Mechanical or Vacuum Secondaries?

Santa is going to bring me a new carb for Christmas but he needs help deciding on which one of the QF Brawler carbs to go with. This is going on a 73 Cuda with'71 340 that is currently relatively stock. J-heads, stock-ish cam, headers, RPM Air-gap, 727 Auto, 2400 stall, 3.55 gears. The motor will be getting new heads and a cam in the future so he's leaning towards a 750 CFM for the extra capacity. There will likely be a swap to a manual transmission but that might be a year or two out. Car is driven aggressively on the street and will see some auto-x and track days, but not much drag strip. Trying to figure out vac or mech secondaries. It seems that in the near term a vacuum sec would be the way to go with the automatic??

Mechanical, regardless of trans type.

https://www.hotrod.com/articles/street-demon-750-carb-review/

A better choice for a street car IMHO considering that the cam is near stock. And it has mechanical secondaries.

hi

i have done a few street 340s and used a 3310-2 750 vs that easily put a 3650 lb cuda in to 12 sec bracket !

I coulda swore that there was a bad thread on street demons here the other day. I ain't in the market for one (& likely wont be) so I didn't pay it no mind.

I like the mechanical secondary carbs. You have to be more careful with tuning so you don't get that lean bog when you floor it.

According to the Holley website, the Street Demon all show to be Vacuum secondaries??

https://www.holley.com/brands/demon/products/fuel_systems/carburetors/demon/street_demon/

The article makes it seem like a pretty easy carb to use though.

I REALLY want to go with EFI but the car needs so much other work and parts that I haven't been able to justify the additional cost.

My rule of thumb says that if a person needs to ask the internet which carb they should get then they most likely need a vacuum secondary carb.

Maybe. But that rule seems to be more oriented to the tuning capability of the user, not the capability/performance of the solution. But what I'm trying to get to the bottom of is which is better for the application. If a vacuum carb is unsuited for how I want to use the car, I don't want to have to deal with that. Most of the discussions are oriented towards drag racing and not road course performance. Just looking for input because I don't have unlimited resources or time to try all the potential combos first.

I am aware that mech carbs typically have more adjustments, race versions even more than street, and it takes some knowledge to be able to utilize the capability. Currently the car has a Holley 6708-1 650 DP on it that I rebuilt. THat rebuild got the car moving from a non-running state but it's an old used POS and has issues that a rebuild kit can't correct so it needs to go. I'm just getting back into carburated engines after a 30 year hiatus so I certainly have room to learn, but I'm pretty sure I have the tools and intelligence to do so.

The Street Demon secondaries are similar to the Carter AVS. Both using an adjustable secondary air door.

I always thought a carb could be made that uses BOTH vacuum and mechanical means to open the secondaries.

Vacuum for mid throttle, with a transition to mechanical that would ensure WOT at WOT.

The 650 DP should work just fine for your application. Maybe buy some new metering blocks or new bowls or whatever it needs and call it good.

Kinda weird the way Holley describes the secondaries. But then, they still say to choose your power valve by half your idle vac, so who knows?

Anyway, the secondaries are actuated mechanically, and the air door is opened by air velocity. Just like the AVSs and Thermoquads of old.
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" Full-Throttle Operation And The Secondary Air Valve;

The Street Demon has a sort of hybrid version of the two. The actual secondary throttle-plate is mechanically actuated, but progressive linkage begins to open the secondaries when the primaries are opened approximately 30 degrees. At this time, the secondaries then open progressively faster until reaching full-throttle at the same time as the primaries. However, instead of an additional accelerator pump, the Demon carburetor uses a spring-loaded air valve to control the transition.

The secondary air valve is nothing more than an aluminum flap that closely resembles a choke plate, which restricts airflow into the secondary venturi. As the secondary plate opens and air requirements increase, pressure drops in the venturi, causing atmospheric pressure to push the flap open. Spring pressure controls the amount of pressure differential that is required to open the valve. The result is a smooth secondary transition, regardless of how fast the throttle is opened."
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https://www.chevyhardcore.com/tech-stori...mon-carburetor/

The Street Demons are easily the best street carbs I have ever installed. Better throttle response with the least fiddling around to get them close. Most other Holleys are way fat on the cruise circuit. For example, the 3310 is usually more than 4 steps too rich for a big block Mopar.

Lots and lots of positive tests and reviews on the 'net if you want to search. About the only ones who dis the Street Demons are people who haven't tried one on the right engine.

