[Christer N (Christer)]

Any particular reason that you use a Demon 750 carburator, and not a Holley or a Edelbrock?
I plan to buy a 750 vacuum to my 383-engine someday and I wonder which make that will be the best choice? Any suggestions?

[Daver (Daver)]

When it comes to a single 4bbl carb...my personal choice would be thus;

My first choice would be the Edelbrock Performer. The secondary opening rate is not easy to tune but other than that a better carb than the Holley. Not as good a carb as the Carter Thermoquad was but the tuning kit and instructions are fantastic. With the one kit you can make this carb suit any engine.

My second choice would be the Carter Thermoquad if I could find a decent one. A great working carb that is easy to tune and also keeps the fuel cool! A better carb than the Edelbrock or Holley.
Tuning parts are hard to come by though.

My third choice would be a vacuum secondary Holley or something based on the Holley. Holleys have the widest range of performance parts available but if you need to do a lot of tuning to get it right you soon get sick of soaking up spilt fuel with a towel and replacing gaskets.

[Christer N (Christer)]

Thanks! I didn´t know that it is possible to tune the secondary opening rate on vacuum carbs.
Is that possible with a Holley carb too?

[Daver (Daver)]

Yes. You change the spring on the vacuum diaphram.
Which means you have to buy the right spring first time or else get a few to play with.

The Edelbrock you have to add or remove material from the air door counter-weights. Bit tricky.

The Thermoquad you just turn a screw I think.

[SteveO (Steveo)]

I chose a Demon Carburetor because I heard a lot of good things about them like "innovative concepts and high-quality manufacturing". Also, on one of the car shows I watch on TNN, they took a tour through the factory and interviewed Barry Grant. I was sold. However, when I installed this High Quality carb on my motor I could not get it adjusted just right. So I went to The Carb Connection and they opened up the air curcuit on the metering block and that made the difference.
It's been working great since then. Anymore carbs I buy will be Holley! Their Street Avenger series sounds like a good product. "Bolt it on and go"

[Daver (Daver)]

I don't get any of the American auto magazines any more. Can you tell me about these 'Street Avenger' carbs? How come they are a 'bolt on and go deal'?

[SteveO (Steveo)]

Here is a copy and paste from one the adds:
http://speedfreaks.safeshopper.com/994/cat994.htm?143
Holley Street Avenger Carburetor
Street Avenger Carburetor
Designed from the ground up, the all new Holley Street Avenger Carb is loaded
with No Trouble features so you get the best in ''Bolt It On and Go'' street
performance. It's 100% wet-flow tested and calibrated for awesome street
performance right out of the box!

No Trouble Adjustable Vacuum Secondary: a 60-second adjustment can give you either 6% more
acceleration - a difference you can really feel - or a 5% improvement in fuel economy - a difference you can
clearly see at the pump.
On-Vehicle No Trouble Electric Choke: Holley simplified the choke adjustment so it is easily done on your
vehicle, and you can't accidentally over-adjust your choke setting to ensure great cold starting and smooth
idle.
External No Trouble Float Adjustment with Sight Plugs. Every Street Avenger carb comes factory set and
flowed. However, you can easily adjust your float level to compensate for varying fuel delivery pressure right
on your vehicle without the risk of over compensating.
Generation IV No Trouble Power Valve with million mile blowout protection to ensure years and years of
reliable performance.

[Daver (Daver)]

Sounds like they still build them as close as they can to the spec for your engine. Then it is just a matter of fine tuning. Still it is a big step in the right direction.

[David Robson (Admin)]

Thread moved here from 'Photos of SteveOs Challenger'.

[Daver (Daver)]

Hey I just noticed. They don't do one big enough for anything more than a warmed over 440!

[Christer N (Christer)]

I have heard that many four-barrel carburators, like Holley, are "bolt-it-on-and-go" items.
I have heard that they work good direct from the box. Is it really like that or have I been misled?

