Vacuum Secondaries or Double Pumper

[Idaho Chris]

Yeah I know there are probably better and more economical carburetors out there, but in my youth in the mid 70's I had a Holley 650 double pumper carb on my 351 Mach 1 and for my own nostalgic reasons I am thinking about going back to a similar carb I had all those years ago.  I am not sure though if I will return to the double pumper.  Any thoughts on vacuum secondaries for anyone that runs that setup?

Car details: Stock 4V heads (open) w Ford HO cam and Ford cast iron intake.  No headers.  Will be swapping my FMX for a manual T5 before hitting the road again.

Thanks. 

 

[rvrtrash]

Just my rule of thumb, I think double pumpers are best on cars with improved air flow (headers, better intake, higher profile cam) and manual transmission. Anything close to stock and/or with an automatic and they tend to bog the engine down when you bury the gas pedal.

Steve

 

[71coop]

I've run both on near-stock engines in other cars, with automatic transmissions. The  double pumper takes a little more time to dial in. The vacuum secondaries are easier/faster  to set up. I believe the DPs are always a little more responsive, but not sure you would even notice it on the street.

 

[c9zx]

A lot depends on the gear and how you want to use the car. The newer Vac Sec carbs have great adjustability and if properly set up will perform as good as Mech Sec carbs. I'd look at the Quick Fuel carbs in the 735-780 cfm range. Buy as much adjustability as you can afford. Chuck

 

[Stanglover]

I'm no expert by far on carbs, but I like what Chuck says. 

I have a Holley 670 with vac secondary's and have an assortment of springs for it. I played around with the different springs until I found the best one for my engine. The 670 has a quick change cap on the vacuum mechanism, so very easy to do. As for Quick Fuel, my preference would be the HR 735 if I were in need of changing carbs again. Downside is the HR are a bit more expensive. 

 

[Don C]

I've had both mechanical and vacuum secondary carburetors, and prefer the mechanicals. However, mechanical secondaries require better gear management than vacuum secondaries do, and will bog if you're too slow in too high of a gear. If your automatic transmission is slow kicking down you may need to downshift manually.

 

[machattack]

I'm no expert by far on carbs, but I like what Chuck says. 

I have a Holley 670 with vac secondary's and have an assortment of springs for it. I played around with the different springs until I found the best one for my engine. The 670 has a quick change cap on the vacuum mechanism, so very easy to do. As for Quick Fuel, my preference would be the HR 735 if I were in need of changing carbs again. Downside is the HR are a bit more expensive. 

I would second the QFT carb. I have one on my 351c.  After rebuild we dyno tested with a new QFT on it. I have a video of the dyno if you would like to see the QFT run.

 

[Idaho Chris]

Thanks for all the guidance guys.  I did not realize how adjustable the vacuum models were.  I do remember it took me a fair amount of effort to dial in my original double pumper back in the day.  With 3.50 gearing and a manual transmission I was leaning toward the double pumper, but the simplicity of the newer vacuum models is appealing.

Question for the QFT carb fans- What is the major advantage over the original 4150 series?

I would second the QFT carb. I have one on my 351c.  After rebuild we dyno tested with a new QFT on it. I have a video of the dyno if you would like to see the QFT run.

Machattack- Sure I'd love to see your dyno run!  Can you post up a link?

 

[71coop]

Quick Fuel carbs, while based on the classic Holley design, are designed with adjustability in mind. They include things like screw-in air bleeds on top of the carb and clear fuel sights on both bowls, for starters.  The vacuum secondary versions have easily accessible diagram springs to adjust secondary opening rate. I am now running QFT carbs on both cars - An 850 DP on my Corvette, and a 670 Vacuum Secondary on the Mach 1. My vac secondary carb also includes 4-corner idle screws. I'm sold on their overall quality and value compared to the standard Holley carbs.

 

[machattack]

Here is a dyno run on the 351c. Its a 4v qft carb with mild cam and 2 v heads. Getting some help from Spike on a set of aussi heads for it.

6k rpm  https://www.youtube.com/watch?v=L_gdlGNSR8c 

 
 idle  https://www.youtube.com/watch?v=mp-JhOshHU0

 

[Stanglover]

Well, I just found out something that to me is disconcerting. It appears that since Holley took over QFT, they have made some changes to the line up of carbs. Notably the HR 735VS that I feel would be ideal on my engine. This carb is/was a based on the 4150. Now in all their wisdom, have dropped that model, replacing it with the SL 750VS which is based on the 4160. This is NOT the same animal at all, not even close. So I guess I'll be looking at other models or makes ( NOT Edelbrock) should I ever decide to make a change. 

 

[69 Rustang]

I would go with the vacuum secondaries.  That said, in the 80’s I had a Motorcraft carb with mechanical secondaries on my 351.  It was way over carb’d, but if you modulated the throttle just right, I could feel the point where the secondaries would open—The car would bark the tires when opened at the right speed/RPM.  Loved that!