Noted

Either can work very good on your combo. Myself I like the Holley Double Pumper carbs best but I am always looking for the best performance also. But I have used and seen others use Holley vacuum secondary carbs that have worked very good. In todays world either will work good as its more of your preference of which you like best. And the mechanical secondary air valve carbs also work good. That's what the Quadrajet , Thermoquad , AVS use and most AFB's even use a weighted secondary air valve. Myself I use the DP on all my cars as we have a 750 DP on my sons Dart and I run an 850 DP on my 63. Both cars are auto cars and both have great driveability on the street and track. Years back I ran a 750 vacuum secondary Holley on my 383 Dart for a bit until I got a 750 DP. And the car was actually ran just about the same et in the 1/4 mile with both. For the most performance out of a car I like the Holley carbs best but that's just my personal preference as I have seen the others also run great. For your car and with your plans I would run the Holley 750 DP carb myself but I would have no problem using the Holley 750 vacuum secondary carb if you prefer the vacuum carb. Good luck which ever you decide. Ron

I can think of very few times to run a VS carb. A Carter/ Edelbrock, Q-jet to TQ is still a mechanical secondary carb, it just doesn't use an accelerator pump. So those are mechanical secondary carbs.


The bad thing about the DP carb that got the bat rep during the 80-90's was that those carbs were almost all calibrated for long cams and race applications. They have emulsion and bleed jetting for more race oriented stuff. Most of them were only 2 corner idle. All of this lends itself to being bad for general use.

All that said, today's carbs are much better calibrated and with a little work, a guy with a reasonable understanding of how to take a simple carb like a Holley apart and put it back together should be able to tune the idle and cruise circuits to make the modern carb about as good as you can anything else.

You can tune the idle fuel jet and the idle air bleed. You can easily install T-slot restricters to clean up the cruise. You can use smaller main jets to clean up the cruise and then open up the power valve channel restricters to correct for WOT. It's surprising how many guys don't realize that on the primary side the main jet is what you change to alter the fuel curve at a cruse and the power valve open timing and power valve channel restrictions are what you tune for WOT.


I say get a tuneable DP and go have fun.

Thanks for the clarification on the Street Demon, AVS, TQ operation. I was struggling to understand how they could be vacuum operated without a diaphragm can. There's a few good nuggets of information here that have been helpful. I'm definitely going to go with a mechanical secondary. I have another Holley that needs a rebuild. It's a List 6711 which was a direct replacement for 71-73 340-440 engines. It's ugly but I think I'm going to dunk it in the ultra-sonic tank, face the body and blocks in the milling machine if they need it, and throw a gasket kit at it. $45 may get me a decent carb that should have good throttle response. It's setup for a divorced choke so I'm going to have to see how I can adapt it to manual operation. That should be fun because Holley said they don't make a kit for it. If this doesn't work out, I'll have a hard look at that Street Demon and Brawler.

Is that the spread bore model? I can't find any info on that carb and I can't find the book. Unless you need the spread bore I think I'd look at something else.

Or...you can use it and test with it. If you have the gumption to work on stuff for fun and learning it may be worth it!

Few people seem to like the 4165/4175 carbs. And that's putting it in a positive light. That said, I have an 800 cfm 4165 and is one of my favorite carbs on a factory spreadbore intake on a stock to slightly warmed up 440.

Yes, it is the spreadbore version, 650 double pumper. Supposed to be a direct fit replacement for the TQ. It was on the car when I got it and was the main reason it wouldn't run. It really needs a rebuild after sitting for a long time. I've got the square-spreadbore adapter already and a kit is $45, so not much to loose. I'm going over it now to make sure it is worth the effort and the shafts aren't worn or something.

I dont' know enough yet to say one way or the other but I'll be able to directly compare it to the 4150 model it will be replacing. I'm not sure what makes 4150 DP perform different from a 4165.

That's why I asked what intake you had. A square intake with a spread bore carb and an adapter is a bad idea. The adapter pisses the carb off and it won't be happy.

Back in the 80's I ran the Holley 6711 carb on my warmed 383 with a ~230 degree cam, headers, and holley street dominator intake. Best carb on that combo that I tried. I tried the stock AVS, 440 AVS, Holley 4777 and 4779 double pumpers. The 6711 idled better, crisp off the line, and really woke up that 383 when the secondaries opened, being a double pumper helped a lot.

It's surprising how many guys don't realize that on the primary side the main jet is what you change to alter the fuel curve at a cruse and the power valve open timing and power valve channel restrictions are what you tune for WOT.


I say get a tuneable DP and go have fun. [/quote]


Actually that's just what I did. I leaned my primary jets a little and opened up the power valve channels and it helped clean up my cruise. But at light throttle cruise you can still be running on the idle and transfer slot circuits. I wanted to clean mine up at even higher speed cruise up to just before my secondaries start to open and it worked very well leaning the primary jets a bit and drilling the power valve channels out some to keep enough fuel at WOT on my 850 DP. Ron

I didn't read all the posts but my opinion is if your great at tuning or if your gonna have a knowledgeable person set up your carb pick simply pick one and enjoy. I've had AFB Edelbrock's in dual quad fashion- single carb vacuum secondary Holley and mechanical secondary Holleys all on 440s. All were razor tuned and excellent for STREET driving. I give them both equal outstanding marks.

3310. If you were not planning on upgrading your engine further, I would say 1850.

Initially the motor has a LD4B with the 6711 and an adapter. It will be getting an Air Gap this summer. Got the 6711 torn down last week and found a crack in the base plate at a front mounting boss. Going to have to see if I can weld that up before I commit to the rebuild.