[David Robson (Admin)]

Some people do not know how to tune a carb so if they can't get it right they buy a new one that has not been messed about with.

When they fit it and adjust the fuel level and idle mixture they find it works a lot better than their old carb. "Gee. It ran great right out of the box!". Then they leave it alone for fear of making things worse.

But in truth a lot of these people do not have a clue if the mixture is right at all throttle positions and under different loads. They don't know because they have no way of measuring it.

That is why I think it is very important to fit a good rich/lean indicator. It is a fantastic tool that tells you a lot about what is going on while you are driving.

Having said all that. Holley will do their best to give you a carb that matches your engine as closely as possible anyway. But as every engine and driver are different (not everybody wants best power) it is impossible to get it perfect just like that.

Just as an example. Did you know that there is a 20% fuel flow difference between best power and best economy?

[Christer N (Christer)]

Hmmmm...Rich/lean indicator? How does it work and how much does it cost?
Where do I buy this equipment?

[David Robson (Admin)]

Look in the Summit Racing catalog.

I did an article on this on the Mopar Muscle Association message board. I thought I had copied it over here too but I can't find it.

I will paste that thread into this board.

[David Robson (Admin)]

From the MMA message board...

I am going to tell you how to build a rich/lean gauge for your Mopar.

It is not hard. You need only an oxygen sensor to fit into the exhaust pipe and something to read voltage. This can be an actual volt meter or a LED device.

You can buy these as a ready made kit if you like. I have found it a great help in getting my sixpack to work and I really, really think Pete would benefit a LOT from one of these.

I will even give you an electronic diagram to show you how to build your own LED indicator if you are the electronics whiz kid type. I gave Jay Gilmore a copy of this a while ago and it has been 'under development' for a while but because Pete is having so much grief I think it is only fair to let everybody in on how easy this is.

The oxygen sensor you need is a part of every day life in modern cars. You can even find them on cars in scrap yards now. They come with either 1,2,3 or 4 wires coming out of them. You only need the sensor wire which is normally black. The other wires are for a pre-heater which you don't need and sometimes one is an earth lead.

The sensor has a threaded end so all you have to do is cut a hole in one exhaust pipe as close to the collector as possible and weld a nut over the hole so that the sensor will screw in and protrude into the pipe. You then need to connect it up to a 12 volt source switched from the ignition switch and wire in a volt meter to join the hot wire and earth (not in series with it).

A digital meter might not be the best thing to use in this case as the display can change very rapidly. A normal 12v gauge is all you need although it will help if it has a good scale.

All you need to know now is what the different voltages you will read actually mean.

Well I left that information at home so you will have to wait!

Get cracking Pete.

By Martin Edridge (Martin) on Friday, July 20, 2001 - 04:35 pm:


Spot on, Dave. Did this a few years ago just using a DVM. IIRC, about .890-.920 was a good value to shoot for if looking for max power. Prone to being very innaccurate at less than full tilt due to lack of heat in the pipes. Guess I should have used a heated 02 sensor. I did ask you a while ago about doing an LED box for this job but you didn't reply.

By Gavin Chisholm (Mrnorm) on Friday, July 20, 2001 - 05:35 pm:


You can use the heated O2 sensor, which improves things a bit (for example if you want to see what your engine is doing when it's cold), but the regular is fine. If you use one with leaded gas though it will not work for very long.

I have a couple of sensors that will find their way into my new exhaust, it's a really good idea. I have some excellent web links which are pretty comprehensive, I will try to post them later.

Some people sell the aftermarket LED meters, and claim that they are 'air/fuel' meters, showing a lamda range from, say 9:1 to 16:1 - don't believe them!! Unless you go to a very expensive UEGO sensor, a normal O2 sensor (an EGO or HEGO) will really only tell you if you are either rich or lean. Still very useful, but more LED's don't make it more accurate. EGO sensors are very non-linear, they transition ('switch') from rich to lean very quickly, meaning that you can't really tell how rich or lean you are with any accuaracy, just that you are either rich or lean - your title is dead right Dave (no surprise).

