670cfm or 770cfm
Thread Starter
Burning Brakes
Joined: Jun 2003
Posts: 1,022
Likes: 6
From: CT
670cfm or 770cfm
Hello,
I have the opportunity to purchase i new Holley street avenger carb. I was planning on getting a 670cfm but was wondering if a 770cfm would be better. I have a 750 Q-jet on the motor now but I am borrowing it from someone.
Here is my set up:
350
10:1 compression
sportsman II heads-2.02 valves, 200cc intake runners
Comp Cams XE268-Adv. Dur. 268/280, @.050" 224/230, Lift .477/.480", LSA 110
Comp 1.52 roller tip rockers
Performer RPM intake
1 5/8" headers w/2.5" flowmasters
Does a 770cfm seem like it would be overkill.
Thanks
I have the opportunity to purchase i new Holley street avenger carb. I was planning on getting a 670cfm but was wondering if a 770cfm would be better. I have a 750 Q-jet on the motor now but I am borrowing it from someone.
Here is my set up:
350
10:1 compression
sportsman II heads-2.02 valves, 200cc intake runners
Comp Cams XE268-Adv. Dur. 268/280, @.050" 224/230, Lift .477/.480", LSA 110
Comp 1.52 roller tip rockers
Performer RPM intake
1 5/8" headers w/2.5" flowmasters
Does a 770cfm seem like it would be overkill.
Thanks
Team Owner
Joined: Sep 2001
Posts: 30,958
Likes: 0
From: Middletown Connecticut
Cruise-In IV Veteran
Re: 670cfm or 770cfm (L-82kid)
a 670 is just about where you want to be with a stock/tame 350, since you've done heads/cam, I'd go with the 770, if it runs too rich, re jet it :thumbs:
Thread Starter
Burning Brakes
Joined: Jun 2003
Posts: 1,022
Likes: 6
From: CT
Re: 670cfm or 770cfm (Corz)
Corz:
Maybe your right, I checked Summit and they recommend the 670 for 300-400hp and the 770 for over 400hp. Desktop dyno said I had like 385-390hp but maybe thats close enough to 400 to go with the 770cfm.
I want to get the most power out of the cam and heads as possible. But I don't really want to put a huge carb on the motor and lose low end torque either.
[Modified by L-82kid, 3:28 PM 11/4/2003]
Maybe your right, I checked Summit and they recommend the 670 for 300-400hp and the 770 for over 400hp. Desktop dyno said I had like 385-390hp but maybe thats close enough to 400 to go with the 770cfm.
I want to get the most power out of the cam and heads as possible. But I don't really want to put a huge carb on the motor and lose low end torque either.
[Modified by L-82kid, 3:28 PM 11/4/2003]
Le Mans Master
Joined: Oct 2000
Posts: 8,897
Likes: 1,357
From: Santa Ana CA
Re: 670cfm or 770cfm (L-82kid)
Don't fall into the "bigger is better" trap. The formula for carb size is:
(ci x rpm)/ 3,456 x 0.85 for a regular street engine or
(ci x rpm)/ 3,456 x 1.1 for a drag strip-oriented vehicle
Your combination sounds like something in between but lets go with the strip orientation. Lets also assume that you can spin the engine up to 6,500 rpm.
(350 x 6,500) / 3,456 x 1.1 = 724 CFM
But in reality, its not a strip engine and you probably can't (or don't) spin much past 6,000 rpm.
(350 x 6,000) / 3,456 x 1.1 = 668 CFM
You may (or may not) sacrifice a couple ponies on the high end but the smaller carb is really the better choice and will give you a nicer torque curve, especially on the street.
(ci x rpm)/ 3,456 x 0.85 for a regular street engine or
(ci x rpm)/ 3,456 x 1.1 for a drag strip-oriented vehicle
Your combination sounds like something in between but lets go with the strip orientation. Lets also assume that you can spin the engine up to 6,500 rpm.
(350 x 6,500) / 3,456 x 1.1 = 724 CFM
But in reality, its not a strip engine and you probably can't (or don't) spin much past 6,000 rpm.
(350 x 6,000) / 3,456 x 1.1 = 668 CFM
You may (or may not) sacrifice a couple ponies on the high end but the smaller carb is really the better choice and will give you a nicer torque curve, especially on the street.
Race Director
Joined: Feb 2000
Posts: 12,431
Likes: 1
From: CT
Re: 670cfm or 770cfm (Corz)
either will work. I use a 750 , but many experienced racers (the old ones with long beards) have told me it is too big for a 350. I tend to believe them after i see them running 7s :)
Team Owner
Joined: May 2001
Posts: 29,745
Likes: 3
From: Exiled to Richmond, VA - Finally sold my house in Murfreesboro, TN ?? Corner of "Bumf*&k and 'You've got a purdy mouth'."
CI 6-7-8 Veteran
CI-VIII Burnout Champ
St. Jude Donor '06-'10, '13
Re: 670cfm or 770cfm (QuickVet)
well, normally I would say 670, but that whole combo you have is made for upper RPM HP.
If you primarily drive street, do the 670. If it's a 1/4 mile car do the 770 and spin that motor for all it's worth.
If you primarily drive street, do the 670. If it's a 1/4 mile car do the 770 and spin that motor for all it's worth.
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Thread Starter
Burning Brakes
Joined: Jun 2003
Posts: 1,022
Likes: 6
From: CT
Re: 670cfm or 770cfm (BSeery)
My car is just a street car. I want to take it to the track and see what it runs but that probably won't happen for a while. My main concern right now is just high performance street. I probably won't be going over 6,000rpms very often. I think the motor can probably pull up to 6500 but i'm afraid of spinning a bearing or wrecking something.
Corz:
I've heard that 650cfm speed demon's are awesome carbs but I can't afford one right now. I'm just a broke kid and only have a little to spend on a carb. I know some guy who can get me a new street avenger for $300 or less.
Corz:
I've heard that 650cfm speed demon's are awesome carbs but I can't afford one right now. I'm just a broke kid and only have a little to spend on a carb. I know some guy who can get me a new street avenger for $300 or less.
Team Owner
Joined: Sep 2001
Posts: 30,958
Likes: 0
From: Middletown Connecticut
Cruise-In IV Veteran
Re: 670cfm or 770cfm (L-82kid)
Corz:
I've heard that 650cfm speed demon's are awesome carbs but I can't afford one right now. I'm just a broke kid and only have a little to spend on a carb. I know some guy who can get me a new street avenger for $300 or less.
I've heard that 650cfm speed demon's are awesome carbs but I can't afford one right now. I'm just a broke kid and only have a little to spend on a carb. I know some guy who can get me a new street avenger for $300 or less.
i picked up my 625 demon for 325 :thumbs:
Le Mans Master
Joined: May 2003
Posts: 5,642
Likes: 1
Re: 670cfm or 770cfm (Corz)
I'm running a mechanical speed demon 650 in my new 383 which is over 400 horses and around 500 lb/ft
it runs awesome, pours fuel like the exxon valdez, kinda cool to watch today in the shop, they had me come over and watch the inside of the bowl when they rev, swirled just like a toilet with lotso fuel and plenty of air.
it runs awesome, pours fuel like the exxon valdez, kinda cool to watch today in the shop, they had me come over and watch the inside of the bowl when they rev, swirled just like a toilet with lotso fuel and plenty of air.