I have ordered my zz383/425hp crate engine recently. I have to decide which carb to get for it. GM tested this engine with a 750 cfm demon carb with mechanical secondaries. Should I stick with this carb with the mechanical secondaries or get the same size carb with the vacuum secondaries? I have did research on this and still really convinced of one over the other. Any suggestions?

You'll probably notice better fuel economy with the vacuum secondary unit, simply because the secondaries don't open via the accelerator, and only come on when needed. The mechanical secondaries seems to be the racers' choice, however fuel consumption goes out the roof if you tend to have a heavy foot.

In my case, I went with vacuum secondaries just because my car is a street cruiser, and I can really benifit from the "automatic" operation of the secondaries. I guess it depends on what you want to do with your car, but honestly, a mechanical secondary carb will offer little noticeable difference in performance over a properly setup vacuum secondary carb. My $0.02 anyway.

DP will make around 10-30hp more on the dyno, and will perform better at the track. So why give up the extra power? To me it would be like stickin 1" exhaust on a car and given up hp - why do it? Also just keep your foot out of the secondaries and your mpg will be about the same. Demons have tunable (screw in) air bleeds, idle feed restrictions, emulsion bleeds, so if you want mpg - you can have both hp and mpg with them. graemlins/thumbsup.gif

The only reason a DP makes more HP on the dyno is the vac. carb usually doesn't get all the way open. I have gone awful fast with a good vacuum carb. The only time I found that you really must have a DP is a trans brake car that is race only. As for the adjustability of the Demon a big DP is going to get lozy mileage unless you disconnect the sec.

Originally posted by 67RS502:
DP will make around 10-30hp more on the dynoI'd like to hear more about that one; given the same venturi sizes, jetting and airflow capability, why would a DP make one more horsepower than a VS on a dyno? Accelerator pumps don't do anything on a WOT dyno run once the throttle is opened. :rolleyes:

John, I think that the DP will make marginally more power because there will be less pressure drop across a DP carb at WOT.

By design the vac secondary carb will typically never flow its full potential where the mechanical secondary carb will go to full WOT and flow its max as soon as the pedal hits the floor. The secondary side on a vac secondary carb will continue to throttle the engine until velocity through the throttle bores is high enough to fully open the secondaries. Unless a very light spring is used this doesn't typically happen until high RPM.

That said, I don't think there is a huge difference between the vac secondary and double pumper if both are tuned correctly. I do believe though that the mechanical secondary carb will outperform a vac secondary carb by a tenth or maybe two tenths in the quarter mile with slightly better mid-range power production. But that's about it.

But back to the original question . . . if you are concerned with mileage the ZZ383 was the wrong choice for a motor ;) Go for the gusto and get the 750 Demon with mechanical secondaries that was recommended. :D I use a 750 Speed demon on my 383 and like it a lot.

I can speed 10 mph over the speed limit every where I go and never crack the secondaries on my DP. A DP will not get worse milage under normal driving conditions.

If you drive like a teenager, full throttle to no throttle every 50 feet, then the DP will get worse milage. But if you drive like that you're obviously not concerned with gas milage.

I like DP's because they are simpler and I don't have to worry about the secondaries being open all the way.

If properly set up I don't believe there will be more than .1 difference between the two. Probably less.

Dynoed a friends mild (440hp) 406, it made over 25tq more in the mid range with a 750DP then a 750vac.
And around 15hp more. I will admit that most of the power loss on a 750vac. is cause by the
secondaries not opening up all the way, and if they would that would usually happen at a really
high rpm which a motor will never see. Now a properly tuned 750vac. will make with in 10hp,
but who wants to jack with one that long, just stick a dp on it and be done. Also my work truck
got 22mpg (not just highway) with a home built HP950, that’s on a 383 with an overdrive, which
aint bad so if tuned right and you keep your foot out of the secondaries you can get the best of
both worlds. Also, don’t take my word for it – check with your dyno operators and see what
theyre getting for power differences between the 2 carbs, I was surprised too. I’ve also seen cars
loose almost ½ sec from a dp to a vac. So the dyno don’t lie. Just sharing what I’ve seen, and
that may not always be the case.

I'll cast my vote with Eric, Lonnie, and RS502.

With a vacuam carb, a spring controls the throttle, with a DP, you do. As long as you're smarter than the spring, you'll be fine.

Vacuam carbs were really only created so GM could use the same carb on a 350 that works on a 454, or even a 262 in the '80's...

Agree... I'm running the mechanical secondaries with the lever for the secondary in the "delayed" opening position. THe secondary doesn't even open up until about 1/2 throttle actuation. There is a "one to one" hole to hook the lever up IF you want to open the secondaries up IMMEDIATELY upon throttle actuation. I hardly ever even open the secondaries... mainly running a "two holer" carb... I really like this Speed Demon line.

Heck, you want to have really lousy fuel mileage? Put a Holley 660 center-squirter on your engine and see how fast you can make gasoline dissapear! I tried it once, lasted a week, before I stuck my 750DP back on. Jeez.....

Wow what a coincidence! I just bought a Demon 750 that will replace my Holley 3310 Vac Sec., after seeing the end results of my dyno session. There was a about 20-25 hp difference between the stock 4779 dyno carb and my 3310. Maybe the difference was due to the fact that the VAC SEC was not opening, but I just need to find the truth within myself before between the carbs on my street rod.
Here is the results of the dyno:
http://community.webshots.com/scripts/controlPanel.fcgi

Thanks for all the input guys. Eric is correct in saying this is the wrong engine to be worried about gas mileage. Im going with the DP!

Thanks again

A vac. carb gives the motor the amount of air it needs not what you think it should have. You guys keep buying those big DP I have a lot of oil stock and I am doing quite well right now.

What are going to do with the carb. That will answer your question. IMHO, 750 is just above what that engine will use at 6500RPM, but I wouldn't say it's over-sized unless the motor is under-cammed or the heads don't flow well. Unless you plan to run the 1/4 mile with it, I'd recommend either a vacuum secondary carb similar to a holley. Better yet would be a spread bore design like a Q-jet. I noticed you're running a THM350. But I'm a street guy. As you can see there are a few folks in here that go for the gusto.

Be honest with yourself and figure out what the car will be used for and what is more important to you. If you want mileage get a Vac secondary, if all out performance is your goal get a DP/mechanical secondary. The use of the car is what will dictate the choice. I personally prefer mechanical but, then again my car/cars are toys when I build them. I drive them for fun and not to commute or stretch out a gallon of gas.

If you go with the DP it's really important to size it right for the airflow needs and intended usage. A DP that is too big won't produce max power, and it'll bog if you hit the throttle too hard at low revs. A vacuam carb can be way too big and not hurt ya.