Anyway, top idea, everyone should use them!
(Yes Sandy, I know you already have one! Maybe even 4?)

By David Robson (Dave) on Friday, July 20, 2001 - 08:34 pm:


Sorry Martin. Jay swore me to secrecy! I will print all the details I have sometime over the weekend.

By David Robson (Dave) on Friday, July 20, 2001 - 08:39 pm:


Just to keep you going...

Under 250mv = Too lean
250-410mv = Good cruise mix for mileage
410-580mv = 14.7:1
580-800mv = Rich but good WOT power
Over 800mv = Too rich

By Gavin Chisholm (Mrnorm) on Friday, July 20, 2001 - 10:42 pm:


There you go boys, this is not a bad article at all, and has a good bit on EGO sensors.....

http://www.bob2000.com/carb.htm

By Sandy Mercer (Sandy) on Saturday, July 21, 2001 - 01:48 am:


Dragged kicking and screaming into the late 20th Century, eh ??

Gavin,

GN has one O2 sensor (pre-OBDII by years) up by output of turbo. It is a one wire sensor, earths through the downpipe.
On the laptop, 750-800mV is what you aim for. Best power comes at just the lean side of rich (if you see what I mean). 470mV is stoich.
Between 800 and 850mV is okay but rich. I tend to launch rich (hence crappy 60 foots), say 850mV then the car leans out through the run. My car has always done this for some reason, but I have a little boost creep, hitting 22psi or so at the end of the track and O2 has dropped to about 750mV. Not too concerned, except when I get detonation.
In the States where the turbo Buicks guys get 116 octane race gas that is leaded they go to heated sensors, or just throw them away after a race day.
They do foul up very badly with lead, and basically go off.
There is no feedback to the management system at full throttle. The system goes into Phase Enrichment, and the injectors go to about 110% duty cycle (hmm, tell me how injectors can be open 110% of the time ) But the car basically run a fixed set of values at full throttle, and the O2 is 'tuned' by upping fuel pressure to richen or upping boost to lean ...
If you guys are tuning for power though, forget O2 and buy a digital EGT (exhaust gas temperature) gauge and aim for 1550F or so.

By MoparKev (Kev) on Saturday, July 21, 2001 - 04:10 pm:


HOORAH! I've been waiting for Jay to pull his finger out for about a year now! Very interested in the circuit diagram Dave. Where do these sensors live on modern scrapyard type motors? Do most injected cars have 'em? What's the difference 'tween O2 sensors and Llambda sensors?

By Gavin Chisholm (Mrnorm) on Saturday, July 21, 2001 - 06:58 pm:


Kev just use a DVM, no circuit required. The sensors are located before the catalyst in the exhaust pipe, if you look at the link I posted it has a good pic of what one looks like.

An O2, or EGO/HEGO just senses the presence of oxygen in the exhaust - excess oxygen = running lean. Of course once you are using all the oxygen (stoichimetric) it can't tell how rich you are, but it kind of acts as a temp sensor in this range. As I said, it won't give you a real air/fuel ratio, at least not with much accuracy, but it will tell you whether it is rich or lean, and just around stoich its good enough to do the job.

A Lamda sensor will actually measure the air/fuel ratio itself - 10:1, 12:1 14:1, etc. Better than an O2 but a lot more expensive. A UEGO does this, and from the name I have to assume that it also works by sensing oxygen.

By David Robson (Dave) on Sunday, July 22, 2001 - 09:59 pm:


Right. Lets get back to this.

Because I did not have the figures on my when I started writing this I did not realise the voltage scales we would be working on. So I think the cheapest (and easy) way of doing this is with a cheap digital meter.

And that is what Mr Ehrenberg suggests!
But I took the easy path and bought a complete kit. much easier to read.
There are several different makes but all work the same. Hooker headers even make collector reducers with a threaded mounting boss for the sensor!

