Max

I've attached a link to an article by David Vizard. It explains why CFM rating is not as important, when choosing a carburator, as most people think. What is more important is that you achieve sufficient venturi booster signal. The article explains it all in great detail:

Understanding Boosters

Let me know what you guys think.

sofakingdom

iTrader: (1)

"There's no such thing as too much carb; only not enough motor".

You could say the same thing about a cam. In fact, I often do.

But yes, that's exactly right; the thing is, most carbs have their boosters designed in such a way that the carb is sort of inherently calibrated into the right range for a given size range of motor. And it often costs as much to change boosters, as it would to change the entire carb (sell and re-buy). So there's an economic aspect to it, as well as the technical side.

Max

I'm running a Holley 670 on a 305 and it runs great except that the off idle throttle response is just a little less than crisp. Even though I have maxed out the pump shot with the largest squirter and pump cam profile.

I called The Carb Shop in Ontario California today. They are considered by many to be leaders in area of Holley carb tunning for high performance applications.

The Carb Shop

They said that if I shipped them the main body of my carb that they would install single stepped dog leg boosters in to the primary venturies of my carb for $45. That's the cost of parts and labor. Not bad in my opinion; even considering the cost of shipping.

A chart in the article from my first post shows a 42 ft./lb. gain at 2000 RPM by simply changing to a better style booster. That's huge. Part of the reason is better fuel atomization at these lower engine speeds. I'd sure like to pick up some low end torque from the better fuel atomization. Plus I'm sure that this will be an effective remedy for my off idle stumble.

contact

about the article,
................
I can understand how a 'too big' carb
can have sloppy throttle response,

but how can a 'too big' carb,
have less power at 2000 rpm,
than something else?


a/f way off, missfire , something else?

Max

Insufficient booster signal causes the the poor throttle response you mention. It may also cause a lack of power output at low RPM for the same reason. Basically it is due to poor fuel atomization. In other words the carb doesn't break the fuel down into small enough particles for the most effecient combustion to take place.



The opposite can happen as well. Especially in a manifold that is heated by the exhaust gas cross over. In that case, if the fuel is too highly atomized too much of it becomes vaporized (turned into a gaseous form) by heat on it's way to the cylinder. Gasoline in the form of a gas takes up a far greater volume that gasoline in a liquid form. This means that you can't get as much air packed into the cylinders. It lowers volumetric efficiency.

The key is to find a balance. That's why carb manufacturers have size recomendations.

But, what the article is saying is that if you want maximum horsepower you should use a carb somewhat larger than reccomended and match the booster size to you combination to get both great throttle response down low and increased air flow on the top end.

HarryG

Is that price for Holley parts or aftermarket? Did they say anything about using annular discharge boosters?

Merlin

Then this Holley 1685 should work just fine on my 283, right?

Fast355

iTrader: (2)

SO what world war II fighter plane did you rob that monstrosity off of?

Merlin

I just need to find a distributor to fit it now.

I guess it doesn't matter. I read somewhere that they are all the same.

(Wright 2880, Grumman F7F/G - The true 'F's originally had PWs like the B-25s. F7F/Gs were supercharged. )

Max

I didn't ask if the boosters they recommended were made by Holley or not. I did ask about the annular booster and the tech said that for a 670 on a 305 it would produce too strong a signal an that would make it difficult to acheive a proper a/f ratio. He said that the dog leg booster with the singel step should be ideal.

Dialed_In

iTrader: (3)

David VIzzard is stuck back in time.

Max

Why do you say that?

Apeiron

Probably stuck on the BL A-series sometimes I think too.

Dialed_In

iTrader: (3)

Because a lot of his techniques are out-dated. The rest of the world has learned from their mistakes and moved on. He on the other hand has found a way to make a living being stuck in his rut - writing and selling books.

HarryG

Can you be more specific?

rx7speed

if anything wouldn't actually improve VE% since most motors don't take in their full capacity in mixture? also the more finely atomized fuel should be able to burn quicker and more efficient I would think. if anything wouldn't the problem with a smaller booster or cfm flow rating cause choking of the intake a little bit and causing a restriction or by causing too strong of a signal that would end up causing the mixture to get a little too rich?

Max

Your right about a couple of key points.

