There will be no power difference as long as the vacuum secondary is properly tuned and opens fully at w.o.t. A double pumper is a great race carb when the right size is selected they are usually better suited to light manual transmission vehicles.

 

But will i notice a huge gas mileage difference?

 

No, mileage will depend on how well you tune the primary metering which is the same between either type.

 

If milage + performance is the goal a Q-jet is hard if not impossible to beat.

 

Mechanical secondaries do not belong on a street driven car unless you own a gas station. There is no performance gain from running mech secondaries on a street engine, quite the contrary in fact.

Vince

 

Mighty

What type of transmission are you running in your car?

 

Years ago (early 80's)I ran a 30 over 327 with 292turbo casting heads 11-1 comp and a holley 750dp which was my "daily driver". And for the sake of saving gas I switched and ran with a 600 vac sec. The diff in mileage was 1mpg. I ended up switching back for the performance factor and tuning able. I eventually then went bbc and was not concerned as much (obviuosly) with economy. On the holley dp the secondary side starts to open at about 1/3 throttle with there standard link bar. If you went 1to1 when the pri and the sec causing them to open at the same time you would certainly keep opec happy. My friends run in super street,super gas,super comp they launch or leave against the limiter so the accelerator pumps are used up because there foot is already to the floor so they are working off the mains.
Here is a interesting read http://www.classictrucks.com/tech/cl_0312_choosing_carburetor/index.html

And as always your results might vary

 

I'm runnin a 379 with a TH 350.

 

Unless you're running a loose converter you're going to get better drivability with a vacuum secondary carburetor.

 

there is always both... no body seemed to mention this, you could buy vacuum seconday carb like the holle 4160 and bend the linkage to make it shorter betwen primaries and secondaries to make it partially mechanical, u can have the secondaries open sooner than the vacuum would if you want more performance or take the linkage out if ur looking for economy and let the vacuum do its job

 

there is always both... no body seemed to mention this, you could buy vacuum seconday carb like the holle 4160 and bend the linkage to make it shorter betwen primaries and secondaries to make it partially mechanical, u can have the secondaries open sooner than the vacuum would if you want more performance or take the linkage out if ur looking for economy and let the vacuum do its job

:nono:

Actually this is a really bad idea....

First, you won't have any secondary accelerator pump shot when you manually pull the rear butterfly's open which can cause drivability issues.

Second, and much more important is that the secondary link on a vacuum secondary carburetor is designed to physically close the secondary butterfly's when you take your foot off of the throttle. If you shortened the link up the secondaries could hang causing all sorts of havoc.

 

383 cubes worth of TQ + 3.79 gears = for normal driving mpg you "should" be operating 100% on the primaries only with a darn good rate of acceleration....

strictly depends on how well you can control your right foot....with either carb type

a simple way to "not" get into vacuum secondaries is mount a vacuum guage on your dash...control your foot and don't let the hg fall below about 10 when accelerating

a simple way to "not" get into mechanical linkage secondaries is rig a second spring on the linkage that adds pedal resistance at the point that the linkage is about to start moving to remind your right foot....

LOL, 90%+ of my driving is in town stop and go....I have both on my car

 

:nono:

Actually this is a really bad idea....

First, you won't have any secondary accelerator pump shot when you manually pull the rear butterfly's open which can cause drivability issues.

Second, and much more important is that the secondary link on a vacuum secondary carburetor is designed to physically close the secondary butterfly's when you take your foot off of the throttle. If you shortened the link up the secondaries could hang causing all sorts of havoc.

im sure your right, but mine seem to be working pretty well the way it is, the only thing is like a half second delay of the secondaries closing, which i have no complaints about

 

im sure your right, but mine seem to be working pretty well the way it is, the only thing is like a half second delay of the secondaries closing, which i have no complaints about

Great way to get dead in an emergency...

 

Mechanical secondaries do not belong on a street driven car unless you own a gas station. There is no performance gain from running mech secondaries on a street engine, quite the contrary in fact.

