I too, used to shift by ear. The doctor told me to start using my hand. That way the ear would heal and I'd also be able to see where I was going. Just thought a little humor was needed here, as this topic has been hashed and beaten to death over and over on this forum and no one ever agrees on anything.
Duane

 

It amazes me how there are so many different opinions on this CFM question still with all this info direct from the pros available at our fingertips. The "old dogs" phrase, I guess. I for one think life SHOULD be simpler with this "Information Super Highway". But, we find a way to always make it more difficult. Maybe that's human nature...

I would love to see someone organize a track rental day, put away our egos, get together, and help each other out with our "tricks" in turning the best eta for each others rides.

-Stroke

 

I'll know for a fact. That when I put mine on a Dyno. The professional will know if I should move from 650 double pumper, to a 780. I didn't know that when I started this thread it would go so crazy. It's great!!! LoL

 

I don't see any difference between now and "back then". Just talk to people, and after a little time and experience you can pretty well figure out who knows what they're talking about, and who's full of crap.

This thread has several examples of both.

 

Well the big difference between now and then is the availability of information. We now have the sum knowledge of all of human intelligence in our pockets at any given time. Access to EVERYTHING. That didn't exist then.

But yes, what's the same is the mentality of people.

Maybe I need to learn to expect less of humans.

-Stroke

 

Or lack thereof. You have guys like Jim AKA 427Hotrod who have a depth of real world experience with what works and what doesn't, and then you have somebody who digs up a link that supports his limited understanding of things and hangs his hat on that. Who to believe....hmmmm, tough choice.

 

Remember, when the engine comes off the dyno and gets installed into the street driven C3 complete with a full exhaust system the engine's need for lots of cfm suddenly drops at least 15%. That's the cold hard facts of life whether you like it or not. People like to brag about their 525 hp 383" or their 450 hp 350" but those figures were based on none of the accessories being used during the dyno pulls. In the end the only thing that matters is how much of that power is getting to the pavement and that's where we get humbled.

 

Well, for me it’s simple really. I take sort of the scientific method approach. Believe neither to be true and test the theories for to try and disprove until the results are clear. If unclear, test again. The problem may lay with the person speaking the answer. They want affirmation of their hunches or maybe just what seems simple to understand.

Eh... philosophy of thought. Maybe we should have a philosophy corner thread.

-Stroke

 

People seem to forget street engines and racing engines are completely different animals that require completely different carburetors. Whereas street engines operate from off idle to around 3500 rpm racing engines operate from 5000 rpm and up. A street engine needs a heavy flywheel or a low stall torque converter to maintain civility and a camshaft that will produce a high torque at a low engine speed. A racing engine on the other hand needs a light flywheel or light weight high stall torque converter, low rear gears, high compression pressures, and a camshaft that will produce power at an elevated engine speed. And street engines need an AVS or vacuum secondary carburetor to prevent the dreaded "over throttle" that kills low engine speed power. Used 800 hp @ 8000 rpm NASCAR engines are readily available for as low as $5000 through E-Bay but they would make pitiful street engines because of their high rpm ONLY nature.

 

As I said earlier....a 650 double pumper will work well for a relatively mild combo...but I'd probably still use a good 750.

Here's a test where they tried to do what we're talking about...but they initially used an old 750 that had issues, only made a few passes but they did some chassis dyno runs. Giving up 14 RWHP is pretty significant...but if you're not using it......

http://www.hotrod.com/articles/how-t...eet-strip-car/

This is also interesting...

http://www.hotrod.com/articles/ccrp-...retors-tested/


Another good one....read the last sentence.....
http://www.rockettbrand.com/download...ing%202009.pdf



JIM

 

Here's four carburetor sizing calculators to help you select the proper size for your STREET engine that requires a full exhaust system:

http://www.gtsparkplugs.com/CarbCFMCalc.html

http://www.carburetion.com/Calc.asp

http://www.holleycarbs.com.au/chooseCarb.htm

http://www.holleycarbs.com.au/chooseCarb.htm

When using these calculators it's imperative you are HONEST about the maximum rpm your engine sees in street use and I recommend you use a VE of 85% which is typical of a high performance street engine with a street legal exhaust system attached. And remember your street engine operates from off idle to 3500 rpm about 99.99% of the time.

