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Demon Carb

9K views 36 replies 12 participants last post by  cobalt327 
#1 ·
Does anyone have experience with a Demon carb? I am now using an AFB (really, not an edelbrock) and wonder if I can gain anything by going to a similiar CFM Demon. The design with the blended and smooth bores seems like it would in itself make more HP. I have shied away from Holleys due to leakage, but it looks like these other folks have solved that problem. I have always liked the fact that one can change jets and metering rods on an AFB type without loosing fuel and removing the carb. Thanks, Uncle Rich
 
#8 ·
I have not used one of these but if I had to buy a new carb for a mild street car this one would be it. I also love the edelbrock 1406's I've used but this Demon seems to have a more refined design.

I've used holley's too and while they have a LOT more aftermarket support and options they really don't preform as well on a MILD STREET engine. A race engine is a different story.

I never saw the appeal of a quadrajet.
 
#4 ·
The Edelbrock/AFB/AVS carbs are somewhat more daily driver friendly than a Holley-type carb. A QJet really is the best daily driver carb but finding a suitable one and then tuning to the particular engine can be a chore even if you are familiar with them. A Holley-type will almost always make the most power for the least effort and are so simple anyone can get one close to optimum.
 
#7 ·
I have an 850 Mighty Demon on my big block engine. I bought it back in 2005 I think.

Things I had to do to it:

Send the metering blocks back for exchange because the idle screw holes were drilled improperly. ( they were offset, they didn't line-up which caused the idle adjuster needles to break when you screwed them all the way in )
Change the power valve.
Change the jetting. ( It was ridiculously pig-rich. Came with 86pri and 94sec jets. )
Add notched float and jet extenders on the secondary side.

After all that.... it works great now!

lol
 
#17 ·
because the venturi needs air speed to atomize the fuel..
the smaller size= faster air movement..
at the same criuse rpm the square bore carb the bigger size hurts this in 2 ways
1) bigger area
2) less throttle blade opening , thats putting a door in the way of the venturi to get air speed..
the q-jet is in it's "sweet"spot at cruise rpm.. atomizing the fuel better than the square bore carb.. in most sizes..
drop a 390 on an engine and you can get very close.. as the air speed at criuse rpm is nice..
but we live in a world that everyone thinks they drive at 7000 rpm all the time and carb the engine for that.. dropping a 750-850 on everything..
when that tiny square bore 390 will work 100% better in the engines usage 95% of the time.. but will choke the standard 350 up top. where most don't drive very offen..
the q-jet is a 600-800cfm carb that works like a 390 and a 750
it's all about air speed, and atomizing the fuel.. and you need air speed through the carb for that..
better atomized fuel = more power, = less throttle =better mpg
 
#18 ·
OK,lets call the throttle blades doors:
You have a 750 spread bore(yours is a quadrajet)
I have a 750 square bore( mines a carter AFB)
we are both driving a constant speed on the highway: Your door( primaries are 1/2 open) allowing 100 cfm of air to flow enough air/fuel to drive at speed,,,

My My bigger door,( primaries are open 1/3) allowing 100 cfm of air to flow to drive the same speed


How do you get better mileage?

question for gearhead
 
#21 ·
Airspeed does NOT equal better atomization. The idiot that started that myth (a very widespread one on the internet) should be beaten with his own crankshaft.

There are many well designed carbs that can do a very good job of metering and atomizing fuel at low air speeds. And many terrible ones that can't even do a good job with high air speeds.
 
#20 ·
If I had a daily driver that was carbureted I would have a Q-jet. I remember back in the day when I had a 72 Monte Carlo with a 400 ci and topped with a Q-jet that my wife drove all the time. I drove a 73 Monte Carlo with a rebuilt 350 and also had a Q-jet. Both of those cars got around 14-15 mpg. Also my dad had an 83 Oldsmobile that had a Q-jet he had me rebuild. There was nothing wrong with it besides just being dirty. I put the rebuild kit in it with a new float and replaced all the vacuum lines. He also had me put new spark plugs, cap & rotor, and spark plug wires on it. Anyway he took a trip to Portsmouth, Va from Hopewell to visit his sister. When he got back he called me to tell me he got 17 mpg on his trip.
 
#33 ·
Jet sizes can be up sized with a drill bit.There used to be special bits sold for that purpose.trouble with drilling is the machine marks left in the hole were not consistent enough to meter fuel flow,or sometimes the increase was huge in comparison to the size difference due to rifling in the hole.
some people drilled the jets from both ends to prevent rifling.or buy jets that are broached,,,
Thanks tech for the use of your vocabulary
 
#36 ·
Jets varying in flow rate such that jets with two numbers apart may flow the same, ie a 70 jet that flows on the high end of the tolerance may flow the same as a 72 jet that flows on the low side of the tolerance. You can buy sets of flow tested jets but they cost alot more.
 
#37 ·
The flow through a jet is determined by the surface finish, the length of the orifice, the entrance and exit angles and their finish, and the size of the orifice. When a jet is drilled oversize, all of these things are changed except the angles of exit/entrance, but even the lengths of the angles are changed.

The number stamped on a jet (Rochester, Holley, etc.) is an indication of the actual flow, NOT the orifice diameter. The manufacturers flow the jets to see what number they get stamped on them. The same orifice diameter jet may be stamped w/different numbers. This tells you they do not flow the same, even though the orifice diameter is the same. Subtle differences in the above parameters account for the different flows.

The difference between the correct and too rich/lean jet sizes may only be 0.002”-0.004”. There are no readily available drill bits that are that close in size to one another. So at best, the changes made to jets by hand drilling are going to be in steps decided by the availability of drill bits. Because drill bits are often available in 1/64” increments (the small numbered bits- which have a finer increment between bits- are too small for drilling the average carb metering jet), this will be on the order of a 0.0156” change between drill bits- which is HUGE (~40%) change in metering area when in the 0.070” jet orifice range.

It is for these reasons there is no practical way for a person to casually drill a jet and have any way of knowing what the flow is going to be. Drilling out jets is a holdover from the dark days when guys were struggling to make “high performance” engines live, and if they got within 10% of ‘right’ they were at the top of the heap. Nowadays, thankfully, we all (should) know better and leave the drilling of jets and other dubious practices to the desperate, the uneducated, and the poverty-stricken.
 
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