More finely atomized (smaller) gasoline droplets do burn more effeciently because of the greater surface area. That's one of the benefits of strong (adequate) booster signal. But, when an intake manifold is heated by an exhaust gas cross over and you combine that with finely atomized gas your going to get more of the fuel vaporizing than normal. Because a gas takes up more volume than a liquid, the fuel takes up more room in the intake and ultimately the cylinders. In the tests I've seen it produces a reduction of 2.5% in air volume capacity. This may not sound like much but if you figure that an aftermarket intake manifold on a SBC may provide about a 5% increase in airflow over stock. The 2.5% represents an additional gain of 50% more that what the aftermaket intake is worth on it's own.

You also said that a stronger booster would cause the mixture to go rich. That's right. Sometimes you have to compensate for this by increasing the size of the air bleeds to the main jet fuel well to create an emulsion that contains a higher concentration of air.

That's why the tech that I spoke to at The Carb Shop recommended that I go with the single step dog leg booster over the annular booster for my application. He told me that the annular syle was too strong a booster for my needs and would produce an overly rich mixture in my application. He said that with the single step dog leg booster I could achieve the increase in booster signal that I need to compensate for my slightly oversized carb and that I should not have to enlarge the air bleeds like I described earlier.

HarryG

I'm assuming that your carb does not have replaceable air bleeds and any enlargement would be by drilling or reaming? Couldn't you get the same result by changing to leaner jets?

Damon

If you got by Vizard's recommendations he specifically states that a stronger booster will work best under all circumstances- IF YOU ARE OPERATING OVER A WIDE RPM RANGE. Intentionally going to less efficient boosters can help if you're looking for ideal atomization in a narrow power band (race motor). They can also hurt slightly if the intake is heated and too much of the mix entering the cylinder turns into vapor (displacing air/oxygen).

In general, bad atomization absolutely kills the bottom end response. But "over-atomizing" the fuel with too strong a booster doesn't seem to hurt much on the top end. For a street motor you probably won't notice the slight loss on the top end but you'll DEFINITELY feel the bottom end soggies if the booster is too weak.

One thing that keeps getting washed away in the flood, however is this: YOU MUST RETUNE THE CARB. And I mean more than just jets. Air bleeds as well. And the idle circuits will require changes to match. Changing the signal strength with a different booster venturi CHANGES THE KEY OPERATING PRINCIPLE OF THE CARB. There is really no other change you can make that will have a more widespread and profound effect on the basic operating principles of the carb. This is not for the casual weekend wrench.

'The Interceptor' - That is the reason The Carb Shop recommended the stepped down-leg boosters. They don't change the signal strength of the booster very much but they improve atomization (and, hopefully, low end response in your application). If you went to annualr discharge boosters you would be increasing the signal strength of the booster considerably and have to completely rework the tuning on the carb. The stepped down-leg boosters are "plug and play" by comparison.

Vader

The evidence presented in some of the very traditional opionions and philosophies not only suggests that statement is true, but perhaps it would be better demonstrated and more obvious if we step back out of the tree line and look at the whole forest - He's STILL talking about carburetors! It's not as if a barometric pressure/differential liquid metering device is the state-of-the-art, last word in fuel management.

Granted, there are some highly technical intricacies in getting a carburetor to work well, and work properly (and many of them do), but it's akin to fine-tuning grids and plates for vacuum tubes in an age of surface mounted integrated multi-layered compatible metal oxide substate semiconductors. That alone should provide a clue to the merits of the opinions presented.

305sbc

iTrader: (3)

EDITED ------------------------

When the first poster slammed Vizard I just shrugged it off as general misinformation or arrogance and let it go.

Vader

At least I didn't try to discredit Mr. Vizard.

I have no doubt that he has substantially more than simply a rudimentary knowledge of general carburetor theory, and probably knows something about about gasoline as a fuel. I'm sure I don't posess even a small percentage of his knowledge on domestic automotive carburetion. No one can take that away from him - Particularly some third-rate babysitter on a web discussion forum, whose only value may be in bleeping out naughty words when someone posts them by mistake.

That notwithstanding, I can still stand on top of a hill and notice the surrounding mountains. His cumulative knowledge likely has some value in some sectors, but it all seems tantamount to someone perfecting the straight axle 60 years after even Ford started using independent suspension, or creating the ideal fabric and doping sealer for aviation fuselage coverings decades after everyone has been using metal alloys and composites as the standards.

