I highly doubt the holley can match the q-jet for mileage, due to the efficient metering and booster design.It has been said many times a properly set up Q-jet is the next best thing to fuel injection on the street.

 

carb

I had both and an Edelbrock Carter style on my big block. The Holley was fasted but when the power valve leaked twice over 100 miles from home off it went. Holley states they now sell leak proof power valves. The Q Jet from my small block worked ok until the typical q jet metering well leak started. I put the Edebrock version of the Carter from my small block on it. It is the most reliable. Out of the box the Holley made more power but it was bought for a big block. With work the correct size Q jet or the Carter should perform well. Mileage was within one or two miles per gallon. My friends big block with a small roller cam and early fuel injected heads and Q Jet in a 73 Dually got 14 mpg bringing a race car from Phoenix to Riverside and back. I could only get 10mpg with rectangle port heads and 268 comp cam.

 

The typical Q-Jet well leak is easily remedied with a well known repair. A properly setup Q-Jet will outperform the Spreadbore in just about all regimes, mileage, drive ability and performance.

Vince

 

Will there at least be a noticable increase in milage from my current set up?

If I keep my foot out of it I can average a little over 15mpg right now, That's with more highway miles than in town though.

 

Would these two perform the same fuel mileage wise? I'm going to replace the street-master intake (single plane) and 600 square bore Holley with a 650 Holley spread bore and factory q jet aluminum intake. I have a few Q jets sitting around but I haven't bought a decent manual on how to work on them yet. Although I do like them.

I ran them back to back on my old 454, 4 speed, short bed, 78 Scottsdale. The Q-jet's primaries died and I couldn't get them back so I put the big Holley spread bore on. The noticable difference was it got 9.5 miles to gallon with the Q-jet and out of the box the Holley got 9.

Bogie

 

My buddy (350 sbc) and I (283 sbc) both switched from a well tuned Edelbrock/Carter to a Qjet in our Bel Airs and we both noted a 10% increase in fuel economy and better cold starts (less cranking). The fuel economy improvement was our goal (with gas now at over $6/gallon no wonder?).

We are both getting 22mpg at 60 mph. Both rev at 2200 rpm at that speed, both have very mild cams and dual exhausts (his cast, mine headers, which is probably why mine doesn't get better mileage than his, mine is probably over-scavenging with the stock-like cam). I'm still trying to get the mileage into the 24-25 range, but its a tough chore without fuel injection. Once I get there I intend to post a long in-depth report here.

 

I helped a friend swap to a Q-jet from a fairly new 670 holley with vacuum secondarys on a car with a 350/350th/2.87 gear/210-216 duration cam/9.5:1 cr/double hump heads/performer intake/ hei/2400 stall/3000 lb curb weight/block huggers/single 3 inch exhaust/50 total degrees of ignition (mech + vac).

Mpg went from 8/12 with the holley to 12/18 with the Q-jet with no other changes.

Yes, we tuned the hell out of the holley (jets, power valves, secondary spring, accelerator pumps, float levels, ect). Just couldn't get it to get any MPG. Ran good just bad mpg. Q-jet and holley had the same power.

Results may not be typical, but that is what happened.

I'm swapping my holley for a q-jet as soon as I can.

 

I don't have a fuel flow meter on my boat, but going from Qjet to Holley spreadbore gained me a couple MPH and it sure did seem to suck down the fuel ALOT faster. Usually if I went to the top of the lake and back I could stuff in 4 gallons, now about 6 can fit.

 

I had a Holley 650 Spread-bore on my '72 Nova for a short while back when it was almost new. Wouldn't idle smoothly, had comparatively lousy throttle response, and if you hit the gas from mid-range rpm, it bogged profoundly for 3 to 5 seconds, all the while sounding like somebody lit a whole pack of firecrackers under the air filter. I let a local carb "guru" diddle with it for a while, and he couldn't make it work, either.

Highway gas mileage at the time was a tossup, but around town the Holley performed dismally. I put the original Q-jet back on it and never looked back. In the Holley's defense, it did work somewhat better on a friend's mildly hot-rodded Olds 455, but he didn't leave it on too long, either.

 

The issue here is not whether or not it is a Holley or a Q-Jet or a Carter. It is what is the list number of the carburetor. As an ex Holley guy, I can tell you that Holley made over 250 different spreadbores. Put the wrong carb on and it will run lousy, put the right one on and it will run like a dream. Many Holley spreadbores were specially calibrated for moderate street performance applications like headers, moderate cam, etc. Look up the list number. It is on the driver side of the front vertical surface of the choke tower. It should be something like R-6210;R-9895, etc. Post it and I will tell you what you have and what you need to make it work.