 

Here's my vote. Perfectly sized carb for a street 350-383ci. Ask me how I know...

https://www.holley.com/products/fuel...arts/HR-680-VS

 

DO NOT EAT THIS CARB!!!

 

A larger-than-needed carburetor will give a false sense of increased power because every degree of throttle opening provides more air flow THROUGH THE CARBURETOR at low engine speeds. As an example a 950 cfm carburetor will provide 475 cfm at only half throttle which is just about all a street 350" engine running at an 85% VE can swallow at 5000 rpm. But after that half throttle the rest goes to waste because the 350" can't use it and because of the horrendously slow venturi velocity the fuel hardly gets atomized. But it will give a false sense of more power in the beginning.

Another example of this is running 2.0" throttle bodies on a 350" CrossFire Injection engine. The pair of stock 1-13/16" throttle bodies will flow 567 cfm but 2.0" throttle bodies will flow 720 cfm so they provide more power up until the engine reaches its maximum consumption which would be around 520 cfm @ 6000 rpm (assuming an 85% VE). So using a larger-than-needed carburetor accomplishes nothing on a street engine other than providing very poor air/fuel mixing and lower power at low engine speeds.

All of the carburetor manufacturers will tell you it's a big mistake to use a larger-than-needed carburetor on a street engine but yet most people do because they get really bad advice from people who think they know more than the carburetor manufacturers. Ideally you want to see 0" HG at maximum power as that tells you the carburetor isn't being a restriction but if you see 1" Hg you're only giving up a tiny amount of power but gaining a much better throttle response and better air/fuel mixing.throughout the entire rev range.

 

My neighbor recently installed a ZZ6 turn-key crate engine in his '73 Chevy pickup; a 405 hp @ 5600 rpm serpentine belt 350" that comes with a tall single plane G.M. intake and then he installed a new 770 cfm vacuum secondary Holley carburetor. First of all that engine makes its 405 hp at 5600 rpm largely because of the single plane intake but it'll give up a good 25 to 30 hp down low where he needs it the most. And then as his 85% VE street 350" can only consume 481 cfm at 5600 rpm 290 cfm of his carburetor's 770 cfm potential goes to waste as his secondaries will only open about 1/4th the way. Had he selected a lesser hp dual plane intake equipped engine and installed a 500 cfm carburetor he'd have a much better running combination for his heavy pickup. Street engine require VERY different parts than racing engines because they operate at much lower speeds 99.99% of the time.

 

The engineer's at GM really got it wrong with all the resources they have to engine design, testing, etc. Better than most mortals could ever dream of. Imagine their mistake folks of putting on a 750 cfm rated QJ on everything from a 283 ci sbc to a 500 ci Caddy. Too bad these forums weren't around in the '50's so they could of come in here and read what the real experts had to say.

 

The irony of your comment is found in how you don't seem to realize you just supported the argument for smaller than Conventional Wisdom Holley Carburetor on most street engines with your observation about Quadrajets.

I feel like a condescending IDIOT as type this because I know most here already know what I'm going to type next......but I'm not at all sure YOU do given what you've written.

The reason the Quadrajet is such a TOUGH TO BEAT street engine carb is found in SMALL CFM.

Those Primaries on the Carb are SUPER SMALL.....far smaller than what you see in the Primaries of a 750CFM Holley.....probably as small or smaller than a basic 600CFM square pattern Holley's primaries.