I'm happy for Mr. Vizard that he can still sell copies of text on his areas of expertise, and I hope he continues to be successful. I wish I could be as successful in a chosen field. At this point, however, I probably would select neither monokote and dope nor bell axles and lever type shock absorbers as those fields of endeavor.

I went back and read my post, and fail to see where I may have been less than complimentary. I also read the posted link about Understanding Boosters, wherin the author takes nearly half the article to describe the very basics of venturi operation and theory. It is basic enough that I didn't even see an example of a triple venturi, nor discussion about the merits and detriments of that arrangement, yet it goes on for half the article. And I thought that I could be verbose.

Again, I doubt where anyone could challenge Mr. Vizard's explanations and the theories he relays in his writings, and that's just wonderful if you're building a carburetor. He's the man.

Dialed_In

iTrader: (3)

I'm not trying to discredit David or "slam" him, but his methods and techniques are out-dated. Port a set of heads and set up a carbereuter based on his advice and you will be giving up HP. If I step back and look at the whole forest I will see that the world has moved on and advanced, that's all.

Misinformation and arrogance? Not hardly. We've had some of his stuff in the shop. What worked like a champ 20 years ago wont cut it now. Thats all.

Untie your panties.

Red Devil

How exactly was that self-discrediting? Because he gave his opinion, or because he pointed out the Vizard is still yapping his gums about carbs when they died off about 20 years ago?

You might be surprised what Vader actually knows about those carbs, they aren't really that complicated. Applying them properly is something else. - That said, I'd take a carb that RB tuned over one Vizard tuned.

Why exactly do you hold this opinion? Anyone that has a published book is King? That's asinine. Is it because Vizard is considered knowledgeable? Great, a lot of people are. WHY exactly can't we consider Mr. Vizard as anything but a G0D of archaic knowledge?

305sbc

iTrader: (3)

You're entitled to your opinions all right, but I still pity you two.
Using hindsight to criticize 30+ year old work is absolutely arrogance at its worst. How do you suppose we got to the point where we are today without past work to discover basic facts?
No answer is needed.
I just wish you'd realize that you're being flat out disrespectful and ungrateful.
Even those old publications have an amazingly high degree of fact reliability even compared to todays knowledge base.

Some looking at the forest type of logic applied to the "no carb is too big" writings, and you'd see that he was trying to point out the precise reason why response and metering ability suffered with larger carburetors... that it wasn't just a direct relation with total carburetor CFM, but was in fact directly related to signal at the metering orafices.
To take it literally that you cannot have a carburetor too big would be mistaken, and I'd bet Mr. Vizard probably wishes he had re-worded that for the sake of those readers who didn't quite grasp the whole concept after one reading.

Making disrespectful posts are also discrediting to the poster. Insults and references to "panties" have no place in a technical discussion, so you've only succeeded in showing your level of maturity and probably your technical competence as well.

Dialed_In

iTrader: (3)

I'm not criticizing his 30+ years of experience, just the fact that he and many of his followers still think that the 30+ year old techniques are the best way to get HP out of a small block. I'll be the first to admit that we are always learning. Working with a flow bench and engine dyno on a daily basis has a way of teaching you how much you don't know, and that most of the common myths and modifications that many swear by are snake oil.

All of you that think I'm discrediting the man or trying to slam him are completely wrong. I simply said that his methods and technology are outdated. I am on the other hand sorry that the word panties got you all excited.

rx7speed

not trying to discredit someone here but rather ask a question. how much do you really know about vizard and with what he says is the truth. how much are you 100% sure what vizard is saying is FACTUAL. are you 100% certain that that he is correct (speaking of vizard that is) honestly? can you back vizards claims up? Can you verify his claims? have you seen his claims proven? Can you show to me that his methods are really that great?


or is this one of those everyone else talks about him and trust him so I should to?


as far as vader goes can you do the same to prove that he doesn't hold knowledge of carbs? or does one really have to have equal knowledge to make any sort of statement? if that is the case then it goes back to proving the vizard has that much knowledge and truth in what he speaks.

