3X2 intake

[Avanti7]

Hi fellas, I got a question. Has anyone put a 3X2 intake on their Corvette 350 engine that's in an Avanti II? I'm wondering about hood clearance or other problems.

Jim Barker

1970 Avanti II

[Gunslinger]

I've not seen it done but I think there would be clearance issues with the hood unless some creative planning was done with the air filters. Even then, the sloping hood could well hit the front carburetor unless you can come up with an intake that's stepped with the front carbs lowered.

My Corvette has 3-deuces and to accomplish that Chevrolet had to use a sunken intake manifold and a modified hood. Probably a similar thing would be required for an Avanti but maybe it can using thinner motor mounts or even adding extra spacers between the frame and body.

Intakes are probably limited between Edelbrock, Summit Racing's house brand and Offenhouser. Barry Grant used to make what they called their "Six-Shooter" intake and carb combination...there may be some available.

If it can be done, 3-deuces can look stunning on an engine. The potential downsides are cost, keeping the carbs properly synchronized and that they change your induction system from a fuel distribution system to a fuel dumping system if everything isn't just so. Realistically, a properly selected and setup 4-barrel carb/intake/cam combination will outperform the 3-deuce setup with less headache. Chevrolet had their 3-deuce setup in the Corvettes like mine...but when they wanted max performance they went with a 4-barrel carb on the L88.

[PackardV8]

Random thoughts:

1. Chevrolet never offered a 3x2 factory manifold for the small block.

2. All the dyno tests have the more recent 1x4 intakes i.e. Edelbrock Air Gap, et al, outperforming the old-school 3x2 and 2x4 intakes.

3. Many of the best-performing 1x4 intakes don't necessarily fit under the hood of the Avanti. Most are too tall.

jack vines

[Avanti7]

Thanks Gunslinger and PackardV8 for your input. I've been kicking it around inside my head for a while now. I hadn't done any research on the subject

but figured there would be a hood clearance problem. I did know that a well tuned 4 barrel will out perform a 3X2 set up. But that 3X2 set up sure looks cool.

Gunny do you have problems with the 3X2 on your Vette?

My 350 has a 750 cfm Q-Jet on it that the PO put on it. In my opinion the 750 is to big. I think I will get better performance and MPG with a smaller carb. At this point

I don't know what the jet sizes are, but I will look into a smaller size.

What do you guys think?

[Gunslinger]

The 3x2 on my Vette is a factory setup and the previous owner sent all the carbs to Holley for rebuilding. I really haven't had any issues with them but i don't push the car anyway. In fact, I have the car sold...the buyer is supposed to pick the car up after I return home from the trip I'm on. I only drove it twice last year so I decided it's time for someone else to enjoy it. I'm not gonna be one of those guys who doesn't drive a car but refuses to sell it and have the car deteriorate.

That Q-jet is a fine carburetor. Most of them are 750 cfm with some being 795 cfm depending on application. With he small primaries and large secondaries it's fine on a small block Chebbie. They were standard on most Corvettes and other GM cars for years. They do have to be matched and setup for individual applications. A Q-jet was probably standard on your car from the factory.

A Q-jet with its spread-bore design will squeeze every bit of fuel economy your car is capable of, but again that assumes everything is in good working order and matched.

Should you decide to use a different carb such as a square-bore Edelbrock Performer or Holley, you'll have to buy an intake manifold that will accept a square-bore carburetor. If that's what you do, you don't need more than a 600-650 cfm carb unless you have a built engine.

BTW...your car is quite close to my '70...RQB1574. They may have shared space in the factory at the same time.

Edited April 7, 2015 by Gunslinger

[PackardV8]

In my opinion the 750 is to big. I think I will get better performance and MPG with a smaller carb.

Very unlikely to happen. The Quadrajet was the last, best iteration of carburetor design. The Rochester engineers used everything known about combining performance and economy. As mentioned, the small primaries give the best economy and the large secondaries give good performance.

Bottom line - if the Q-Jet is right, changing it will result in lower performance and/or more fuel consumption. That plus the cost of a new intake, carb and lines will sink the equation by several hundred dollars one would never recover.

jack vines

[Avanti7]

Fellas I'm not bad mouthing The Q-Jet. I like em. Iv'e had them on a GTO and Corvettes and once tuned right they preformed well.

This is not the carb that was on the car from the factory. The PO bought this one from Summitt Racing, it's called a "Street Master".

According to the serial number on the carb this one was originally made for a marine application. The 350 was rebuilt and bored

30 over just 10K ago. I've had the car for two years and last summer drove it to a Zone Meet. Tranny is a FMO 3-speed with 3:31

gears in the rear. I averaged 15.5 MPG overall for that trip without the A/C on. Does this seem like reasonable MPG's for what I have?

[Avanti7]

Gunslinger with our cars being so close together, back in 1970 we could have had a "Kodak Moment"

[Gunslinger]

I had a carb rebuilder once tell me that most any carburetor was rebuildable as long as it wasn't physically damaged. True or not I can't say but he was a whiz at carb rebuilding.

Most Q-jets are essentially the same...it's the jetting and other internal pieces that change for specific engines that makes the difference. A guy that understands Q-jets can tailor it to your engine...but he needs to know certain things...cam profile, headers or standard manifolds, etc. Just sticking a Q-jet with a general, all-purpose set of jets, etc., may make the engine run, but not optimally.

By "FMO" do you mean Flite-O-Matic? It should have the BorgWarner PowerShift...similar but not the same. The mpg you have is in the ballpark.

[mfg]

I'm 'on board' with the 3X2 setup..IF it can be squeezed under the stock Avanti hood!

Reasons?....3X2 carb setups have great 'eyeball appeal'....and....Nothing sounds as nice as when the front and rear carbs open up when you 'floor it'.....You just CAN'T beat that 'Howl'!!!!!

[Avanti7]

Gunslinger, by FMO I mean Ford Motor. It is a Borg/Warner case with ford gears. The rear tail seal is larger then the Stude Power Shift and larger

then the GM 350. Mine is a first gear start but shifts almost immediately, between 5 and 7 mph. This also was rebuilt 10K ago. Some day I will put a 700R4

in it. This will help with MPG.

[Gunslinger]

No problem...just wondering what you meant. I have a 700r4in my '70...definitely helps with mpg but you have to get used to the very low first gear and the abrupt 1-2 shift.

[Avanti7]

Gunslinger at what MPH does your 700R4 shift from 1st to 2nd?

[Gunslinger]

I couldn't tell you right now...but it's pretty quick and abrupt. The car hasn't been out since last Fall and the Winter that won't go away has kept it in the garage. My car does have 3.31 gears as well but a different engine...350HO 330hp crate engine with fuel injection.

When I do get the car out for the season I'll take a look at the speedometer when it shifts from 1st to 2nd.

[Devildog]

Gunslinger at what MPH does your 700R4 shift from 1st to 2nd?

I have a 700R4 mated to my 400SBC and it shifts at about 10 MPH. I think you will find the stall speed on the torque converter effects the first shift. As Gunny and others have said the first is a firm shift, but not worrisome. General street driving is comfortable, it does not down shift into 1st, only when brought to full stop.

When I have it out tomorrow, I will try to catch the MPH and RPM's on the first shift.

Joe

[PackardV8]

GM makes about twenty different governors for the TH automatics. They can be changed to control full throttle shift RPMs. The TH700R4 also has adjustable line pressure and throttle valve cable to control part throttle shifts.

Bottom line, they don't all shift out of 1st at the same RPM.

jack vines