Carb confusion help
 

So im trying to Figure out what size carb I need. Motor is a 400 stroked to 461, 4x heads, old faithful cam with 1.65 rockers and edelbrock performer RPM intake. 9.8:1 CR. Now when I run my motor though a Carb calc. it shows I need a 624 cfm (this is with an assumed redline of 5500 and .85 VE) But when I search threads for similar size motors it seems like the 450+CI motors are getting recommend 800 CFM or higher carbs. So am I just missing something? Am I inputting the wrong numbers? Just want some help before I drop cash on a new carb. Thanks guys.

I would run a 800 on those heads if they`re not ported. They should be with that cam. If ported, run a 850, 4 corner idle.

Get a vacuum secondary carb for street use, Most engines are over carbed but vacuum secondary type carbs are forgiving, whether its a qjet or holley. On these type big inch Pontiacs, I don't have any real world experience to give a # from experience. I don't think anyone can make a solid recommendation on cfm without more info.

what type of carb are you leaning towards? factory Q-Jets were 750 and 800cfm.

Many opinions and preferences out there, but a properly built qjet is tough to beat for street use, and works well for occasional strip runs. As a general novice with the qjet and Holley, I find the qjet to be easier to reach good street manners. But then again it seems that my Holley ' s are always some old piece of crap someone else butchered up.

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Why not just go with a WELL BUILT 800 Q-jet, and don't even give it another thought. They can be tuned to run great, as a daily driver, or run 10 sec ET's at the track. It's a no brainer. IMO--But then, opinions differ. :)

I would recommend an annular 850

X2 on the annular deal or even the good downleg booster.

Cliff R had a very nice running Holley 850 Double Pumper carb on his engine and there
were minimal mods to make it run as well as his $$$ Q-jets. The carb might use a bit more gas (but gas is cheap these days) and you can also use a lot better square bore manifolds without mods to them to make them work with the 850 carb.

You would be on your game with a properly built Shaker 455 850 DP Holley.

Tom V.

So, there you have it. Some like vanilla--some like chocolate. Some like Q-jets--Some like Holley DP's--Some like street tuned Dominators--Some like Holley vac sec--Some like other brands of Holley type carbs--Some say FI is the only way to go nowadays. It's no different than a cam thread.

So, take your pick. :)

I don't have much experience with a carb so brand is not a big factor to me. I have no prob with a q-jet or holley. I had to do some research as I did not know the difference between annular and a downleg. I would prefer the annular. Im not worried about the fuel cost either as this will be a weekend fun car and already had 3.73 and a M22. So I don't expect good fuel mileage. everyone seems to agree about the 800-850 CFM range but the question I still have is why is the formula I see everywhere so far off? Is it a Pontiac thing?

Choosing A Carb Size. Yes, lots of opinions.

One of the most asked questions we receive at PRO-SYSTEMS is, how big will my carburetor be, how much will it flow? Most customers are expecting to hear big numbers.

http://www.pro-system.com/pjames011900.html


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Lots of guys like the Quick Fuel brand.

http://www.gtoforum.com/f178/opinion...tml#post624025

Here's some of their street carbs.

http://www.quickfueltechnology.com/carburetors/street/

We build Qjets here too.. no bias

I've found properly jetted & modified they do good on gas.

I know for a fact that Pro Systems buys the same parts from Holley that I (and Shaker 455) buy so then it comes down to who knows more about how to tune a Holley carbed Pontiac Engine. Without a lot of research, the only two Pontiac names that come to mind were the carb Jim Hand bought from them and a carb used by a second guy.

Jim sent me the PS carb to evaluate and I found nothing in the carb that was anything special in the way of putting it together or the calibration.

The other gentleman actually tried several times to use the PS carb and actually had better luck with his well tuned BG carb.

I think his background with PONTIAC engines and carbs is limited.

Tom V.

Off topic..but who did the head work?

Noting that Patrick James (Pro-Systems) formula to determine carb size is different than the common or in his opinion 'old' formula. A fwiw for interest.

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heads are stock right now. Just cleaned, fluxed, and a valve job. Precision engines in Illinois did the heads and the block. Im thinking of having bier brothers here in ofallon port them.

Excuse me for revisiting this 'older' thread.
While there were many recommendations and info on which carb to use, the OP seemed to be interested in (as am I) as to why he would need an 800 cfm, or more, carb when the calculators he used shows he needs about 624 cfm.

Using what is now considered to be an outdated formula, 624 cfm.

Read the link Steve C posted above for a more modern and accurate take on carb sizing:http://www.pro-system.com/pjames011900.html

CID x 2 gets you pretty close for very well thought out engine project.

I've dyno'd all sorts of carbs on 400's and 455's, on quite a few different intakes. At the 500-550hp mark a 455 will want around 850cfm to make the most peak power. Even with that said a 750 powers them just fine, and the numbers will be equal to at least 4400rpm's.

From there on up the larger carb will make a few more HP everyplace.

At the track the differences will not be very much, as a 455 uses it's huge torque production to produce good ET's.

Back to back to back I tested carbs from 750cfm to 850cfm on my last 455 making right around 514hp. The difference from best to worse was less than .15 seconds and 2mph. Interestingly that for some reason a real 1971 HO carb rated at 827cfm ran virtually the exact same ET/MPH as my 1977 Pontiac carb rated at "800" cfm. We also ran a big Holley carb on the same outing, 850DP, and it ran just a tad slower than the 1977 Pontiac carb, .03 seconds and .30mph.

So FWIW it's about a wash at the track at 500 or so HP from 800-850cfm. The smaller 750 Pontiac Ram Air carb we tested ran the same in short times, but slower everyplace on the entire once the car shifted out of low gear, so it netted almost a tenth and half worst et and almost 2mph slower........Cliff

How much does an annular 850 flow? Assuming the annular boosters are a restriction?
This question seems on topic