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Carb CFM rating for 454

windrider50

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Oct 1, 2002
Messages
51
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NJ
Corvette
1996 CE LT-4 6 spd; 1972 BB, T Top "Boomer"
I'm about to dive into fine tuning the carb on my mildly moded 72 454 with a LM-1 Wideband O2 AFM setup. A couple of local sages have opinioned that the Holley Squarebore 600 CFM carb might be too small for the engine. As we discussed it, they focused on how much the throttleplates would need to be opened at idle to flow the air required, and they thought that too much of the transition slots would need to be exposed.

Any thoughts??

BB NJ Ken
 
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7,246
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Washington, Michigan
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'67 Marina Blue Convertible
There's probably sufficient authority in the idle system for it to idle OK, but the 600 is definitely on the small side for a 454 in terms of performance; it came originally with a Q-Jet that was 750-800CFM. Assuming you have a manifold on it that takes the Holley, a vacuum-secondary 3310 (750CFM) or equivalent Avenger would be a better choice.

:beer
 

jim coulter

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Dec 3, 2002
Messages
157
Location
covington,tn
Corvette
71 big block, 84 coupe
I checked a couple of speed shops when I was ready to put one on my 454 in my 71. Not sure whats in yours, but with a 282 duration cam, 10.25:1 compression and hi-perf heads they recommended a 750-800 for mine. I went with the edelbrock 800, but haven't worked on it any since I bolted it on so not sure how it will do. A 600 seems small to me. I'm sure others with 454's will come along with their set-ups which you can use to help decide.
 

minifridge1138

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Feb 2, 2007
Messages
908
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USA
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1982 Black Fastback
600 CFM is too small for a 454.
750-800 would be better for that big block (yes I am jealous).

Carb size is just a math problem. Using the engine displacement, rpm's, volumetric efficiency, and some other numbers, you can accurately calculate the size of the carb you need.

A ballpark formula is this:

Carb size = ( (cubic inches X max RPM) / 3456 ) X Volumetric Efficiency.

Here is a useful website with a calculator that will do the math for you:
http://www.4secondsflat.com/Carb_CFM_Calculator.html

You enter the size of your motor, the max RPM you want your engine to reach, the volumentric efficiency of your engine (normal motor is around 83% avg), and it calculates the appropriate size carburetor.

BIGGER IS NOT BETTER!! (for carburetors)
My 350 cid motor had a Holley 750 cfm carb on it when i bought it. In order for my motor to actually use 750 cubic feet of air per minute, i would have to be running at about 9,000 RPMS. So I installed a 600 cfm carb. Now i get better throttle response and better gas mileage.

Good luck!!!
 

hoosierdaddy

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Messages
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Bluffton, Indiana
Corvette
2000 Black Coupe; W/6-Speed
I stand to be corrected but I remembered the 454CI came with a dual line/dual feed Holley 780; but that was a 454CI/450HP with the cowl induction. 1970 was the only year for the 450HP from the factory; your 1972 must have 370 HP or so is that correct?
I had a few 454CI but only one 450HP and it has the dual line/dual feed Holley 780. This is what I rember and had anyway.

Bill
 
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Messages
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'67 Marina Blue Convertible
I stand to be corrected but I remembered the 454CI came with a dual line/dual feed Holley 780; but that was a 454CI/450HP with the cowl induction. 1970 was the only year for the 450HP from the factory; your 1972 must have 370 HP or so is that correct?
I had a few 454CI but only one 450HP and it has the dual line/dual feed Holley 780. This is what I rember and had anyway.

Bill

The '72 454 had hydraulic lifters, 270hp, and came with a Q-Jet. The last Holley-equipped 454 was the '71 LS6 (solid lifters, aluminum heads, 425hp).

:beer
 

hoosierdaddy

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Messages
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Bluffton, Indiana
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2000 Black Coupe; W/6-Speed
The '72 454 had hydraulic lifters, 270hp, and came with a Q-Jet. The last Holley-equipped 454 was the '71 LS6 (solid lifters, aluminum heads, 425hp).

