holley 600 on it. Out of the box it would barley run, and tuning it was a pain! I'm very limited on tuning carbs, I changed it out for a edelbrock 650 thunder series. Out of the box it ran great and with a little tuning the carb performs perfect. Go edelbrock 650.

[1969_Mach1 333]

This is not an easy question.  I have no Cleveland experience but read they like larger carbs than Windsors. The 770 should be about the right size for your Cleveland.  Plus its a vacuum secondary and they are more forgiving with regard to being slightly oversize.  Edelbrock carbs, use to be Carter, and are more friendly out of the box but do not have the potential to perform as well as Holleys, QFT's, etc. (Holley style carbs).  If your partial to Edelbrock, use the Thunder series like 970fizz has.  But I don't know if 650 is the optimal size for a Cleveland (definitely not smaller).

 

As far as a Holley 600, everybody that has some experience with Holley's will say that carb is terrible.  I don't know why it's so popular because it rarely works well.

 

Holley's are not difficult to tune and adjust.  The most simple and critical areas that are often overlooked are adjusting the accelerator pump linkage, selecting the correct accelerator pump nozzle and linkage cam, and getting the throttle plates positioned correctly at idle.  I usually suggest getting a book on how to tune Holley carbs.  If your carb is physically in decent shape, I would by a Holley, QFT, or AED brand rebuild kit, two new floats, a book for tuning Holley carbs, and cleaner (preferably the gallon containers to soak the parts in).  You will probably save yourself a lot of money and be surprised how simple they are.

[Shep69 149]

When selecting a carby you can use a carby calculator for the size. Holley has one on there site and most other brands will do as well. A lot of people over carb there cars. Especially if it's a stock set up.

I'm running a edelbrock 650 thunder series on my 351w and it's been a pig straight out the box . I purchased the edelbrock spring and rod tuning kit and it has made it slightly better but it's not perfect . I was thinking about swapping it for a quick fuel carb.

[shaun071 62]

I've had several 351c's and had mixed results with 600 holley carbs. One was great but the others were worse than the carter thermoquad that was the factory carb here in Australia. Crap economy and so-so throttle response. I do think the 770 is too big for a stock 351c, something in the 600-650 cfm range would work well. I've been looking at the Summit 600 cfm carbs and will probably get one for my 351c 2v. A few of my friends are running the edelbrock 600 carbs with good results on 351c's.

[1969_Mach1 333]

Holley use to have a calculator that asked more questions than simply engine size and maximum RPM.  If the 770 vacuum secondary carb is too large, the secondaries simply won't fully open.  As long as there are no low speed stumbles or surges that cannot be eliminated by tuning the size should be fine.  The Quick Fuel carbs like Shep69 mentioned have nice tuning features and are gaining popularity.  Even though, I believe, QFT carbs are assembled in USA with imported parts.

[MikeStang 247]

That 750 should be fine. Think about this.

Chevy Used Q-Jets on just about all their v8s from the lowly 305 to the 454 and Q-Jets only came in a few sizes.... 700, 795, and 800 cfm if I recall correctly.

Any yes I know its a different carb and is tuned much differently, but generally speaking the same carb was on the 305's and 350s and they ran great and got pretty damn good mileage if you kept your foot out of it.

And yes a Q-Jet can be easily built to perform with a Holley and be twice as reliable.

His Vac secondary 770 will be just fine.

[Rsmach1 71]

I have a couple questions before recommending carbs.

 

1. How do you plan on using the car, is this a street car used for cruising, trips to shows/car hop, occasional commute? Will it see the track? If yes how often? Is gas mileage a concern? What do you consider your redline rpm?

 

2. When you say basically stock, does it have the stock intake? If not which intake? Cast iron exhaust or headers? Cam? Manual or auto?

 

One thing I will say for sure is the 770 is definitely to much carb for a stock 351c unless it spends most of it's time at the track. IMHO.

[Rsmach1 71]

That 750 should be fine. Think about this.

Chevy Used Q-Jets on just about all their v8s from the lowly 305 to the 454 and Q-Jets only came in a few sizes.... 700, 795, and 800 cfm if I recall correctly.

Any yes I know its a different carb and is tuned much differently, but generally speaking the same carb was on the 305's and 350s and they ran great and got pretty damn good mileage if you kept your foot out of it.

And yes a Q-Jet can be easily built to perform with a Holley and be twice as reliable.

His Vac secondary 770 will be just fine.

You can't compare a Q-jet to a standard flange carb, 2 different animals. The sm block Q-jet had a tab on the secondary side to limit the amount it opened, effectively reducing the cfm.

[1969_Mach1 333]

If you only consider the total CFM rating and compare it to the CFM rating he needs, you can probably say the 770 is too large.  But don't forget, the vacuum secondary is very forgiving and will only open far enough to meet the engines demands.  Sometimes, if a carb is too large there will be surges and stumbles during mild acceleration that you cannot tune to eliminate.  If that does not occur, his 770 should be okay.  The stumbles or surges occur when the air speed is not high to start the main circuit thus delaying main circuit startup.  The A/F mixture gets too lean without the main circuit being activated and the motor is trying to run only on the idle and transfer circuits.

