My transition slots are .020 initial then turned in 1/10th turn to achieve 850RPM hot idle, my 4-corner A/F are all 3/4 out, my jets are 64front/72rear. All my Demon problems turned out not to be the carb but air leak under the intake (hard to diagnose) and bad seals on headlight actuators.

P.S. what's an electric choke?

 

I resealed the intake a little while ago, symptoms were similar before. Next week college is out for the summer, so I'll actually have time to diagnose and fix things. I haven't even had time to read a magazine in a month! Don't ask about showering Ordering the parts tonight to rebuild the t-t column I got for $150 let the summer projects begin!

-Chris

 

can you post photos of the t/t rebuild? thks bob

 

Plan to. Here's all that I've done so far, just checking out my steering wheel: http://forums.corvetteforum.com/show....php?t=1048763

 

Baloney. It all PHYSICS. If you don't understand the physics and requirements for a system to operate correctly, then you'll keep butting your head against a wall....

His problem is quite simple: Insufficient CFM to get the idle circuit vacuum signals needed, for the carb to operate correctly.......

AKA "physics."

 

Not enough vacuum signal for the idle circuit sounds plausible but not knowing a ton about carbs I will defer to Those Who Know:

"I'm running a 750 DP on my 357, and others are running 750s on 350s and 355s. With the big carb, you just need to be aware of the minimum rpm at which you dive into the secondaries: don't flop the secondaries wide open at 1700 rpm..."

Can you guess who on this forum I am quoting?

http://forums.corvetteforum.com/show...126&forum_id=3

Also Cali's '68 above has an identical carb on a 327, I think that's proof enough.

 

A quote from that thread you posted the link to... Of course, I'm quite certain you didn't actually follow that tidbit of advice... If you did, I equally certain that you would have been told what I already know- that bigger is NOT better, especially in your situation...

But then again, considering the rest of your combination as posted in your signature, it's not surprising that you would ignore common sense, and go with the "bigger is better" theme... High-flowing heads, intake and carb, oversized exhaust.... And then choke it all off with a camshaft that is marginally better than OEM specs...?

Clearly, you haven't thought your engine combination through. The rest of your engine is high-flow, high-RPM.... except for where it REALLY matters...

Choosing the wrong size of carb, is just par for the course.... Eh???

 

HAHAHAHAAAAA. OK, I'll play along and pretend you are right. How does one determine carb sizing, oh great carb-Breath? The ubiquitous "formula"? Cmon, inquiring minds want to know.

Brett

 

You quote someone who is less of an authority than the person I quoted and count it a victory?? No offense to MILO because I'm sure it is sage advice.. for someone who is purely driving on the street. I have never denied that a 750 may be excessive for my engine's current configuration, but that does not in itself prove that it cannot work just fine.

Want me to explain my engine's mis-matchedness?

#1: I'm a freakin college student! As I mentioned earlier I barely have time to feed myself these days, much less money and time to devote to my car. When the cam went in, I did not have the heads, carb, headers, or exhaust I have now. A higher spec cam would've been stupid at the time. It will be changed, but as it runs perfectly outside of the idle issues, it's not my main concern at the moment. Same goes for my suspension, it's not ideal and I am very aware of that, but it has been a fun learning process that gets ever-nearer to its goal.
#2: I race quite a bit. Autocross and drag racing, but I plan to start road racing this year. It's no the most reasonable daily driver, and it can be a bitch in the intermediate stages, but it's fun. You're only young once and I'm not going to be young in a Honda. Effeciency is for motorcycles

In conclusion, please don't preach physics to the engineers, that's me but Lars is also one with a huge amount of experience. If I have a Managerial Economics question, I'll be sure to let you know.

By the way, my dad worked with nuclear sub design (he's an ME) while we lived in New Jersey quite a few years ago. Very cool machines.

-Chris

 

Efficiency is THE name of the game... Any beginner engineer worth his salt will tell you that... LOL...
With regard to physics...? As I said, physics is the name of the game... Of course, hydrodynamics also figures in, but physics is where it's at... Keep reading the books, and one day it'll all click, how it works... That's what happened to me, about 15 years ago...

Oh, and BTW... before I went back for the business degree, I spent 15 years in nuclear power and robotics... Where physics will make you successful or kill you...

