Carburetor experts please look!!

[1955V8]

ps- Holley also doctored CFM rating with subtle changes.
3310-1 780cfm gained 30cfm from slightly 1/16" larger secondary venturi than 750DP.
3310-2 750cfm lost 30cfm from a more restrictive straight leg standard secondary booster, compared to 3310-1 downleg booster.
Holley 800 cfm gained 20cfm over the 780, by using a skinnier, less restrictive secondary downleg booster.
750, 780, 800 vac. sec. carbs all were the same body, with slightly larger secondary venturis, a half-assed spreadbore of sorts.
750DP had 4 equal size venturis. if you get all these together and measure them, ephiphany will strike.

Demon vs. Holley
Demon 750 1.40" venturi vs. Holley 750 1.375" venturi
the Demon "750" venturi size was already opened up .025" over a stock Holley 750 venturi size
again, the proof is in the measuring. Demon carbs were souped over Holley sizing,
to give added WOT performance for same rating. smart marketing.

[JES]

Most all of the calibrations on those carb are jacked up. Go to racingfuelsystems.com read and learn like I did. Listen to Mark Whitener and you will have such a great journey and have a car that will perform great. BTW. Thanks Mark, Tuner and Shrinker - RIP

[1980RS]

Most all of the calibrations on those carb are jacked up. Go to racingfuelsystems.com read and learn like I did. Listen to Mark Whitener and you will have such a great journey and have a car that will perform great. BTW. Thanks Mark, Tuner and Shrinker - RIP

What happened? did Mark pass on?

[JES]

No, Shrinker

[1955V8]

I'll check your link, thanks.

when Holley took over the Demon line, they dropped the BG flow ratings for the most part.
That is misleading in some cases, because a Holley with choke horn and standard top casting,
is not going to flow anywhere near a Demon with the patented inlet sculpting bowl shape on top, with a bored oversize venturi.
The Demon with no choke horn, larger venturis, and toilet-bowl like entry will flow more.
Holley dropped the ratings back down to match the rest of their other brands.
That's because they didn't want to devalue the rest of their carb lines, Holley, Quikfuel, Brawler, etc.
a BG Race Demon carb on a 750 throttle plate, had the 1.375" venturi opened up to 1.42", will be stamped "42", and had a wet flow rating of 825cfm, logic dictates it won't just still flow 750cfm. It has a sculpted bowl entry, no choke horn, and .045" larger venturis.
75 more cfm from those mods is not an outrageous claim.
according to BG in their old carb manual, all their ratings were done at 1.5" HG, wet flowed, all 4 barrels open at once.
they had a new high end wet-flow bench that BG designed and spent over $250k on, to test all the carbs.
that would be the same as 20.4" H20 rating rating.
some other carb companies were flowing one barrel of the 4bbl carb, dry flow, then multiplying by 4, to get the 4 barrel rating.

[1955V8]

Most all of the calibrations on those carb are jacked up. Go to racingfuelsystems.com read and learn like I did. Listen to Mark Whitener and you will have such a great journey and have a car that will perform great. BTW. Thanks Mark, Tuner and Shrinker - RIP

I usually try 3 or more carbs on an engine, then use the one that performs the best overall actually driving it on the street.
typically they lose idle signal, blacken the plugs the larger the cfm gets, but have more top end at WOT.
and they have better idle, part throttle, cleaner plugs the smaller the cfm,
a spreadbore being a very flexible, driveable, most economic street carb, with the big secondaries for power.
the Holley 750-800 is usually the best compromise for a cammed street pump gas motor, doing everything good.
better idle/bottom end than a Dominator or 850, better top end than a Qjet or Thermoquad.
the difference between primary/secondary size on a spreadbore causes a flow velocity difference in the bores and plenum.
Edelbrock attempted to address that with their old Torker 1 intake, had what's called a "veloci-step" in the floor of plenum,
and the front port runners were smaller than the rear runners.
if I had to choose one carb to be stranded on a desert island with, it would probably be a 750 Holley