 

I have rebuilt a lot of Q-jets when I was younger working as a mechanic.  I don't recall seeing anything limiting the secondary opening.  That doesn't mean it didn't exist.  I just didn't see any.  The Q-jets were typically larger than needed and still worked great because of the small primaries with extremely sensitive boosters and also the secondary air velocity valve which only permits enough air flow through the secondaries to meet the engines demands (like a vacuum secondary).

[JD08 42]

Thanks to an offer from a member to rebuild, I'm going to go with the rebuild option. Thank you all for your assistance

[Rsmach1 71]

Glad to hear you got help from a member, and kudos to the member for helping, there are great people here. Sad that this ends the cfm debate with one of my favorite posters, (1969_Mach1), next time bud, lol.

[1969_Mach1 333]

This topic is always interesting.  I tend to prefer a slightly larger carb than smaller which is usually opposite of most others.  Unless something is broke or extremely worn throttle shafts his carb should work.  I'm sure we'll all be back on this subject again Rsmach1.

[RogerC 136]

I posted the following in another thread (old site) which the link seems dead. Applies to 351Cs, sourced from "Sticky #3: 351C Performance Tuning and Re-Building" by George Pence on the Pantera forums

 

Four Barrel Carburetors

At 6000 rpm a 351 cubic inch motor would theoretically inhale 609 cubic feet of air per minute if the volumetric efficiency were 100%. At 7000 rpm the same motor would inhale 710 cubic feet of air per minute assuming 100% volumetric efficiency. Assuming 90% volumetric efficiency a 351 cubic inch motor will inhale air at the rate of 548 cfm at 6000 rpm or at the rate of 639 cfm at 7000 rpm.

However, as the volumetric efficiency of a motor improves the intake manifold vacuum at wide open throttle shall decrease. The intake manifold pressure of a motor with 100% volumetric efficiency is theoretically equal to atmospheric pressure at wide open throttle. The airflow rating of carburetors is measured at a fixed depression, such as 1.5 inches of mercury in the case of Holley carburetors. If the depression across a Holley carburetor is less than 1.5 inches of mercury at wide open throttle it will not flow the amount of air it is rated at, the motor shall require a carburetor with a larger rating than what we calculated in order to supply adequate airflow at 6000 or 7000 rpm. The reason for this is not because the motor demands more air flow than what we calculated but because the carburetor, which is rated at a depression of 1.5 inches of mercury, flows less air if the depression is less than 1.5 inches of mercury; in other words the flow rating of a carburetor as determined at 1.5 inches of mercury becomes less relevant as the volumetric efficiency of a motor increases.

Both the 351C 2V and the 351C 4V have higher volumetric efficiency than the popular in-line-valve V8s people are more familiar with; at wide open throttle the vacuum in their intake manifolds will drop lower than it does in those other V8s if the carburetor is large enough to allow it. This is the reason larger carburetors are recommended for the Cleveland engine series. If an owner allows the popular literature to influence their choice in carburetors, they shall end up strangling their Cleveland motor with a carburetor that is too small.

On top of that the 351C 4V is capable of operating over an extraordinarily wide power band, certainly wider than any other OHV engine from its era. The first 351C 4V performance manifolds designed by Ford were designed for list #4575 Holley Dominator carburetors (1050 cfm)! Ford’s earliest carburetor recommendations also included the Holley 850 cfm double pumper. The 351 Cleveland engines require carburetors designed for engines having higher volumetric efficiency and in the case of the 351C 4V a wide power band too. The usual carburetor choices for a 351C 2V usually range from 650cfm to 750cfm; for the 351C 4V those choices usually range from 750cfm to 850cfm. None of these carburetors are too big for a 351C street motor, especially if they are equipped with annular booster venturis. With a 351C 4V street motor it is a challenge to find a carburetor that performs well at low rpm while also being large enough to take advantage of the WOT (wide open throttle) volumetric efficiency of that motor.

Annular booster venturis atomize fuel better and provide a stronger fuel metering signal at low air velocity. In other words, annular booster venturis benefit the low rpm and mid-rpm performance of a motor in the same manner as the smaller primary throttle bores of a spread bore carburetor. These attributes make annular booster venturis popular for improving the low rpm operation of performance engines, where they have earned a reputation for improving torque, horsepower and throttle response at low engine speeds. However the improvement in fuel atomization distributes fuel more consistently throughout an intake manifold, resulting in more consistent fuel/air ratio from cylinder to cylinder, therefore annular booster venturis actually improve torque and horsepower across a motor's entire power band; and they improve fuel economy too! The only drawbacks of annular booster venturis include their larger physical size (which reduces the airflow capability of a carburetor by a relatively small amount) and their greater cost of manufacture.