305sbc

iTrader: (3)

I don't have the time to explain why I feel the way I do right now, or the will to write it all out about where I see Vizard's credibility come from. I will say that I think Vader is a VERY credible guy, but I was surprised at someone with his knowledge figuratively trashing carburetion, and supporting the disrespect of one of the originators of the knowledge base we now enjoy.
As far as Vader's experience with carburetors, I honestly don't know all the facts there as I've never even met him in person. I do feel based on what I've seen him post that I have a better working knowledge of carburetors than he. Next month I will be 40 and have an advantage of time & experience over most I see on this forum. I'm admitedly old-school and conservative, but don't make the mistake of misjudging me to be limited to only knowledge of carburetors. That's not even close to true. I started working with MPFI around 1990 and by 1994 I had experience with DFI systems. I have a background in machine language programming which lends itself handy to understanding routines in the hacks. I'm not ignorant of the advantages of fuel injection and computer controls. I am also aware of the downsides and limitations. Right now I have approximately 50 carburetors in my garage along with approximately 10 MPFI systems, and quite a collection of nice test equipment.
I can say that I have verified so much of what Vizard has written that I have developed a huge confidence in his publications. I own at least a dozen of his books with some special ordered from the UK and long out of print. I have been consistently impressed with the information published, and out of the vast amount of information he chose to share in his publications, I've only found maybe 3 things total that I do not agree with 100% or have conflicting data from my own tests.
I would only guess the number of carburetors Vizard has worked with and learned from are just uncountable. I know enough to pay attention when he shares information about carburetion even though it may be from 30 years ago.
People should be thanking the pioneers for getting us where we are today and sharing the information. Anyone can walk into a bookstore and read many of his publications for free. I for one am very thankful. I also do not think he is behind the times. He was way ahead of his time 30 years ago, and he probably still is.
This is a man with great ability that could probably use carburetion to put to shame the best MPFI effort from this site, and on half the budget. I wouldn't doubt that at all, especially after having observed for years the caliber of best efforts showing up on here. There are members here with a great knowledge base, but as the above poster points out, none have anywhere near the amount of publications as Mr. Vizard. Remember that when you publish something, you have exposed your work and opinions to the scritiny of the world. His publications have stood the test of much time. That's far more than I've seen from here. I'm going to continue to enjoy the benefits of Vizards shared knowledge.


A quick yahoo search of Vizard and credibility turned up something from this site even. I'm quoting it below to save my having to write out why I have seen him to be credible from my own experiences. This is obviously from someone else.

"David Vizard -Technical Editor

by Chuck McLean

It's pretty apparent that I have more to say about David Vizard than our other contributors. Why? Because, as a practicing work-o-holic he has packed probably three lifetimes work into his already 42 year long career and this technically makes him the oldest of our team with 126 man years of experience. Think I'm exaggerating? Well, try this for size.

Apart from a full time career as a designer and a performance consultant he has a full time journalistic career. In this capacity he has written over 3000 feature magazine articles. This alone makes him one of the most recognized and, more to the point, one of the most respected and read automotive writers in the world. This volume of material as far as I can determine, is more than the next two most published automotive writers put together. During one 20 month period in the early 80's he appeared in no less the 12 magazines a month with often two articles in some of these. With the aid of a 90 words a minute professional typist he once produced 11 feature articles in just 7 days! As a writer of performance stuff he is unique in that he is the only writer to have produced books that have allowed complete beginners to go out and win against pro builders. How did he get so far ahead of the game? A lot of determination and 50,000 dyno pulls just might have something to do with it.

Along with articles he has had 29 books published of which 5 have been publisher's best sellers. As much work as that may seem we are not done yet. The material for the articles and books was not from some outside source where all the work was already done. About 95% of the porting, engine builds, machining processes, dyno testing etc was done personally by DV.

Usually people who shine at theoretical stuff are a little short on the practical side. Not so DV. Apart from his academic qualifications in the field of both mechanical and electrical engineering he is also a qualified toolmaker and can expertly run just about any machine in a machine shop. On one occasion he made a camshaft using only a lathe and a file to form the lobes.

David Vizard's engineering background is the aerospace industry. He won an apprenticeship with one of the worlds leading aerospace companies and after attaining degree level mechanical engineering went on to become an R&D engineer for that company. During this time he worked on some pretty exotic supersonic aircraft and many of the contents of his desk drawers were in folders marked 'Top Secret'.

