Ok guys i just picked up myself a screamer. a fast car that is... with some serious stuff in it... its a 355 with comp's 280 Cam, 2.02 intake/2.60 exaust on Aluminum Bow-tiw heads, 556 Max lift springs, Forged aluminum magnum roller rockers, forged pistons, performer rpm intake...
engine revs to about.. 6500 RPMs when i nail it, i dont go further, that combination should only be good for about 6K maybe 6500...
Now the biggest pisle of crap on it is the carb.. its an ancient 650Cfm Carb (vaccume seconds) that kinda leaks onto the intake and never can be tuned to make it idle right. Anyways, im gettin a new carb and i ran the numbers like so:
6000 X 355 = 2,130,000
2,130,000 / 3456 = 616.3
So, at max performance my engine would only need 616 Cfm, and since my 280 Cam pulls "0" Vaccume at idle (when you hit 1200 RPM is finnaly registers on my guage) i will need mechanical secondaries for cruising since i cant pull any vaccume. this comes down to me needing a 600 Carb with mech. secondarys, and that will give me max perf. and max gas mileage...
So, whats my question? Well, theres no question, im just checking my math and making final sure im puttin the right carb on it :-)

 

Are you gonna start this arguement again! ROFL

 

Oh no. Didn't we already argue this one to death just recently?

You're gonna start a running gun fight in the Carb section here asking questions like this!!

Let me summarize how this is gonna shake out:

1. Most people will agree that a ~750 carb will give you considerable performance over a 600 or 650, especially when that motor starts really singin'. I would be one of those people

2. Others will say that BY THE NUMBERS a 600 or 650 will flow enough to feed the engine.

3. Then we'll start arguing about how carbs are flow rated at a 1.5" Hg vacuum level and that's not really the best for performance- too much vacuum, costing power.

4. Then we'll argue about how the real question is how much is ENOUGH, not what's best for maximum power. Questions of fuel economy will thrown into the mix for good measure. The ture nature of the application will be batted around for a while.

5. THings will degenerate from there. Board memebers various levels of intelligence will be called into question. The old "experience" versus "theory" guys will come out, guns blazing. People's mothers will be insulted. In short, feelings will be hurt.

Did I get this about right, guys?

 

750 would give you some performance on the top end at that much rpm. Vacuum secondaries don't rely on manifold vacuum. As air flow increases as the rpm goes up under acceleration, the primary's start becoming a restriction. A vacuum is then created. This vacuum is what overcomes the vacuum secondary spring and the secondaries open up. You will find lots of opinions on this board and I won't push my idea on anybody. My opinion is that if you drive on the street more than the track I would lean towards a vacuum secondary(750 for max performance, 600 for best mileage). If you race it often, manual tranny/higher stall auto, deep gears, a double pump will give you some additional performance.

------------------
82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

well I found something that will help you decide for yorself www.engineprosoft.com/
Download the cfm calculator its free!

 

so i take it this is nothing new to be asking about eh? Well as it is, Im getting 10 MPG on this crappy 650 Cfm Carb, and im hating it.it kinda leaks and im sure it pushing way to much gas. you can tell when the secondaries open, the car "wakes up" when you start the press in on the gas pedal. I'm hopeing a new carb will give me better gas mileage. maybe theres more i can do to improve it? i put over 50 miles a day on this mother F****** and im droping $70.00 a week in gas. 11:1 compression, so its premium only baby.

 

Why don't you put a rebuild kit in it? That's probably all it needs, along with some bigger squirters and jets.

 

Well as i read further into your message i see you already know how much cfm you need.
616. a vac secondary carb is really the only way to go as far a fuel economy is concerned.
I usually recomend to people a holley "80457S" 600cfm. ive used one on my 85 "350/350 3:73" with comp magnum 270 and p&p 58cc heads. kinda the same setup you have. just a step down on the cam. I pull 14in of vac at 1000 rpm idle thats advanced 1 d. i was getting about 18/21 mpg with this setup "light foot of course "
I need a little more info to help you more.
are you sure about the vac readings?
what gears do you have?
do you have a 350,700r4, standard if so 4,5speed?
how heavy is your foot?
basicly if you have no vac something is really wrong you shouldent have any brakes if the guage reads correctly "0" until you stomp it. Im very curious about this.

