Hi, Have found a lot of information online but still need help with a plan. 71 convt project car, getting a 383 motor running on a stand and put in the car this weekend. Have a Holley 4150 700 dp/dual feed on Edelbrock EPS. Car is T400 automatic.

Vacuum needs: PCV, brake booster, auto trans, distributor, then all the other corvette elements that will connect to the vacuum reservoir such as headlights, switches, wiper door, etc.
Vacuum sources: Large port on back of carb base plate. Small direct vacuum port on front of carb base plate. Timed vacuum on carb front metering block.

Short on ports, so made a 90 degree fitting (arrow below) to squeeze under the secondary bowl and the ridiculous vacuum port on the runner of my EPS. Know many of us have cursed Edelbrock for ensuring double pumper Holleys have this problem. I bought the $16 fitting that was advertised to solve the problem, and that was a joke. Here is my solution and it still barely fits - maybe 1/16" clearance. Not pretty, but will give me vacuum to the reservoir and inside the car via the check valve and tee. This will also need to provide vacuum to the transmission.


The large port on the back of the carb base plate will provide vacuum to the PCV valve. Know I cannot tee that line and run to the brake booster as well, so tapped into another spot on the manifold, in place that would clear throttle linkage. Also can't go into the top of the runner as the aluminum is really thin, and wouldn't have enough thickness for proper threads. After drilling/tapping, it is now capped (at arrow below) as someone advised me against having it so low in the one side of the plenum, but didn't have real justification. Does anyone know if this would be a problem? If not, this can be for the brake booster.



If this doesn't work, then I am still short a port for brake booster, which cannot be teed for consistency. There is this spot on the front of the carb base plate, where some Holleys have a large PCV port. Here is the spot below.



And here is what it looks like with the port.


Has anyone drilled and tapped into the base plate of their Holley to get an extra port for PCV? If this is possible, may be a better solution that going into the side of the manifold. Thoughts?

For the last consumer of vacuum, the distributor will get the un-ported vacuum from the small port on the front of the carb. That should be everything. Guessing the timed vacuum on the metering block would just be capped.

Any help on this would be greatly appreciated.

Cheers,
Brian

 

How about a spacer with the vacuum port in it? How’s your hood clearance and are you using a drop base air cleaner?

 

I am using a drop base air filter, but that's exactly why I chose the EPS (vs. the taller RPM Air Gap) and can't use a 1" carb spacer. Using the stock non-LTI/BB hood and don't want to go that route with my small block.

Tried finding a 1/2" spacer with a vacuum port, but quite sure those don't exist, nor can be fabricated...

 

I often wondered how drunk the Eddy engineers were when they choose that spot for a vacuum fitting. Stevie Wonder could see that was poor design.

I Googled "Low Profile" Vacuum Fittings and found some promising units, but was still skeptical with using a Holley DP. Some were a little pricey, stainless, and had three 1/8" NPT ports with it, but came with cheap plastic fittings. Switching to three brass fittings, all-in-all came out to $50 for the stupid thing. It was clever, but still searching for a better unit.

In order of vacuum importance, your PB Booster is number one. The handful of others Vac needs are just accessories (ya right).
Number two priority is your dizzy Vac. Your Timed Port as you know is to be blocked off. So that leaves a few more needs as you stated.

On my Ed 2101 the vac port is blocked off and have a maze of rubber hoses going everywhere off the carbs port. (Will be swapping to an Air Gap but with the same problem of DP clearance)

Last winter, a forum member posted a picture of a Eddy Intake with a drilled & tapped port in the back, just under the carb base plate. That was really slick, that any machine shop could do for chump change. The owner installed, I believe a 3/8 NPT, maybe, brass-barbed fitting. That new vac port fixed all his issues about the pump and supplied vacuum to all those acc. Wish I had a picture to show you where the new port was drilled.
Something to think about anyway.

Add: Drilling a additional vac hole in the carb throttle plate sounds like a bad idea.

 

Apparently, they learned something about that and tapped my BBC Airgap on the lower runner. I still needed their low profile fitting that is a 90 degree one.

 

Install the brake booster as planned and move on.

 

The auto trans modulator and the dist. vacuum can may be connected together. Those should be connected to the 'manifold vacuum' port on the carb baseplate. The brake booster should connect to the rear outlet and the PCV line to the front fitting. You need one other fitting off of the manifold (the one you added should work fine). That should connect to a vacuum reservoir tank/can. The other vacuum systems should connect to that vacuum tank reservoir.

I would not recommend drilling more holes into the carb baseplate. That would not be a good thing, IMO.

 

I would purchase another baseplate before I drilled and tapped that one. Holley and AED have them of course.

Jebby

 

Thanks 7T1, Understand all but the PCV to the front fitting. The only front fitting on the carb is the manifold vacuum on the base plate going to the dist/trans. Believe still one short for the PCV. Did I understand you correctly? Were you referring to the timed vacuum?

 

Considering that you have a port in your #8 runner and the plenum of the intake, I would have no issue with running the brakes off of either one.

 

If that is a pipe plug on intake manifold left side, under the carb, that would be the best place to connect your PCV 'purge' line, IMO.

 

I don't believe any of the after-market plates have more than three Vac ports though. But maybe.
Timed Port is useless to most of us.

 

Thanks. That is my real question, if anyone sees an issue with putting my barbed fitting back in this plugged hole and either running PCV or brake booster from here. Again, someone with dubious credentials informed me it was too low and would cause all sorts of issues, but I was skeptical as it is vacuum in the plenum as it is in the runners.

Know the PCV is more of a "controlled vacuum leak," so will likely run that off the back of the carb, and use this one for the brake booster, as it doesn't require too much actual flow and perhaps less likely to lean the mixture going into that side of the plenum, and hence the cylinders fed from that side. My EPS doesn't have a relief or ridge in the plenum divider, as I want to keep the torque up, so whatever is pulled in here affects only the one side.

 

The only issue I see is interference with throttle. The PB Booster uses a huge thick-wall rubber line to it.
But if you can manipulate a line from that port, to around the Holley linkage, under the throttle cable and not touching the return springs, go for it.

Myself, I think I would leave the carb port to booster hose, well enough alone considering the bulk of the booster hose.
Then use your hidden add-on port for HVAC, Headlites, Trans, etc.

 

I would not run the brake booster from the 'side' port. The lowest vacuum point is at the base of the carb. It might be OK to put it on the side of the manifold; but it makes more sense to me that the PCV purge line go there, is the vapors are to be ingested and burned with the air/fuel charge. At least, that's the way I'm seeing it.

 

Yes, that sounds like the right approach. Thanks again.

Interesting thought that boats often run their PCV lines to bungs in the headers to pull crankcase fumes and pressure. But can't imagine the oil and gunk that would end up in the mufflers of a car. The thought had crossed my mind as a potential solution, but only for a split second.

 

Those type of systems usually only work with open headers that produce lots of air velocity & vacuum to pull the fumes out of the crankcase.
In other words, off road / racing apps.