[HQM383]

this is an old topic, but I've run into this Demon nomenclature confusion myself,
when it comes to Demon carbs, look for the stamped mark next to the rear bowl vent tube on top.
what these numbers mean, is venturi size. 28-1.28" 42=1.42" etc.
28=650cfm
40=750cfm
42=825cfm
56=1000cfm
56 w/annulars=1000cfm

if the carb had removeable sleeves, the cfm changed slightly, with more options.
28=675cfm
40=775cfm
42=825cfm
50=975cfm
56=1025cfm
56 w/annulars=820cfm
59=1050cfm
59 w/annulars=950cfm

if the OP had 2 Demon carbs, we'd need to know what the stampings were. if the stampings were not identical, they are different CFM.
Holley used up all the existing BG Demon cores and parts, when they obtained all the Demon equipment, stock from the BG bankruptcy estate. At first they sold off the complete or nearly complete carbs to the general public, then started modifying them, by stamping a punch mark next to the number on top. That would typically mean, a Screamin' Demon or Mighty Demon that may/may not have screw in top air bleeds, or screw in emulsion bleeds in metering blocks. When all the Demon stamped main bodies were used up, Holley started casting and stamping their own with a new system, a long part number along the air cleaner base rail, and the rated CFM.
so- if you have 2 digit numbers stamped next to rear top bowl vent, that's an original BG core.
if there's no numbers, or numbers stamped along the side air cleaner rail base ring, that's a Holley core after the buyout.

FWIW, the best idling carb with clean plug readings, for a street car, is the "40"
as you go higher to the "42" "56" etc. the velocity/signal slows down,
fuel comes out of mixture, idle deteriorates, plugs start turning black
42 and up Demon carbs were really RACE CARBS. idle down to the staging lanes, put a run on it.
if you drive them around on the street in traffic, they will blacken the plugs.
the same goes for any Holley BTW. there's really no need for anything beyond a 3310-1 780cfm on a street car.
going bigger in venturi size, you'll pay for it with black plugs.

Some good information there. I tinker with these all the time and currently have the ones above in

[HQM383]

this is an old topic, but I've run into this Demon nomenclature confusion myself,
when it comes to Demon carbs, look for the stamped mark next to the rear bowl vent tube on top.
what these numbers mean, is venturi size. 28-1.28" 42=1.42" etc.
28=650cfm
40=750cfm
42=825cfm
56=1000cfm
56 w/annulars=1000cfm

if the carb had removeable sleeves, the cfm changed slightly, with more options.
28=675cfm
40=775cfm
42=825cfm
50=975cfm
56=1025cfm
56 w/annulars=820cfm
59=1050cfm
59 w/annulars=950cfm

if the OP had 2 Demon carbs, we'd need to know what the stampings were. if the stampings were not identical, they are different CFM.
Holley used up all the existing BG Demon cores and parts, when they obtained all the Demon equipment, stock from the BG bankruptcy estate. At first they sold off the complete or nearly complete carbs to the general public, then started modifying them, by stamping a punch mark next to the number on top. That would typically mean, a Screamin' Demon or Mighty Demon that may/may not have screw in top air bleeds, or screw in emulsion bleeds in metering blocks. When all the Demon stamped main bodies were used up, Holley started casting and stamping their own with a new system, a long part number along the air cleaner base rail, and the rated CFM.
so- if you have 2 digit numbers stamped next to rear top bowl vent, that's an original BG core.
if there's no numbers, or numbers stamped along the side air cleaner rail base ring, that's a Holley core after the buyout.

FWIW, the best idling carb with clean plug readings, for a street car, is the "40"
as you go higher to the "42" "56" etc. the velocity/signal slows down,
fuel comes out of mixture, idle deteriorates, plugs start turning black
42 and up Demon carbs were really RACE CARBS. idle down to the staging lanes, put a run on it.
if you drive them around on the street in traffic, they will blacken the plugs.
the same goes for any Holley BTW. there's really no need for anything beyond a 3310-1 780cfm on a street car.
going bigger in venturi size, you'll pay for it with black plugs.

Wouldn’t let me edit before or I timed out so new post.

