I'll admit I have very little experience messing with Holley-style carbs. So it is possible to tune a DP-type carb for good mileage without sacrificing throttle response? I was under the impression it was pretty difficult to have both. I want a DP on my car because it is more tunable and there's no delay from waiting for a secondary air door to open up but again I haven't messed with one yet myself, I don't know. I did test an old Mighty Demon years ago but it was a "race" carb with no choke, PCV or vacuum ports so I sold it before I tried tuning it. Also did an initial setup on a 770 Street Avenger (vac secondary) for my cousin years ago but never got to drive and tune it to run its best as the car was basically a roller with a 440 in it.
Ok this is, I wanna say, you have been misinformed. Ima trying to be nice.
1) ANY carb can be tuned for economy. They ALL work the same. Some are just easier to access the circuits. And some, were just never designed for economy which does NOT mean that you can't change that.
2) there is no good reason to run a DP carb on the street, unless your engine has enough power to spin the tires below say 3000; then the DP carb is good for showing that off.
3) the Vacuum-Secondary , or Air-Valve Secondary, or any such-type, carb, is well able to be over adjusted to spin the tires too. And if your engine is of a decent compression design, and the carb-size well-matched, the supposed "lag" will be nonexistent.
4) in both cases or any case, a great deal of how a carb performs will be compression and ignition timing related. If you throw a too big carb Vacuum Secondary onto a low-compression slug with stock ignition timing and a lo-stall convertor, yes, you will have a dog. It's not the carb's fault, and you can spend hours and hours attempting to tune with no good results. Then you install a DP anything, and the POS combo wakes up.... sorta. But hang on, you only think the DP solved your problem. You just band-aided a pos combo, is all.
5) What's cylinder pressure got to do with it?
Nothing; it's Not the pressure. It's the sharp vacuum-signal created by the falling piston in the small combustion chamber. All cams have overlap, yes even the lowly factory 318 cam. During overlap, BOTH valves are open to some degree. At low rpm, the falling piston can suck exhaust back into the chamber. If you have a big chamber, and a semi-closed throttle, yur gonna have a good amount of EGR, before the plenum starts feeding. If you then put a too big carb on this crappy combo, guess what, the darn thing is super lazy during this condition. The cure is NOT a DP carb, that's not the problem, the problem is the super-slow air thru-put. Getting fuel into the engine is NEVER the problem. It's ALWAYS about getting the air moving.
6) for a streeter, a Spreadbore design is the bee's knees, and that is why anybody who has ever run a Thermoquad, loves them, me included. The small Primaries respond so much faster and on less signal. Being a metering rod carb, they can be, with appropriate cruise-timing, be tuned for a very lean cruise. And the Secondary Air-door is supremely adjustable for any sized street engine.
7) if you marry a low cylinder-pressure engine, to a lo-stall convertor, that's just silly, no amount of tuning is gonna get you the results you crave in terms of low-rpm throttle response, and torque. Once you have run hi-pressure, you will never go back. Sure your 8/1Scr 318, with a 340 cam, and a 3500 stall TC goes like a raped ape, I get that. But go ahead and pump the pressure to the max and you can run at least two sizes smaller a cam on the street and drop to 2800stall, and you will be a much happier guy in the lower 1/3 to 1/2 of your rpm band. and along with the way-better throttle response, and the stronger low-rpm, will come the potential for way more fuel-mileage.
8) I suppose a guy could break carb-type selection down to combo's but for a streeter, the DP carb, would rarely be the go-to design.
9) I have run every type and several sizes of carbs on my combo, with excellent results. I have a manual trans with 4.10 type gears, so they all run very similar. The 750DP on it today is mostly there just to spin tires.
My secret weapon is that no matter what cam was ever in it, I always adjusted the TotalChamber size to produce about the same 180/185 psi CCP. Yes this required the teardown of the engine and adjusting the deck heights. So what/ that's HotRodding. ..... and I was never sorry about the extra effort. BTW, yes, the Thermoquad was my favorite. But my 600VS got the bestest fuel-mileage......... but my other secret weapon is the overdrive. My combo cruises like it would with 2.77s.
The biggest cam in this combo was the 292/292/108 Mopar. The smallest was a Hughes offering 270/276/110(my favorite). Currently it has a Hughes 276/286/110. All were hydros.
The carbs I tried were; the "big" TQ, the Holley 750VS, the AVS , the 750DP and the 600VS Holley. Someday I hope to find a free Quadrajet to try.
just trying to help; this is not an attack.
