mechanical or vaccum secondaries

badazz70z28 · 04-09-2011, 09:53:01 PM

355 flatops, bowtie heads, .480 lift cam, 2400 stall, 373 gears in a lightweight vega
the 600 holley i have is too small, the holley speadbore i have was on a drag motor and def. been worked so it is a bit much, but it really wakes the motor up with the mechanical secondaries.probabaly going to get a 650 or 670, if i go 650 should i go with vacuum or mechanical secondaries

351maverick · 04-09-2011, 10:13:59 PM

I've never trusted vacuum, and not trusting it makes life easier i.e. manual brakes, mechanical advance & mechanical secondaries

Air_Adam · 04-09-2011, 11:02:09 PM

I always prefered vacuum secondaries myself. 'Vacuum' is kind of a misleading word in this case, as it doesn't work based on engine vacuum like say, power brakes do. It functions off airflow demand from the engine.

COPO · 04-09-2011, 11:19:20 PM

If your not racing at the track then you don't need mechanical. Plus the price of gas, why waste more.

dubs74camaro · 04-10-2011, 01:32:11 AM

That 600 is enough cfm. The other carb is probably working better and feels like it wakes the thing up because something is not quite right or out of adj on the 600. Coming from someone that is a DP freak, you can get a vacuum carb to come on just about as hard as a mech if setup right. The 670 avenger is pretty hard to beat. Adj vacuum housing and a rear block makes for nice tuning. A lot of times a spread bore carb will feel like you are getting a "punch" of power because you actually go through a lean spot or hesitation with the secondary transition before the A/F ratio get correct and the thing takes off. People talk about that with Q-jets from time to time.

K5JMP · 04-10-2011, 02:46:19 AM

I like mech secondaries...
and as dubs eludes to.. there is no way around learning to tune a carb if you want maximum performance from it.

well, there is.. but it amounts to paying someone to tune it for you.

and yeah.. the 600 should be plenty for that motor.

hhott71 · 04-10-2011, 02:47:37 AM

You are better off with a 650 DP than anything else.
I used the SAME combo and was running 11.30's with it when I got a decent 350TH and 10" converter in it.

AS far as mileage goes.
If you stay out of the secondaries it'll be the same as any 2 bbl or 4 bbl vac sec. driven normally.

Marks71BB · 04-10-2011, 02:56:40 AM

All things considered, yer right foot has more to do with it. Assuming they are tuned properly.

K5JMP · 04-10-2011, 03:19:00 AM

a 650 DP is actually my favorite carb.. particularly the spread-bore model.

AJ_72 · 04-10-2011, 05:06:20 AM

I have a mech secondary 650 DP on my car and I love it. Wouldn't have it any other way.

The response is right now! And as far as MPG, the opening of the rear throttle plate is based on how much throttle you give the engine. A vacuum diaphragm won't open quite as quickly (obviously), but once it is open the MPG drops anyway. Stabbing the throttle is stabbing the throttle.

Simon@London · 04-10-2011, 07:16:31 AM

Quote:

Originally Posted by badazz70z28

355 flatops, bowtie heads, .480 lift cam, 2400 stall, 373 gears in a lightweight vega
the 600 holley i have is too small, the holley speadbore i have was on a drag motor and def. been worked so it is a bit much, but it really wakes the motor up with the mechanical secondaries.probabaly going to get a 650 or 670, if i go 650 should i go with vacuum or mechanical secondaries

How many RPM's are you spinning and where does it begin to fall off. I ran a 4777 650 DP on my 355 and it really woke that motor up. Tuned properly the 600 Eddy/ 650 Holley were night and day difference when stabbing pedal.

Eddy made better mileage but that is to be expected.

DesmoEd · 04-10-2011, 08:40:21 AM

I'd go vac sec on an auto car with a mild cam and 2400 stall - Holley 3310 worked well on my '73 with TH350/2200 stall/3;73 gear

650/750 DP would be my choice on a stick car with good gearing

dubs74camaro · 04-10-2011, 09:35:53 AM

One thing you haven't told us is that this car is being used for. It's the good ole "V8 Vega". Is this a weekend car? Track time? Tell us what you are trying to do with the car.

I want to re-address the fact that I'm a DP freak even though I said the vacuum might be better suited. I had a 460 BBC in a '68 GMC 3/4 ton with a 650 Holley DP. Goes against all common sense. Too Heavy and not the greatest gears on tall tires.

Your converter might not even be stalling at 2,400. Stall is based on torque and a converter rated at 2,400 will stall lower behind a mild SBC than it will a BBC. What you really have is going for you is gears and a light car. The reason why these guys all like the 650 DP so well is because they may have been the ones to resist the urge to go with a 750 like so many do and picked a lower cfm carb. At least with the 650 you will be staying with a smaller venturi and a straight leg booster. The signal is not a strong with a straight leg as apposed to a down leg due to it's higher position in the venturi. They are a little more forgiving.

With that combo I also hope you are someone that can care less about gas mileage!!

badazz70z28 · 04-10-2011, 09:43:08 AM

Quote:

Originally Posted by dubs74camaro

One thing you haven't told us is that this car is being used for. It's the good ole "V8 Vega". Is this a weekend car? Track time? Tell us what you are trying to do with the car.

I want to re-address the fact that I'm a DP freak even though I said the vacuum might be better suited. I had a 460 BBC in a '68 GMC 3/4 ton with a 650 Holley DP. Goes against all common sense. Too Heavy and not the greatest gears on tall tires.

Your converter might not even be stalling at 2,400. Stall is based on torque and a converter rated at 2,400 will stall lower behind a mild SBC than it will a BBC. What you really have is going for you is gears and a light car. The reason why these guys all like the 650 DP so well is because they may have been the ones to resist the urge to go with a 750 like so many do and picked a lower cfm carb. At least with the 650 you will be staying with a smaller venturi and a straight leg booster. The signal is not a strong with a straight leg as apposed to a down leg due to it's higher position in the venturi. They are a little more forgiving.

With that combo I also hope you are someone that can care less about gas mileage!!

it is just a street car, i might try it in the 1/8 but not sure yet.as one guy commented, the 650 spreadbore is what he likes and i can get a rebuilt one w/ warranty for $150.00.as far as gas milage,not my daily driver, so i really don't care if it gets 15mpg or 5mpg,
ps thank you guys for all your input,dummies like me really do appreciate it

dubs74camaro · 04-10-2011, 10:02:42 AM

First of all....you're not a dummy. It's just an area you are not as stronge in. I build Holleys in my shop but don't know squat about transmissions.

What kind of intake do you have? If you have a square bore intake, you will have to run an adapter. If you have a spread bore intake, pretty easy to run the spread bore carb. Depending on the intake, you can saometimes even run a plate that necks down the mounting area but doesn't use the 1" spacer. I'm not a fan of running a spread bore on a square intake due to the adapter you HAVE to run but square carb on spread bore intake is not a big deal. Also remember that spread bores come in both mech and vacuum secondaries.

Choice is yours and how you "drive" might come into play a little. If your right foot is pretty light and you don't get into the secondaries that much, neither style will make that much difference. If you like to play and maybe go sideways or paint black marks, that's a different story.