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I have a Pontiac 455 bore 0.30 over.. It has 11.5 to 1 compression with comp 270 cam with Edelbrock intake idle to 5500 what carb would you put on it...
What carb to use
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I would run an 800 to 850 DP carb.
The annular carbs would be best. The 830 CFM #9381 or the 850 CFM #9380.
The annular carbs would be best. The 830 CFM #9381 or the 850 CFM #9380.
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Just wondering wouldn't that be to big for just street use!!! Now I don't know much about carbs.. Thanks
SpeedCrazy,
Take a look at one of the Demon carbs, They have a application guide on their site.
The reason you could get away with a big carb for the street like lluciano77 suggests is that he is also suggesting annular boosters which have a better signal and atomize the fuel better at lower rpm's.
I disagree with big carbs on the street but that is one way to make them work.
Take a look at one of the Demon carbs, They have a application guide on their site.
The reason you could get away with a big carb for the street like lluciano77 suggests is that he is also suggesting annular boosters which have a better signal and atomize the fuel better at lower rpm's.
I disagree with big carbs on the street but that is one way to make them work.
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Thanks I will take a look..
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I talk to Edelbrock they said a 750 cfm #1406 and Holly said to use a 670 street avenger part # 0-80670, But something about power valve size I need to check my engine vacuum @ idle (in gear if auto trans) then cut my reading in 1/2 for a power valve size. Mine is a auto.. So what do you think let me know ... Thanks
Well I could be wrong, but it seems like the static compression would be lowered. The velocity will be lower and incomplete cylinder filling will occur. It will also run richer helping cool the cylinder. Maybe "bleeding off" is the wrong choice of words.
Well the cutting of the vacumm in half for the power valve is a baseline guess. That will get you in the ball park and maybe right where you want to be. You will have to try it out.
I don't know much about the street avenger as I have never owned one.
I have had a Edelbrock carb and they usually run lean out of the box. BTW the edelbrock 1406 is a 600 CFM carb.
What is your cam duration @ .050?
NXS,
A big carb will usually run leaner as the area in the venturi is larger and therefore has less of a signal at the boosters.
Now if you run a annular booster then you may have to jet it down a bit as it may be on the rich side.
I don't know much about the street avenger as I have never owned one.
I have had a Edelbrock carb and they usually run lean out of the box. BTW the edelbrock 1406 is a 600 CFM carb.
What is your cam duration @ .050?
NXS,
A big carb will usually run leaner as the area in the venturi is larger and therefore has less of a signal at the boosters.
Now if you run a annular booster then you may have to jet it down a bit as it may be on the rich side.
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Speed, if your gonna stay within 5500 RPMs, wich sounds about right, I think Id use a 770 CFM Holley Street Avenger
Well there may be less velocity leading to a weaker signal to the boosters but they always seem to run rich to me. I still think a big carb helps ease high compression on the street. Or you can reverse it and say high compression helps an oversize carb.
what carb?
I would go with a carb between 700 and 750 CFM. Thats what my magic formula calls for.
displacement times desired rpm divided by 3456. Figuring with 5500 RPM, you need 725 CFM.
Either one, a 700 or a 750 would carry you well past 6000 rpm as well.
I would go with a carb between 700 and 750 CFM. Thats what my magic formula calls for.
displacement times desired rpm divided by 3456. Figuring with 5500 RPM, you need 725 CFM.
Either one, a 700 or a 750 would carry you well past 6000 rpm as well.
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A big carb will increase compression.NXS said:
Less restriction = more cylinder fill. Imagine a 1 barrel carb. It would take for ever to fill the cylinders, where as a Dominator would fill them immediately.NXS said:
That is a big misconception. You will always need to jet a carb that is too big UP. A carb that is too big will have less intake velocity passing through it. That will make the signal lower. Less signal will draw less fuel. That is why bigger carbs come with relatively larger jetting.Tmod said:
You can throw that formula out the door.Max Keith said:
Some of the STOCK Pontiac 400s came with 800 CFM Quadrajet carbs. We are talking about an 11.5:1 455.
