650dp or 750dp on 347?

[brianj]

I would be curious to see what i have for vacuum under the carb at full tilt. The same rules should apply, actually.

[xctasy]

Work, work work the throttle body...don't slow down ya gonna hurt somebody.....

LMFAO @ the QFT MALfunction... gee, what a surprise. All moot... would I suggest a vacuum secondary in most all cases? Yes, I would, because they can be dialed in to administer exactly what the engine wants. The bottom line is this: drive-ability and cruise efficiency is completely about the idle/transition circuit... and so, double pumper or scary named newer avenger or slayer BS or what have you, dialing in the primary idle feed restrictions and idle air bleeds (and maybe adding transfer slot restrictions if the camshaft is ridiculous radical) is where perfect function is at, NO MATTER. Besides giant Dominators, there is NO SUCH THING as too much carburetor... the only danger there is is making too much torque and horsepower, AND average torque, area under the curve. Anybody needing help in this department, don't hesitate to PM me.

Sometimes... QFT base plates have far too wide or long transfer slots... transfer slot restrictions installed is a CURE.

I like Mikes stuff. Thinking again.

Persoanlly, I believe in mechanical secondary Thermoquads, and 4M Quadrajets. My experience with Vac Secs hasn't been as good as others, but I did sit as one of six people in a 3500 pound THM 350/350 powered 63 Chev at 110 mph up a 8% gradient in 1995...under the bidge in my home town .



One simple 780 cfm VS Holley 4bbl carb really can erupt into 8 cylinder symphony. It dynoed at just 265 hp at the rear wheels.



In the Group A 100% production based regulations in Australian and New Zealand for 1985, the only Group A XE Falcon V8 and all the Group A carb 4-bbls Commodres won races with Mec Sec Spredbore type carbs. The Holden womn international 24 hour races with a stock emissions era Quadrajet, the 1971 429 CJ carb really.


Fords 750 CFM carb was developed over the next model years of 1972-1978, then they axed it for the Vac Sec 4180 600 cfm carb.


I like the 650 Mac Sec Holley. The 350-390 hp Group A Mustangs ran it, and it was very awesome too.

The 750 cfm Q Jet.... The other 5 liter Holden V8's were making 469 hp at 6700 rpm in 1987.


The vac sec 750/780 3310 series has more info on it than any other carb. It was the undisputed winner before emissions compliance crushed it. In the 350 to 470 hp area, the Spreadbore style Themos and Q jets are very good, but you've gotta read old tech from the Mopar and GM guys to unlock it.

[Walking-Tall]

This is very useful for me. I have a quickfuel 650 slayer DP mechanical secondaries on my mild 347. The car has always had a hesitation at low rpm, basically anything below 1800 to 2000. Almost feels like a misfire but it runs great at high rpm. Also, the car always runs too rich but if I lower the fuel level in the bowl it doesn't have quite the kick when I jump on the gas. I have a Ford racing 5 speed with 3.55 rear end in my 65 mustang. City driving is very irritating because of this. Do you think I should go with VAC secondaries to solve this?

Any information I could find for that (or similar QFT) 650 DP carburetor does not include primary idle feed restriction size, and on a mild combination and 1-11/16" throttle bores, if the primary idle feed restrictions are bigger than 0.031" (quite likely), install that size in the primary metering block, relocated to the traditional old school Holley down low location if in the new-fangled up high position (also usually likely), in combination with 0.070" primary idle air bleeds to start... should get you into non pig rich, sane idle and transition cruise AFR territory. ... WITH correctly adjusted float levels, and primary idle setting exposing an approximate square of primary transfer slots below the primary throttle plates (with the secondary idle position setting backed off or increased in order to have that with the primaries), and primary idle mixture screws initially set at 1-1/2 turns out from lightly seated.

All lower rpm main function and/or lower rpm WOT AFR that is rich down low and "runs great at high rpm" is an indication of either/both main air bleeds that are too big (greater than 0.026") or "emulsion" bleeds that number more than 2 per barrel and/or at a size greater than 0.028". Both/either of those new math scenarios don't work worth a hoot, and encourages lower rpm main circuit fuel flow and leans out up high... so when jetting is right for high rpm WOT, it's always too rich down lower in the rpm band... and vice versa if you dare to decrease the primary main jetting to try to lean the mixture at lower speeds. All of this isn't just a worst case scenario for potential engine damage at WOT, but follows no such established 100 years ago AFR to be load dependent, not rpm dependent, and at a constant approximate 12.5:1 AFR for high load over the greatest rpm span, not a "curve" rich to lean, and neither a primary idle/transition circuit that wastes all sorts more fuel than is necessary in the majority of driving that is at lower speeds and light acceleration, the vast majority on the primary idle/transition circuit.



With your mild combination, you shouldn't have to re-calibrate any more than the primary idle feed restrictions and idle air bleeds for best function. Transfer slot restrictions can be a useful addition if the camshaft is wilder, such that off idle and cruise and even light acceleration produces a higher vacuum signal to the idle circuit, drawing more from it, than the manifold vacuum that the engine develops at idle.


An example of where transfer slot restrictions will quite likely be necessary: maximum street-bound effort SBC with large dart pro heads and super victor and 11:1 CR and a solid roller camshaft with 250+ degree intake duration at 0.050", that I'm building an 800 DP for. It's going to have adjustable primary idle feed restrictions and idle air bleeds and transfer slot restrictions for the above mentioned reasons... and will require a bit of dialing in, since no 2 combinations are ever the same... and it will be non pig rich, but fuel efficient at idle and off idle and at cruise, and provide a safe and consistent bottom to top WOT AFR. Because of the grumpy camshaft, I'll be heading straight to big cam territory for 1-11/16" throttle bores with 0.035" primary idle feed restrictions, and probably 0.070" primary idle air bleeds and whatever diameter transfer slot restrictions that are about 95% of the area (width x length) of each primary transfer slot for a decent ballpark initial calibration.




Passages in the main body that lead down to the primary transfer slots drilled and tapped for 10-32 thread (only to a depth where 10-32 x 3/16" brass socket set screws sit flush):




10-32 x 3/16" brass socket set screw transfer slot restrictions installed, not drilled yet. When the new Holley DP throttle base comes in I'll measure it's primary transfer slots' width and length, and then size these:



... but again, if a certain throttle base happens to have primary transfer slots that are wider or longer than traditional, transfer slot restrictions can help... as well as make for recently unheard of fuel efficiency, WITH A CARBURETOR.