Hi, I just got a new 350 engine, 330 HP and will be installing it soon. I didn't want to use to old Q-jet from the old engine and was wondering if a 750 cfm Q-jet would be too big?

I think a 650 would be about optimum for that engine, unless you change the cam and plan on racing it.

A 750 will give you better high rpm performance, but your car will suffer at the low end with stumbling on takeoff and somewhat less torque. Maybe not a lot, but I think a 600-650 cfm carb will give you better performance overall with a better idle.

Thanks Gunslinger, I guess the 750 would be better for a automatic trans, compared to a 4 speed.

You need to SHRINK your picture,WOW

Come on Ron! Maybe he needs the bigger carb to fit the pic!! (I sent him a P/M on the pic)

I would stay with the same size carb, unless you are planning on modifying the engine extensivly.

And the original q-jet can work very well for street applications.   Just have a good rebuilder do the job.  You will be happy with it.

As long as we're talking carbs, how 'bout some advise for a 390HP 427 in a '68 Convertible?  I bought it a year & a half ago & it has what apprears to be a Barry Grant "Speed Demon" 650 CFM carb.  I only get 5.8 miles per gallon & the spark plugs on inspection are nice & powdery tan with NO black soot like the fuel isn't burning.  Is the carb the reason for the 5.8 MPG?  If I could find the 7028209 Quadrajet that BELONGS on this car (per the NCRS judging manual) would my MPG improve?  To WHAT??  I'm gradually restoring this vehicle to judging quality since it's truly numbers-matching & in GREAT shape.  ANYTHING I can do in the meantime to improve my MPG??

Weinie...

If you found the correct Q-jet, your mpg MIGHT improve...primarily because the Q-jet's spread bore design is more conducive to better economy than a square bore carb. Do you have a different than stock cam in your car? What's the rear axle ratio? They likely have a bigger effect on fuel economy than simply changing the carburetor.

I have a '69 427/435 and my fuel economy is about the same as yours. The previous owner had the engine professionally rebuilt...bored .030" over, had the three Holley carbs rebuilt by Holley, and what most likely had the biggest effect on how the car runs is an L88 camshaft was installed. Very little engine vacuum because of that. The specs on the cam show its power band is from 4400-7000 rpms...higher than I drive the car at. Combine those specs with the car's factory 4.11 rear axle ratio and you have a car that can pull tree stumps and pass anything but a gas station.

I considered installing a 5-speed transmission, but ultimately didn't. I did install an MSD 6A capactive discharge ignition...hid it behind the passenger side of the dash where it can't be seen. It helped a bit with fuel mileage, but not much.

If you want to install a correct Q-jet, feel free to do so to make your car correct, but don't expect a big difference in fuel economy. These cars weren't designed for fuel economy to begin with.   There are incremental things you can do to help...make sure you have no vaccum leaks, properly inflated tires, don't lead foot it, etc.

Short of a complete engine overhaul optimized for economy rather than performance, about the only subtantive things you can do is change the rear end in the car and/or install a 5- or 6-speed transmission.

Gunslinger: Thanks for the response.  There are no vacuum leaks.  I don't know what's inside the engine (I bought the car a year & a half ago) but the idle on this car is too smooth for it to have a radical cam.  I too, have the MSD 6AL ignition box with a high energy coil.  The rear axle is 3:36 Posi-traction which isn't TOO high.  Finally, KEEP MY FOOT OUT OF IT?  WHY??  Seriously, my local Corvette Club did a road tour a month ago and the mileage probably only jumped to 8 or 9... 

I understand exactly what you're saying...it's a Corvette! Why would you WANT to keep your foot out of it?

With your rear axle ratio, your mileage, at least to my way of thinking, should be better.

To be honest...8 or 9 mpg is probably about the best you can hope for short of changing the transmission or rear axle with any kind of sporting driving habits. It comes down to...if you want an econobox, buy one and drive it.    The Corvette is what it is...a balls to the wall performance car. The 427 is and always was a performance engine, pure and simple...not a sedan/station wagon engine, not a truck engine, not a motorhome engine.