ez2cdave

I have some questions about the Quadrajet. My application is a street-driven 383 or 400 with 5.7" rods. DCR will be 8:0 - 8.25.

I would like to know at what point a 750 cfm Q-Jet becomes detrimental to MPG when driving at a steady 70-80 MPH on the freeway, due to the Secondaries "tipping in". caused by engine demand.

At that "tipping in" point, would the use of a larger Q-Jet restore highway MPG, since the CFM flow would be higher and the engine wouldn't try to draw on the Secondaries, killing MPG ?

Obviously, Gear Ratio and Tire Size comes into play and I'd like to try to figure out the "tipping in" point for 3.08 / 3.42 / 3.54 / 3.73 gears, both in 1:1 and .70:1 (OD) . . . Tire Diameter is 26.6" +/- .

Mirror Image

iTrader: (2)

I have a 377 sbs with 342 gears and a 700r4 with a 725 qjet and I get around 18-20 miles per gallon

ez2cdave

Is that driving on the freeway at a steady 70-80 mph ?

Tire diameter and cruising rpm at highway speeds?

What's the maximum MPH you can achieve, without the Secondaries activating ?

Thanks !

Mirror Image

iTrader: (2)

That is at 70 mph and my tire is 25.5 tall and I I'm at 1800 rpm at 70

ez2cdave

Excellent data . . . Thank you !

sofakingdom

iTrader: (1)

The Q-Jet will support a 500 CID motor (Cadillac) on the primaries at that speed; shouldn't be a problem for a small block. The one on my 454 (turbo 400, 5000 lb car, 3.42 gears, maybe 2800 @ 65 but I don't know for sure) doesn't have any problem with running out of primaries under cruise conditions.

novaderrik

every qjet except for the ones on the 500 Caddies and one of the BOP 455 variants (forget which one.. thinking the stage 1 Buick..) was rated at 750.. the bigger ones were rated at 800.. Edelbrock had an 850 cfm one, but that might have been rated differently since aftermarket carbs were rated by a different standard than the oem installed ones...

anyways, as already stated, you are going to have to move a lot of air at part throttle to get the secondaries to tip in at highway speeds- which isn't going to happen in a 3300 pound Camaro with a small block.. either that or you have the air door spring tension set way too loose..

ez2cdave

Very interesting . . .

Given the information posted by "Mirror Image", does it sound like that is the case, in his setup?

He indicated 70 mph maximum in OD ( .70:1 ? ) at 1800 rpm with a 25.5" tire, before the secondaries tip-in, on a 377 SBC.

Thanks for your input !

ez2cdave

I'm trying to maximize mpg on a mild 383, byr using a large enough Q-Jet where the primaries are large enough to allow high-speed highway cruising (75-80 mph) without activating the secondaries.

novaderrik

if your secondaries are kicking it while cruising, you've got a problem... the throttle linkage shouldn't even be moved open far enough to mechanically open the butterflies, let alone the air door that actually meters the air and fuel on the secondary side...


what makes you believe that your secondaries are opening?

call Edelbrock... tell them that you just picked up a used 1910 quadrajet and you'd like a manual for it.. the book that came with my 1910 is a pretty damn good manual that explains how the different parts of a carburetor work, how to tune them for power, throttle response, and economy, and how to adjust other stuff like the ignition system to work together as a system...

ez2cdave

Was that intended for me or "Mirror Image" ?

My questions are based on the desire to prevent the secondaries from tipping in, while cruising at highway speeds of 70-80 mph with up to 3.73 gears and 26.5" tires. The goal is to maintain mpg.

Ozz1967

iTrader: (4)

I have 26.1" tall tires (235-50-15) which is a stock tire size for 84 T/A. I have a 700R4 with 3.73 gears with my Qjet and I run 2600 RPM at 70mph. I run 18-19mpg even at 75-80mph on my trip through Germany on the Autobahn.

ez2cdave

What other modifications does your car have? Engine? Cam? Headers?

