Hot Rod Forum banner
1 - 20 of 37 Posts

·
My fourth Firebird
Joined
·
24 Posts
Discussion Starter · #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
 

·
Registered
Joined
·
4,397 Posts
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.
 

·
Bye Jerks
Joined
·
101 Posts
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
 

·
Banned
Joined
·
9,993 Posts
Are you referring to the new "Street Demon" 625 carb? That's the only Demon that uses metering rods. The rest are much like a Holley.

Here's a quick review I did on the 625-- http://www.hotrodders.com/forum/new-demon-street-demon-625cfm-carb-223299.html
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.
 

·
My fourth Firebird
Joined
·
24 Posts
Discussion Starter · #11 ·
Demon carb

Actually, I was looking at a Speed Demon 650 cfm electric choke. I was hoping to improve the performance of my 400 Pontiac over what the AFB provides.
 

·
Grumpy Old Goat Herder
Joined
·
1,397 Posts
I, like gearheadslife, think that the Quadrajet is the best street carb ever built. Because it has an air valve controlled secondary, you can't be over carburated. Once you get it tuned, it will do an excellent job!

Bill

 

·
Banned
Joined
·
9,993 Posts
I, like gearheadslife, think that the Quadrajet is the best street carb ever built. Because it has an air valve controlled secondary, you can't be over carburated. Once you get it tuned, it will do an excellent job!

Bill

as does every carb with a vacuum secondary... the edelbrocks and the new demon 625 has secondaries that work off of airspeed instead of vacuum signal (like a holley), but either can be tuned to deliver the same results.
 

·
Banned
Joined
·
2,474 Posts
as does every carb with a vacuum secondary... the edelbrocks and the new demon 625 has secondaries that work off of airspeed instead of vacuum signal (like a holley), but either can be tuned to deliver the same results.
the q-jet small primaries allows better mpg and cruise than you'll EVER
get from a square bore carb
 

·
Banned
Joined
·
2,474 Posts
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
 

·
Registered
Joined
·
12,338 Posts
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
 

·
Registered
Joined
·
1,684 Posts
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.
 
1 - 20 of 37 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top