:beer

I knew the HP dropped substantially after 1970 and I never had a 454CI after that either. That was the good ole days for sure; dream it and you could order it from the factory for the most part. This is what insurance companies :crazydid to all of us and of course the cost of fuel climbing; remember fuel in the double digits. I use to have a service station and sold fuel for as I remember 28.9 during the gas war days :ugh

Thanks for the update, have a great week and life :)

Bill :beer
 

baxsom

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Joined
Jun 29, 2008
Messages
388
Location
Cocoa Beach FL
Corvette
72 454 convertible, 2000 C5 Z51
600 CFM is too small for a 454.
750-800 would be better for that big block (yes I am jealous).

Carb size is just a math problem. Using the engine displacement, rpm's, volumetric efficiency, and some other numbers, you can accurately calculate the size of the carb you need.

A ballpark formula is this:

Carb size = ( (cubic inches X max RPM) / 3456 ) X Volumetric Efficiency.

Here is a useful website with a calculator that will do the math for you:
Carb CFM Calculator

You enter the size of your motor, the max RPM you want your engine to reach, the volumentric efficiency of your engine (normal motor is around 83% avg), and it calculates the appropriate size carburetor.

BIGGER IS NOT BETTER!! (for carburetors)
My 350 cid motor had a Holley 750 cfm carb on it when i bought it. In order for my motor to actually use 750 cubic feet of air per minute, i would have to be running at about 9,000 RPMS. So I installed a 600 cfm carb. Now i get better throttle response and better gas mileage.

Good luck!!!



please please please forgive me for bringing this dead thread back to life but it is totally appropriate to my situation. i am desperatly trying to learn all of this engine building stuff.

the reply i am quoting here says a 600cfm is too small for a 454 but then provides a formula that gives an answer of 599cfm at the 5500rpm 83% VE limit of a stock 454.

well my question is why do all the carb forumlas say to use a much smaller cfm than people actually use. i am thinking about a brand new quick fuel carb to replace the frankenstein holley i have on my 454 now. while researching carb choice people say at least 750. when doing the math 100% VE says 750 should work. my stock engine will not do 100% VE. at 85% VE math says exactly 615cfm:W so the 680 quick fuel should theorically be perfect.

what part am i missing that makes people go to the bigger carb. i dont want to spend that much on the wrong carb.
thanks
 

minifridge1138

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Feb 2, 2007
Messages
908
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USA
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1982 Black Fastback
I got 2 questions out of your post:
1) why do people say 600cfm is too small for a 454, yet the formula works out to 599.1 cfm?
2) why do people go bigger?

Answer 1)
If your engine is running at 83% VE and you never go beyond 5500 rpm, then a 600cfm should work. But if you are running above 83% VE, you'll turn the carb into a restriction. Going to a 650 or 680 gives you a nice buffer in case your engine is more efficient and gives you room to play with performance parts. I would hate to buy a 600 cfm carb now, install a performance camshaft in a year, and then have to buy another carburetor to match the new VE.

Answer 2)
Lets say you have an engine that uses a maximum 500 cfm. So if you put in a 500 cfm carburetor, then you will get 100% of the engine's power at 100% throttle.
But now install a 1,000 cfm carburetor. You get 100% of the engine's power at 50% throttle. You get the same power for 1/2 the pedal movement!!! (sounds cool to me).
It also means that half of your gas pedal does nothing. Anything after 1/2 throttle doesn't really do anything except maybe dump more fuel and run rich (not so cool to me).

I hope this helps, and Good Luck!!!
 

baxsom

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Joined
Jun 29, 2008
Messages
388
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Cocoa Beach FL
Corvette
72 454 convertible, 2000 C5 Z51
thank you for the reply, sometimes i think i am searching too much because the more i read the more questions i come up with.

i can understand the going beyond 83% thing. i did read more and found that even race engines wont go much above 90% unless they go into forced induction. so lets say i go to a hot cam and better flowing heads and achieve that 90% VE that means at a cool 6500 rpms i need that 750cfm carb.

i did however read that the best way to tell is to go WOT with a vacuum gauge. if the reading is under 1.5 in/hg then your carb is the correct size. any truth to that.
 

minifridge1138

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Messages
908
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USA
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1982 Black Fastback
I hadn't heard about the trick with the vacuum gauge.

