Tom, I think you are the only person I've every know to equate a QuadraJet with economy!

As I said, I can make them work pretty well, but for economy you need something else, and as for performance, no one in racing that I know of every used anything but a Holley.

Q-Jets are notorious for big clouds of black smoke on sudden acceleration. They have mechanical secondaries (HUGE ones) with a vacuum operated air valve on top to prevent stalling on sudden throttle opening. They almost always stick shut, causing high manifold vacuum (instead of venturi pressure drop) to be applied for several seconds to the main fuel jets, sucking quite a bit of fuel out of the carb into the manifold with no air. Since the main venturi are about the size of your index finger, this happens quite often, resulting in lousy milage. Remember, the Q-Jet was the Queen of GM fuel systems in the days of 4 mpg land yachts. Ford and Chrylser similar cars got 18....

Q-Jets are also, for some reason I don't understand, usually grossly oversized for the application. 750 cfm rating on a 350 ci engine that tosses a rod at 4500 rpm is just silly, secondary venturi vac will never rise enough for the air valves to open all the way. For road use, vacuum secondaries are MUCH better. In fact, in this applcation where you have a slow speed engine (due to the overdrive tranny), a Holley 2300 series 2-barrel will give better low end performance and even better milage.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Wow! That car looks familiar.

I've the same GM drivetrain in my W108. Around 325hp 350 sbc with a 700R4. I average around 21mpg. It will easily do 135+mph. That was my limit. Looks stock, hubcaps and all...

When are we going to see that Dan?

__________________
With best regards

Al

My FIL's 68 caddy with a 472 and a qjet could get 18 running 65 and 70. I thought that the q jet was known for good economy if you kept the large secondaries closed and ran on the little primaries.

The HOlley car is known for high quality and great adjustablility and is the preferred carb by drag racers for the same reasons.

Of course these opinions are twenty years old since nobody except nascar runs carbs any more.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I'm with Tom on the q-jets. Fantastic carbs IF you set them up properly. They wouldn't flow the CFM's like the Holley's so everybody turning any kind of rpm's dumped them.

For the stock set-up they were fantastic if you stayed out of those back two barrels. But hey, what's a four barrel for if you don't use all of them?

Q-jets will flow 750-800cfm. More than any sbc street motor ever needs.
They are an excellent "one size fits all" carb if set up correctly.
Extremely versitile to just about any engine from 250 to 455 cu in.

Al, I still have the AC out of the W108. One of many projects I need to get back to.

Here ya' go...

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I'll just chime in and say I've got a q-jet working perfectly on my 1973 280 (w114)... starts instantly, runs very very smooth. I get 16 mpg, not bad at all. Oh, and

Dan, are you going to the GTG in Oct? Let me know if you need some R12 for the A/C system.

__________________
With best regards

Al

I'm planning on it...

Might have to take you up on the R12 offer. At the moment I'm putting the '66 2 ton chevy dump truck back together. It might still be warm enough to inspire me to get back to the 108's AC when I'm done. Otherwise, It can wait until next spring.
I have the W116 front end rebuild and seats to do. Then finish painting the 123.
Somewhere in between I have Tab's 914 to put back together as well as my 914GT V8 project. Bad case of 'ADD' got me in this mess. That and JimmyL sending me CL links. He's like the MB "pusher".

I hope I have time for the GTG.

That sounds exactly like my weekends. So many unfinished projects that nothing ever looks like it's getting done.

Right now as the weather is cooling down, I want to focus on getting the engine back together on the MGB and making it drivable. I also bought another gas tank for the W108 that needs to go on as the other one got bent in the rear end accident and then there is the 59 Ford Pickup.

__________________
With best regards

Al

I had a q jet on my bmw 3.0 bavaria. Using the metering rods from the pontiac sprint of '67 ( a six cylinder) the 183 ci bimmer engine ran like a scalded cat as long as you didn't push open the secondaries below 2500 rpm.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Tom, That was an easy cure. The Q-jet is a "variable CFM carb" supplying as much air/fuel as the engine requires when set up properly.

The secondary airvalve on top of the carb has a windup spring holding it shut. You can wind up (or unwind) the spring as needed on pretty much any engine. As the engine rpm rises the SAV the is pulled open as needed. A cam located between the SAV lifts the secondary metering rods from the jets enrichening the mixture.

Most 'tuners' set the spring to soft giving the characteristic Q-jet pause when the secondary opens. Set properly, it can be smooth.