Some good information there. I tinker with these all the time and currently have the ones above highlighted in blue and the sleeves highlighted green but have not used those sleeves. Some things to add for anyone interested. In the Barry Grant days when the Race Demon and Mighty Demon were sold side by side the Mighty Demon had a dimple partially over the second digit on the cast airhorn number that denotes venturi size. A clean, full cast number, ie ‘42’ without a dimple made by the touch of a drill taking out part of the ‘2’ meant it was a Race Demon. This was consistent across venturi sizes. This was a sure bet it had 5 hole threaded emulsion metering blocks (A-12320) and removable threaded boosters. Race Demons were not only removable sleeve (RS) and it’s not uncommon to see old non RS are being sold these days as Mighty Demons by people that are not the original owner and unaware of the idiosyncrasies. The buyer gets a bonus. Race Demons also has a two letter stamp on the other side of the vent tube adjacent to the venturi size cast number. This denoted calibration and examples are ‘GC’ for General Competition, ‘DR’ for Drag Race and ‘TR’ for tunnel ram. All removable sleeve bodies have ‘56’ venturi number cast as there was less machining to create the necessary snug fit for the sleeves. General Competition had pcv, timed and full manifold vacuum ports and all other Race Demons had no vacuum sources. Mighty Demons all had the three vacuum sources. Mighty Demons were swedged in boosters only and received C-12320 metering blocks that had three emulsion holes at the positions of 1-3-5 of the A-12320 block, they are fixed hole non-threaded. They did however have threaded idle feed restrictions in the upper position. This I find is the number 1 reason they were disliked as an overly rich carb no matter the cfm rating. The IFR were sized at .037” and .038” with .070” iab for the 1.75” throttle bore carbs - the 825cfm even had .067”. 750cfm had IFR of .036” and mab of .039” and 650 IFR of .029” and mab of .041”. I always lower the IFR and find between .031” and .033” IFR provide the AFR I’m looking for no matter the cfm rating (650 the exception - .028”). Mighty Demon started as a true in-between the Speed and Race Demon line with their calibration but eventually adopted Race Demon calibration. An 850cfm downleg in the early days had an mab of .028” and IFR of .035” but that grew to .031” mab and .038” IFR. Other cfm sizes similar jumps in dimensions. It would be safe to assume it was to make the Mighty Demon the flagship line to replace and phase out the multi-faceted Race Demon.
Demon Carburetors were cast by Empire Castings and were of a very good quality. It’s only when it came to machining time did the Demon factory make a mess of things in their later years. I’ve seen some horrid quality issues on what would be later Barry Grant era Demons. Cross drilling of air bleed chanels barely intersecting is one that comes to mind. I prefer the older Demon carbs with the screw in sight glasses as opposed to the circlip style. This era always proves to be of better quality overall (also pre-idle eze). When Holley took over and ran out of Barry Grants stock they must have had their own cast. These were not of the same quality and can be easily detected by a less than sharp, defined venturi size number on the main body without the dimple along with Holleys extensive part number laser etched into the air cleaner ring. Bowls similar reduced quality and chromate finish more blingy. I have worked on these bodies fitting threaded boosters and the material is second rate compared to what Empire was using. Metering blocks are machined differently too with different part numbers but essentially same calibration. Holley made a specific primary and secondary metering block with no provisions for pv in the secondary. Holleys base plate is a little better than the BG though.

Anyway, enough from me. Thought I’d share what I have found about these problem child carbs over the years.

[1955V8]

I don't have a flow bench, however what this guy found out, is what I found out,
by trying many different carbs on the same engine. furthermore,
it's all in the measuring. the dimensions of the carb dictate the airflow.

Re: Holley,quickfuel,etc flow numbers
Post by Ken_Parkman » Sun Jul 08, 2012 5:54 pm
As an exercise I've had about 10 carbs rated at 750 cfm of various types from various manufactures across the flow bench. My bench is not big enough to accurately measure a carb, but it can very repeatably tell which carb flows more.
The difference in actual flow on 750 cfm carbs is ASTONISHING!. The variation in flow is huge, about 20%.
Top

Bob Hollinshead
Post by Bob Hollinshead » Tue Jul 10, 2012 7:56 am
Ken, which carbs flowed the best?


Ken_Parkman
Post by Ken_Parkman » Tue Jul 10, 2012 9:14 am
Within a carb style the flows followed common sense, ie if the venturi or bore was larger it flowed better. Also non choke horn versions flowed better for a given set of dimensions. The BG 750 had the largest venturi and no choke horn so it was the highest flowing, with the pro series Holley right behind (tiny bit smaller venturi). I had a large number of different Holley carbs, and the flow ratings mean little. Check the dimensions; for example a 3310 750 cfm carb flows significantly better than a 4779 750 cfm carb, which makes sense when you check the dimensions. The 3310 actually flows better than a 4780 800 cfm carb; they have the same dimensions and the 3310 has a slabbed secondary throttle shaft. The Edelbrock 750 did flow noticably less than the Holley style 750s.