Keep studying, kid... One day you'll finally get it...

The first symptom of a losing argument, is to digress into personal attacks, without actually substantiating anything said... That being said, BRETT, I will actually follow through on your challenge...

While there is no "scientific" formula to size a carbuerator, there are some basic thumbrules that apply for the required sizing of the carb, that have been proven to work quite well for many years.

The basic thumbrule is as follows:
[(cubic inches X RPM)/3456] X volumetric efficiency.

In the kids' case, he has a 350 with the tiny cam and lost flow velocity due to oversized intake, gas reversion due to large exhaust system, etc. So max RPM will be somewhere around 5500 (for the sake of argument), and we'll assume an efficiency of 90%.

Therefore, we come up with [(350 X 5500)/3456] X 0.9
= 501.3 CFM. Since his carb is rated for 750 CFM, the carb is [750/501] = 149.7% of actual required capacity...

Here is where it starts to get sticky... BG carbs tend to be under-rated, with regard to flow capacity... A 750 CFM BG carb will flow much more than a 3310-3 Holley... The venturis are much cleaner, the overall flow is SO much better... This is why the BG carbs make much better power than the "equivalent" Holley...

But for the venturis to work properly, you have to maintain a minimum amount of CFM for the idle circuitry to work properly... Since the carb has only one main function- to mix gasoline into an airstream at a specific ratio- it stands to reason that (since we rely on the Bernoulli Principle) there is a minimum CFM required for the proper operation of the carb, completely separate from the max CFM capabilities of the carb. Since this is determined by the design of the carb, I'm sure you could call BG, and they could tell you what that minimum CFM is for a specific carb....

Let's say that (again, for the sake of example), the BG carb idle circuitry doesn't function below 10% of rated capacity, and the Holley won't work below 7% of rated capacity (makes sense, since the more restrictive nature of a Holley carb will tend to increase the vacuum signal at all RPM levels, yes?)... I clearly don't know the numbers, and it will vary from carb to carb... As I said, just for discussions' sake...

Anyway, we therefore, we come up with a 3310-3 Holley 750 requiring at least 52.5 CFM to operate correctly... And the BG 750 requires 75 CFM to operate correctly...

Well, we go back... The 350 SBC is turning 600 RPM at idle, so (350 X 600)/3456 = 60.764 CFM, assuming 100% efficiency... I.e., the Holley carb WILL function at idle, the "same" BG carb will not... If we again factor in the same efficiency of 90%, the engine still draws 54.7 CFM. Still enough CFM for proper operation of the Holley carb, but not the BG...
If you narrow the difference to only 1%, with Holley needing 7% of CFM for the idle circuitry to work correctly, and 8% for the BG, you are right on the ragged edge of the lower limits of the BG, but then the inefficiency of the engine kills it...

If you're an engineering student, familiar with the Bernoulli Principle and how it applies to a carbuerator, do you STILL stand by this statement???

Since I have gone through all of this, and wasted a lot of money trying to make a BG carb work on an engine that was simply too small for it, I have already learned these lessons... After the fact, I did a LOT of reading on BG and Holley carbs, so I'm quite familiar with the issues...

You think I'm wrong on this, then call up BG, and they'll tell you the same things...

One more thing, Chris... I'll bet $100 in honest money, that if you put a generic 1850-0 Holley on your car, it'll run like a scalded cat, with fantastic off-idle response, good economy... I bought two 1850-0 carbs off of Ebay for $80 (including shipping), rebuilt them to supercharger-specs, installed and dialed in... Total cost, ~$300... You can find a nice carb, build it and run it... for about $80... it will make a good interim carb, until you have the camshaft which can utilize the flow potential of the BG...

 

The efficiency comment was tongue in cheek, hence the wink.

A minimum-necessary rule of thumb does not prove the carb will not work. Why is not every 350 equipped with a 500-550cfm carb? Why in the world did my car come with a 750cfm carburetor from the factory? Well, GM did do alot of stupid things to the mid-70s Vettes, but you get the point I hope.

Your invented minimum CFM for idle circuitry math isn't very convincing without something to back it up. Just how different are two closed barrels of different CFM rating? It seems to me that the venturis are barely even involved at such low airspeeds.

