79tubeblzr:
cant i just re jet the carb to make it equal to a 600

It does not work that way. Your mistake is classic. Many think bigger is better with carbs when it is not. It has to do with venturi bore size and mixture velocity through it. Your SB cannot draw enough air flow through it to be fully effective at most RPMs. It would about be right for a warm 454. Even a 600 would be a bit big but better that current carb. Somewhere around a 550 would be close to ideal.  

rejet to around 58-62 primary and close to or 72 on the secondaries. smaller than about 58 starves them and the surge from the power valve.

thats a ball park for a 260 cam, performer intake and headers..

the 550 comment is kinda funny there...
I have a bone stock 71 monte carlo with 245hp 350 and a two barrel carb, It runs 15 flat in the quarter mile every time starting from drive....no chrome either.  

the sucky thing is with the truck avengers the next lowest carb is 470 cfm.  I would like to keep my current carb because it works flawless on near vertical climbs and descents, my Q-jet would die on very small hills. What kind of horsepower range would you say it would require for 670 cfm.  

It is not a HP range, it is a RPM and displacement range. A 670 would play well on a warmed up 454 with a power peak around 5000 RPM. (it would feed it well about 6000  but no point in winding it that tight)  With a SB. You would need to be power peaking past 6000 RPM and cranking to 7000 RPM and more to really need a 670. Smaller carbs do give much better WOT low and mid range torque. No better way to cut low and mid range power than to install headers and a oversized carb. It might sound cool but it will hurt performance. I think that some feel that a big carb is like a mini super charger on a engine so they buy them when it is quite the opposite.

BTW, a 470 would feed a SB fine up to 5000 to 5500 RPM and would be a far better choice than a 670 for off road.  

sounds like you're into high speed, hil climbing type stuff...stick with your 670cfm...anything over 300hp the , 470 will not be suficient, a 260 grind can make around 350hp with the right components and run well over 5500 rpm.

you need the higher rpm hp, mid range tq and stall for that kind of wheeling. hill climbing is alot like drag racing. if you set it up for crawling it aint gonna make it to the top.  

headers even on a stock engine will increase the hp and tq....at a higher rpm range. so it may feel less responsive in the lower rpm range the gain is a fair amount in passing gear. getting back the high tq launch takes a combo of gears and stall, at that point a stocker has no chance at keeping up.

a 470 cfm, stock exhaust and mild cam will do a nice burnout then fall on its face.  

mudb8-.:
sounds like you're into high speed, hil climbing type stuff...stick with your 670cfm...anything over 300hp the , 470 will not be suficient, a 260 grind can make around 350hp with the right components and run well over 5500 rpm.

you need the higher rpm hp, mid range tq and stall for that kind of wheeling. hill climbing is alot like drag racing. if you set it up for crawling it aint gonna make it to the top.

It is NOT about HP. It is TOTALLY about displacement and desired power curve. A 670 is gonna kinda stink in low RPM and not shine at mid RPM either on a SB. With a 90% volumetric efficiency at 5500 RPM (which you will be lucky to get that high at that RPM on that engine)you need in thoery  500CFM. a 470 will easily flow that at most a .1 inch increase in pressure drop which is nothing. NOTHING is gained going to big but many seem to think otherwise. Many years ago I had a 396 with a 600 CFM on it and it was more than enough carb for it and my needs. Nothing is gained trying to buzz it to 5500 and 6 grand and more to try to use big carb. Your truely desirable power is in the 1500 to maybe 4500 RPM range and that is where you want it off road.  THis does not equal a 670.  

your idea here will make and economical and strong running engine...

however it will not run with the big dogs.....the small carb and poor breathing of the stock exhaust manifolds will not get any where in the performnance world. flat out dusted...

BTW.. my 396 came stock with a 830cfm holley spreadbore  

mudb8-.:
your idea here will make and economical and strong running engine...

however it will not run with the big dogs.....the small carb and poor breathing of the stock exhaust manifolds will not get any where in the performnance world. flat out dusted...

BTW.. my 396 came stock with a 830cfm holley spreadbore

No my idea is to out run the big dogs, while they are bogging and trying to get wound up you are gone and when they shift and have to get back to max RPM again to get carb to flow, you do not have to because you will have strong midrange. Give me a nice 500 or so CFM on a fairly SB off road any day and I will eat up one with a 670 overall and have much stronger low end and have to downshift less too. BTW, I downsized my carb on 396 and it ran sharper and stronger with it than stock carb. BTW I have been messing with carbs since 60's and know them well. Most today now little about them or what works best and only think about the numbers. I do question the "stock" 830 on a 396 too.  