[1955V8]

this is an old topic, but I've run into this Demon nomenclature confusion myself,
when it comes to Demon carbs, look for the stamped mark next to the rear bowl vent tube on top.
what these numbers mean, is venturi size. 28-1.28" 42=1.42" etc.
28=650cfm
40=750cfm
42=825cfm
56=1000cfm
56 w/annulars=1000cfm

if the carb had removeable sleeves, the cfm changed slightly, with more options.
28=675cfm
40=775cfm
42=825cfm
50=975cfm
56=1025cfm
56 w/annulars=820cfm
59=1050cfm
59 w/annulars=950cfm

if the OP had 2 Demon carbs, we'd need to know what the stampings were. if the stampings were not identical, they are different CFM.
Holley used up all the existing BG Demon cores and parts, when they obtained all the Demon equipment, stock from the BG bankruptcy estate. At first they sold off the complete or nearly complete carbs to the general public, then started modifying them, by stamping a punch mark next to the number on top. That would typically mean, a Screamin' Demon or Mighty Demon that may/may not have screw in top air bleeds, or screw in emulsion bleeds in metering blocks. When all the Demon stamped main bodies were used up, Holley started casting and stamping their own with a new system, a long part number along the air cleaner base rail, and the rated CFM.
so- if you have 2 digit numbers stamped next to rear top bowl vent, that's an original BG core.
if there's no numbers, or numbers stamped along the side air cleaner rail base ring, that's a Holley core after the buyout.

FWIW, the best idling carb with clean plug readings, for a street car, is the "40"
as you go higher to the "42" "56" etc. the velocity/signal slows down,
fuel comes out of mixture, idle deteriorates, plugs start turning black
42 and up Demon carbs were really RACE CARBS. idle down to the staging lanes, put a run on it.
if you drive them around on the street in traffic, they will blacken the plugs.
the same goes for any Holley BTW. there's really no need for anything beyond a 3310-1 780cfm on a street car.
going bigger in venturi size, you'll pay for it with black plugs.

Wouldn’t let me edit before or I timed out so new post.

Some good information there. I tinker with these all the time and currently have the ones above highlighted in blue and the sleeves highlighted green but have not used those sleeves. Some things to add for anyone interested. In the Barry Grant days when the Race Demon and Mighty Demon were sold side by side the Mighty Demon had a dimple partially over the second digit on the cast airhorn number that denotes venturi size. A clean, full cast number, ie ‘42’ without a dimple made by the touch of a drill taking out part of the ‘2’ meant it was a Race Demon. This was consistent across venturi sizes. This was a sure bet it had 5 hole threaded emulsion metering blocks (A-12320) and removable threaded boosters. Race Demons were not only removable sleeve (RS) and it’s not uncommon to see old non RS are being sold these days as Mighty Demons by people that are not the original owner and unaware of the idiosyncrasies. The buyer gets a bonus. Race Demons also has a two letter stamp on the other side of the vent tube adjacent to the venturi size cast number. This denoted calibration and examples are ‘GC’ for General Competition, ‘DR’ for Drag Race and ‘TR’ for tunnel ram. All removable sleeve bodies have ‘56’ venturi number cast as there was less machining to create the necessary snug fit for the sleeves. General Competition had pcv, timed and full manifold vacuum ports and all other Race Demons had no vacuum sources. Mighty Demons all had the three vacuum sources. Mighty Demons were swedged in boosters only and received C-12320 metering blocks that had three emulsion holes at the positions of 1-3-5 of the A-12320 block, they are fixed hole non-threaded. They did however have threaded idle feed restrictions in the upper position. This I find is the number 1 reason they were disliked as an overly rich carb no matter the cfm rating. The IFR were sized at .037” and .038” with .070” iab for the 1.75” throttle bore carbs - the 825cfm even had .067”. 750cfm had IFR of .036” and mab of .039” and 650 IFR of .029” and mab of .041”. I always lower the IFR and find between .031” and .033” IFR provide the AFR I’m looking for no matter the cfm rating (650 the exception - .028”). Mighty Demon started as a true in-between the Speed and Race Demon line with their calibration but eventually adopted Race Demon calibration. An 850cfm downleg in the early days had an mab of .028” and IFR of .035” but that grew to .031” mab and .038” IFR. Other cfm sizes similar jumps in dimensions. It would be safe to assume it was to make the Mighty Demon the flagship line to replace and phase out the multi-faceted Race Demon.
Demon Carburetors were cast by Empire Castings and were of a very good quality. It’s only when it came to machining time did the Demon factory make a mess of things in their later years. I’ve seen some horrid quality issues on what would be later Barry Grant era Demons. Cross drilling of air bleed chanels barely intersecting is one that comes to mind. I prefer the older Demon carbs with the screw in sight glasses as opposed to the circlip style. This era always proves to be of better quality overall (also pre-idle eze). When Holley took over and ran out of Barry Grants stock they must have had their own cast. These were not of the same quality and can be easily detected by a less than sharp, defined venturi size number on the main body without the dimple along with Holleys extensive part number laser etched into the air cleaner ring. Bowls similar reduced quality and chromate finish more blingy. I have worked on these bodies fitting threaded boosters and the material is second rate compared to what Empire was using. Metering blocks are machined differently too with different part numbers but essentially same calibration. Holley made a specific primary and secondary metering block with no provisions for pv in the secondary. Holleys base plate is a little better than the BG though.

