There's a mechanic around here who thinks the Q-jet is the best carb ever made. He says I should dump my Holley dp and get a junkyard Q-jet for like $100. According to him, Holleys don't get as good gas mileage, and, to some degree, dump raw fuel in, without fully atomizing it, which hurts the engine. He says that a lot more money and research went into developing the Q-jets than the Holleys. He also says that 600cfm won't be enough for a 350, even if it is a double-pumper; but that's an old debate we don't need to get into .

So, is he right about the Q-jet? I'm keeping my Holley either way, but what do you guys think about this?

 

The Q jet is a capable design..it does exactly what it was designed to do well.

That is
A. Meet new government emissions requirments.
B. Work acceptably on a wide range of vehicles/motors with minimum changes to keep manufacturing costs down.
C. Work dependably for as long as possible to keep costs from warranteed repairs to a minimum.

As far as performance, sure they can be made to work. I saw a completely worked one once on a 9 second vette. But thats the only one I've ever seen..and it was sprayed for the bulk of it's power. I've seen a ton of 9 sec plus cars with Holleys....you do the math. Before anyone says it..I've NEVER seen a 9 second car with an Edelcrap


------------------
"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

Ah, but who is concerned with making a 9 second car around here?

Most of us would be very happy, indeed, to be sitting in the 13s! And for that a QJet will work quite well

 

lol..even at just the 13 sec level, even a fully profesionlly prepped (not stock) Q jet will give up some power to even a basic Holley 1850 Vac sec; the bottom of the Holley product line. I've posted dyno tests to prove it


------------------
"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

If you pay more than $25 for a Q-jet that hasn't been freshly rebuilt then you're a sucker... chances are that you'll get more power with the apropriately sized holley, but the Q-jet can be modded to work just as well with better mileage. If you don't like computers running your car though, then don't give the Q-jet a second thought... at least not a Q-jet made after 1981.

------------------
1984 Firebird - Daily driver, 305 LG4, 3.23 rear, T5 - Hurst short shifter, Edelbrock 600 CFM 1406 carb, Performer intake - Edelbrock Pro-Flow air filter, gutted cat, IROC 16x8 5 Spoke Wheels, AIWA bargain basement (from Sears of course) CD player, Eight-ball shift ****
Check out my ride here

"IT'S SNOWING! TO THE HIGH SCHOOL PARKING LOT!" - The Mustang and Firebird face off in the dead of winter.

 

The Q-jet has been used on millions of GM and some Fords since 1966. It is infinately
adjustable with thousands of jet rod hanger combinations. When it is working right it is hard to beat on the street and on track.
There are tons of Stock and Super Stock Eliminator GM's running real fast with then. The factory carbs are generally set up quite lean for emissions and gas mileage.
Therefore people run into trouble when they
try to bolt them onto a performance motor
with a larger than stock cam, heads or intake. With some basic carb knowledge anyone
can bring a Q-jet into tune for there car.
They are actually are quite simple once understood.
Once set up and adjusted for a perormance motor they really scream. With superior cruise and throttle response and equal top end to a holley of the same size.
They come in three airflow ratings 750, 795
and 850 cfm and can be sized to any engine with the air-door I perfer the early pre '74
chevy ones with the angled fuel inlet. and richer idle circuits out of the box.

 

That brings to mind a debate on the subject I've brought up before but never really found a good answer too. I don't think any of those numbers are right. I have a Holley 6210 double pumper siting on my shelf. If you're not familiar with that carb, it was designed to be a bolt on replacment for Qjets. I've never seen a real measurment of the Q jet throttle size, but they look to be about the same as the 6210. They're only rated at 650cfm. Thats on a double pumper where the secondaries actually open the full 90 degrees.

I have a hard time believing that Q jets flow even as much..let alone a minimum 100 cfm more.

------------------
"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

First off, I don't think there's any variance in the CFM of the ECM controlled Q-jets. Second, I've seen it printed about 9 dozen places that there are two different CFM Qjets, but I can't for the LIFE of me remember if it is a bigger primary bore or secondary bore that is responsible for the change. I don't know where that third size came from, but I don't think it exists... the difference between the 750 and the second size was 25 or so cfm (so I suppose the 790 might be right).

At any rate, unless you're concerned about emissions or fuel mileage, why would you want to rip off a perfectly good Holley and replace it with a Qjet anyway? I'm one of the biggest Qjet supporters on this site and even *I* think that's a stupid move. Holleys make more power, period. They make a little more torque, but the difference isn't as dramatic there.