The same applies to all brands of carbs. If you have the right carb, it should work great. I say "should" because just like anything else these things are developed by humans. When I worked at Holley in the '70's, we had over 150 development engineers and technicians...much bigger than Holley today. The point is that a carburetor is only as good as the guy who developed it. We had some great development people and we had some lazy, "waste of time" guys. Just like everything else. Post what you have and maybe we can get you some good info!

 

The 650 spreadbore Holley I had was jumk IMO, it had reverse idle circuits and I couldn't get a good mixture through the idle circuits, wide open it pulled just fine. A friend of mine had an older Holley spreadbore that worked like a dream, bolt on and go with no tunability problems. The Holley economizer 450 was also an awesome carb IMO and fixed many quadrajunk problems for many people-why Holley discontinued them I have no idea, the parts stores sold hundreds of them in this area for years.

 

Thanks for all the replies!

The list number is 8679
I've already bought the Holley trick kit but haven't rebuilt it yet.

Fall classes have started and I haven't had the time.

 

Holley Spread Bore VS Q-Jet

Anyone remember the Thermo-Quad? I thrashed the hell out of a 454 in a 67 Camaro using one of these things and it never missed a beat. I don't know how much extra power it made, but it made lots of noise. In 1972 if it made noise it was fast. :thumbup:

Bob

 

Anyone remember the Thermo-Quad? I thrashed the hell out of a 454 in a 67 Camaro using one of these things and it never missed a beat. I don't know how much extra power it made, but it made lots of noise. In 1972 if it made noise it was fast. :thumbup:

Bob

Whaaaaaat, Thermoquad, now I feel really old. That's like going around Lake Washington on the 405 about 20 years ago and my son starts up; "hey dad, look at that old car". I looked left, I looked right, I looked up and down and didn't see any old cars! So I said "where?" he said, "you're next to it"! I looked beside me at the 57 Ford and felt really ancient.

Bogie

 

Holley Spread Bore VS Q-Jet

My wife reminds me all the time how much of an old f@^t I am. My first car was a 57 2 dr hrd top Belair. I paid $400 for it and it was clean. Anyway I wont reminisce any further.

Bob

 

Thanks for all the replies!

The list number is 8679
I've already bought the Holley trick kit but haven't rebuilt it yet.

Fall classes have started and I haven't had the time.

The 8679 is a good carburetor but needs a little tweaking. I'm pretty sure that carb has a "reverse idle circuit" where the mixture screws in the primary metering block adjust air instead of fuel. They are not as sensitive as fuel adjusters. Turn the mixture screws all the way in until seated and then back them out 1/8 turn and forget about them. Stock primary jetting was 59 and that was for the emissions controls before cats. Jet it up to 63s in the primary and leave the secondary plate stock. Secondary diaphragm spring should be the plain color. Note: you should never be able to see the secondary throttles open under a free rev. You can never get enough airflow through the primaries on a free rev to create enough venturi vacuum to open the secondaries. Don't worry about them! If there is a check ball in the secondary diaphragm lower housing, throw it away. Put a .037 pump shooter in it. If you really want a sweet driver replace the stock #85 power valve with a #125-206 two stage power valve. Put brass floats in each end. When you set the floats invert the bowls and measure the float level between the seam of the float and the top (its on the bottom when the bowls are inverted) of the bowl at the center of the float. Primary float level should be .180" and secondary float level should be .240". It's easiest to measure with a drill bit. If you don't have decimal drills use 3/16 on the primary and just less the 1/4" on the secondary. Clean all parts in mineral spirits, rinse in hot water and blow dry. Squirt some WD 40 on the throttle shafts and you should be fine. The 8679 has a divorced choke so if you need the choke you will have to rig up a hand choke. One last tip, there is a small transfer tube with two o-rings between the metering block and the main body. Be sure to use the smallest o-rings in the kit on this tube. There are often an extra set of o-rings that are slightly bigger in the Holley kits. They look like they go on that tube and that the small ones are too small. Use the small o-rings. If you use the bigger ones, you will chip them installing the tube and you will get an internal fuel leak that you will swear is a blown power valve. The two stage valve is expensive, but it is probably worth at least two MPG over the single stage and you never have to worry about one blowing out! Good luck!

