dgc333
Well-Known Member
Essentially yes but just remember 1/2 of a 600 cfm 4bbl (300 cfm) has more air flow capability than all of a 300 cfm 2bbl by close to twice.
Vacuum And tuning Help
- Thread starter 805moparkid
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Essentially yes but just remember 1/2 of a 600 cfm 4bbl (300 cfm) has more air flow capability than all of a 300 cfm 2bbl by close to twice.
Charrlie_S
Well-Known Member
Dave the numbers are correct, but the reason for the differnce is not. Back in the day, before 4 bbl carbs, when a 1 or 2 bbl carb was put on a flow bench to determine the cfm rating (as you said) a 3 inch pressure drop was the standard used. When 4 bbl carbs came into being, the same flow bench could not flow enough air to get to a 3 inch pressure drop, with the increased air flow capability of the 4 bbl carb. So rather then build a lot of larger flow benches, it was easier/cheaper, to just change the measurement standard to 1.5 inch pressure drop. Just a FYI
If you put a 600 4bbl carb on a flow bench, that could develop a 3 inch pressure drop, it would flow a ton more then 600 cfm.
1wild&crazyguy
Banned
500cfm 2 brl circle track racing..'keith dorton'???
300 is 300.
meanwhile ol'dave is trying to make sound like dudes 600 flows 1200...STOP!!!!!
300 is 300.
meanwhile ol'dave is trying to make sound like dudes 600 flows 1200...STOP!!!!!
Charrlie_S
Well-Known Member
300 is 300 is true, if you use the same pressure drop measurment.
A 500 cfm 2 bbl will only flow 353 cfm, if measured at the 4 bbl flow rating of 1.5 inches pressure drop.
A 500 4 bbl will flow 707 cfm, if measured at the 2 bbl flow rating of 3 inches pressure drop.
That same 600 carb would be 884 cfm at the 2 bbl spec.
Here is a good reference to the comparrasons
http://www.rasoenterprises.com/inde...arb-flow-rating&catid=45:carburetion&Itemid=4
1wild&crazyguy
Banned
right, but were talkin what it flows on his slant, and it ain't 884cfm on this motor.
Charrlie_S
Well-Known Member
OK, I'll put in my 2 cents worth. The carb may or my not be too big. It is not going to make much of a difference at idle, with the throttle plates closed (make sure they are). I agree tailor the spark curve. You need to get the centrifical advance to not change during changes in idle speed. Use an electronic distributor and change the springs to get to the point, where the timing does not change from the lowest rpm, you can keep the engine running to about 1500 rpm. I would shoot for about 12-15 degrees initial, at that rpm. thenweld the slots in the governor, to allow for about 15 degrees centrifical advance. That should be a good starting point. I also think you need more compression.
Charrlie_S
dgc333
Well-Known Member
Dave the numbers are correct, but the reason for the differnce is not. Back in the day, before 4 bbl carbs, when a 1 or 2 bbl carb was put on a flow bench to determine the cfm rating (as you said) a 3 inch pressure drop was the standard used. When 4 bbl carbs came into being, the same flow bench could not flow enough air to get to a 3 inch pressure drop, with the increased air flow capability of the 4 bbl carb. So rather then build a lot of larger flow benches, it was easier/cheaper, to just change the measurement standard to 1.5 inch pressure drop. Just a FYI
If you put a 600 4bbl carb on a flow bench, that could develop a 3 inch pressure drop, it would flow a ton more then 600 cfm.
Charlie, thanks! I had read somewhere along time ago the reason or maybe I read the reason into what I read but anyway the capacity of the flow bench makes perfect sense. In my 30+ years as an engineer I have seen the way you measure things change because the equipment wasn't capable any longer.
dgc333
Well-Known Member
We were talking about the flow ratings of 2bbl vs 4bbl carbs and why you can't compare the number directly.
A 225 slant at 6000 rpm that is 100% effcient can pump approximately 390 cfm of air. In reality the best you could hope for is 90% effcient or ~ 350 cfm. And any rpm lower than 6000 it's going to be less.
Putting a carb that is way oversize on an engine does nothing for power and will be difficult to tune to run acceptably at anything other than idle and WOT. For a good all around performing engine sizing the carb correctly is very important and it's better to error on the small side than big.
