Who's Running Mechanical Secondaries on an Auto???

[72step]

Well, I've decided on a 650 Speed Demon Carb for my truck, and was leaning towards mech. secondaries. Some people tell me I'll lose performance with mechanicals on an auto. I've also been told by performance specialists that I will be more than happy with mechanicals on an auto. What's yalls real world experiences? Thanks....

[poorboy]

I don't think the it will make a difference. You can tune both mech and vac carbs to open the secondaries when you prefer. I personally like mech sec. Unless you like driving at full throttle all the time you not using the secondaries anyways.

[Longhorn Man]

Unless you like driving at full throttle all the time you not using the secondaries anyways.

huh?

[Back in Black]

I put a 650 holley with mechanical sedondaries on my truck. If you have enough motor, the auto won't be an issue. Otherwise, you may need to set the secondaries so the open a little slower. I have not done this, but my mechanic buddies say its no problem.

The thing I like about mechanical is you know when they are open all the way.

If you are running a mild motor, you may want to go with vacuum.

Have fun.

[72step]

I don't think it will be a problem, I have plenty of motor. Everything in it is high performance as I put it together. According to the cam specs, and the Demon Website, the 650 Speed Demon is the right carb for my application. I'm anticipating huge differences with this new carb, especially off the line. Mechanicals it is. Thanks....

[panhandler62]

I had a 350 in my 72 Camaro with camel hump heads running 2.02/1.94, a Torker II manifold, cheap headers and the stock cam that came in the motor (was a 2v from the factory) and 350 turbo witha shift kit.

I ran a 780 with mechanical secondaries. I adjusted the secondaries so they opened a bit late and only got to about 3/4 open at WOT. It worked well.

[gotchevys?]

Its all in the tuning. An automatic will tend to stumble or cough off the line with mechanical secondarys if not tuned properly. The most common cause of this if the butterflys coming wide open all at one but there in not enough fuel to compensate. This can be corrected by properly tuning the accelerator pump or pumps to provide the proper amount of fuel at the appropriate time.

[piecesparts]

I have had no issue with the mechanical secondaries. It is true that the tuning is the trick, but the drastic tep to the secondaries is usually due to the jets and needles on the secondaries being setup with a large difference to the primary jets and not aligned together well.

[1968SWBBigBlock]

Read thru the Holley & Demon site Tech areas and let that be your best judge.
I have run Mech secondarys with out issue, but the information I have been reading that explains how a carb works it looks like the Vaccuum is the way to fly for a street motor. Unless you meet the criteria for stall speed which i think they list as 4000 + cam, manifold, headers and some heafty rear end gearing.

[piecesparts]

ALL of this discussion on the thread and a couple of earlier ones about the vacuum or mechanical secondaries; REALLY is just sematics. To each his own and to everyone that has a setup on their car (It will be the best in the world, if you don't believe them--just ask them). The mechanical actuation of the secondaries is direct and if not adjusted to a fine transition, them also dramatic. The vacuum secondary actuation is from what the engine feeds back to the carb and when it gets to a popint that is low enough on vacuum, then the feedback allows the spring to bring in the secondaries. If that spring is heavy or the vacuum feed is mis-calculated---It too can be dramatic.

I believe that if you set your spring or your jets right then you will rarely notice the change over and it works well. On my truck the cam has enough lope that the mechanical transition along with the needle springs and jets works well for me. I drivwe my truck on the street and YES--I do have somewhat of a hard hitting engagement of my secondaries, but I also control that some with the AVS setup on my carb.

Buy what you want, and work with it. Don't go for the hype on either end. Also call Edelbrock, Holley, and Demon for input for your decision, but in the end buy what you believe will work. We won't m ake fun of you ---MUCH.

[68C15]

from my personal experience I have found 3 factors that favor a vacuum secondaries. heavy car-truck, auto trans with low stall, or low numbered gears. the reason for this is the normal tendancy for the VS to be more gradual while the mechanical ones are more of a gunshot engagement (which usually translates to instant power when tuned right)
you also need to check if it is 1:1 or staged or staged progressive opening.

[rage'nrat638]

get ready to suck the gas...

[piecesparts]

Originally Posted by rage'nrat638

get ready to suck the gas...

So what does that mean? I believe that there is no difference in the functions, just the actuation of the secondaries. It takes a certain amount to make a vehicle go, no matter what brand of carb. Just approach it with common sense.

[panhandler62]

My general experiance has been that carbs with vacum actuated secondaries are easier to tune for the neophye.

As far as total performance; very few street cars run at low enough midrange vacume to make mechanical secondaries neccesary but when properly tuned, as noted above, they will work fine on street cars.

[72step]

I'm confident on Mechanicals, and that's what I'm buying. Gas mileage means nothing to me. I didn't build this truck for MPG's, it's strictly fun and adrenaline. What I don't like about my VS Carter AFB-same as an Edelbrock, is that it doesn't perform off the line, and is just okay through the rest of the power band. When I 've run mechanicals on two other cars in the past, a nearly stock 350 on a 63 Impala, and a very mild 327 on a 62 pickup, when you punched it you could actually feel the power and sound when the 4 barrels were open, and on mine you can't really feel the power, and the sound. It almost seems like a decent 2-barrel. Oh well, thanks, and the mechanicals it will be. If i have problems, I'll sell the TH400Turbo, and convert it to 4 speed, or tremec. It's actually something I've thought about.