Anyway, enough from me. Thought I’d share what I have found about these problem child carbs over the years.

great info...never be bashful or timid about posting stuff like this. I look at it this way, we're building an online reference encyclopedia, that'll still be here long after we're gone...that's a good thing. I noticed the 750 Race Demon idles cleanest on my engine, the 825 Race Demon makes more wide open power but idles rich, blackens the plugs. I've tried the G.C. and T.R. models. also the Road Demon, Street Demon- these 2 are not quite good enough for the bigger cam I'm using. If you look at a later Demon pamphlet that came with the carbs new, you'll see a short 2-page rundown of their product offerings. BG rated those carbs by engine size/cam duration. In that respect he was decades ahead of the rest of carb market. Nobody sold carbs based on cam duration to my knowledge prior to that. Now they do, finally. There's an 850 Mighty Demon on the way should arrive tomorrow here, with dimple. It's a BG carb, not the later Holley. My question to you is this- how exactly did the "56" Race Demon manage 1000 cfm flow ? That's 150cfm more than a Holley 850 with the same dimensions. Was the sculptured inlet and improved casting/boosters really worth 150cfm more than a stock Holley 850 DP ? Or did BG change his flow pressure from 1.5"hg/20.4"h2o, up to 28" h2o ? reason I ask, if you do the conversion from 20.4" up to 28" h2o, the flow rating would be around 1000cfm. I have heard BG started flowing his carbs at a different pressure drop, and one of the owners manuals from Demon said why flow heads at 28" and carbs at 20.4" when they should both be flowed at the same pressure drop. but nowhere did it say what he changed the pressure drop to. it did say he wet flowed them at 1./5" HG in his early manual. So what exactly happened there ?

[1980RS]

I'll check your link, thanks.

when Holley took over the Demon line, they dropped the BG flow ratings for the most part.
That is misleading in some cases, because a Holley with choke horn and standard top casting,
is not going to flow anywhere near a Demon with the patented inlet sculpting bowl shape on top, with a bored oversize venturi.
The Demon with no choke horn, larger venturis, and toilet-bowl like entry will flow more.
Holley dropped the ratings back down to match the rest of their other brands.
That's because they didn't want to devalue the rest of their carb lines, Holley, Quikfuel, Brawler, etc.
a BG Race Demon carb on a 750 throttle plate, had the 1.375" venturi opened up to 1.42", will be stamped "42", and had a wet flow rating of 825cfm, logic dictates it won't just still flow 750cfm. It has a sculpted bowl entry, no choke horn, and .045" larger venturis.
75 more cfm from those mods is not an outrageous claim.
according to BG in their old carb manual, all their ratings were done at 1.5" HG, wet flowed, all 4 barrels open at once.
they had a new high end wet-flow bench that BG designed and spent over $250k on, to test all the carbs.
that would be the same as 20.4" H20 rating rating.
some other carb companies were flowing one barrel of the 4bbl carb, dry flow, then multiplying by 4, to get the 4 barrel rating.