First you need to aim for stoichiowhatsit all round. Then once you have that settled down you will want to increase the part and full throttle mixture to 12:1 for best power.

Out of interest. That equates to a fuel flow increase of 20%. No wonder we don't get many to the gallon!

Looking at it the other way. Best economy (without getting too lean) is 15.75:1. That equates to a decrease in fuel flow of only 7%.

[David Robson (Admin)]

[David Robson (Admin)]

[David Robson (Admin)]

By MoparKev (Kev) on Friday, July 27, 2001 - 09:43 am:


What is stoichiometry, please?

By David Robson (Dave) on Friday, July 27, 2001 - 11:14 am:


Stoichiometry is when you have exactly the right amount of oxygen to burn the amount of fuel in the mix.

The theory behind the sensor is that little or no oxygen detected by the O2 sensor means that the engine is not burning all the raw fuel being delivered to it. An excess of oxygen means that the mixture is too lean.

In theory the best all-round performance occurs when the exact amout of air and fuel exists for complete combustion. For petrol that is 14.7 parts of air to 1 of fuel (by weight).

This is from Hot Rod magazine;

"For best power, all the oxygen in the mixture must be burned, because the power output of any engine is limited by the amount of air inducted into it. To ensure that all available oxygen combines with the available fuel, extra fuel should be provided. Combining this fact with inevitable exhaust gas dilution of the incoming air/fuel charge, lack of mixture homogeneity within an individual cylinder, and mixture variations between different cylinders, for all-out performance it is best to have a slightly richer than stoichiometric A/F ratio under full-throttle conditions (as opposed to idle). A slightly rich mixture also ensures against top-end overheating, lean misfire, and detonation."

In general an A/F ratio of about 12:1 produces the best power.

[David Robson (Admin)]

I should add that the oxygen sensor has to be as close to the exhaust header/collector as possible without getting in the way of the nuts and bolts.

[David Robson (Admin)]

Looking at the figures above I see I did not remember something as well as I should. There is a 27% difference in fuel flow between best power and best economy! Not 20% as I stated near the top of this thread. That is a lot of fuel and it goes to show how anybody could say "it worked fine right out of the box" because as long as it falls somewhere between best power (rich) and best economy (lean) then you will not have any problems and everything will seem fine.

[Christer N (Christer)]

I am overwelmed (and grateful) with your answers.
I have learned a lot! (I have been busy installing a 6:71 blower atop of mine 383! ....just joking!)

Many years ago I was a Che****et Cam**o ´67-´69 fan (how could a sensible man like me like them??:-) and my guru at that time was a guy called Smokey Yunick. (Does it ring any bells? Does anybody know if his "best damn garage in town" still exists?)

Anyway, he is a master in getting more horsepower out of a smallblock Ch**y, and his book
"Power secrets" is warmly recommended (inspite the make of the engines). What I am getting at is that he does not agree with you Dave, that there is a GREAT need to tune carb´s. Maybe have I misunderstood either you or him. Have you read this book that I refer to? If it is of intrest I can post some intresting parts of it.

[Dave R (Roppa440)]

I used to like those same cars when I was a young teenager in the 1970s. A guy not far from where I lived had a '70 Z28 which I thought was cool. There was also a red '68? SS and a 71 Firebird in the same area. I used to walk well out of my way just to pass these cars!

I do indeed know of Smokey Yunick. He died not long ago. A great character. I have not read that book although it is very famous. But I understood (I may be wrong) that it was aimed at race cars and not street driven cars. Race cars tend to be wide open throttle or closed throttle with nothing much inbetween. Much easier to tune. But in any case you would still need to get the mixture right to make the most power or economy or whatever you want from the car in a race.

Carb tuning is very important on a street car in particular because of the different loads at different throttle positions.

[Christer N (Christer)]

Smokey is dead. The world has lost a great man!

Yes, I also think that there are horsepower to gain in tuning the carburetor but I sometimes wonder if the horsepower gain is worth all the efforts? How much is it possible to gain? 2 hp or 20 hp? I think it is a relevant question and can be of intrest to discuss.