Vince

Completly untrue and way too simplistic.. It seems to me that most remarks like that are based on the assumption that we are comparing typical "performance (holley/demon) carbs.

The bottom line is that vaccuum secondaries are reccomended for the average Joe because they are automated.

There are lots of different carbs in both categories, an old Carter 450 with mechanical secondaries will get a lot better mileage than a Holley 850 with Vaccuum secondaries on nearly any application.

For most situations I agree that Vaccuum secondaries are hard to beat for street use, but that doesnt mean that mechanical is a bad idea.

It mostly depends on the specific application and how it is tuned and driven.

If there were no performance gain to be had from using mechanical secondaries then they wouldnt be the dominant carburetor for strip use.

 

There are lots of different carbs in both categories, an old Carter 450 with mechanical secondaries will get a lot better mileage than a 850 double pumper with Vaccuum secondaries on nearly any application.

A double pump would be a mechanical secondary. A Vacuum secondary carburetor only has one pump.

For most situations I agree that Vaccuum secondaries are hard to beat for street use, but that doesnt mean that mechanical is a bad idea.

It mostly depends on the specific application and how it is tuned and driven.

Very Well Said!

If there were no performance gain to be had from using mechanical secondaries then they wouldnt be the dominant carburetor for strip use.

Exactly...

 

A double pump would be a mechanical secondary. A Vacuum secondary carburetor only has one pump.



...

Doh!!! (homer simpson smile)

 

Great way to get dead in an emergency...

what do u meen ?

 

what do u meen ?

A throttle that doesn't close all the way is a great way to run through your brakes and possibly crash and or kill someone. Which with a modification like that a good lawyer would make your life VERY bad.

 

A throttle that doesn't close all the way is a great way to run through your brakes and possibly crash and or kill someone. Which with a modification like that a good lawyer would make your life VERY bad.

i see what ur saying, i havent had this problem ever, maybe it has something to do with driving a stick, cuz i could just put in the clutch and brake or w/e . but if anything close to what ur talking about happens im goin back to original

 

If you are forcing a vacuum secondary carb open by cobbling up the linkage, you really DON'T have a clue as to what you are doing :nono: , you will go faster if you take the time to tune it correctly. Can't tell you the # of times I've seen that bonehead move , although I've purchased several used carbs this way for cheap knowing I could fix them.

 

If you are forcing a vacuum secondary carb open by cobbling up the linkage, you really DON'T have a clue as to what you are doing :nono: , you will go faster if you take the time to tune it correctly. Can't tell you the # of times I've seen that bonehead move , although I've purchased several used carbs this way for cheap knowing I could fix them.

its working fine for me, but im new to the carb thing, i just rebuild me holley 4160 and have been fine tuning it since, its super responsive right now and has a good mix, but what is the "by the book" way to do adjust the vac secondaries ? i appreciate the criticism and comments, i am learning a lot from them, but thats just how i do things (trial and error)

 

This was a long time ago and i ended up goin with a vacuum holley 650. Works real nice except i cant get rid of a small hesitation. BTW, i'm only getting like 10-12 mpg so i guess the fuel efficiency difference between the two doesnt really matter. But then again, every mile per gallon counts these days.

 

This was a long time ago and i ended up goin with a vacuum holley 650. Works real nice except i cant get rid of a small hesitation. BTW, i'm only getting like 10-12 mpg so i guess the fuel efficiency difference between the two doesnt really matter. But then again, every mile per gallon counts these days.

hesitation is probably wrong air/fuel mix off idle (ie too much air or too much fuel) , which could be a simple change like a different accelerator pump cam (30 dollar holley kit comes with a whole bunch)
mine did the same thing till i changed this

 

Correct way to adjust Holley vacuum secondarys is to buy the vacuum secondary diaphram spring assortment from Holley or The Carb Shop, and then install the best spring for your combo by road testing it, then track testing.