Dad's Carb Choice
 

I'm thinking ahead to what carb I'll use on the new "stroked 351M, which will be a 408 with a roller cam, Trick Flow heads, Weiand intake, and Tim's pistons which will yield 10.5:1. So, I'd like to have a discussion regarding carbs.

However, I don't want to have a wide-open discussion as I'm only interested in Carter or Rochester-based carbs. IOW, that would mean mean something like an AFB or Edelbrock Performer, AVS or Edelbrock Thunder, QuadraJet, or AVS. I'm open to a side discussion of why I've already settled on carbs like these, but I'd rather not in this thread or we will get sidetracked.

My goals are set-and-forget reliability and excellent drivability and economy. Note that I didn't say horsepower, as I'm building the engine for torque not RPM. Also, I want an electric choke as the intake won't have a hot-air well. And toward those goals, here are my thoughts on several carbs, but I'm not saying these things as done deals but to start discussions:

• AFB/Performer: I like these carbs, but they only come in 500, 600, 750, and 800 CFM sizes. Tim felt that 650 is a bit small in the Thunder series, so I can't see a 600 being big enough. And, a 750 is too large from an economy standpoint.
• AVS/Thunder: This is the carb that Tim recommended - specifically the 650 CFM 1806, which is the one he said he uses on most engines. I really like and understand these carbs as they are basically an AFB/Performer with the adjustable air valve.
• QuadraJet: This is actually my favorite carb as I've tuned lots of them and do understand how to make them work. But, they require an adaptor to fit on a square-bore manifold and most don't have an electric choke.
• Thermoquad: These carbs have always had an appeal to me as they are odd balls. But, they should be really good carbs - even though they were bad-mouthed extensively in the 70's and 80's. However, they also take an adaptor to fit on the square-bore manifold. And, I'm not sure about e-chokes.
• Street Demon: Then there's the new Street Demon, which has the small primaries/large secondaries of the spread-bore carbs, but fits a square-bore manifold. And, has an e-choke. It comes in 625 and 750 CFM, but since it has the small primaries even the 750 should give good economy. It appears to be a cross between a ThermoQuad and AVS. And, it looks cool!
  
  http://www.demoncarbs.com/Assets/ima...medium1901.png

So, what do y'all think?

Gary, as far as bolt patterns, Q-Jet bolts space the same as the inner AFB bolts, TQ space the same as the outer AFB and Holley pattern. As long as the plenum will accept the throttle openings (most aftermarket manifolds are clearanced for Q-Jtet/Spreadbore and TQ secondaries) there shouldn't be a problem.

FWIW, later Q-Jets had electric chokes. You being a connoisseur of Rochesters should remember the nomenclature, 4Mx = Quadrajet, 4M being manual choke, 4MC hot air choke, 4ME electric choke and 4MV vacuum break divorced choke. Later Oldsmobile engines used a hot air choke along with Chevy. E4ME is feedback electric choke. As long as a Q-Jet has a vacuum source near the choke, you can stick a hot air choke on, without it you can still use an electric one.

The Thermoquad is built in two main variants, solid fuel and air bled. Only the early aftermarket models are solid fuel which may be what the Street Demon is copied from. All the OEM versions are air bled which emulsifies the fuel better.

Bill - I guess I'm really confused. I think what you said about patterns means a TQ will bolt on using the same bolt holes as the manifolds provide, which takes a Holley - right? And, I'm apparently using the outer holes on the AFB. But, if the QJ takes the inner AFB holes do I just drill and tap the manifold?

I'd forgotten the nomenclature on QJ's, but now that you mention it I do remember that. So, I'd be looking for a 4ME. But, you said "without it you can still use an electric one" - does that mean I can graft on an electric choke to a 4M or 4MC? And, don't I want electric instead of hot air?

Read it again, it has to do with a vacuum source to pull the hot air through the choke housing. Once I get back over to Newport News, I may have some Q-Jet pieces and I can annotate some pictures for you.

I think I gave my quartered QJ to my nephew. I'll see if he still has it. But back in the 80's I cut one up to see where all the passages went.

So, are you thinking I could use hot air off the exhaust, just like for the air cleaner?