You are right on the Demons for sure. The new Demons are just watered down Holley's

[HQM383]

My question to you is this- how exactly did the "56" Race Demon manage 1000 cfm flow ? That's 150cfm more than a Holley 850 with the same dimensions.... So what exactly happened there ?

I believe a carb with 1.56” venturi and 1.75” throttle bore without choke horn flows that 1000cfm+. The Mighty and Speed Demon lines got the 850 name to better target their intended market. I have long given up on what names marketing departments call their carburetors. Take a look at the dimensions of ‘950cfm’ offerings from Quickfuel, Holley HP classic and Holley Street HP (and various others). Airflow cannot be the same. When a carb company bring out a line of calibration for an intended market they have to give each carb size in that line a name that will appeal to the end user, reduce competition with another one of their lines and eliminate confusion. Our hobby is notorious for rejecting products that are too different or not easy to understand. Hypothetically if carb manufacturers were accurate would say an ‘923cfm’ or ‘884cfm’ or ‘1003’cfm carb sell as well as the neat ‘950’ ‘850’ and ‘1000cfm’ offerings. It’s all marketing. Due to that I now ignore cfm names and focus on venturi and throttle bore sizes and take note of venturi to throttle bore percentage. What changes have you made to the carbs that blacken plugs? I wouldn't attempt to use an 825 on the street with the .037" ifr and .067" iab.

This thread covers some Demon flow #’s

viewtopic.php?t=17060

[1955V8]

certainly, I agree...I'm not asking for an exact cfm number to the last digit on a carb.
however, Holley was calling an 830cfm carb, a 950 at one time.
that's quite a stretch and false advertising, misleading. off by 120cfm.
someone should plop an old 1000cfm Demon "56" on a big assed SF1000 flow bench,
lock the throttles open, and flow it at 20.4" h2o, then turn it up to 28" h2o,
and take the readings at both drops. end the mystery.
what did I do ? you mean what DIDN'T I do...lol...
truth be told, I purchased nearly a dozen carbs new/used, returned the defective ones,
tried many of the rest. so far the winner is the 750 Race Demon
quite a few didn't last 10 minutes on the engine, that's how far out to lunch they were.
oddly enough, a close second was the factory GM stamped L88 427 Chevy 850 vacuum secondary carb.
in that batch I also bought new baseplates, main bodies, metering blocks and built 2 carbs from scratch,
then tried them. including 3 different pairs of metering blocks, with 3 different idle feed restrictors.
and tapped quite a few blocks for adjustable screw in idle feed restrictors as well.
after all was said and done, so far none can beat an old box stock Race Demon 750,
that I got on Craigslist for $300/free shipping
I found that anything above a 1.40" venturi size, slows down the velocity at idle,
the fuel begins to fall out of mixture, and the plugs go black.
because then there's rivers of fuel running through the bottom of runners,
with the air above it, going into the head ports- at idle.
going from a Race Demon 750 to a Race Demon 825, is like day and night.
I tried an 825 with a 35 idle feed, still rich. and lean spot. it's a mixture issue from low velocity with a bigger venturi.
the engine loves it at WOT, but at idle it's a rich idling slug.
not matter how you jet it, or what restrictors you put in it.
it needs the smaller diameter venturi to operate as a street driven car.
if it was a towed in race car, it wouldn't matter, the 825 would be better.
or...I could drive it to the track, swap the 825 on in the pits, then swap the 750 back on to drive home.
there's no way around it. it's an old 1960's oem solid cam with lots of seat to seat duration,
with moderate duration at .050", and a nasty idle. sounds great...but those are a bitch to tune.
lots of overlap. later modern cams use much less seat to seat duration, to get the same .050" duration spec.
with only 9.3 compression, and 93 octane fuel, there's only so much you can do there.
beyond changing the cam. that would change everything, easier.
the heads are ported, Victor intake, 4 tube headers.

[Walter R. Malik]

Some are actual air flow numbers with fuel, some are actual air flow numbers dry and some are simply PROJECTED air flow numbers.

[1955V8]

HQM383, check your messages, I sent you one
re: Demon removable sleeves