725 CFMs will hurt performance all around.
Well I seem to be wrong from one end to the other in this post.
Just my personal experience. small carbs feel like they increase compression and large carbs feel like they lose compression. Small carbs run lean and spark-knock and large carbs leave you with black plugs.(personal experience...jets alone?)
What you guys say is what I read when I search.
Vacuum advance aside why do some engines spark knock at light throttle but stop when the carb is cracked open? if compreesion is increased why does an engine bog at low rpm with a too big carb(even a dbl pumper supplying adequate fuel)? And why does high compression help a too big carb?
Just my personal experience. small carbs feel like they increase compression and large carbs feel like they lose compression. Small carbs run lean and spark-knock and large carbs leave you with black plugs.(personal experience...jets alone?)
What you guys say is what I read when I search.
Vacuum advance aside why do some engines spark knock at light throttle but stop when the carb is cracked open? if compreesion is increased why does an engine bog at low rpm with a too big carb(even a dbl pumper supplying adequate fuel)? And why does high compression help a too big carb?
what carb
As for the formula, Thats the one the carburetor companies and the auto manufacturers go by for their decisions. Its been working for nearly 100 years, so I think Ill stick with it.
So far, theres been nothing to prove that formula wrong.
NXS, the reason smaller carbs seem to react that way to you is because the smaller venturis are receiving a stronger vacuum signal than will a carb with larger venturis, on any given engine.
Its not uncommon for any engine to ping a bit under light acceleration, but not ping under full throttle. This all has to do with fuel flow through the carb. Under light acceleration, due to the stronger vacuum in the intake, its possible that you may have a slight leaness in your mixture, that wont be there when you stand on it. There are so many variables involved in that, and it all depends on the type of carburetor as to what will eliminate it.
As for the formula, Thats the one the carburetor companies and the auto manufacturers go by for their decisions. Its been working for nearly 100 years, so I think Ill stick with it.
So far, theres been nothing to prove that formula wrong.
NXS, the reason smaller carbs seem to react that way to you is because the smaller venturis are receiving a stronger vacuum signal than will a carb with larger venturis, on any given engine.
Its not uncommon for any engine to ping a bit under light acceleration, but not ping under full throttle. This all has to do with fuel flow through the carb. Under light acceleration, due to the stronger vacuum in the intake, its possible that you may have a slight leaness in your mixture, that wont be there when you stand on it. There are so many variables involved in that, and it all depends on the type of carburetor as to what will eliminate it.
It seems like more vacuum will result is more fuel being drawn thru the venturi.
[email protected], where are you?! i obviously have something to learn here!
[email protected], where are you?! i obviously have something to learn here!
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Most carbs should actually be jetted pretty darn close to start with, and even if you get a used carb, you should always use stock as a baseline. And, stock for stock, the larger carb will have larger jets, so that's not an issue.
However, if you're shooting in the dark, and using the same size jets in two different carbs, yes, the larger carb will run leaner.
Actually that's why I brought up a larger carb "reducing compression" which I am obviously wrong about (but it sure seems like it does to me.
).
Unless there is a cam with monster overlap I don't think it will be real happy on pump gas.
Ok, I think is what I was trying to say is that a larger carb will decrease COMBUSTION PRESSURES and that feels like a loss of compression. And isn't early or excessive combustion pressure what actually causes spark knock?
http://www.wauknet.com/douthitt/compression.htm
).Unless there is a cam with monster overlap I don't think it will be real happy on pump gas.
Ok, I think is what I was trying to say is that a larger carb will decrease COMBUSTION PRESSURES and that feels like a loss of compression. And isn't early or excessive combustion pressure what actually causes spark knock?
http://www.wauknet.com/douthitt/compression.htm
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