As far as the 750cfm rating vs. the Holley 650cfm rating, I don't know. Did you measure the diameter of all four bores on both carbs and calculate the total cross-sectional area, or are you just eyeballing it Jester?


I paid $25 for two used E4ME Qjets. I happen to like them for street use. They are also required to pass emissions here. You can bet if they weren't I'd be running regular Qjets rather than the electronic ones.

Also, as a last note:

Qjets are extremely messy carbs at WOT. The emulsion tubes on the secondaries throw out a LOT of fuel/air mix. So I wouldn't say they are more efficient at WOT! It's only the primaries that are more efficient due to their tiny size, multiple boosters, and lean fuel mixture settings (remember, Qjets are emissions carbs first).

Honestly, that's how I like my carb--good mileage unless the foot is to the floor. I think it's a decent compromise. But a well tuned Qjet will have trouble beating a well tuned holley on a dedicated race car.



[This message has been edited by 99Hawk120 (edited November 07, 2001).]

 

I can speak from experience. I had a Q-jet with all the trick mods done to it... it worked very well. I got about 25 mpg on the highway and performance was good. I would say it was tuned about as well as one could tune it for my engine.

With that said, I put on a Holley 750vac and the performance improved at the expense of mpg. I will be the first to admit that it is not tuned right for this engine and while I am not sure I can get too much more power out of it, I am sure I can get better gas milage out of it (currently about 18mpg)... thought I'll bet power will increase too.

The cost/effort of swapping may not convince me to do it again... but it shouldn't be a detering factor either. Kind of like changing the shocks on a car that doesn't really need it.

------------------
1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

Just eyeballing, thats my problem. I sold the Q jet I had before I thought to measure it. I can measure the holley anytime, but don't have a Q-jet to stick a mic on.

------------------
"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

built correctly a q-jet is about the best carb you can get, but notice the key word "built correctly" they are a major pain in the *** to work on and and do anything to, if you have them sent out then they can do some amazing things with them, with the tiny primarys they will just sip on gas, then take off when you hit it and the huge secondarys open. theres a guy around here running low 11's with a q-jet. he says he wont run anything else.

 

A 650 dp Holley is enough carb. for most any 350 streetable sbc,it's standard practice to down size when using a dp on a street engine. Having said that,think about what you want from your engine is it all out performance or performace with economy? The Holley DP is basically a race carb. detune a little for the street,forget economy. On the other hand a Q-Jet "properly tuned" can give very good performance and economy. As for the universal Holleys,600/750 they are usally installed right out of the box with no regard for tuning, they are jetted rich for a variety of applications and in my opinion have no performance/ecdonomy edge over a Q-Jet. You can pick Q-Jets for $10 to $25 and tune them with about 3 set of rods/ jets.

 

How about a spread-bore vs. a square-bore carb debate. Square-bore has more equal fuel/air distribution, spread-bore is more efficient at low-rpm through the smaller primaries.

------------------
350 with stealth intake, holley carb, 470 lift cam. 700r4 with .5 boost valve, vette servo, tci lock-up kit, B&M megashifter. Richmond 3.73 gears, powertrax locker, timkin bearings, synthetic lube. Custom 3 inch single into 2 2.5 pipes. 1 1/2 drop springs, 1 5/16 solid front sway bar, 1 inch rear bar, custom subframe connectors, custom LCA relocation brackets. Kobel ground FX, currant red metallic paint. Lots of other stuff...
82camaro

 

I went from a Holley square bore, to a Holley spreadbore, and then back to a square bore. I hated the spreadbore part of it. In theory it provides better response and all that, but it practice I found it gave a very unbalanced feel to acceleration. It felt like even at full throttle I was only at half throttle all through the primaries, then when I diped into the secondaries it would surge forward like I just hit a very small spray shot.



------------------
"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

Just my 0.2 but duh! a qjet will get better milage than any DP tunned properly or what have you. Now as the 600 cfm holleys go nothing will beat thier milage not even a tunned qjet 740+ cfm. Now if we were to compare a 750 cfm holley to a 750 cfm qjet the qjet will make better milage and if tunned properly will have more power than the holley.
I like qjets but only run holleys.
SSC

 

your right about the fuel distribution, the q-jet is more of a economy carb that can be made very well thats why they use the speadbore, the sqaure bore does give better distibution as all 4 ports and equil size and intern get equil amounts to all cyls. the speadbores will tend to give too much in the rear cyls and too little in the front, but this insnt really a probley untill you get into more street/strip race engines. for all out power the sqarebore is the way to go, but for good mileage and some power the q-jet is the one.