Manifold vacuum port question

[e rock]

Vehicle: 71 K20 350/465

The motor I just pulled out had just one (5/16"?) port on the rear of the intake that fed the power brake booster. The new motor going in has a vacuum port (is that even the right word? I've been searching for pics, but apparently I'm using the wrong word.) that has 2 smaller tubes out the side (1/8"?). From recent threads, I think I'm convinced to using the manifold vacuum for the dizzy's VA, but will the other one be ok to use for the brakes?

It sounds like I'm going to have to buy a vacuum reserve canister anyway due to the cam I'm running, so maybe the small port size isn't as big a deal?

Thanks,
Eric

[AR15DEFENSE]

you should be able to remove the fitting and install a new one to use for your BB.. you want the booster to have vacuum at all times, bit the distributor should go to ported vacuum at the carburetor. there should be 1 port on you carb that has no vacuum at idle, and has vacuum when you throttle it.

[e rock]

Originally Posted by AR15DEFENSE

you should be able to remove the fitting and install a new one to use for your BB.. you want the booster to have vacuum at all times, bit the distributor should go to ported vacuum at the carburetor. there should be 1 port on you carb that has no vacuum at idle, and has vacuum when you throttle it.

That's what I thought too, but then I caught this thread: http://67-72chevytrucks.com/vboard/s...d.php?t=241086

I honestly can't remember if this motor/carb combo was set up with ported or manifold vac when I pulled it so I'm kinda starting from scratch.

[e rock]

Bump for a voice of experience.

Can I run a brake booster off the little vacuum nipple? Does the size of the port really make a difference?

[jhow66]

What kind of carb?

[e rock]

Originally Posted by jhow66

What kind of carb?

Holley 3310

[jhow66]

Just reread you question. Do as AR15' says.

[AR15DEFENSE]

I read that thread and honestly it don't make sense to me... goes against everything I've learned in the past 10 years. I would hook it up how I know it works... PS, kinda related.. GM had a service bulletin out a few years back on the late model vehicles and faulty crank sensors. They won't always throw a code but when they go bad and cause too much initial timing, it will cause hard starting at the least, and can kick the starter back hard enough to break the engine block.

[e rock]

Originally Posted by AR15DEFENSE

I read that thread and honestly it don't make sense to me... goes against everything I've learned in the past 10 years. I would hook it up how I know it works... PS, kinda related.. GM had a service bulletin out a few years back on the late model vehicles and faulty crank sensors. They won't always throw a code but when they go bad and cause too much initial timing, it will cause hard starting at the least, and can kick the starter back hard enough to break the engine block.

I didn't mean to sound like I didn't appreciate your input, I'm just trying to sort through a lot of info to get this thing set up with a minimum of 'do-overs'.

I've read a lot about vacuum in the past couple days, and I can see why it would work both ways, but I just don't have enough experience to decide which is best for my situation. I do like the idea of not worrying about changing the VA unit and/or the MA weights to get it tuned right.

Thanks.

[KFreddy]

If you are using a spacer between the carb and the manifold, you can simply drill two holes in the spacer, tap it with a 5/16th thread then screw in two 5/16th brass plumbing fixtures. Use these two lines for you brakes and Dizzy. Not only is it trick but you will then be pulling the vacume more evenly off of all 8 cylinders instead of just the #8 cylinder...

[bigjimzlll]

Ahh..The ported or manifold vacuum deliema.
My take on it is if you have a bigger than stock cam use manifold vacuum, but to do this you need to make a few mods to the distributor.

If you limit the mechanical to around 15 degrees of advance then you can set the initial timing to 20 or so degrees. Then with a vacuum advance of 12 or so you will have an idle advance of 32 or so.
This will clean up the idle with the big cam an give alot more throttle response. There will be no hard starting because there is no vacuum until its running.

Now this is for a performance tune up...I have no idea about fuel milage.
I have run a locked out distributor for years. It sure makes it nice when you eliminate one more variable from the tuning process. By controlling the vacuum advance you also limit the variables.

[76bonanza]

bigjimziii does it hurt your throttle response when you lock out the VA?

[bigjimzlll]

I not only locked out the VA but all advance. So it really helps throttle response. This is for a full performance only deal. Limiting VA and Mechanical really helps throttle response on a daily driver.

Have you ever noticed how much better the idle quality is with a higher initial timing? Without the mods I discribed if you increase initial to get a good idle you would have too much total.

These mods really help with a big cam that has low vacuum.

[e rock]

Originally Posted by bigjimzlll

These mods really help with a big cam that has low vacuum.

That's what I'm dealing with.

For the time being, it sounds like I should run the VA off ported vacuum until I buy a new VA that I can limit.

[Billla]

Originally Posted by bigjimzlll

I not only locked out the VA but all advance. So it really helps throttle response. This is for a full performance only deal. Limiting VA and Mechanical really helps throttle response on a daily driver.

Have you ever noticed how much better the idle quality is with a higher initial timing? Without the mods I discribed if you increase initial to get a good idle you would have too much total.

These mods really help with a big cam that has low vacuum.

You're suggesting this on a street-driven 350? Or that 700 HP monster you drive?

If you're recommending this for a street-driven egine, I'll respectfully disagree.

[bigjimzlll]

No..on a hot street driven ride, with a big cam(poor idle quality and vacuum) I like to run manifold vacuum with it limited to 12 degrees. I also limit the mechanical to 12 to 15 degrees with it coming in at the rpm the stall converter stalls to.
This allows you to set the initial timing to 20-22 degrees, add in the vacuum advance and the effective initial timing 32-34 degrees and as the mechanical comes in the vacuum goes away.

With a cam with a lot of duration the high initial timing really cleans up the idle quality and really speeds up the throttle response.

Again this is for a "hot" street or strip engine with maybe too big of a cam. It is not for fuel milage or long distance freeway jaunts.

[AR15DEFENSE]

drank so much tonight I cant type, but lets think about the basics,,, as RPM goes up.../ time for combustion to occur goes down. optimal power is made when total combustion happend at 0 TDC... so the point is, to get it close to that . at lower RPM, detonation can occur if timinf is too advanced. i fyou are retarded, power is lost because said combustion is happening after TDc when the piston is already on its way down, ok good night

[JameslovesJammie]

Originally Posted by KFreddy

If you are using a spacer between the carb and the manifold, you can simply drill two holes in the spacer, tap it with a 5/16th thread then screw in two 5/16th brass plumbing fixtures. Use these two lines for you brakes and Dizzy. Not only is it trick but you will then be pulling the vacume more evenly off of all 8 cylinders instead of just the #8 cylinder...

Is this a common practice? I am having the dilema that the carb I put on doesn't have any vacuum ports on it and I would like to hook it up. Can there be more than one hole in the spacer, such as one going to the vacuum advance and one going to the vacuum canister? I am using the manifold vacuum port on cylinder 8 currently for power brakes.

[jhow66]

As many as you want.