Vacuum secondaries vs mechanical secondaries

[Lou's Comet]

I have been looking at carbs and intakes. I am a little unsure on the vacuum vs mechanical secondaries. From what I have been reading I could go either way. But the vacuum secondaries seem to be the safer bet?

Engine 363 cu in, 4 speed, 4:11's, and guessing around 3100 lbs? Shooting for fun streeet car with couple trips to track per year.

I would kind of like to get the mechanical secondaries, don't like the look of the vacuum diaphragm on the carb and also think the mechanical will give me more adjustment. But if the vacuum secondaries are going to work better thenI will go that route.

As always thanks for any and all advice!

Lou

[Boss/Cyclone]

If you are running an aggressive cam go with mechanical. Aggressive cams generally don't produce a lot of vacuum and won't allow your secondaries to open correctly without changing the diaphragm spring. Ask you're engine builder what he suggest also.

[Joe Travers]

How much track time will your Cyclone see?
0= go vacuum. >0 go mechanical.

I installed a DP last year. Thinking about going back to vacuum because I rarely open the secondaries.
They crack open at half throttle.

Joe

[A/FX]

In the past most of mine were mechanical secondary’s, sometimes using a double pumper. On the Comet I now have two 390cfm vacuum secondary carbs that seem to work fine. It took some tuning with different jets and springs to dial in the progression of the secondary circuits but no carb is correct right out of the box. I did find that using Holley OEM parts is the best way to go, in a pinch I bought some Summit brand springs and found that the tension did not match the Holley units and the colors on the springs did not coincide with Holley either.
Jim

[Joe Travers]

in a pinch I bought some Summit brand springs and found that the tension did not match the Holley units and the colors on the springs did not coincide with Holley either.

Good to know. Thanks Jim!

Joe

[poboyjo65]

I want to get away from vacuum secondaries. I had an edelbrock on this 347 (which has more of a low end mid range cam) in a truck with a C6 trans & everything was hunky dory ,only one slight bog if you stomped it. hardly noticeable. but when I put it in this comet with a 4sp ,it was alright just cruising but if you went thru the gears @ WOT it had 4 major bogs,bogged every gear. it was so annoying & sounded like chit. then I put an autolite 4100 on it & was able to tune some of it out but I still have a bog sometimes,& I want it to jump when I stomp it,,,,& not throw me in to the steering wheel then back to the seat like it does when it bogs . but sooner or later I'm going with a mechanical double pumper so it gets the proper amount of squirts for instant power when I nail it!

[Joe Travers]

I want to get away from vacuum secondaries. I had an edelbrock on this 347 (which has more of a low end mid range cam) in a truck with a C6 trans & everything was hunky dory ,only one slight bog if you stomped it. hardly noticeable. but when I put it in this comet with a 4sp ,it was alright just cruising but if you went thru the gears @ WOT it had 4 major bogs,bogged every gear. it was so annoying & sounded like chit. then I put an autolite 4100 on it & was able to tune some of it out but I still have a bog sometimes,& I want it to jump when I stomp it,,,,& not throw me in to the steering wheel then back to the seat like it does when it bogs . but sooner or later I'm going with a mechanical double pumper so it gets the proper amount of squirts for instant power when I nail it!

The reason I got away from single Edelbrock carb. Two of them would cure that problem

Joe

[A/FX]

Another point of interest. Not that I am into economy or fuel mileage with the Comet but on the hiway cruising at traffic speeds I have been able to achieve 17mpg. This is with the Boss engine, Hefty roller cam, and a 3.50 gear.
I run primarily on the front carb using progressive linkage to pull in the second carb and vacuum secondary's with un matched springs to activate the back barrels of each carb.
Jim

[robib]

Vacuum secondaries do operate on vacuum, but not engine vacuum. They operate on Venturi vacuum, which is vacuum that is created by the passage of air being drawn into the venturi (carb) and as the engine speed increases, so does the amount of venturi vacuum. The venturi vacuum then reaches a point which then overcomes the spring that keeps the secondary throttle plates closed. It does this as needed by the engine, via the engine speed and venturi vacuum. A mechanical secondary carb does not know when the engine needs or "can use" the secondaries.The secondaries are opened by the operator's foot and that may not be at the appropriate time. Starting at 1500 RPM, "floor the gas pedal" and a vacuum secondary will not open until the engine RPM and venturi vacuum reach the point of the overcoming the spring, only because the engine can not use the secondaries at the low RPM. Try that with mechanical secondaries and the engine will probably bog, as the carb is trying to give the engine too much fuel for the low RPM. The vacuum secondary carb is, in my opinion, superior to the mechanical secondary carb for street use.

[poboyjo65]

Here's a pretty good article on it;
https://www.motortrend.com/how-to/vacuu ... arburetor/

[Lou's Comet]

If you are running an aggressive cam go with mechanical. Aggressive cams generally don't produce a lot of vacuum and won't allow your secondaries to open correctly without changing the diaphragm spring. Ask you're engine builder what he suggest also.

Thanks Boss, Don't know the cam specs yet. But will ask builder his recommendation also.

Lou

[Lou's Comet]

How much track time will your Cyclone see?
0= go vacuum. >0 go mechanical.

I installed a DP last year. Thinking about going back to vacuum because I rarely open the secondaries.
They crack open at half throttle.

Joe

Probably see the track a couple times a year. Leaning toward the mechanical.

In the past most of mine were mechanical secondary’s, sometimes using a double pumper. On the Comet I now have two 390cfm vacuum secondary carbs that seem to work fine. It took some tuning with different jets and springs to dial in the progression of the secondary circuits but no carb is correct right out of the box. I did find that using Holley OEM parts is the best way to go, in a pinch I bought some Summit brand springs and found that the tension did not match the Holley units and the colors on the springs did not coincide with Holley either.
Jim

Another point of interest. Not that I am into economy or fuel mileage with the Comet but on the hiway cruising at traffic speeds I have been able to achieve 17mpg. This is with the Boss engine, Hefty roller cam, and a 3.50 gear.
I run primarily on the front carb using progressive linkage to pull in the second carb and vacuum secondary's with un matched springs to activate the back barrels of each carb.
Jim

Thanks Jim,,,Originally was going to go with dual quad, but decided it would be a lot simpler to go with a single carb. And I would really like to be driving it this year. Not worried about fuel mileage with 4:11 gears.

Lou

[Joe Travers]

Probably see the track a couple times a year. Leaning toward the mechanical.

I would run this one- https://www.holley.com/products/fuel_sy ... ts/0-4779C
Holley sells remanufactured ones for less. Probably the way I'd go.

Joe

[Lou's Comet]

Here's a pretty good article on it;
https://www.motortrend.com/how-to/vacuu ... arburetor/

Thanks John

I read that! Before I read that I was going to go with the vacuum secondaries, after reading that and looking at the bottom of the article where it tell you use this type if etc, etc, and use the other type if etc ,etc, I may have more of the requirements for the mechanical secondaries . Then I got to thinking lol.

Will get in touch with the builder this week and maybe get more info and what my cam specs will be. Short block is suppose to be done and ship at end of week.

[comethead]

Hey Lou the Falcons 302 is running a smallish .540 lift mechanical flat tappet cam with 290 duration. It’s pretty lumpy and I run a Holley 650 HP double pumper. No choke and a “race” calibration. Driving around town with a dual point distributor, mechanical secondaries, race valved Bilsteins, and a 4 speed is actually pretty fun
And of course mashing the sandal down on the track gets you all the gas you need NOW.
I’m ok with the user controlled secondaries- street or track.
I like this one https://www.holley.com/products/fuel_sy ... 0-80803BKX

Joe