Then why did our 360 pick up almost 2 MPH going from any 4150 based carb to a 1050 dominator?
According to all the CFM calculators, the biggest 4150 based carbs were more than required, in fact, they were about 10% too big? Hmmmmm.
It clearly did something for power.
carry on.
dgc333
Well-Known Member
Easy, you went from a poorly set up 4150 to a better set up Dominator. When a NASCAR Sprint Cup 358 cid engine that makes 850+ HP, turns in excess of 9000 rpm and does it with a 4150 model 750cfm carb you are kidding yourself if you believe you really "need" a 1050.
805moparkid
Slant and AFX Guy
im likeing all of the info floating around! this is a good topic to stir up!
but as far as my set up i can down shift @ 34mph or 2900rpm from second to first and nail it and it takes off! no poping or bogging. i can down shift @ 65 or 2750rpm from 3-2 and bounce the speedo way to fast (flat ground). again no poping or bogging.
the cam starts to pull like a stall converter at like 2700rpm and havn't found where it ends yet.... 5800rpm is the highest ive gone!
but as far as my set up i can down shift @ 34mph or 2900rpm from second to first and nail it and it takes off! no poping or bogging. i can down shift @ 65 or 2750rpm from 3-2 and bounce the speedo way to fast (flat ground). again no poping or bogging.
the cam starts to pull like a stall converter at like 2700rpm and havn't found where it ends yet.... 5800rpm is the highest ive gone!
805moparkid
Slant and AFX Guy
forgot the pic! and it was bouncing off the end of the end of the speedo! it is accurate within 5 miles an hr
1wild&crazyguy
Banned
put that 750 on the bench...
uhhh....the heads flow 400cfm or so.. the carb is a cork.
They've slowed cars down over the years and continue to do so....the carb is a cork...put a 1050 race carb on the same mill and watch happens.
ha ha
Charrlie_S
Well-Known Member
That is certainly true. The smaller carb did act as a restrictor. You need to campare apples to apples. The carb size table/formulers are for a street type car, where you need "driveability" not max power. Remember we were discusing pressure drop across the carb (1.5 inches and 3 inches). Anytime you have a pressure drop, you have a restriction.
For max power, you want the least restriction possible (BIG carb) for driveability you need air velocity thru the carb at lower rpm (this will give a pressure drop at higher air flows) to establish enough signal to operate the main metering system at street speeds.
1wild&crazyguy
Banned
....which brings us back to cylinder heads and how they effect the cfm demand/usage/requirement.
small or big motor with relatively small ports will have plenty of velocity/speed to activate the larger carb, and will make more power with it.
I think we know what happens with a large port on either...
small or big motor with relatively small ports will have plenty of velocity/speed to activate the larger carb, and will make more power with it.
I think we know what happens with a large port on either...
Charrlie_S
Well-Known Member
Not true. A given motor with small ports, will not need as big a carb as the same motor with big ports. The small ports will be the restriction, not the carb. But again, what type of motor are we talking about. All out race motor or street motor? Makes a difference. Try putting your dominator on a stock slant six. Might be able to get it to idle, but nothing else. There would not be enough air velocity thru the carb to active the metering circuits.
1wild&crazyguy
Banned
Not true.
what does ''bigger carb'' mean to you, dominator????? lol come on man...
''the larger carb'' is what I said, not the ''largest carb you can find''.....
most everything is app specific and I did not intend to sound as general as you took it.
It's port volume in relation to stroke/cid/....& rpm
in the stock world...look at the super six....look at what a 750 on a stock 340 does to et's.....look at the 440 and what an 850 does for et's....
heck look at the thermo quad, the added cfm helped those low comp 340's, let alone had to pass smog=tiny 240cfm primarys with like 600cfm secondarys
what I'm saying is 'reasonably speaking' for example...you take a sb stroker and put smog heads on it, that air speed is flying and makes plenty of signal to run the larger carb to an extent[as in not talking about dominator carbs or the farthest reach of debunkment to this] thats stay in the relm...say a 700cfm was recomended where really you could get away with a 850-900cfm carb.
A good friend of mine ran an 850 on his STOCK 340 4spd car, it idled fine, was a lil blubbery if you stomped it but once above 3000 [not hard in stick car or if you have a 3000 stall convertor...] ran lke a bat outta hell.
you know where I was going with this, you understood enough to reach.
and hey we don't want guy's going out a buying dominators to throw on their 318's.