The invention: "Let's say that (again, for the sake of example), the BG carb idle circuitry doesn't function below 10% of rated capacity, and the Holley won't work below 7% of rated capacity"
Your logic up to this point is correct, surely at near-zero velocities fuel will not atomize into the airflow. However the statement above is entirely arbitrary, how do you know 10% from 5, 2, or .05%? Slow airspeed makes for very difficult to predict air/fluid(not hydro)-dynamics. Certainly not a time to be pulling random numbers out of the air.

I'm sorry that you were unable to successfully tune your Barry Grant carburetor, the simple fact is, no carb is necessarily going to work straight out of the box. If it does, that's called luck. Some may require more work than others to tune. You still haven't said a word about the many examples of large BG carbs running without fault. Again, Cali's 327 with a carb identical to mine. There is no disproving success I suppose. Go ahead with telling 'the kid' how much he has to learn. I know enough to value the right authorities on the subject.

-Chris

 

In any case, this is getting us nowhere. I will work on the car soon and let you guys know how it goes. Lets stick to tuning advice, carb theory needs its own thread.

 

I chose my 650 because my ole redneck engine builder said you can put too much carb on an engine. The temptation is there to go bigger but you just have to fight it. Or fight tunning one too big for street use. Race track may be different I think. Regardless, these spirited debates are fun to read when you can't sleep!

 

The flaw in this statement, is that GM used the Rochester Quadrajet... Actually rated up to 850 CFM... but notice that the primaries are tiny in comparison to the secondaries, due to the necessity to maintain a sufficiently high rate of flow at low RPMs to allow proper operation of the idle and off-idle circuitry.

Square-bore carbs, OTOH, are usually rated much lower on OEM applications, because if the primaries are too large, the velocity drops to a point where there is insufficient vacuum to correctly operate those circuits... (which is exactly your problem).




Common sense backs it up. But again, you're not using common-sense, instead adhering to the "bigger is better" mistake that every rookie, wannabe hotrodder makes... If you DID use common-sense, you'd start with a smaller carb, and study the results... Instead, you're after the "sexy" aspect, not thinking things through...

SO CALL BG... Call them, and ASK them... their techs are very good at what they do. Their livelihood relies on knowing their product... Tell them your combination, and your problems... And they'll tell you the exact same thing: your carb is too big, won't function at low RPMs, because your engine is TOO SMALL...

Call BG. Or do you refuse because you are afraid of the answer you will receive??? You can email them at tech-sales@barrygrant.com
Of course, you can also look at their recommendations... Such as the following data:

Application: Mild Perf, Muscle Cars, Street Rods & Towing
Carb Size: 750
Secondary Actuation: Vacuum
Engine: 393 - 477
Cam duration at .050": Up to 240°
Intake manifold: Dual Plane, or small single plane
trans/stall speed: Less than 3,000 RPM
Initial timing: 14 to 16

Doing a cross-reference of their minimum engine sizes versus rated capacity of airflow, the 575 and 650 CFM carbs require at least 7% of rated airflow to function, the 750 and 850 carbs require at least 8%... Do the math yourself, and you'll see what I'm talking about....

They recommend a 575 or 650 CFM carb for your engine...

I certainly seemed to nail THAT one, now didn't I??? And I didn't need an engineering degree to figure it out...

Yeah, and he has the same engine as you, the same cam, the same mismatched heads, the same oversized exhaust??? What's his cam profile? What does his engine idle at??? What is his compression, vacuum readings at idle???


You have no idea how much you don't know... Typical kid mistake, typical ROOKIE mistake... But that's ok, keep on getting getting a bloody nose... And while you follow your so-called "right authorities" on whatever subject, you'll keep wasting money, sounding foolish... and not getting the results you're after...

But that's what happens, when you're just another sheep, following the rest of the herd... If you're this way already, going for your undergrad in engineering...? You might want to change majors...

 

Clearly, your "redneck" knew what he was talking about...

I built a Buick 455 two years ago... Did a 268 cam, mild lift, a dual-plane manifold, stock ignition... And then I stuck on the old Holley 1850-0... Sucker ran HARD, and the new owner still comments on how strong it pulls...

He was quite skeptical, of course, when he saw what I'd put on... But when we went for a ride, he was a believer, and bought the car that afternoon...

