Carb needs vacuum

[69clone]

So I found out the cam in my 498 is only going to make about 8in of vacuum at 1000rpm I am now worried because that is not going to be enough to run a vacuum secondary carb? I was planning on buying http://www.jegs.com/i/Demon+Carburetion/333/1563010VE/10002/-1 (demon 850). If it doesn't make the vacuum needed would my next step be to buy a vacuum pump? or could I just turn it into a mechanical secondary? Sorry I'm a bit lost on this.

 

[RustyRatRod]

The vacuum secondaries will operate fine. The engine has to be under a load to produce the kind of vacuum it takes to open them. Use whichever carburetor you want, it will run fine.

 

[69clone]

Thank you Rusty, I will do. I figured it would and never questioned it until I was talking with Comp cam tech who said the cam would only make 8in of vacuum which wasn't enough for what I needed. Oh well if it comes down to needing more vacuum to open I will just have to get a pump.

 

[RustyRatRod]

Did he actually say it wouldn't work the vacuum secondaries? Dammit. Who are they hiring these days? What a moron. It will work fine. No need for a vacuum can. You'll see.

 

[D575]

My car produces 8" and it operates a six pack just fine.

 

[coloradodave]

Vacuum to operate power brakes and vacuum to open the secondaries on a carburetor are an apples and oranges discussion. I'm with Rusty on this. The sales guy mislead you. Your carb will operate fine, as long as you don't expect a 'kick in the pants' when the secondaries open. You shouldn't feel them open if everything is working right.

 

[69clone]

Did he actually say it wouldn't work the vacuum secondaries? Dammit. Who are they hiring these days? What a moron. It will work fine. No need for a vacuum can. You'll see.

Yup here is part of the transcript from our conversation.

Ryan: Hi Nick, do you have the part number for that cam?
Nick: 21-226-4(Single Bolt)
Ryan: probably around 8 inches at 1000 rpm maybe
Nick: Do you think that will be enough to run a vacuum secondary?
Ryan: how many inches are you needing?
Ryan: Is it for brakes?
Nick: For the carburetor
Ryan: no, it wouldn't be enough. The 21-224-4 would work

 

[snakeoil24]

The cam guy doesn't know what he is talking about Rusty and Colorado are right on. The carb will work fine with 8 inch vac. The vacuum comes from the throttle bore which always will create enough signal. As these guys said power brakes is another story.

 

[RustyRatRod]

Right. I am hoping beyond all hope he meant P/B, but from the context it doesn't look like it. lol

 

[69clone]

Thanks, more peace of mind. Yeah I'm not worried about power brakes if I upgrade I'm going with manual disc.

 

[Meep-Meep]

Yep, the secondaries of a vacuum secondary carb use venturi vacuum as a vacuum source to operate the secondaries. If your engine pulls a strong enough vacuum through the primaries, and that signal is strong enough to overcome the secondary diaphragm spring, the secondaries will start to open.

Having a cam that only produces an idle vacuum of only 8" will require very high initial advance and low vacuum power valve settings. Low speed mixture control won't be good either. Also a high stall converter and deep gears to help make it all work will be required.

 

[lewtot184]

i like to refer to these carbs as "velocity controlled". the secondary opening has nothing to do with manifold vacuum, but rather the velocity of air thru the primary venturi that has a passage connected to the diaphragm pod.

 

[RustyRatRod]

i like to refer to these carbs as "velocity controlled". the secondary opening has nothing to do with manifold vacuum, but rather the velocity of air thru the primary venturi that has a passage connected to the diaphragm pod.

You should call Comp and get a job. lol

 

[lewtot184]

You should call Comp and get a job. lol

rusty, i really hate comp cams. i truly believe if it wasn't for all the free magazine ink they get they'd be out of business.

 

[RustyRatRod]

It's entirely possible.

 

[kb67mopar]

i like to refer to these carbs as "velocity controlled". the secondary opening has nothing to do with manifold vacuum, but rather the velocity of air thru the primary venturi that has a passage connected to the diaphragm pod.

The velocity is what actually creates the vacuum. Its also how the fuel system on the carb works...no velocity no fuel (except accel pump). if it draws fuel, its pulls vacuum which means your vac secondary should work like the others have said.

 

[RustyRatRod]

That's correct, but Lew hit the nail on the head. The vacuum secondaries are not really operated on manifold vacuum. They are operated on the air rushing through the rear venturis on the carburetor. Technically, that's a little different. If it WAS operated on manifold vacuum, the secondaries probably would NOT open because vacuum falls off almost to nothing when you stab the gas and get into the territory where the secondaries open. I knew how it worked......but damned if Lew didn't just come in here and sum it right up with a great post. Good job Lew.

 

[lewtot184]

velocity controlled holley 4bbl secondaries begin their opening thru a passage in the passenger side primary venturi that is connected to the vacuum pod. there is a pressure differential in the venturi vs the pod that draws air out of the pod. as the secondarys begins to open/function additional air is drawn out of the pod thru the kill bleed. there has to be a pressure differential between the air in the venturi and the vacuum pod for air to move in or out of the pod, but the differential is above the throttle blades and manifold and isn't considered manifold vacuum. as long as a sufficent amount of air is moving thru the primary venturi somethings going to happen in secondary movement. the springs in the pod provide a resistance to control the rate of opening. there should be some good holley tech out there that would provide a more detailed explanation of whats happening with secondary actuation. personally, i like vacuum holleys and they seem to get an undeserved bad rap. on another note here, carter afb, thermoquad, and q-jets use a different method of secondary opening.

 

[RustyRatRod]

I think the reason they geta bad rap is because you caint feel them "kick in". .....and if you can they are not tuned properly.

 

[69clone]

Thanks guys, not that I did not trust you guys but I contacted Demon carbs today and explained my engine build/combo and they assured me that the Demon 850 with vacuum secondaries would be a great match for my engine and confirmed what you guys said about it using the air speed moving through the carb and not the engine vacuum.
Thanks again guys, will be ordering one this next couple days and move onto the next thing on my list.