It appears to me that when Smokey speaks carburetors, he speaks Holley double-pumpers. I think it is important to have that in mind. The book is printed in 1983 (19 years ago! Time flies!) and, of course, a lot has happend during that time.

Here are some cuts from Smokeys book “Power Secrets”:

“The carburetor is really a pretty simple device. It doesn´t know or care how big the engine is. It is strictly an air-fuel mixer that is calibrated to meter a specific amount of fuel according to the airflow. As more air passes through the carburetor, it simply meters fuel into the airstream. And I don´t think there´s any reason in the world to get tricky with a carburetor.”

“And I don´t think the average racer can do much to improve it. If you get the right model, take it out of the box, and bolt it on the engine, the job´s 95% done. The only thing left is to get the fuel mixture shaped up, but this is not nearly as tricky as a lot of guys would have you believe.”

“The Holley double-pumper carburetors that are built for racing usually come with a jetting that is about 5% rich for most applications-Holley does this to be a little on the safe side-but I´ve never seen a totally stock Holley that would produce a brake specific fuel consumption figure very near 0,50 pounds per fuel per horsepower-hour. And I think this is about right on the number that most weekend racer should be running. If you´re an expert racer and you have the right equipment, you can run leaner than this-if you´re real good you can run at about 0,45 Ib/horsepower hour-but if you´re on the lean side and things get a little out of hand, you´re going to burn something up. So as far as I am concerned, the best bet is to just run what comes out of the Holley box. If it´s not right on the money, it will be damn close.”

[Christer N (Christer)]

I have read that there are idle circuits in all three carbs in a six-pack setup. I didn´t know that until a couple of days ago. The reason is that the secondary carbs get fresh gasolin that way, even if you drive the car slowly (using only the center carb) for some days.

[Dave R (Roppa440)]

Not the ones made after 1972.

[Dave R (Roppa440)]

Just to comment on your 'Smokey' quotes.

If you were Holley and your customer was Smokey and he gave you an exact spec of his engine he is racing at the next BIG meeting on National TV and he wants YOU to supply a carb for it....

You would make damn sure it was the right one for the job. Yes?

[Dave R (Roppa440)]

But most of us don't get a new carb every time we change something on the engine. Changing something like the cam or even the exhaust can completely change the carb requirements.

Take my 6-pak.

Worked fine as standard on my mates slightly hot 440. Mixture seemed about right too. But would not idle at all on mine. Had trouble getting it to fire at all. Ran lean on the center carb too. I have gone up from #62 jets to #66. Plus I have had to play with the squirters and pumps as well as fit a different type of metering block.

Not exactly off the shelf is it. But if I had given my exact engine spec to Holley I am sure they could have sold me a 4bbl carb that worked ok near as damn it.

[Christer N (Christer)]

Yes, I admit that Smokey sounds like he is paid by Holley....

I come to think of my Weber-carburated hot four-banger I had many years ago. There are long lists with jetting sizes and so on depending on camshaft, heads,....I think that we have a tradition here in europe to fine-tune things meanwhile the americans make things big. That is why they don´t have the same need to tune things, I think. They make lots of power without the fine-tuning.

I wish I could help you with your carburators, but I am not experiensed enough. The carburators I owned were a pair of Weber 45:s. (I love these carburators...) Don´t laugh at me but I never managed to fine-fune them so I decided to sell them and when I loosend them I saw what was wrong! One diffuser had been mounted the wrong way by a previous owner. The result was that one cylinder run too lean, so it "dry-banged"! What can we learn from this? Be thorough when you troubleshoot and it is a advantage if you do it with an open mind.

[Dave R (Roppa440)]

Don't worry!

My carbs work fine now. And I have learned a LOT about tuning Holleys in the process!

[Christer N (Christer)]

I saw in one book that the center carb of the six-pack has different number, depending on if the car has automatic or manual transmission, but I guess you already knew that. Any idea about the difference?