No, not off the outside of the exhaust. A lot of aftermarket intakes have the provision for the factory choke stove in the crossover. That's where Ford put them on the 335 and 385 series engines. Windsor and FE series have them in the exhaust manifolds, Y blocks did both depending on when they were built.

But the aluminum heads don't have the crossover. So, that's why I've been thinking electric choke.

Gary,
I don't know a thing about these carburetors and don't want to side track this conversation, but I wonder why you would shy away from a larger carburetor for the sake of "economy"?

The engine is only a positive displacement pump, right?
It will always require X cfm of properly metered A/F to produce X horsepower.
So this would simply mean less throttle opening.

Are the primary bores that much bigger in a 750 vs 650 carb that vacuum signal is compromised?

Gary, talk about building a cold natured beast, have you ever driven a vehicle with no manifold heat in cold weather? You can do it with port injection, but carburetors and TBI systems need the heat to keep gas droplets from condensing on the runner walls. Minimum you might want to do what the old MEL and HD truck engines did, run coolant through the manifold crossover.

My understanding is that, within reason and comparing the same style boosters, the smaller the better the vacuum "signal". And, just like with electronics, if you have a better input signal you have an easier time determining what to do as a result. And that means you can tune more precisely. IOW, all else being equal, a smaller Venturi means better economy. Which was the reason for being for the spread-bore carbs, such as the QJet and TQ - tiny primaries to provide good economy but large secondaries for power when you need it.

However, all things are not equal between the carbs I've listed. There was technological progression in the booster area in the 20 years or so between the design of the AFB and the QJet or TQ. And then there's the Speed Demon, which was "designed" much more recently and has what they call the Triple-Stack booster.

The 1 7/16" primaries of the Edelbrock 1806 650 CFM carb's primaries aren't much different than the 1 3/8" primaries of the 625 CFM Street Demon. But, the Eddy has 1 3/4" secondaries, so a rough guess is that the 650 CFM would be split 300/350. However, the web site says the Street Demon's secondaries provide "over twice the CFM of the primaries". So my math says that carb'd flow splits something like 200/425. So, the Triple-Stack boosters apparently restrict the flow - and must provide a good vacuum signal.

Gary, you are correct, good example, Ford wanted a smaller more responsive primary system on the 428 CJ. Holley took their basic 780 CFM R-3310 and used "umbrella" boosters in the primaries. This dropped the flow from 390 CFM down to 345 CFM giving a total of 735 CFM and real good throttle response.

Q-Jets are nominally rated at 750 CFM, but the primaries only flow 260 CFM giving them a 260/425 split. Some very late performance Q-Jets, notably the Pontiac SD 455, did away with part of the booster stack to raise the CFM to around 800.

My Holley has annular boosters purported to increase signal.


So I guess you are saying the 750 AFB primaries are too large, and the 650 carb with its 1 7/16" primaries will allow it to meter fuel more precisely?

Will it help that much if all that fuel ends up puddling in the intake manifold?

Jim - You are hinting that w/o crossover heat there will be little atomization of the fuel.

Bill is saying it will be a cold-blooded beast.

So, you rascals are saying that having gotten down this road thus far the aluminum heads are the wrong choice? Didn't we have this discussion before? Wasn't it said that the aluminum head will conduct heat rapidly to the intake and obviate the need for the crossover?

I'm out and about so can't easily search, but I tend to remember asking something like that some time ago. But, I've not bought the heads yet so now is the time to sort this. Please tell me what you are thinking as I value your input.

No, the carb will atomize just fine. :)
Bill said that the atomized fuel will condense on the cold intake tract.

Don't hesitate with the TFS heads.
I have no idea how you would run coolant through the manifold but would assume it would have no effect until the engine warmed up anyhow.

You chose the operating parameters and goals for your engine.
Compromises will (and must) be made. }>
Maybe you can stuff an electric resistance heater into the intake crossover before you install the manifold?

Gary, I have no idea who told you that piece of information, but the cold winter air coming in will more than cancel the heat in the intake until everything is real hot. For a performance engine, not a truck, you could probably live with it. I don't ever remember any discussion I participated in regarding not using a heated intake, but if I did, I would have suggested running coolant through it then.