It's just some free-thinking curiosity this morning, but what are the factors one would use to decide between the two carb styles?
Pros & Cons of vacuum vs. mechanical secondaries?
- Thread starter halfafish
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QuickDart360
Well-Known Member
I suppose vacuum secondaries don't work too well with big cams? Not sure though.
K.O. SWINGER
Meeting in the alley since 1976
Vacuum secondaries can be set to just about any camshaft unless you're going solid roller or huge flat tappet. I love vacuum secondaries for the street when working properly they're a thing of beauty like a six pack, I've also run mechanical secondaries with great luck especially with high rear end ratios which allows the motor to rev quickly and adjust to quick air changes better. Then you have carbs like a Carter Thermo which controls the air with a air door that's weighted. Sizes important more so with mechanical secondaries. Either can be made to work. For speed cars are more forgiving especially with high gear ratios.
Locomotion
Well-Known Member
If tuned correctly and the car combo can handle the double pumper, it will launch harder & be faster than a vacuum secondary. I tested quite a bit and I saw at least a tenth advantage in a 750 4779 double pumper over a 750 3310 vacuum carb on my particular 360 race only application. But I had lots of gear, converter, cam duration, etc.
A vacuum carb will be more forgiving and drivable on the street and many track applications. I think they are easier to tune.
A vacuum carb will be more forgiving and drivable on the street and many track applications. I think they are easier to tune.
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33IMP
Well-Known Member
My car (well leave out the brand for the moment) currently has a very mild truck big block in it. Current carb is a 600 afb style edelbrock. Vacuum actuated secondaries. It probably needs a 750 vacuum holley or eddy.
When it gets the 496 or 540, an 850 dp will be the minimum carb, it will probably get a 1050 dominator.
My street strip 440, has a 850 dp, would not even consider a vs.
As for factors that determine choice, I would list rearend gear first. 2.76s won't like a double pumper much.
When it gets the 496 or 540, an 850 dp will be the minimum carb, it will probably get a 1050 dominator.
My street strip 440, has a 850 dp, would not even consider a vs.
As for factors that determine choice, I would list rearend gear first. 2.76s won't like a double pumper much.
j par
Well-hung Member
Just put a simple Edelbrock 1406 on it...
And if we're talking slant 6 a nice Factory two barrel...
And if we're talking slant 6 a nice Factory two barrel...
dano
Evil Handy Man
Whatever carb or efi your used too and comfortable tuning.
I've only tuned a mechanical once, the 650 Quick Fuel I have now. I think VS are more forgiving. My biggest problem is lack of tuning information, as in accurate consistency. Seems there is 10 different way to tune them and deciphering which is the correct way can be a real pain for a daily driver.
I've only tuned a mechanical once, the 650 Quick Fuel I have now. I think VS are more forgiving. My biggest problem is lack of tuning information, as in accurate consistency. Seems there is 10 different way to tune them and deciphering which is the correct way can be a real pain for a daily driver.
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j par
Well-hung Member
Agree.. the op needs no tuning headaches... Just getting them on the road in a timely matter seems to be a hurdle...
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Treblig
Well-Known Member
I always thought that the vacuum secondaries were manufactured to give you better fuel economy? Mechanical secondaries are actuated every time to press on the accelerator too far but vacuum secondaries only open if you have enough vacuum to open them, like when you stomp on the gas??? Mechanical would be best suited for the strip or aggressive street drivng when you don't care about gas prices!!
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Actually, it is airflow through the venturis that actuate the VS, not engine vacuum. There is little engine vacuum when mashing on the gas, but as airflow increases through the venturis, that causes the VS to open. Changing the VS spring brings the VS in earlier or later, and is the primary way to tune them. It is a very efficient system for street engines.
If it's a properly matched combination, there are ZERO cons to a mechanical secondary. Ok, well perhaps just one. Decreased fuel mileage in comparison.
I just installed a 750 dp onto my 408 and am impressed by the improved low and midrange response compared to the 750 VS I was running from BPE. Getting it "all in" when slamming the throttle open down low is amazing. Yes, fuel economy will be less, but I did not build my car for economy, but fun!
