Hi, I know what they do of course but what would be best for my street car. Its a 66 Malibu 396 auto. It might make it someday to a strip, but mostly nasty street pounding. I would like to look at the Barry Grant Speed Demon. thanks

vacuum carbs work just fine on the street or strip.I have used both.
What gear and convertor are you using?

I suggest vacuum secondaries for the street. Mechanical secondaries can make a car very cantankerous to drive on the street with an automatic tranny where you never know what gear it might choose. If you had a manual tranny and deep gears, the mechanical secondaries work fine on the street, or if you have this car only for drag racing, then mechanical secondaries are just fine with either tranny because you aren't going to be in a position where the tranny shifts unwanted.

I have had great luck with the Holley Street Avengers. I would recommend a 770. BTW its a vacuum secondary.


Dave

It has 373 in the rear, I have not added a converter yet. Its a 350 trans and I'd like to try 2000 to 2500 stall.

I did a s week long search, and it seems that vacuum secondaries in a street car are lazy compared to mechanical secondaries, in the lower rpms.

I did a s week long search, and it seems that vacuum secondaries in a street car are lazy compared to mechanical secondaries, in the lower rpms.

don't mistake laziness for not being able to have the secondaries open when you want them to..just because they open sooner mechanically doesn't mean the car is actually going faster.Been there,done that and proved it at the track :)

don't mistake laziness for not being able to have the secondaries open when you want them to..just because they open sooner mechanically doesn't mean the car is actually going faster.Been there,done that and proved it at the track :)

Im sure that they can be tuned to run the exact same at the track, but even you will agree that mechanical secondaries "feel" faster on the street.

Holley even says that mechanical secondaries make more power until wide open throttle, then they would be equal.

but, I read a test that tested a mechanical 750 vs a vacuum 750. the mech made more power. something to do with the vacuum secondary still having some manifold vacuum at wot.

Street W-auto Trans = Vac Sec For Most Cases.

Scott

but, I read a test that tested a mechanical 750 vs a vacuum 750. the mech made more power. something to do with the vacuum secondary still having some manifold vacuum at wot.

For high performance engines a carburetor
with mechanical secondaries
has an inherent advantage over a carburetor
with a “controlled” secondary
system (air valve or vacuum
diaphragm). This is possible because
a controlled secondary carburetor, until
it reaches wide open throttle, will not
have as great a pressure drop below
the throttle plates as would a mechanical
secondary unit. The greater the
pressure drop below the throttle plates
the more dense will be the fuel/air
charge to the engine and, hence, the
more output.

http://www.holley.com/data/TechService/Technical/Selecting%20A%20Carburetor.pdf

OK It would seem the more popular choice would be vacuum controlled. thanks, Dave427 suggests Holley street avenger 770. It does have a good price, its under $400 at Jegs, anybody prefer Demon or Holley

Holley wrote that? It's backwards. VS carbs have more pressure drop across the throttles exactly because they're vac sec. Mech sec carbs flood the manifold with atmospheric pressure air, no pressure drop, reduced velocity, hence the need for big accelerator pumps to cover the inherent lean spots.

Chris said: "just because they open sooner mechanically doesn't mean the car is actually going faster.Been there,done that and proved it at the track" Perzackly and all that.

If you have a street car use a carb designed for a street car. It's extremely unlikely that bolting on a traditional 750 double pumper will be optimal. Holley DPs are normally pig rich, especially on the transition circuit that in use most of the time in normal driving.

Now watch, somebody will chime in with how his 283 ran great with an 850 DP in a 4000lb car with an automatic and 2.73 gear.

How many times has this been beaten to death?

Jerry, truth is that you would probably be much better off with a 670 than a 770 for your car and usage. It'll be sharper and more responsive, which is really what you want for street pounding. How much time does your engine spend above 5000RPM?

Holley wrote that? It's backwards. VS carbs have more pressure drop across the throttles exactly because they're vac sec. Mech sec carbs flood the manifold with atmospheric pressure air, no pressure drop, reduced velocity, hence the need for big accelerator pumps to cover the inherent lean spots.

Chris said: "just because they open sooner mechanically doesn't mean the car is actually going faster.Been there,done that and proved it at the track" Perzackly and all that.

If you have a street car use a carb designed for a street car. It's extremely unlikely that bolting on a traditional 750 double pumper will be optimal. Holley DPs are normally pig rich, especially on the transition circuit that in use most of the time in normal driving.

Now watch, somebody will chime in with how his 283 ran great with an 850 DP in a 4000lb car with an automatic and 2.73 gear.

How many times has this been beaten to death?

Hey Royce - did you see the article titled "Secondary InFOURmation" in the October Chevy High Performance? They compared 7 different carbs at the track. They compared Demon and Holley, vac and double pumper models on two different cars. Here's the results (best runs after mucho tuning):

'66 Chevelle 350 engine 375HP

Holley O-4779 750cfm DP
13.39@103.69mph

Holley 3310 750cfm vac
13.43@103.57mph

Holley O-1850 600cfm vac
13.59@103.48

'68 Camaro 377 engine (3.75 stroke)

Speed Demon 750 vac
12.94@105.44mph

Speed Demon 750 DP
12.80@107.5

Speed Demon 650 DP
13.08@103.37

Road Demon 625cfm vac
13.15@104.40

Thought this was interesting especially after our vac vs. DP debate. It would have been nice to see them compare the Holley to the Demon, but just comparing the vac to the DP, it seems to confirm our 1 to 2/10th guestimate.

http://www.camaros.net/forums/showpost.php?p=114173&postcount=31

too bad they they didn't have the cojones to risk pissing off an advertiser. They could have swapped the Holley and BG carbs on the same car, that would have been interesting.

Holley VS vs DP, 13.43 - 13.39 looks like .04, not really a tenth or two. The 600 VS was only down 2 tenths, that's great. the 600 would would run better everywhere on the street, get better gas mileage, keep the plugs clean. How many people could tell the difference between 13.39 and 13.59 without dragstrip clocks?

too bad they they didn't have the cojones to risk pissing off an advertiser. They could have swapped the Holley and BG carbs on the same car, that would have been interesting.

Holley VS vs DP, 13.43 - 13.39 looks like .04, not really a tenth or two. The 600 VS was only down 2 tenths, that's great. the 600 would would run better everywhere on the street, get better gas mileage, keep the plugs clean. How many people could tell the difference between 13.39 and 13.59 without dragstrip clocks?

Amen, Tom!

too bad they they didn't have the cojones to risk pissing off an advertiser. They could have swapped the Holley and BG carbs on the same car, that would have been interesting.

Holley VS vs DP, 13.43 - 13.39 looks like .04, not really a tenth or two. The 600 VS was only down 2 tenths, that's great. the 600 would would run better everywhere on the street, get better gas mileage, keep the plugs clean. How many people could tell the difference between 13.39 and 13.59 without dragstrip clocks?

thats one sided because the bg dp was 1.5 tenths faster and 2 mph faster.

one can pick up a good 750 dp for less than $150. if its .15 faster and 2 mph faster its more than worth it. there are people who do a lot more than swap a carb to try and pick up those numbers

Hi, I know what they do of course but what would be best for my street car. Its a 66 Malibu 396 auto. It might make it someday to a strip, but mostly nasty street pounding. I would like to look at the Barry Grant Speed Demon. thanks With automatics and converters under 3000 typically recommend staying with a vacuum secondary carburetor. One thing to keep in mind with the Speed Demon though it is only rated for up to .240@50 .