I'm one of these guys that has it in his head "double pumper or nothing". I ever ran one on my 5,500 lbs 3/4 auto and loved it. I see a lot of these big inch, big horsepower crate motors coming with big vac carbs. I'm going to be running two 600 1850's on my tunnel ram truck and know that you can put the quick-change top on the vacuum housing. I'm just wondering when do you really want to have the secondaries kick in?

Next question is that I'm going to be running the carbs sideways with that cool linkage so it won't be progressive. Is there any benefits to running the vacuum secondaries progressive? In other words....having different springs in them so they open at different times?

(now that's thinking outside the box huh?) :yes:

If you can feel the vacuum sec's open they are opening too soon. Vacuum sec carbs are an on demand carb so if you have too much carb to begin with they won't open as much. You also can't test them with no load on the engine. Rev it all you want in the garage the sec's won't open. My thoughts are if you tried to set them up to open progressively you would only ever open 1 set of them.

If you can feel the vacuum sec's open they are opening too soon.

That's quite interesting Dennis....but what do you mean by feel? I always thought you "heard" the secondaries open. Wouldn't or shouldn't you "feel" the secondaries open if they are supplying more gas to your motor and increasing HP?? Lay your wisdom on me MR!

Unlike a manual sec carb where the sec's kicking in is felt (where the term "kicking in" comes from) the vacuum sec is a smooth transition and any kick felt is actually a bog. Go back to the basic's of how a carb works and is designed. Air is moved much faster through the carb than fuel so an accelerator pump and squirters are added to help the carb transition off the idle curcuit as you open the throttle without a bog or hesitation. The same applies to the secondary side of the carb although you typically are beyond the idle curcuit when they start to open. Mechanical sec's have an accelerator pump and squirters that over come the air fuel imbalance when you go wot. A vacuum sec carb doesn't have the accelerator pump and by design only provides additional fuel if the engine can use it. If you feel vacuum sec's "kick in" it's actually a bog or hesitation.

Can you be more specific??? LOL

Just kidding Dennis and that was a great explaination. Would it be safe to say when choosing a vac spring to go as light as you can without feeling the "bog"????

Wouldn't or shouldn't you "feel" the secondaries open if they are supplying more gas to your motor and increasing HP??
Dennis is pointing out one of the old wives tails about feeling the secondaries kick in
If u can feel it 1 of 2 things is happening
1/the sec are late, the engine leans till a slight drop in power, thu it still pulls, then sec opens, mixture corects and u feel the surg of power
2/Sec open early, engine still pulling thu, as the load/rpms increase it comes up to meet the correct mixture and the car gets more power.

The basic principal applies to both vac and the correct setup of the duoble pumper

Can you be more specific??? LOL

Just kidding Dennis and that was a great explaination. Would it be safe to say when choosing a vac spring to go as light as you can without feeling the "bog"????

Yes, if you feel it go down a spring, don't feel it go up one. Most books on Holley's will give a chart of which spring color opens at what rpm. They usually show two scales, one for a 350 ci and another for a 400 ci, the more cubes the engine the lower the rpm a spring works at. Use the color chart as well to identify each spring, I know white is lowest, then there are 2 yellows, the plain one is about mid and the black is not going to be the highest rpm opening point. I believe there are 7 different springs...

... I believe there are 7 different springs...

8 - if you count useing the 'old-racers trick' of a Corvair Fuel inlet spring :thumbsup:

Here's a link to Holley's springs in their parts cat. - http://holley.com/20-13.asp

After all the good info, I just got a great deal on a 9776 off eBay to go with another one that a guy gave me. Think I'm gonna take Holley's advice and run the 2 450 mechanical carbs that are made for tunnel rams. If I'm not happy with them, I still have the 2 600 vacs. Thanks for the good info guys.....I definately learned something.