Exactly! Every time I see "MPG" in a post here about a hot rod I think "pussy!". lol Because in the end, if you tune properly for best performance, "whatever" your combination is will be getting its best mileage. People just cannot understand that.
If it is a driver and you drive in cold weather, mechanical secondaries suck. If you push down the gas a little too far before it is warmed up you lose your choke & it will likely die. Pretty scary if you are half asleep pulling out in traffic…..
Then don't be a moron and let it warm up. This ain't EFI we're talkin about. lol
Gas mileage consists of a few things and one’s build may not be suited to it no matter what you do, but, in the end, it really is a state of tune of the carb, easy breathing in and out, a complete burn and most importantly…… a light foot.
Of course things like engine size, camshaft size and gear ratio are going to limit what can be done per gallon, but getting the best of what ever you have should be what your after no matter how hat the combo is.
OK - race vehicles may not care…
The pro’s and con’s? Hummmm ….. that’s a bit like splitting hairs. On a mechanical secondary carb, I will stick with the Holley DP as the example, the above complaint of to much throttle when cold has happened to me but that was short lived as you learn that curve real quick. IMO, it’s just a minor hinderance for a minor amount of time.
On a VS carb, it has to be getting the opening of the secondary side timed right and full open at its earliest without issues. That’s right foot controllable with a DP carb.
After that, it’s splitting hairs. Nothing to complain about between them. I find the VS a good carb to use on anything except a racing vehicle or a serious street machine. The higher end of performance. And just the flip for a mechanical secondary carb. Like said above, no sense of one on a car that’s a mileage master. But ether carb will work on ether end of the spectrum. Just not as great as it could be.
Of course things like engine size, camshaft size and gear ratio are going to limit what can be done per gallon, but getting the best of what ever you have should be what your after no matter how hat the combo is.
OK - race vehicles may not care…
The pro’s and con’s? Hummmm ….. that’s a bit like splitting hairs. On a mechanical secondary carb, I will stick with the Holley DP as the example, the above complaint of to much throttle when cold has happened to me but that was short lived as you learn that curve real quick. IMO, it’s just a minor hinderance for a minor amount of time.
On a VS carb, it has to be getting the opening of the secondary side timed right and full open at its earliest without issues. That’s right foot controllable with a DP carb.
After that, it’s splitting hairs. Nothing to complain about between them. I find the VS a good carb to use on anything except a racing vehicle or a serious street machine. The higher end of performance. And just the flip for a mechanical secondary carb. Like said above, no sense of one on a car that’s a mileage master. But ether carb will work on ether end of the spectrum. Just not as great as it could be.
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That really comes down to tuning and the driver understanding the quirks of the engine. Again, the VS carbs are more forgiving, but driving too deep into the throttle when the engine is not fully warmed up is not the carbs fault!
Winner winner chicken dinner! lol
thesiren74
Well-Known Member
I will never run a vacuum secondary carb again after having mechanical.
K.O. SWINGER
Meeting in the alley since 1976
I agree that tuning is 95% of the setup the Mopar Bible still lists the 750 Holley vacuum secondary as the go-to performance carburetor for all around. It's hard to argue with the initial hit of a double pumper when set up right with the right combination.but a vacuum secondary is set up correctly can be all in so quick that you're not losing much under most circumstances it's equal. My six pack is already into the outboards/vacuum operated coming out of the hole. I think most controversy is that people have poor setups crappy tunes and then blame it on the carburetor.
thesiren74
Well-Known Member
I had a 3310 with the qft jet plate and qft adjustable secondary. My street q-series 750 mechanical secondary blows it away.
Am I off base here, is the secondary pump not actually working UNTIL the secondary linkage is coming in..?? (Mechanical) secondary. At which point if your driving normally why would your mileage change with a proper tune..?? Seingn’
thesiren74
Well-Known Member
Yeah you are only cruising around on the primaries but it's so much fun hitting the secondaries on a mechanical secondary.
Morons have the most fun. I wouldn’t have any fun at all if I only did things normal people do….
Treblig
Well-Known Member
That's the way I look at it................I like breaking rules and regulations.......I tell my kids, "Don't be afraid to break the rules!!"
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