 

Is a 600cfm carb enough for a 383 PART 2. Once again . Going with a 750 HOlley is not bad for fuel mileage if you got it tuned. That is a Big IF.

------------------
Big 454 72 Chevy Nova
383 86 Camaro Z28
10.5:1compression 186 casting Heads,234dur.488lift exh&int,Edelbrock performer intake, Holley750 vac.sec 400 crank,flat top pistons,edelbrock valve springs and lifters,tranny shift kit,kickdown kit and tranny oil cooler. Will install 3.73's later on and might go with a more aggresive cam.

 

I should know better, but, I used to race my 72 Mach 1, it ran 12's, 351C with Fireball 302 cam, 3.89 gears, high stall converter and a 3 speed automatic (no overdrive) and I got 16 mpg on the highway at 65 mph. I ran a 750 Holley, vacuum secondaries that I tuned for the car.

 

I used to get 25mpg with my Q-jet, but I may have lost a few mpgs after I swapped to a Holley 750 with vacuum secondaries. I have a pretty similar engine to the one you described too...

I am assuming it is a 700R4. What kind of stall converter does it have? That will make a huge difference off idle with a cam that big no matter what carb you are running.

------------------
1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

holy crap thanks for all the responses, at this point im hoping to get up to the 14 MPG but thats hoping quite a bit. my friend says ill be lucky if i get 12 MPG at this point. I have a 4-speed manual Super T-10 (borg warner) freshly rebuilt and race ready so its no problem. my stall converter is my left foot :-).
Currently this carb wont stay tuned, im going with a 600CFM Holly with MECHANICAL secondaries, since its a stick and im a big stickly on quick throttle response. not to mention my carb guy doesnt have vaccume carbs, only mecanical and hes got one for me for about $120.00 and i trust him hes got me alot of stuff and is the manager for "horsepower sales" although he was my friend before he became the manager there.. long story... anyways...
this car has 3.23 posi rear end, 4-speed T-10 like i said, flowmaster delta (not that it matters but im providing alot of info for ya) um.. and about BRAKES...:
your correct i have no braking power at all below 1K Rpms, the result being when i slow my car i have to leave it in gear and glide it down, and had to adjust the idle higher than i would've liked since it kills the cam sound alot...
and somthing else I've noticed, my engine will sound different ever other d*** day, first it will be radical cam soundy, then the next day it will have a consistant double fire lope to it, you know like 2 pistons firing right after one another, then a pause, then 2 more, and other days it will idle all over the place.... really strange. I'm in dire need of help with this stupid 7-10 MPG, today i put 30 miles on my car and used 4.5 gallons of gas. its making me sick. i drop $20.00 every 2-3 days in my gas tank. 3.23 GEARS!!! and i drive it REEEEEALLY EASY. with this kind of gas mileage i NEVER hammer the thing maybe ONE time a day to clear it out, i dont like carbon clogged engines. has new plugs, so those arnt the problem. headers yes, 650 Cfm carb that leaks slightly, uh performer RPM intake, K&N filter of course, 2.02 / 1.60 valves on Aluminum fully ported heads, 355 Cubic inch engine, um... you already know the cam is comps 280 magnum, the roller rockers are comps most expensive magnum roller tipped forged aluminum things, eh... what else? 11:1 compression i dont know if they are flat tops, i didnt build the thing but i delved into it to check it out before i bought it so i know this much for sure, but i DONT know about the pistons flat tops or domed, and its a double roller timing chain, and and and i cant think of anything else now...
why the frik am i getting 10 MPG???? i shift quickly and smoothly at about 1.9-2.4K rpms usually maybe less somtimes, and in 4th gear i dont go over 2K i cruise at about 45-50 MPH, although the engine tops out around 7K rpms im led to beleive, ive red lined it at 6900 rpms and still had plenty of breath left but i was afraid of valve float you know because i dont remember what springs I've got, for all i know they could be singles (yuk) because of the .466 lift of the cam, maybe doubles. yukkie. 7-10 MPG
where am i going wrong? im changing the carb asap with a 600 mech. carb. ill let you know the details after its done, in the mean time...
suggestions????? please?