Since then he has been a consultant to many well known companies from top Formula one manufactures across the board to some of the best known names in the cam, induction and exhaust business. Just one example of his work is the range of cams he designed for a popular European car. These cams outsell all others combined. He also has his name on over 40 patents ranging from fuels to 4 valve cylinder head designs. Heading a team of four engineers he developed what could have well been the first 50/50 car. That's 50 mpg at 50 mph. I can't say for sure this effort was first but it was before Honda did it and done on a really skinny budget. This project not only showed DV's engineering talent to good effect but also the quality of the team he chose to work with highlighted his ability to surround himself with some of the sharpest brains in the business.

As if the forgoing was not enough David is also an accomplished race driver. He has driven his way to drag racing championships but really shines at road racing where he has set class lap records on every major racetrack in England. On 4 separate occasions he has taken a team to a 100% season win record by virtue of the speed he has found for them. Two of these occasions were at international level! In his best season his motors achieved a combined 169 track records, first places, and championship wins - from just 8 engines!

On each of two seasons when DV decided to devote time to a complete season of racing, his cars, by the end of the year, were not simply competitive, but totally dominant. How dominant, in the supporting race of the British Grand Prix at Brands Hatch, driving in the Sedan manufactures championship, he took his car from 28th on the grid to joint first by the first bend! Millions of British race fans watched that on TV. At Prescott hill climb, one of England's oldest tracks, David stormed up this 1000 yd. course on a damp asphalt surface 7 seconds faster than the current track record holder and a time 0.2 of a second faster than the fastest F1 car! These are not isolated events!

Although setting lap records is not unusual in itself there can hardly be many who have achieved it in cars where they developed not only the engines but also the chassis, including designing the shock absorbers, the aerodynamics and even doing the paint job.

DV is a man who seems constantly ahead of his time. Just one example of this is the 'tunnel' ground effects car he built in 1971, that was a full 7 years before we saw it at Indy. Most tech writers would like to be up to the minute with their material. Very few achieve it. DV is not only up to the minute most of the time but ahead of the game. Often , in his capacity as a consultant, he is doing research that will result in components that will be seen on F1 cars in a couple of year's time.

Unlike most effective sources of race technology DV is very generous with information being prepared to give out almost anything that is 'declassified'. Couple this to a renowned ability to simplify complex subjects and you have an information transfer formula second to none. This is in stark contrast to most engine builders who are 'paranoidly secretive'. Bearing all the factors in mind it is easy to see why DV's lectures to Pro engine builders at Universities such as Denver and Charlotte have proved unparalleled in popularity. Such popularity though is hardly confined to the US. In England he is known as Vizard the Wizard. During his one and so far only visit to Australia for a lecture tour he signed over 2000 autographs. Try naming one other writer that would be called upon to pen his name that many times?

Put all this together and I seriously doubt you will find a person with more overall qualifications to make your car go faster than the next guys or get it past the finish line first. This, my friends, is why we have David Vizard as the kingpin of our publication."

EvilCartman

iTrader: (1)

meh, my 750 Demon worked great on my old 305 compaired to the 670 that was on it

Rick King

well this is a take on things-- I have a victor jr on a 333 (327)cubic inch small block solid cam about .55 intake and about .598 ex 220 psi per cylinder with a 650 holley dp (4150)

called holley told them the power band -- we uped the jet sizes to there speck- increased the needle and seat size and Run it and 6 psi presure under full load and a stock power vavle for now.
yes at idle it is a slouch and the motor only likes to run at a low idle of 2200 rpm, but only winds out well when the idle is set at 2800 rpm, holley said that is ok because of the application of the motor-
-the point is the motor runs stronger and it is faster and the tunning is not done--we are still looking into cams (for the carb) and boosters and larger pumps, the other point is if using a holley get there tech department and have them teach you so your carb will do what you need it to do with out going out and buying a 1200 cfm carb for a stock little motor


also i have a stock roller 305 with a750 cfm elderbrock carb- a little tunning and it works well I spin the thing to 6500 rpms well it will blow sooner but who cares- i go 75 down the freeway at 2300 rpm with a trubo 350 tranny and 373 gears with 255 by 60 tires on it


like stated just my 2 cents take it or leave it both theories are true, and yes if I buy a larger carb it will be a 950 or 1200 as the motor will then spinn to 9000 rpms