LMAO.....somebody help me up!


as I mentioned before...I agree, the smaller cfm is a plus but there is a point that it becomes insufficient. from the cam profile, induction, and exhaust systems.  

I do not dispute that cam profile and induction have a roll but generally many over carb engine think bigger is better. You want to basically reach rated flow capacity at or slightly after your FP peak (RPM it occurs at, not how high you can wind it) a 260 cam in a SB is going to peak by or before 5000 RPM so a 470 would work great, not a 670. What get a big truck moving is strong low end and midrange power so that when you take it to power peak and upshift you are smack in middle of power curve. Many lift truck, install big tires with stock gears and then get magic air cleaners, bigger carbs and dual exhaust thinking it is going to cure it and make it a "Big Dog" when their efforts would have been better spent on proper axle gears. I have waxed many "Big Dogs" over the years with family sedans on the road as many are all show and no go because they over carb and under gear. BTW, dual exhaust might help a bit on top end and "bark" but they actually reduce low and mid range torque/response and remove some "bite".  

its about the right combo's not just a breather or exhaust...your right, in slapping on some things isn't gonna work, heads cam intake and exhaust make a "big dog" run...I'm talkin in the 350+ hp range here..not gonna happen with a small carb and stock exhaust..
if you take your family sedan's engine and slap it into the properley geared 4x4 you think is racing you you'll find out what a big dog really is.

many do over carb for sure...I wonder what the actuall cfm on those carbs is..compared to what they're advertised as..for the sake of making the sale..  

If I really wanted a Big Dog that would run and be a big sleeper too I would use a SB with a large custom 3 to 3.5 inch single exhaust with a good flowing quiet muffler, some good gears, and a roots type supercharger and a intercooler running about 8 to 9PSI with 93 octane and maybe some water/alchol injection too if it need it. It would be quiet and go like a bat out of "H" and shock a lot of wannabes. You could have your cake and eat it too with awesome power and excellent low and mid range torque as well top end and good MPG (for HP potential) when you keep foot out of it. Supers when set up and feed right give very broad and flat torque curves. You can keep the noise and lack of low RPM power. BTW you do not need a 670 carb to make 300 HP plus either. I have seen dyno runs of engine making well over 400 HP with 600CFM or less carbs.

BTW, 4 BBL carb are flow rated at a 1.5 inch (inches of mercury or about .5 PSI) pressure drop across carb and 2 BBL's are rated at a 3 inch drop. (about 1 PSI) Been that way for many years. There is no brick wall on flow and 4 BBL's will flow more than rating with a slight increase in pressure drop. The added  turbulence on/in carb bore and in intake below it from running at or slightly over rated capacity mixes fuel and air better and it gets a better burn and response.  

I have heard all the stuff about keeping stock exhaust for good low end power but i have never seen it as true, my ex had a 78 ford with a big block 400 and 33 inch tires and dual exhaust, if you stomped it the tires lit up, her dad decided to have the guy at the muffler shop convert it back to single exhaust and it couldnt get out of its own way, and the same with my truck, junked the stock exhaust manifolds for some flowtech headers and flowmaster 2 -chamber thunder series mufflers and it made a huge difference in power off the line, maybe having an x-pipe would help low end but i think keeping the stock manifolds is hog wash.  Btw what is the cfm on a stock Q-Jet.  

Big Block tend to act a little different than small blocks with dual but if you use the right single setup, you will get more low and midrange torque. It has to do with letting expanding gasses escape too quickly while there is still some effort for it to be transfered to piston stroke. One upon a time there was 2 sizes of Qjets. One was good for around 800 and the other around 630 or so. Anyone you find now will likely be the smaller one. Nice thing about a Qjet is the vacuum operated butteryflies over rear bore throttle plates that limit amount of air dumped into engine vs what it can use. There is also a spring on those plates as well and you can tweak it for right setup (tighten it if it tends to bog) I used to work with them a lot from late 60 thru 80's and was very found of that carb on and off road. Good sturdy unit and very tunable. It's only real weakness was float tended to "gas log" and cause it to flood or go rich. (a VERY common problem with its foam float) I used to keep extra new ones on shelve and change them every year or two or at first sign of trouble.