 

Sounds to me like a combination of problems:

1) Cam too big for rpm's you are driving in
2) Carb set-up wrong for your setup
3) ignition curve problems? What's your curve?
4) vacuum leak(s) somewhere?
5) vacuum gage hookep to Ported vacuum instead of manifold vacuum

EDIT: Just so everyone sees this, I though you had the LT4 Hot Cam from your TOPIC choice, but you have a MUCH bigger COMP cam than that...

[This message has been edited by FastBroker (edited April 05, 2001).]

 

oh yeah the cam is the right choice, i know those low *** rpm's arnt high for it but beleive me i get on it and shift around 7K rpms when i do... that cam has to stay :-) but around town and stuff i shift quickly cause i dont like high rpms it eats gas.

 

Shifting quickly (low rms) will KILL your mileage with that cam. You nee a mellower cam, you will be faster with it. If you choose not to listen to me, that is cool.

 

so wait, your saying with this cam i should keep my rpms high? like how high? im all ears but a cam swap just isnt possible at the moment.

 

COMP 280? real high! Like 3000rpm highway cruise rpm. Very inefficient at lower rpms = bad mileage. You basically can't get good mileage with a cam like that. Good power, YES!

 

ok so wait, instead of cruising around at 40 MPH @ 1400 RPMS i should keep it in 3rd gear and cruise @ about 3000 RPMS?? that will save me gas??

 

that makes no sense. you say highway are you refering to 70 MPH speeds? in 4th im running about 3200 RPMS @ 70 MPH this is o.k. im assuming. but what about 40 MPH? most of my driving is @ 40 MPH. 3rd gear or 4th? I assume lower rpms is at least a LITTLE better on gas. i dont have a heavy foot. and im getting 7 MPG. no, no, i will change my carbeurator to a 600 Mechanical secondaries, and dusconnect the secondaries. 300 CFM carb is what ill have. wont go past 3500 RPM but oh well, ill save gas. hope i dont hurt the engine....

 

I am still waiting for you to post your distributor ignition "curve".

I am not saying that cruising at 3200+rpm will give you better mileage, I am saying that your engine is more efficient at those higher rpms. ie, you'd get much beter mileage at 2000 rpm with a mellower cam, like an XE262 Comp, that's all.

Still, it sounds to me like your ignition curve is MESSED up bad and/or your carb is totally out of whack.

You gotta post some numbers, like jet sizes, initial advacne on the distributor, max centrifugal advance (vac advance disconnected) and at what rpm does the advance top out, and the total available vacuum advance available to you from the vac canister and at what vac does it start and end.

Then, we can help, possibly. I will try.

 

well i changed the carb. the new one is a 600 double pumper with mechanical secondaries, like i said, and it has 28 size squirters on both ends. now my problem is air flow, the car studders at high rpms because it cant suck enough air, and the rpms should shoot up alot faster, but they dont because my exaust Y-pipe is all bent up, its maybe 1.5" thick due to hitting speed bumps too hard. im gettin a new air cleaner 11" by 3" this should fix my air intake problem, and im going with a new set of headers (metallic red coating, like the rest of my engine) and a new y-pipe. i was thinking hooker competition Y-pipe and headers. im still not sure how much it costs to have them coated in mettalic red like everything else on my engine is. except carb and intake as of yet, but im paintin my intake that color. the carb doesnt look like it can be painted... hehe..
On another note, my engine now has tons more power than it used to with this carb. 28 squirter size seems to big, im goin down to like 25 size. i think it floods at low RPMS when you floor it. but it pulls REAL hard from 3K - 7K rpms and burns my tires all the way thru 1st and 2nd even alittle thru third. Im still learning how to shift with this 4-speed though, i cant imagine the car being any faster than this though.
and im not sure about my ignition curve anymore. my friend took his timing light and decided it was too advanced (like at 30 degrees or somthing i forget) he decreased it alot and changed some stuff around. ill let you know all about it when i can ask him about it, hes the one who helped me gett his carb on and tuned and fixed all my problems so far.