Hopefully someone else can confirm or bust that idea.
 
Joined
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Messages
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Location
Washington, Michigan
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'67 Marina Blue Convertible
You also need to consider how you drive the car and how you plan to use it. A bigger carb than you really need may be of benefit when you're at wide-open-throttle and high rpm, but is that a realistic view of your "normal" driving routine? A more realistic 600-650CFM carb will have more efficient metering at "normal" speeds and crisper throttle response due to the higher air velocity through the smaller venturis providing a stronger metering signal.

:beer
 

TimAT

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Sep 8, 2007
Messages
708
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Gladstone Missouri
Corvette
1969 LS-7 BB
I'm going to put a little "real world" experience on this one. Your results may vary. I started with a Z-28 engine- 350/245hp model in 1973. Added a GM "off-road" cam (3927140) lifters and springs, then a Edlebrock Torker intake with a 780 Holley (3310). Got to playing with carbs had several different sources so no $$ involved. Put a 650 DP on there and seat of the pants performance was fantastic- off the line was just short of amazing. But at 5000 RPM it was done, no more pull. Put an 850 DP on and the off the line was crap, almost impossible to get it rolling. Started to come to life about 3300 RPM and would pull until 7000 (where I quit) but still felt like it would keep going. Back to the 780 vacuum secondaries and got a hell of a bog when the secondairies started to open (manual trans). Ended up with a 750 DP. No bog, bottom end was ok and still had the top end.
Still have that engine today and it runs as good now as it did back then. Hope this helps a little.
 

baxsom

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Joined
Jun 29, 2008
Messages
388
Location
Cocoa Beach FL
Corvette
72 454 convertible, 2000 C5 Z51
I'm going to put a little "real world" experience on this one. Your results may vary. I started with a Z-28 engine- 350/245hp model in 1973. Added a GM "off-road" cam (3927140) lifters and springs, then a Edlebrock Torker intake with a 780 Holley (3310). Got to playing with carbs had several different sources so no $$ involved. Put a 650 DP on there and seat of the pants performance was fantastic- off the line was just short of amazing. But at 5000 RPM it was done, no more pull. Put an 850 DP on and the off the line was crap, almost impossible to get it rolling. Started to come to life about 3300 RPM and would pull until 7000 (where I quit) but still felt like it would keep going. Back to the 780 vacuum secondaries and got a hell of a bog when the secondairies started to open (manual trans). Ended up with a 750 DP. No bog, bottom end was ok and still had the top end.
Still have that engine today and it runs as good now as it did back then. Hope this helps a little.

thats pretty much what i was looking for. considering that the car redlines at 5500 i dont see it near that rpm often

the 750 range carbs sound from what i have been reading as to what i am looking for. since i have an auto trans the vacuum secondaries will probably work better.
 

TimAT

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Sep 8, 2007
Messages
708
Location
Gladstone Missouri
Corvette
1969 LS-7 BB
Go with vacuum secondaries and an auto tranny. The little "bog" that you get with vacuum and a manual gives the tranny time to downshift and be ready. I've found that the auto trans/vacuum secondary combination works very well, as the DP/Manual. Crossing the combinations takes some real good tuning to cure and get right. And you still might not make it 100%


And another FWIW-- I'm running an 850 Demon DP on Ol' Red now. 454 LS-7 crate engine complete with "XCH" stamp. It's got a ZL-1 cam and oil pump with GM's C-port heads. 4 speed and 4:11's.
 

Rodney Gold

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Jul 28, 2003
Messages
200
Location
South Africa
Corvette
1986 White l98 coupe, 1995 LT1 coupe
The CFM rating is the max a carb can flow in perfect conditions with all butterflys full open not what it actually does flow with air cleaners and so on in the chain.
Forget the "formula" , if you got an auto , stick on a 750+ vac sec , A 600 will strangle a 1/2 decent rat unless you have it in truck that never sees 4500 rpm.
 

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