The argument you just tried to make......just blew up in your face as an argument that actually makes a case for the smaller carbs on a Street engine. Ultimately, anyone looking to REPLACE a Q-jet or the stock issue Holley on an engine is probably looking for more performance with different considerations vs, the compromise riddled agenda of what the factory was attempting deliver. If this isn't the case......then yes......it's awfully TOUGH to beat a good running Quadrajet. The stock carb won't deliver the absolute best best performance at the drag strip unless you're awfully good at reworking them..... but if MPG still matters to you and if you're running a vehicle that rarely gets driven hard for anything more than the few times you need a little extra passing power....that Q-jet is almost impossible to beat as best "all around" performer. It'll beat a Holley on MPG most every-time.

The reason it's such a good street carb found in how the Quadrajet spends virtually all it's time running on it's extremely small primaries. This delivers terrific part throttle response and those GIANT secondaries only open as needed as vacuum from the engines of various sizes ask for it. This is why everything from a 283 to a massive 454 big block could run the carb. The smaller engines probably never see the secondaries fully open or what you'd experience is a giant BOG and fall on it's face performance.

But ultimately your trade offs are all in favor of street performance and MPG when you choose the Q-jet. This is no doubt a big part of why GM's hottest small and Big Blocks ran Holley carbs instead......but even in the case of factory installed Holley Carbs the Vacuum secondary was always the first choice for a street engine.

 

Nope. Your love of the QJ is muddying reality of carbs. The QJ is a 750 cfm carb, period. If the small primaries were such a hot deal, which they aren't, why did the other big two, Ford and Chrysler, not use that same design? They just threw Holley's on their engines. Damn the gas mileage back then as it was all about performance. The engine still requires an AFR ideally of 14.7:1 and the cylinders don't care what the primary size is. Any carb can be tuned to do this. Don't kid yourselves that the QJ had some huge advantage over other carb designs with regards to fuel economy. The QJ is not an all out performance carb. It can be tuned to be one though up to a point and match the performance of Holley's et al again, up to a point. GM also used a lot of two bbl carbs on their run of the mill engines, more so than using a 4 bbl. So where are we at with this? QJ's filled that gap for a transition between a 2 bbl or a 4 bbl for some added performance. For their serious performance engines, they did what Ford and Chrysler did - Holley. Anyway, the point of the post was carb sizing. Obviously the car manufacturers did pay heed to some extent by throwing on more two bbls than 4 bbls because their cfm ratings were indeed lower to better match their engines/useage. QJ is a good all around carb. So is my Holley 650 Street HP double pumper. But neither one is meant to be on a 283 Chevelle station wagon.

 

The name is HOLLEY. Not Holly. Believe me, when you do a search for a center-squirt holly you will get some very interesting links you should not follow.


4224 Holley.

 

I get the impression that from what you've written here....you have me and my thinking on the rifght carb for most performance SBC confused with the post I was responding to when I typed what you've "quoted".

I have no great love for the Q-jet beyond it's use as a passenger car Carburetor where MPG and crisp part throttle performance is desirable even if comes at cost to all out WOT performance to red line.

I disagree with your assessment of the small primary/large secondary design......it does make a difference on a street car that is an advantage in terms of crisp responsive throttle.....I stand by that.....and if you can keep your foot out of it....the ability to do better MPG is there too.

I find little to quibble with in your thinking about the Holley 4150 in a 650 size.......it's the carb I've run with best results most consistently too as I've already posted in response in this thread,

Next summer I'll be swapping in for testing at the track a 750CFM 4150 because I'm running a new 383 with a big cam and lots of breathing capacity in the heads.............if it does better I won't be too terribly surprised but if it works out to be the better choice it'll be the first time ever vs a 650 on 2 different 350 SBC I've run in my Corvette over the last 30 years since pulling out that SUPER WEAK and pathetic L82, The Q-jet on that original motor was no doubt the right carb for it's just over 200HP rating, barely 9:1 compression and pretty flat cam and really crummy heads.

The 650 Double pump worked really well on that first ZZ4 crate motor replacement and then even better when I swapped in better heads and cam just about 10 years ago........but this time, with a new engine, there is more Cubic inches...even more cam and the heads flow bigger air so I may find the 750 is my better choice,

I'll book mark this thread a post again in the early summer when I know for sure and can back up what-ever the results of one vs the other are with actual time slips.