Like the title says, does a spreadbore to squarebore adapter on a dual plane intake hurt economy? I'm trying to sety 78 Fury up with a 6322S Thermoquad and just can't get the mileage dialed in. WOT it pulls like a freight train, no bog, no hesitation. It runs like EFI. I'm running a 9.25:1 360, Magnum heads, XE256 cam, dual plane M1, headers into 2.5" exhaust, A500, 3.92 gears. The carb has 1966 rods, .095" primary jets, and .143" secondary jets. City and hwy mpg isn't great. I've tuned the rods from the point of a lean cruise miss, to about 1 7/8 turn up. Just for testing purposes, I put on a Carter 9635SA AFB, with .098 primary jets and 71/47 rods, and .100" rear jets. The car lost a ton of power for some reason with the AFB, but gave huge gains in mileage.

I don't prefer one over the other in the long run, but if I can't tune the TQ, I'm gonna have to use the AFB. Is the 625 AFB too small for a build like this? It's a very tame engine, and if I can get the AFB to keep its mpg, but pull like the Thermoquad, ill keep it. Any ideas?

in theory,the t/q should show better mpg,and low end power/tq,,from its small primarys,as with the q-jet from gm.but if either carb is not up to specs,or its engine,that will screw with the mpg right quick.have you tried jetting and timing advance inc vacuum advance too.check and make sure the engine is al up to snuff w/ timing ign ect,and try and customize each carb to the engine,w/ jetting,ect.oh the dual exhaust dont help the mpg's also....mike

I'd be interested in your AFB if you end up getting the TQ to work... I still can't get over 14.5 mpg with my old 625 AVS on a 318, and I'm exactly one step above a lean stumble at cruise.

I think, if you measure the primary throttle blades on both carbs., you may be suprised The venturi boosters on the Thermquad should be the limiting factor on the CFM on the primary barrels, not the throotle blade sizes What do you mean when you say you tuned the rods up 1 7/8 from a lean cruise miss? If I'm remembering correctly the Carter 9635SA is a 500 CFM carbs, isn't it?

I mean I've tuned the metering rods on the Thermoquad from all the way down to a lean cruise stumble, to about 1 7/8 turns above that point, in 1/8 turn increments. The 6322s is a 1.5" primary carb off of a 73 440, that shares the same primary and secondary size in addition to identical rod/jet sizes as some of the early 340s and 360s. I don't have a wideband yet, so for "plug tuning", should I be a light brown color after a steady cruise? This TQ just runs so strong, I'd love to make it work. And no, the 9635sa is advertised as a 625cfm

What are you calling "not great MPG"? That statement can vary a lot. Might be normal for all we know.

Well with the Thermoquad, I'm averaging 12/16. With the AFB, I'm at 16/19. I don't mind the AFB. It's great for daily driving and mileage, its just nowhere near the Thermoquad in terms of performance. This is my daily driver, so I'd really like to iron it out

AFB needs a 4-hole spacer for proper operation.

What advantage does the 4-hole have over an open insulator gasket? With the AFB, I run the thick open insulator gasket that Edelbrock recommends. With the Thermoquad, its just a cheap Mr Gasket spreadbore/squarebore adapter

Have you tried limiting when the secondaries kick in with the TQ???

The reason I ask is you indicate great power, with the TQ, and the AFB has a much more limited "over-center" weighted mechanism for its secondaries.

You may just want the secondaries to come in later with the TQ.

Here's a pic of the #1 spark plug after a steady 10 minute cruise at 1600rpm with the Thermoquad. All eight plugs look the same, with just a slight spot of tan on one part of the porcelain, and the rest of the porcelain is white as can be, all the way down. They are the correct Champion RC12YC plugs for the Magnum heads. I know being too lean could hurt economy. Does this plug look like it needs to be fattened up a little bit? I know I need to get a wideband setup for it when funds permit, I'm just trying to get it in the ballpark for now.

Any opinions based on the pic?

An open spacer definitely can hurt performance and economy on a dual plane manifold. On my truck I thought I'd try cutting out the divider on the holley base gasket on my action plus topped 360. I could not figure out why it was saggy on the bottom/midrange and why my gas mileage sucked. I finally bought a four hole spacer with the proper four hole gaskets and the problem was solved. Since then I've worked at a machine shop where we had a dyno and I just happened to be blessed enough to run the dyno. We had some HVH spacers, those CNC'd deals that look like some kind of hill is milled into them. On an open plenum manifold I would run the flat side of the spacer up against the base of the carb as it was designed to be ran. On a split plenum I would flip it over and use a four hole gasket against the manifold with the flat side against the manifold. Upside down in other words. It didn't make more peak hp but on a 480 hp 360 it made 16 more ft lbs in the midrange. I was shocked. Since then I've told people to use four hole spacers on their dual planes with the appropriate gaskets and they couldn't believe the improvement in drivability.

MPG w/ a 440? if that is your goal get a 6-pack. What is your set-up? Most of the time a open hole spacer is a good idea for race applications. You might see an improvement w/ a 4 hole 1/2" spacer, it will help keep the carb cool and give you some low end grunt. Yes you rplug color should be a light tanish brown, best to check them after a had pass, not after a casual putt around town.

440? The question was regarding my Fury, which is running a Magnum head 360. The details are up at the top of this page, in my first post