Oh 1 more thing, my ignition died a few days back. i thought it was the starter so i put a high torque mini starter on it, wired it up and it didnt work. i took a long peice of 10 Guage wire and hooked it from the starter to a button inside the car, and the other wire to the fuse box for a 12V signal to start the car. funny thing though, i have to turn my ignition on now, then hit the button to start the car. but my trunk stopped working!!! the problem is intermitten, somtimes i can use the key it sends the signal to the starter O.K. and i can use the trunk button, but somtimes the ignition just doesnt work, and my trunk opener button doesnt work anymore.... very odd. im goina wire my trunk to the same 12V signal, butt hat means the cars gota be "on" when i go to open the trunk. i wired the starter button thru the fuse box just in case, cuz without a fuse i can start a fire or worse, turn my battery into an acid bomb (!!!) anyways, im just wondering what peice i have to replace now that i lose my ignition and my trunk.

 

oh ya, brakes suck real bad at idle, no vac. canister to reserve vaccume, and that cam pulls NO vaccume at all even with the new carb. i have to hit the gas and let the rpms fall so i can brake... heeh

 

ouch... running WAY too rich. my plugs are freaking wet. car wont idle anymore... especially after a run. dies right as you clutch it after a run down the track or 1/8 mile run too. wonder if its spitting too much gas in after the 6K+ rpms i have no clue. its weird. going to smaller squirters ASAP.

 

The car HAS to have vacuum at idle. You have your vacuum gage hooked up to the "ported" vacuum (ie, turned off at low throttle settings) plug on the carb. That is the wrong connection to read engine vacuum. Make sure your distributor is not getting "ported" vacuum, either, as your cam needs as much idle advance as you can give it and the ported connection will give it NO vacum advance at idle.

You gotta tune the carb to get rid of the excess richnes. It's gonna take a while but do it right the first time and you'll be ok. You should use an LED O2 meter to get tihngs in the ballpark and then use exhaust plug removals to finish it up, if you like...

 

bah no matter where you turn the screws it spits smoke.. trust me the jets are too big!! doesnt matter if the screwes are all the way inside. then it doesnt even run right. but still smokes.

 

Don't forget that the higher the V. E is then the pore effiecient the engien will be. if you run at 1400 rpm then the VE isn' t that high instead you are wasting gas. I agree that you should either run it in third or drive faster in fourth. You could also put a smaller cam in. I find with my truck that if I keep it aorund 2800-3200 rpm then i get the best gas mileaage and this is around 15 mpg.

------------------
82 Firebird with a 2.8 but a 350 is on the way

87 Chevy K20 4x4 TBI pickup

71 Olds Cutlass S My ongoing project. Olds
350 with a th350

 

My Uncle used to live in Newtown.

 

Kingtal0n,

While it's true that your engine cannot possibly flow more than 616.3 cfm at 6000 rpm under any circumstances (except in the case of forced induction), it is not true that using a carburetor larger than 600 cfm won't make more power up top. Friction lowers the volumetric efficiency of an engine greatly, and the biggest restriction to be found in the intake can be found in the carburetor 0-- in its venturies, in its barrels, and across its throttle plates.

Here's an extreme example: A typical Pro Stock engine displaces 500 cid and redlines at 9000 rpm. Using the max cfm formula, the engine should not need more than 500 * 9000 / 3456 = 1302 cfm. The maximum possible flow rate of air into that engine is 1302 cfm at redline, yet Pro Stock cars typically run dual 1050 cfm Dominators to maximize horsepower.

Dyno results have shown time and time again that naturally aspirated street-strip engines make more power with a carb that's 125% to 150% larger than what is predicted mathematically using that "dynamic displacement" formula.

You'd do better to run a 700 cfm or 750 cfm double pumper for street driving.

 

well either way it matters not anymore!!! my tranny has officially gone "boom" and now im out ALOT of money to either repair or get a new one. it was a super T-10 Borg warner 4-speed manual. should ihave it rebuilt, or buy a new one with, say, overdrive? or a 6-speed? or go with an auto?? i have about $1000 to spend i think, maybe more if i have to, although i would like to spend ALOT less.... suggestions would be nice?