[vinniekq2]

gearhead,please explain how small primaries gets better mileage? The more details the better? Thanks

[gearheadslife]

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

[vinniekq2]

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

[gearheadslife]

air speed through the venturi = better fuel atomization..
less fuel needed for same power/rpm as it burns better(completely)

[cdminter59]

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.

[ap72]

Originally Posted by gearheadslife

air speed through the venturi = better fuel atomization..
less fuel needed for same power/rpm as it burns better(completely)

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.

[cobalt327]

Originally Posted by ap72

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.

Not all Edelbrock squarebores have an adjustable secondary, just the Thunder series. Unless you call grinding/drilling or adding weight "adjustable".

Has anyone actually seen the new 625 Demon in use??

[gearheadslife]

Originally Posted by ap72

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.

wow..

guess you know more than g.m/ford/mopar/toyota

[ap72]

Originally Posted by gearheadslife

wow..

guess you know more than g.m/ford/mopar/toyota

I'll admit I'm baffled by your ignorance. You win.

[ap72]

Originally Posted by cobalt327

Not all Edelbrock squarebores have an adjustable secondary, just the Thunder series. Unless you call grinding/drilling or adding weight "adjustable".

Has anyone actually seen the new 625 Demon in use??

There was a link to a first hand account of it in use.

And I would call that adjustable considering it takes all of about 10 minutes to pull it out for "adjustment". Air bleeds and jets on almost all carbs are adjusted in a similar manner- by drilling a bigger hole or filling it in.

But you're correct the AVS series is easier than the AFB.

[gearheadslife]

when you can get 19mpg out of a olds350 in a 4300 lb tank on hyway trips with anything .
tell me..
we got 19mpg every time going from ma to fla. 2000+ miles each way..
mighty q-jet.. you won't even get close with a square bore carb..

[cobalt327]

Originally Posted by ap72

There was a link to a first hand account of it in use.

And I would call that adjustable considering it takes all of about 10 minutes to pull it out for "adjustment". Air bleeds and jets on almost all carbs are adjusted in a similar manner- by drilling a bigger hole or filling it in.

But you're correct the AVS series is easier than the AFB.

Jets are most certainly NOT "adjusted" by drilling a bigger hole in them. Unless you live in east bum**** or are a hack. Drilling a jet can reduce flow. Not to mention holes are easy to make bigger but not so easy to make smaller.

The way a Performer secondary (or an air bleed or a metering jet) is adjusted is you go quicker (remove weight or bigger hole- easy) until it is too quick, then you back up. Adding weight back/filling in holes is NOT easy. Adjusting the secondary is easy w/a Q-jet. Not easy w/a Performer. And to say it IS easy is just to argue for argument's sake.

[ap72]

Originally Posted by cobalt327

Jets are most certainly NOT "adjusted" by drilling a bigger hole in them. Unless you live in east bum**** or are a hack. Drilling a jet can reduce flow. Not to mention holes are easy to make bigger but not so easy to make smaller.

The way a Performer secondary (or an air bleed or a metering jet) is adjusted is you go quicker (remove weight or bigger hole- easy) until it is too quick, then you back up. Adding weight back/filling in holes is NOT easy. Adjusting the secondary is easy w/a Q-jet. Not easy w/a Performer. And to say it IS easy is just to argue for argument's sake.

how do you think those holes get in the jets that you buy out of a catalog? And I've been drilling jets since I built my first engine- often the local parts house doesn't have the jet I want so you get the next smallest one they do carry and bust out your drill bits. MOST carb shops drill their own air bleeds and those are even smaller holes in smaller brass inserts.

This **** really isn't rocket science.

and adding weight is not that hard with some epoxy and scrap lead, just like in the pinewood derby days.

I know a LOT of people prefer to buy their parts out of a catalog and pay someone else to do the work, and there's nothing wrong with that, I buy parts too. But there's also nothing wrong with fixing your own stuff.

[Motochris]

Originally Posted by gearheadslife

when you can get 19mpg out of a olds350 in a 4300 lb tank on hyway trips with anything .
tell me..
we got 19mpg every time going from ma to fla. 2000+ miles each way..
mighty q-jet.. you won't even get close with a square bore carb..

I'm getting 18mpg on the hwy in a 4500+lb car with the new Demon 625.

I think the carb is an ideal mix and super easy to tune. Adjusting the secondary is via screwdriver. It's basically spring preload.

[Motochris]

Originally Posted by cobalt327

Not all Edelbrock squarebores have an adjustable secondary, just the Thunder series. Unless you call grinding/drilling or adding weight "adjustable".

Has anyone actually seen the new 625 Demon in use??

Link-- New Demon "Street Demon" 625cfm carb

I'm by no means a carb guru, but I've had and tuned a few different brands. Up until this carb, I've always had the best overall street manners/performance blend from the Edelbrock carbs. I'm digging this one...