Thanks!

GICATA

It's not going to happen. The secondary doors will not just fall open, he will have to push the pedal a long way down to get the carb that far open. Not going to happen just rolling down the expressway. A 750 version will work just fine as long as it is jetted correctly (jets, rods float height, air valve tension, etc).

ez2cdave

OK ... Thanks !

I haven't driven a carbureted car since 1987 ( that was a Holley 750 DP on a 454 Chevelle ) and the last Quadrajet car I drove was a 1979 T/A 6.6L (403) in 1984 ...

I may be "overthinking" things, but I want to "get it right" !

Ozz1967

iTrader: (4)

I've replaced the 305 with a 350 bored .040, world Sportsman II heads, Edelbrock Performer intake, Lunati Voodoo 60121 cam, Speed Pro pistons and Scat Rotating assembly. I also have Hooker 2055 headers and a 3" catback with a hooker Aerochamber muffler.

The 3.73 gears are factory stock as is the 700R4 but I've upgraded to Moser 28 spline axles, poly bushings, sub frame connectors, wonder bar and new sway bars, an Aluminum drive shaft and a Zexel Torsen differential.

The dyno reading from before I tuned the carb said 240 rear wheel horsepower, so I'm sitting right about 300base horse power if you figure 15-20% drive-line loss.

novaderrik

it was a general statement- you are only going to have the throttle open far enough to open the big butterflies in the secondaries when you are playing- you need to get past about 1/2 throttle before the butterflies even start to move, and then the air door will only open and let air and fuel flow into the secondaries if the engine is moving enough air to demand it...

like i said- get the Edelbrock tuning manual for a quadrajet... read it... study it... apply it..

my real world experience with an Edelbrock 1910 quadrajet on a my 71 Nova with a vortec/HOT cammed 355, T10 4 speed (no overdrive...), 3.70 gears, and 245/50/16 tires resulted in 17mpg in regular daily driving and 19mpg when i would jump on the interstate and cruise along at 75mph for an hour and a half at a time.. the secondaries were most definitely not opening under any cruise situation and in fact i had to slow the air door opening way down from the factory settings to get it to not bog when i went to WFO- i had people that rode with me that didn't believe that i had a quadrajet because it didn't do the "quadrabog" thing when you hit the pedal..

ez2cdave

Is this the manual ?

http://www.florida4x4.com/tech/quadr...nersManual.pdf

http://www.florida4x4.com/tech/quadr...910%20Inst.pdf

ez2cdave

I found a link to a GM Service Manual for Quadrajet carbs, too !

http://www.tocmp.com/manuals/Carbs/R...20complete.pdf

novaderrik

that looks to be it... that should go a long ways towards figuring out the quadrajet, but i'm old school and i prefer good old fashioned paper books for stuff like this...

ez2cdave

Me, too . . . I'm 52 years young !

I printed it out . . .

STREETDEMON

as stated above, it is a moot point.

you're really going to have to push your car on the interstate to open up the secondaries. if the top door is opening at cruise speed/light throttle, you need to tighten your secondary spring up!

most will be the 750cfm as stated, gm did put 800cfm carbs on a few of the big blocks. keep in mind the 750 will feed a 454.

if you want a good q-jet book, check out cliff ruggles. people who badmouth the q-jet haven't taken the time to understand them. it's as close to fuel injection as you can get with a carb.

ez2cdave

Thanks for that info . . . !!

ez2cdave

The "QUADRAJET SOUND" . . . I forgot how much I LOVED it !!!

STREETDEMON

no worries....

on a side note, you can also send your q-jet in to cliff ruggles or smi and have it custom built to your application.

ez2cdave

Really ? Do you have a link ?

Thanks !

STREETDEMON

http://www.smicarburetor.com/

http://www.cliffshighperformance.com/

ez2cdave

Great . . . Thanks !