Mechanical secondary/annular booster carburetors featuring street calibration and electric chokes

*Demon Carburetors #1282020 - 650 cfm - Speed Demon, mech. secondary, elec. choke kit #421440
*Demon Carburetors #1402020 - 750 cfm - Speed Demon, mech. secondary, elec. choke kit #421440
*Demon Carburetors #1563020 - 850 cfm - Speed Demon, mech. secondary, elec. choke kit #421440
*Holley #0-9379 - 750 cfm - Competition Series, mech. secondary, choke horn equipped
*Holley #0-9380 – 850 cfm - Competition Series, mech. secondary, choke horn equipped
*Quick Fuel Technologies (QFT) #SS-650-AN – 650 cfm – SS Series, mech. secondary, electric choke
*Quick Fuel Technologies (QFT) #SS-750-AN – 750 cfm – SS Series, mech. secondary, electric choke

Vacuum secondary/ annular booster carburetors featuring street calibration and electric chokes

*Demon Carburetors #1282020VE - 650 cfm - Speed Demon, vac. secondary, electric choke
*Demon Carburetors #1402020VE - 750 cfm - Speed Demon, vac. secondary, electric choke
*Demon Carburetors #1563020VE - 850 cfm - Speed Demon, vac. secondary, electric choke
*Summit Racing #M08600VS - 600 cfm – vac. secondary, electric choke
*Summit Racing #M08750VS - 750 cfm – vac. secondary, electric choke

Vacuum secondary/down-leg booster carburetors featuring street calibration and electric chokes

*Quick Fuel Technologies (QFT) #SS-680-VS - 680 cfm – SS Series, vac. secondary, electric choke
*Quick Fuel Technologies (QFT) #SS-735-VS - 735 cfm – SS Series, vac. secondary, electric choke
*Quick Fuel Technologies (QFT) #SS-780-VS - 780 cfm – SS Series, vac. secondary, electric choke

[barnett468 418]

.

all the dominator carbs use annulars also and they make a 750 cfm version.

 

holley makes a 670 vacuum, sedcondary called a low rider carb.

 

summit racing has the old holley shoe box style with annulars and vacuum secondaries in a 600 and 670 version.

[shaun071 62]

I'd read that before Barnett, it was one of the reasons I was looking at the Summit carb for mine. I was leaning toward the 600 as I would rarely see 5000rpm.

[JD08 42]

I suppose I could also throw a wrench in the works here and mention that it has a Victor Jr intake. Considering this car might see the drag strip a few times, but will mostly be used for the occasional cruise, I'd have preferred a dual plane manifold.

[fvike 173]

I've read somewhere sometime that 351C's isn't very fond of Carter/Edelbrock type carbs. I just can't remember why. It might have something to do with the fuel "bowls" not being large enough for that engine. Also, the fuel tends to get warmer because of the "bowls " being inboard, and resting on the manifold heat, as opposed to Holley type carbs.

[Rsmach1 71]

I suppose I could also throw a wrench in the works here and mention that it has a Victor Jr intake. Considering this car might see the drag strip a few times, but will mostly be used for the occasional cruise, I'd have preferred a dual plane manifold.

Can always pick up a dual plane for street use with the $ saved by rebuilding your carb vs buying new, and swap back to the Victor Jr on track day's. That's what I did.

[barnett468 418]

I suppose I could also throw a wrench in the works here and mention that it has a Victor Jr intake. Considering this car might see the drag strip a few times, but will mostly be used for the occasional cruise, I'd have preferred a dual plane manifold.

 

you would be better off with a blue thunder intake.

 

what gears do you have?

 

do you have headers?

[JD08 42]

3.00 open differential and shorty headers.It needs an H/X pipe though.The rear end swap is down the road as money permits. Gotta get it on the road before I worry about down the road.

[barnett468 418]

3.00 open differential and shorty headers.It needs an H/X pipe though.The rear end swap is down the road as money permits. Gotta get it on the road before I worry about down the road.

 

unless you drive from 4000 to 7000 rpm all the time, get rid of that intake . . it is a ridiculous thing to have on there with a stock engine and 3.00 gears . . or you could put 3.50 - 3.73 gears in it and keep the intake..

[1969_Mach1 333]

I agree with barnett468, a ring and pinion swap would be a priority.  I would do a gear ratio change regardless of what intake you ultimately use.  Your car should have a 9" rear axle.  You don't need to swap the entire rear end or even the entire center section for a gear change.

[Shep69 149]

Barnett what quickful carb do you recommend for a warm 351w

[Max Power 74]

The problem with those cfm charts is that they measure cfm required to achieve an rpm, but they really don't measure rate of acceleration, which, with performance cars, is kinda the point. Back when Holley made mostly just 600 and 750 carbs in vacuum, I would swap back and forth, and in spite of charts telling me I only needed a 600, the 750 was quite often a dramatic improvement, from off idle through the rpm range. I think those charts are better suited to non-performance use, and Holley used them because back in the day, if you wanted to make a car fast, every shade-tree wrench wanted to throw a 750DP on it.

 

A vacuum secondary carb is in extent a variable venturi carb, similar to QJets. You can get away with a lot with them. Not only does the 770 Avenger probably not flow enough in wet test to actually be an actual 770, but it will not open the secondaries all the way to bog you down, as the vacuum signals will not demand it. I have used a 750cfm Holley on a stock 351W and after jet changes and idles adjustments, the thing ran great. I would like to do more research in the boosters though, I bet there might be some gains there.

 

Thumbnails below not mine. Why haven't they fixed this yet?