 

Oversized exhaust? Now are you going to tell us all to run 2" exhausts, or go back to the Y-pipe??

Cali's engine:
Matching # 4-bolt L-79, 327/ 350 HP 11/ 1 compression
Holley ceramic coated headers, 2 1/2" custom exhaust

from his website and

"850RPM hot idle"

from his post on this page, which you just might've seen if you were half as observant as you seem to think you are.

"Or do you refuse because you are afraid of the answer you will receive?"
I've never been afraid to admit a mistake when someone who knows proves me wrong. Now I've been a contributing part of this forum community far longer than you have (3x years, 10x posts), so I'll forgive your ROOKIE mistake of not knowing who lars is. Lars is an internationally known tuning expert who travels as far as Europe tuning forum members cars and teaching the facts of tuning. In short, his fanatical fan base on this forum is half a step short of religiously devoted (couldn't find the photochop of him being blessed by the (prev.) pope...)

As much as I am VERY aware I do not know, I am very good at knowing who to follow like a sheep. Any foolish things I have done with my car, I have researched in depth and chosen to do. I have done several things for the fun of it, to see what it would be like (i.e. 400lb rear spring). For the record, despite the idle problems, this car is very fast. You could benefit from calming down a bit, noone's going to listen to you if you freak out like this everytime someone disagrees with you or questions your logic. You may also want to try to learn from those who know more than you, there are many.

"I'm afraid I wasn't able to deliver your message to the following addresses.
<tech-sales@barrygrant.com>:
user is over quota"

Barry's overloaded, I'll try again tomorrow.

 

OK, then are you going to admit making a mistake NOW, when I prove out the flaws in your thinking????

OK, here we go: the 327 you're referring to, with a hot-idle at 850 RPM, is significantly different from what you're running, a cam that should be idling at 600 RPM... This is WHY I asked you for a quickie rundown on your camshaft profile and basic engine configuration... Because it ALL matters...

Now, follow the math here...

[(cubic inches X RPM)/3456] X efficiency equals CFM, right???
You: [(350 X 600)/3456] X .9 = 54 CFM
Him: [(327 X 850)/3456] X.9 = 72.4 CFM
Minimum CFM of the BG carb (going by the quickie spreadsheet I put together) = 68.23 CFM.

Therefore, his carb WILL function- barely. Yours will NOT... The difference is the cam profile- his is clearly more radical, requiring a higher RPM at idle... Therefore, your assumption that the setups are the same, is clearly flawed...

But hey, what do I know, right??? I don't have a degree in mechanical engineering, so I'm just some fool that doesn't know diddley... RIGHT???

I've been on Corvetteforum since 2000, when I had my '88 Vette that I bought as a beater.... If I remember right, there was a database issue, and my account had to be reactivated... But you probably weren't here for that, were you??? We won't discuss the value of your.... "contributions," given what I've seen on this thread...


Like I said, I'm not a follower.... I prefer to think for myself. You might try it, sometime.... As I just proved above, following the advice of others does NOT always work...

You'd better sharpen your research skills...

Kid, you have no idea what a fast car is.... You might be at 400 HP (optimistically), which works out to something like 7-8 pounds/HP... reasonably quick?? Yeah.

But FAST??? Nope.

Yes, there will always be someone who knows more in some particular niche or other... But not very often. Does it sound arrogant??? ONLY if I can't deliver the goods... And I always do... And with regard to the tendancy to "freak out," it is YOU that turned this into a whizzing match... And looking rather foolish, because you don't have the maturity to think for yourself...

Somebody asks for help, I offer... I always have. I try to give sound advice- and if I don't know the answer, I don't offer opinions... I offer SOUND advice, but only when I am absolutely certain. Other forum members understand this, and don't get personal about it...

But why do you bother to ask for advice, when you are determined that you already know the answer??? What is the POINT??? I mean, I have explained to you in painful detail, why you're screwed with your current setup...

And you act like I'm the fool, for explaining why you made a mistake....?

Now, as to whether I can deliver the goods....?