805moparkid
Slant and AFX Guy
ok guys so i played with the distributor today, just messing with adv. so using a dial timing light i found i was running 17* @ idle (1050rpm) and a Total of about 27* @ 2100-2200.
so i undid the other adjustment bolt on the dizzy and bumped the timing up to 22* @ idle (1050rpm). with a total of about 30-32* @ 2200-2300.
the car runs alot better just bumping up the timing!! on hot starts i used to crank for about 3 seconds will it would kick and start! but now it starts on about the 2 revolution!!
still need to change the springs but do u think i need more total? or just bump up my intial to get my total more?
thanks for the advice on the timing and i'll get a vacuum gauge on it and see what its at.
so i undid the other adjustment bolt on the dizzy and bumped the timing up to 22* @ idle (1050rpm). with a total of about 30-32* @ 2200-2300.
the car runs alot better just bumping up the timing!! on hot starts i used to crank for about 3 seconds will it would kick and start! but now it starts on about the 2 revolution!!
still need to change the springs but do u think i need more total? or just bump up my intial to get my total more?
thanks for the advice on the timing and i'll get a vacuum gauge on it and see what its at.
1wild&crazyguy
Banned
Run as much as it likes 'total/full advance'
but as a rough, 34* sounds good 'total/full advance'
I would still see how initial it''l take before starter kick back, then work on getting the distributor advance curve dialed in so that you can have that high of an initial time setting without ending up with too high of a total/full advance. . example- 25* @ idle 34* full advance in by 2200 rpm, get it?
805moparkid
Slant and AFX Guy
yea i'll bump up the timing some more like u said till it kicks back just to see. so is the idea to get the timing (intial) as high as possible that it will still start "easy"??
1wild&crazyguy
Banned
yep, and also have crisp tire ripping response.
805moparkid
Slant and AFX Guy
well i'll tell u! it did already today, just a little smoke :snakeman:
this is slant just keeps getting faster and faster! can't wait to get the rest of the timing figured out and the comp up! maybe even some 1.6 rockers to get a nice lil valve lift boost!
dgc333
Well-Known Member
You are missing a very import law of physics. As air flows through a restriction the velocity increases and when the restriction is reduced the velocity decreases.
I agree your example of smog heads on a stroker will likely result in a very high velocity air flow in the ports of the head but that has no impact on the velocity of air flowing through the carb. Appling Bernoulli's principle; the air flowing through air cleaner is moving at a slow velocity, as it enters the carb the venturi's in the through cause the air velocity to increase, when it enters the plenum of the intake manifold it slows down again, depending ton the ports of the intake it may increase or decrease and in your example it increases moving through the ports of the head. The CFM does not change in this example.
A big carb on an engine that can not use it results in the air velocity through the boosters and venturi of the carb not creating enough vacuum to draw fuel from the bowls properly that results in driveability issues. With a street car you spend 95% of your time running on the idle and transition circuits so it only makes sense to size the carb to perfrom best in this range. Worrying about .1's of a second when you go to the strip once or twice per year just doesn't make common sense.
1wild&crazyguy
Banned
and yer going over board so give it a rest einstein.
all talkin low seeking high, but forget the inevitable air pump part & momentum..
he has a 600 holley on a slant, and should idle fine regardless of weather you or others think it's on the edge of too big.
yer too busy trying to sound smart instead of helping this guy go fast.
typical engineer you are.
I'm not shooting down everything you say but it's too 'copy and paste' for me, you google tech you.lol
all talkin low seeking high, but forget the inevitable air pump part & momentum..
he has a 600 holley on a slant, and should idle fine regardless of weather you or others think it's on the edge of too big.
yer too busy trying to sound smart instead of helping this guy go fast.
typical engineer you are.
I'm not shooting down everything you say but it's too 'copy and paste' for me, you google tech you.lol
Charrlie_S
Well-Known Member
Dave is totally correct.
In general the 600 holley is too large, for a street driver slant six. They do work on slant six race motors, I do agree with you, however, that it should be able to idle OK.
PS: In general MOST slants do not like more the 30 degrees to advance (without the vac adv connected).
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