 

Thanks QuietTahoe,

What does the two stage power valve do? I understand what a single stage does never had anyone refer to one. I've rebuilt a few standard Holley's and never had any problems. Will all these mods be acceptable to my engine:

350 with 305 heads, dished pistons, gmpp 330 horse cam, 1 5/8 headers with 2 1/4 true duals with flowcrapper's, HEI super coil and 8.8 wires. Nothing big just a decent daily driver. thanks again for all the input.

 

Thanks QuietTahoe,

What does the two stage power valve do? I understand what a single stage does never had anyone refer to one. I've rebuilt a few standard Holley's and never had any problems. Will all these mods be acceptable to my engine:

350 with 305 heads, dished pistons, gmpp 330 horse cam, 1 5/8 headers with 2 1/4 true duals with flowcrapper's, HEI super coil and 8.8 wires. Nothing big just a decent daily driver. thanks again for all the input.

A single stage power valve is just a vacuum switch that opens two extra jets (called power valve channel restrictions) in parallel with the main jets when the engine is loaded and manifold vacuum drops below a certain point. Lean air/fuel mixtures (15:1)burn relatively slowly. When you are at a cruise this lean mixture is enough to maintain speed. When you need to climb a hill or accelerate and the throttle is opened manifold vacuum drops until the power valve opens(approx.6-8" Hg) and then the mixture shifts to the full-rich WOT mixture of approx.12.5:1 (this is usually 8-10 jet sizes richer than cruise). This works OK, but many times you didn't need a full-rich mixture to accelerate moderately and the extra fuel is just wasted. A two-stage power valve accomodates this. It has a first stage that opens up and provides 3-4 extra jet sizes (providing a ratio of about 13.7:1) at about 10-12" Hg vacuum and a second stage that opens up at 6-8"Hg and provides the remainder of the fuel necessary to shift to 12.5:1 AFR. When you are out on the highway cruising and all you need is a slight increase in power to get up a moderate hill or to accelerate moderately, the two-stage power valve provides the power without wasting fuel. BTY, I just checked my old records. Stay with the 63s in the primary with a single stage PV, but go with 62s if you use a 2-stage PV.

Your combination looks fine. The 305 heads give away some power on top end because of flow limitations but they should give good throttle response and acceleration up to about 45-4800 RPM. That is where you will do most of your driving on the street. If you want larger valves go to 350 heads. Putting 1.94s in 305 heads doesn't give much of an improvement because the valve is so shrouded. For top end power flow and cam timing are most important. Compression helps but you if you have 8.5:1 CR, you can cam around compression to some extent. I have made 1.13hp/cubic inch with 8.5:1 engines with only bolt ons several times and they were all very driveable. Good luck!

 

Finally have spreadbore and intake installed. Seems to be working pretty well.
I don't care for the idle mixture screws (air) it seems to limit adjustability. It actually idles better with them completly screwed in!

Trick kit had dual stage pv in it so I used that and ran the 63 jets. The stock squirter has 25 on it and seems to pump a boat load of fuel whem I burp the throttle. Is the .039 nozzle smaller? Also the squirter seems to seep a little
when I shut the truck off. Does this mean the float level is too high?

I don't like the float arrangement either for that matter. Backed inital timing back off to about 9* and it seems to run pretty good, I have one light and one medium spring the the HEI but haven't installed timing tape yet to check total timing. Yes, I only have a cheap timing light.

any input or ideas would be great

 

get Doug Rowe's book, the Quadrajet!
He's the cat that invented it and GM bought'em out and GM used these on every muscle car they ever built ( don't know about the YANCO's )
I read the book like a bible, and it's dead on....
U'll B a Master on these carbs and luv the hell outta'em when U understand'em
sure their complex but thats the beauty, once U get it, U'll know why!
it''ll show U the dark secrets that unlock this carbs real potential...
like taking any stock Q-Jet and N 5min gain 50cfm.....
I don't want to start a who's got a bigger Johnson contest, :boxing: but these R the best spreadbore ever, period, hands down.....
List all the diff carbs on every car, and all the secondary Rod hangers & Jet Rods so U can grab'em up at the junk yrd. and fine tune the WOT.
How to tweak the Powervalve spring..... gain more juice from the acc.plunger......so easy a caveman can do it!
if U ever bought ur-self a how2 book that will blow U away..... this is it
this book is a treasure-map...just follow the map Bro.
U'll B richer by far.
Edelbrock knows, thats why they sell'em :thumbup: