Holley question, what do think of Vacuum Secondary vs "Double Pumper"?

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69_340_GTS

69_340_GTS

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With a very mild 10:1 340, 727 automatic, 2,500 converter, 3.55 gears: What Holley carburetor do you think would be better, a 750 vacuum secondary (#3310 or equal), or a 750 Double Pumper type Holley? I may consider Holley clones too, like the old Demon, or something else of that basic 4150 Holley design. Thanks!
 
krazykuda is correct, a 3310 has better street manners that a double pumper with minimal fuss. I've pretty much always run Holley carbs all of my life because I had a good teacher as far as rebuilding them & tuning them for a specific application. On my street cars I always ran vacuum secondary carbs & double pumpers on my race cars.
 
Why
It is less violent when kicking in, you may need that spot for daily driving and with the vacuum it will help
 
Vacuum secondaries only open as the engine needs more air& fuel, rather than you just opening them by mashing your foot down. A double pumper style mechanically opens the secondaries based on your foot, and can be a lot more than the engine needs right then.
You can though, adjust when and how fast the secondaries open by changing the springs in the vacuum canister. Much easier to do if you buy a quick change kit.
No shortage of information in the googles. Such as;
It is important for the secondary barrels to open at the right time. It is also important that they open at the optimum rate to precisely meter the air and fuel the engine needs.

If they open too slowly—or too late—it will starve the engine of air and fuel. The end result is less top end power.

If they open too fast—or too soon—the engine will get bogged down. This lets in too much air and not enough fuel. This lean condition will result in poor throttle response and slow acceleration.
 
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69_340_GTS said:
But why?
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A properly tuned double pumper will MOP THE FLOOR with any comparable vacuum secondary. However, balls out performance is not what everybody is always after.

A vacuum secondary with an automatic on the street is usually a better match because they have a much smoother transition to the secondary circuit. Tuned properly, you can barely feel the secondary circuit come in, but simply feel smooth acceleration.

Conversely, you will feel a double pumper's transition into the secondary circuit. It is a "harder" hit if you will due to the extra shot of fuel. This is why for performance, the double pumper is considered superior.

However, the double pumper will in all likelihood get worse fuel mileage in the same situation. Sometimes by a good little bit. I've always looked at that by saying the engine "probably needed it".

I looked at the decision the same Larry Shepard talks about it in the Mopar Engine Manual.....which you should already have, btw. With a mild street build with an automatic, mild converter and street gears, the vacuum secondary is a better choice, for the reasons I outlined above.

With a more hot build with a higher flash converter, or a manual transmission, lower gear ratios (numerically higher) the double pumper will have better performance.

The double pumper properly tuned will have better performance in either situation simply because it is a higher performance carburetor. It simply boils down to what you WANT.
 
This: "A vacuum secondary with an automatic on the street is usually a better match because they have a much smoother transition to the secondary circuit. Tuned properly, you can barely feel the secondary circuit come in, but simply feel smooth acceleration." Thanks Rusty.
 
I have the book, RRR. Thought maybe some things had changed over the years. Since this is solely street-driven, I guess I'll stick with the V.S. type carb. Thanks to everyone who responded. Great info & insights!
 
While we're here, any thoughts on this relatively new company?
FST Performance RT-X Series Carburetors 41750XB
fut-41750xb_vc_xl.jpg
 
69_340_GTS said:
All that underhood wiring is just plain ugly. Also, not a big fan of all that high-tech stuff on a 50+ year old car. But the thought has crossed my mind...
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The Sniper has a less wiring. I have a Fast 2.0 on my car and love the way it works.

engine.jpg
 
3310's are pretty forgiving. You might consider an AVS or the Street Demon carbs as well. I think it comes down to what your comfortable with tuning and how much tinkering your willing to put up with. Regardless a wide band o2 will really help in tuning any carb.

I have a Quickfuel 650cfm SS Mechanical secondary on my GTS and it is the first mechanical secondary carb I've ever used. I've not had the chance to drive the car yet due to wild fire smoke air quality here, I need to wire my wide band and it needs aligned. So far I like the carb.

With today's fuels that light off quickly its getting more involved to use a carb, and the cost of a good carb is getting up there too. Going to a Sniper has its advantages and draw backs. I friend of mine has installed 5 of them and loves them.
 
The fuel around here can be a crap shoot, I like to run 93 no alcohol, but it is getting harder to find, some 91 no alcohol, but mostly the 91 with alcohol. There is a couple that does carry the 93, the one go to is next door to the Mercedes dealer. Their high end cars have to have the 93.
 
69_340_GTS said:
While we're here, any thoughts on this relatively new company?
FST Performance RT-X Series Carburetors 41750XB
View attachment 1715597132
Click to expand...


First, I’ve made it clear I have zero use for a VS Holley or clone. I’m not going to type out why, but I don’t use them.

Second, even in the cold winters where I live I never use a choke. I learned a long time ago how to start an engine in single digit weather without a choke and the change in metering with the choke horn out of the way is much better.

Three, I just did an FST carb (with a choke but a double pumper...damn choke) and they are very, very nice for the price point. They use all nice stuff, and they come with hex headed jets instead of the screwdriver slotted ones, notched floats and they are well machined.

Like any other Holley or clone, it needs to be taken apart and pinned. The FST I just did had a wacky tune up in it, and it came to me because the owner couldn’t even get it to run.

I wasn’t surprised when I pinned it. Why it was sent out like that I can’t say, but FST isn’t alone in strange tunes.

Everything was easily corrected. It is now back on the car and runs very well.

FST, QF, ProForm...I’m forgetting some...all make nice carbs. You just have to get into them and at least look at the tune up.

You can certainly clean up any Holley or clone and make them get fuel mileage with anything else. It’s just a matter of getting all the brass (air bleeds, emulsion, restrictors) where they should be, and not some over emulsified, super pig rich tune up.
 
Pinning a carb is taking it apart, and using number drills (at least that is what I use) to determine the opening sizes of internal passages and air bleeds. Newer high end carbs have some,or all, openings adjustable now, via brass screw-in plugs. Older carbs need drilling and tapping for plugs to be able to change them.
Edit: number drills come from #80 to #1 usually. 80 is tiny (if memory serves, .015) and #1 is .228. Use the shank and a caliper to measure, drill end to change sizes.
 
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69_340_GTS said:
I have the book, RRR. Thought maybe some things had changed over the years. Since this is solely street-driven, I guess I'll stick with the V.S. type carb. Thanks to everyone who responded. Great info & insights!
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I believe you will be happier.
 
FWIW, I ran both 750 vacuum (3310) and double pumps(4779 + Quick Fuel) for many years in race only 10-11 second 360/auto/footbrake applications. The closest I ever got the 2 was about a tenth of a second ET in the 1/4 mile. Not surprisingly, the advantage went to the double pump. They are harder to tune. I also found the Quick Fuel SS aluminum carb to be better overall - idle quality, response, ET, etc. than the older Holley zinc castings. But I would go with the majority & recommend a vacuum carb for the street.
 
halfafish said:
For those of us who read all kind of stuff to learn things...........what is pinning a carb? I've never heard the term before.
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Post 20 nailed it, but I use pin gauges, which are pretty inexpensive because they go up in .001 increments, where the number drills (which I have because you need them) go up in .003-.004 or something like that.

You can’t be quite as accurate with number drills but you can certainly get the job done and done well pinning with them.
 
always herd VS for a automatic and a DP for straight gear. but i like the DP better, itll help build tuning skills tho'! for street use with a auto if i didnt wanta toy with the tune id go sniper!
 
Fuel economy VS and DP is all in the right foot.

If you hammer the pedal at every light and complain about fuel consumption with a DP, then buy a VS carb. That's the only place that a VS makes sense. Tune ups on all the holleys can be interesting, even the VS carbs can be a long way from good OOTB.
 
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