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I've got a holley 4150, 2818-1 that only offers the timed/spark advance port. No manifold vacuum port exists on the carb. Should I pipe into the manifold to get manifold vacuum for the distributor vac canister? Thanks in advance. Bill
EDIT: But in doing so, take it all the way and verify/revse your distributor advance curve and vac can. They all work in harmony. I could post up the articles for you to follow, but we have gone over them many many times here already.
Last edited by Powershift; Dec 10, 2019 at 11:00 AM.
Yes. It's pretty easy to tee off the vacuum port for the power brakes if you have them.
The metal line shown in front of the choke is ported and where my vacuum can was connected when I purchased. I capped the line and moved the hose to the tee on top of the manifold where the brake booster is connected.
Last edited by 65GGvert; Dec 10, 2019 at 11:04 AM.
EDIT: But in doing so, take it all the way and verify/revse your distributor advance curve and vac can. They all work in harmony. I could post up the articles for you to follow, but we have gone over them many many times here already.
It's that time of year when I don't have much to do but tinker with my toys. Looking at one of Lars postings, he corrected an owners connection with the t/s advance port just as mine is hooked up too. He was fortunate to have a manifold port on the carb base plate to switch to but my plate doesn't have. That's why I'm asking. Yes, I'll take it all the way! Thanks a bunch. Merry Christmas too!
Last edited by Brumbach; Dec 10, 2019 at 11:33 AM.
Yes. It's pretty easy to tee off the vacuum port for the power brakes if you have them.
The metal line shown in front of the choke is ported and where my vacuum can was connected when I purchased. I capped the line and moved the hose to the tee on top of the manifold where the brake booster is connected.
I've got manual brakes but where you're connected is exactly what I"m targeting. Thanks
Most carbs can be internally modified by tracing the VAC signal passages with a thin wire. The horizontal passage in the throttle bore usually intersects a vertical passage that leads to manifold vacuum. If you drill another horizontal hole below the throttle valves to intersect the vertical passage, then plug the original horizontal passage above the throttle valve, you have full time vacuum advance.
If the choke has an external vacuum break diaphragm, you can tee into the signal line, which should be full time manifold vacuum.
Most manifolds have a pad behind the carb for an nipple for the power brake vacuum hose, but if not originally equipped with power brakes it may not be drilled, or maybe just plugged, so a NPT to 1/8" nipple can be easily installed.
EDIT: But in doing so, take it all the way and verify/revse your distributor advance curve and vac can. They all work in harmony. I could post up the articles for you to follow, but we have gone over them many many times here already.
I've always wondered why they're set up with ported vacuum at the factory if full time vacuum works better. No doubt there's a thread for that here somewhere
With a stock VAC, would simply switching to a full time vacuum source improve low rpm response, or does the factory advance curve need to be changed to see any benefit?
Yes there is a thread here on the Forum that completely answers your questions. Do a search on past posts by Duke (SWCDuke) and you will find it. In brief, ported vacuum makes the car run hotter and raises the engine temps and burns more of the to be expended gasses at higher temperatures. This was decided and done by GM engineers, especially to accommodate emission requirements in California. That move had a domino effect and they ended up using ported vacuum across the board for all 50 states. They did not do it because it made the car run better but likely because it was easier and cheaper to do them all except for just the Vettes for sale in California.
Fast forward 50 years. Most of us are/were clueless about this fact (well known to Duke, John and Lars) and so Duke has been enlightening all of us here on the Forum about the switchover (since emissions are not an issue for us). And no, simply switching to manifold vacuum from ported vacuum would not necessarily improve low end response. It might actually make things worse. There are multiple adjustments to be done to your engine including establishing a baseline, adjusting your fuel idle mixture to obtain maximum manifold vacuum, replacing your stock vacuum cannister with a cannister that will meet Duke's two inch rule, possibly replacing your distributor springs to either stronger or lighter springs, adjusting the rpm and the timing etc.
Duke, John H, and Lars G. have written extensively on this subject and there are several highly detailed papers here on the Forum if you just search for them. Making the switch is surprisingly easy to do, inexpensive other than your time, and is the best bang for the buck in the C2 world for making your car run better, cooler and more efficiently. All of this information has been provided to us by Duke and John and Lars. If I have misspoken or explained in error, they will correct me but that's kind of the answer to your question in a nutshell without getting into detail. And it saves them from having to type it all again.
If the 2818 has it's original metering block #4099 (I think), that port is full manifold vacuum so test it before making any changes.
Someone with an original 2818 can confirm that metering block part # that is stamped into the top.
I don't have any way of knowing if the metering block is original, but the port on the 2818-1 carb on mine (pictured) is ported. That was my reason for moving it to the manifold connection.
Last edited by 65GGvert; Dec 10, 2019 at 07:16 PM.
I always assumed (yes i know the riddle), that even though the chrome vacuum line is going into this upper port above the throttle blade that it was casted/drilled with this port actually entering below the throttle blade to be full manifold vacuum. I have yet to check it on my new to me 65 coupe 327/365 but that’s what I thought?
I've always wondered why they're set up with ported vacuum at the factory if full time vacuum works better. No doubt there's a thread for that here somewhere
With a stock VAC, would simply switching to a full time vacuum source improve low rpm response, or does the factory advance curve need to be changed to see any benefit?
Optimum total idle advance is in the range of low twenties to low thirties, and it's basically a function of valve overlap. Low overlap cams are good in the lower half of this range and high overlap cams should be in the upper half. On pre-exhaust emission controlled engines, this was usually achieved with the sum of initial timing and full vacuum advance.
As total idle advance is retarded from the above range, EGT increases, which helps burn HC and CO in the upper exhaust system, especially with air injection. So ported vacuum advance is an emission control strategy, but it results in higher idle fuel consumption and more heat rejected to the cooling system.
Most OE ported setups will need a different VAC than OE in order to pass the Two-Inch Rule when converted to full time
More details are in my tuning seminar in a thread started by me... easy search.
My 2818-1 is full manifold vacuum at that port, as all 2818 and 2818-1 should be.
I pulled my metering block and traced the vain from the port provided on my 2818-1. It appears that the vain stops just short of the throttle plates. Possibly my block is incorrect. To be on the safe side, I'm going to connect directly to the manifold.
From: At my Bar drinking and wrenching in Lafayette Colorado
Be aware that many of the OE Holley carbs with the vacuum port in the metering block (as shown in the photos above) were internally drilled to make that port a manifold vacuum port - not a ported port. Before you change anything, check it and verify that it's not already a manifold vacuum port. I was very surprised the first time I ran into an OE Holley with manifold vacuum at that nipple (L79's had manifold vacuum at that port). I've seen several others since. Check it and verify before you make any assumptions...
My 2818-1 is full manifold vacuum at that port, as all 2818 and 2818-1 should be.
I stand corrected. After closer examination, the port nipple DOES provide full manifold vacuum. The vain might stop short of the throttle plates but the hole at the end of the vain has a passage through to the carb base and below. The hole in the base is clear of the base gasket. Could've saved myself $40 had I examined more closely. Also sorry I didn't heed your advice!
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Manifold vacuum question