Vader

You give me too much credit. You probably have a lot more experience with carburetors. I can only think of a few types I've worked with, rebuilt, tuned, and modified - Holley, Weber, Mikuni, Carter, Rochester, Autolite/Motorcraft, and a few off-the-wall ones like Tillotson and Marvel/Schebler, and a handful of updraft and variable venturi aviation types, with external enrichment/enleanment controls and heat. Of all of them, Holley occupies a special place in my heart. It's just my opinion, but they are among the least sophisticated, most crude, least elegant examples of fuel metering devices available. They can be tuned fairly well in a RANGE, but that's all I can hope for with one. The transitions seem to kill them, and that's WITH Holley technical assistance. That's sad, too, since the original Holleys actually were decent, capable devices. As I said, just my opinion.

And I still can't see where I slighted Mr. Vizard's knowledge base.

I know a little about his work. I knew he had worked with fuels and additives. I'm not just pulling this out of the air, after all. It turns out that his background is not so different from mine, although I've never work FOR an aerospace company, only IN them. And not with airframes or piston engine, but with turbines and APUs, governors, axial compressors, and the like.

That seems to be the case more and more, but it never used to be. It used to be that most decent engineers could walk out into the shop and PRODUCE what they designed. That doesn't seem to be the case much any more. Our loss. Still, somehow, I doubt he can/could operate a surface shaper, form grinder, program/operate a seven-axis geodesic frame mill, set up and operate a multi-slide former, and rewind a resolver/tachogenerator in his spare time (We all have our moments), but perhaps he could if he tried. We'll leave the hand-forming of cam lobes with a file alone for now. That probably works great on European cars.

There is no doubt he's been around the block, and learned a few things along the way.

Looks like between both of us (not Mr. Vizard, you and me) we have about 86 years under our belts. I'm fairly conservative as well. I cannot read where I slighted Mr. Vizard's base of knowledge. Many of the conventions and ideas he presented are still valid. Basic physics don't change very often. I just find it a bit odd that texts on carburetion still sell so well and are so revered in an age when nearly all facets from daily driving to exotics to Formula 1 have long since moved on to injection, and with better results. Some are even using direct cylinder injection. I know that there are still a lot of carburetors in use in a lot of applications. I have a few myself. And comparing aftermarket, properly tuned carburetion to the 20-plus year old technology of the crude old factory EFI systems is hardly fair. Start digging into the Delphi systems used on more modern systems, and you'll see some real sophistication. Even current OEM systems are light years ahead of what was in use as late as 1995. It's like comparing a modern Holley Demon to an original 1600-Series variable venturi diaphragm type, with full range configurable mixture control cams, separate mixture override control, and tunable fuel for every opening angle of the venturi.
Oh wait - I got that one backward...

Apeiron

I think you might be looking at the past through rose-colored glasses. There has always been a vast surplus of engineers who couldn't operate anything more complicated than a drafting table. After all, the expression "any idiot can tighten a nut with a pencil" has been around forever. (See the Kansas City Hyatt Regency for a somewhat recent prime example). It's been the more competent engineers that are memorable though, and those tended to be the ones that have the practical sense that comes from working hands-on. The difference nowdays is that engineers no longer need to get as involved in their projects.

David Vizard has certainly been prolific in many senses. He has conducted a great deal of research in a number of areas, and has published much of his results. Some of what he has to say though is application-specific, and seems to get misapplied.

Vader

Ape,

I did write DECENT engineer... Granted, there are plenty of adequate engineers.

Apeiron

And always more than enough inadequate ones.

305sbc

iTrader: (3)

Vader I'm sorry. You weren't trashing Vizard like the other two guys, and my reply was meant more for them. You say you prefer Holley as a carburetor brand and you obviously appreciate the sophistication of electronic controls as well. I can respect that.
Thanks for clearing up your stand on Mr. Vizard. We aren't related or anything, but he's still someone I also respect.

For a glorified throttle body that can also meter fuel into an engine, you have to admit that a carburetor can make a very nice showing at a race track. I'm more awed by that than the approximately 30 years of EFI development that has got us where we are today, which IMO is in a lot of ways barely ahead of the carburetor. The code has gotten plenty fat, but the practical results become harder and harder to distinguish unless you're running an emissions tester. IMO custom code is where the benefits lie for what I'm looking for. That doesn't make me like carburetors any less though.