I'm running a 427 BBC, 6-71 blower with dual 600 CFM carbs... A mild camshaft, 7.8:1 compression, 10 lbs of boost.... I've only about 40 minutes of run-time on it- just enough to break in the cam, and (only tonight) finish solving the cooling issues I had... Wednesday the TKO-600 shows up, and it'll be moving under it's own power by the weekend... One of the key factors in this build-up was that it would have to be sufficiently reliable to make a cruise to Reno or LA without a hitch... And be sufficiently mild-mannered that my 65 year-old mother could drive it to the store (10 miles each way) with no worries about whether it would take care of her...

Of course, the trans really blew the budget, but since the M-21 was toast, I stepped up and ordered it... All told, I'm into it for about $12K. Not bad for a engine that'll put out 650+ HP on pump gas (desktop dyno says 714, but I prefer to stay conservative on such estimates until a chassis-dyno session)... BTW, only $4500 in the entire engine (of course, the fuel-injection system I'm already building for the blower should add another $1K onto the price, all told)... It runs, it breathes, it's BAAAAAAAAAAD.

Yeah, I can deliver the goods...... Pics HERE.

 

Hmm...who do you trust? Lars, an actual rocket scientist, who's written literally dozens of tech articles about tuning Quadrajets, Holleys, and Demons? Or some guy who claims to know everything?

 

Please show me where I asked for someone to critique my choice of carb... no it's not there is it? If you had nothing to add to the tuning question, I was not seeking your opinion on the matter. Do you seem arrogant? Just a little! Anyone who is knowledgeable and also WISE will tell you that there is always more to learn. By deciding you know all there is to know on a subject, you have no chance of learning anything new. If you hadn't noticed, hotrodding has changed in the 30+ years since you began. Rules of thumb change because technology changes. Such a simple formula has many assumptions behind it. But anyway, lets look at your numbers again.

My engine should idle at 600rpm? I don't know why you think I would expect the factory specified idle from a modified engine. This is not a stock camshaft and other changes to the engine will also affect where it will idle best. You have not even mentioned ignition timing!!! Is this one of those niches you don't know about? Because it can make a huge difference as to where the idle sits. I am not running the factory 8* advance

L48: "compression ratio is 7.75 - 8.75. The duration at .050 lift (intake/exhaust) is 195/202 degrees, and the valve lift is .390"/.410"." Lobe centerline 112*

my cam: Hydraulic- Good street performance with stock convertor, choppy idle 1500-5500rpm Duration at .050 lift (I/E) 218*, valve lift .454 LSA 110*

Seems pretty damn different to me. So lets say that raises my ideal idle a WHOPPING 200rpm, there I am above your little limit. Can you not see the assumptions behind these numbers and formulas you're using?

It's amazing how clearly you can see my flaws and completely miss your own. I've had the maturity to remain calm and not attack you besides a few very light jabs. You on the other hand take every opportunity to tell this kid what an idiot he is and rub his nose in how little he knows. You'd make a great father, but unless you've erased the ex-wife from your website, it doesn't look like there's much danger of that anyhow.

You also don't seem to care that I race my car (what do you do with yours, shows and cruises?) and am willing to accept a little street unfriendliness for the added performance at 10 10ths. That said, you should notice that my cam's useful RPM range matches the safe redline of my bottom end. Most importantly this is just a step, there is a stroker coming, I KNEW I didn't NEED a 750 and 200cc heads when I bought them, but I also KNEW they would work fine in the meantime. I can see by your projects and equestrian activities that money is not a big problem for you. When I'm not scrounging off $8.50 an hour I will put together a well balanced engine all at once.

Sure, I don't know fast. I work with dedicated race cars every day and I've seen, heard, and felt alot faster than anything you've created. These are racecars that turn by the way, I don't know if you're into going more than one direction. My car isn't a top fuel dragster, it isn't a blown big block, yes it is "reasonably quick" and that is all I meant it was. We have a customer with an ex-Formula 1 car... but I'm just a 'kid,' I wouldn't know fast if it bit me...

-Chris

 

I will take massive amounts of experience over generalized formulas, thank you.

breathial, by ignoring the knowledge of those far more experienced than you you are missing out. "Thinking for yourself" is a very good thing, but it is something to be done on top of knowledge gleaned from those who have been there before you. Please do a search for lars' "Tour," you will see the dozens of forum members he has helped. These followers seem to have very well running cars...

Maybe in another 15 years it'll "click."

-Chris