Carb Spacer Material [Archive] - Team Camaro Tech

: Carb Spacer Material

Aug 29th, 00, 03:54 AM
I'm installing a 383 with a Performer RPM manifold and a Holley 4150 in my 67 Rag. On the RPM manifold, the plenum vacuum port I want to tap off of for the power brake booster is right under the rear carb fuel bowl, and I have been unable to find a fitting that will fit under the carb with the approximately 1/4" thick gasket that the engine shop used when building it.

In a previous post on this subject, several suggested using a thicker carb spacer. The spacers I have found are either aluminum, phenolic, or plastic. Can anyone tell me the pros and cons to these materials? Any impact on electric choke performance if an insulating material is used?

Would also like to hear opinions on whether I should go with a open plenum or a 4 hole in this application. The engine dyno'ed at 435 HP and almost 500 ft. lbs of torque.

I'm also concerned about hood clearance. I am using a GM open air cleaner that has the low profile base that fits over the carb. As I have the front end off the I need to keep the top of the air cleaner below the top of the firewall as a rough swag?

Mark M.
Red 67 Convertible
Red Deluxe Interior
383/Tremec 5 Speed
4 Wheel Disk

Aug 29th, 00, 06:28 AM
I am running a phenolic spacer on one of my cars right now. I bought it as I was having a problem with vapor lock (gas boiling in the bowls of the carb). In another spool on this site, someone did a study with his kid for school science and they found that some gas boil at a temp as low as 110 degrees. Since installing the phenolic spacer, the car has never vapor locked (five years). This material is very good at insulating the carb from the tremendous heat coming from the intake that causes this boiling.
As far as your choke, because it is activated by electricity and not by engine heat, the phenolic spacer will have no effect on it's operation.
I will be very interested to hear any input others have between the four hole vs. open plenum. What are the advantages of each?

Aug 29th, 00, 06:37 AM
Use a 4 hole spacer for street & mild strip use, gives a better signal to the carb circuits, even with an open plenum.

I make them out of good wood, but the phenolic or plastic work as well, and no fabrication is needed.

Might look on the carb rear, under the fuel bowl, some of them have a vacuum oprt that is same-same as the port you are attempting to use on the intake manifold. Some carbs have them, most don't though, worth a look at least.

Just FYI, Dominator users have a little (actually-big) trick they use for better performance. They relocate the throttle shafts and plates upwards in the carbs, as much as 5/8 inch. Makes the carb have better transition and better signal at the different circuits. Really helps the intermediate idle circuits in the Dominator. Point is, a good 4 hole spacer does the same as this on the non-Dominator carbs, which cannot use the higher throttle shaft/plate mods without a different (never produced) base. Just fuel for thought.

Aug 29th, 00, 07:07 AM
I-man hit the nail on the head. Wood is THE BEST thermal insulator you can use, but is sometimes difficult to work with.

I used to run a 4-hole 1/2 inch phenolic spacer on my car for the exact same reason you need one and had a slight rub on the front of my Moroso drop base air cleaner with the standard flat hood. I had a Holley 300-36 (same as Performer RPM), 750 vac sec, and a 3 inch air filter.

Hugger Orange & white 69 Camaro with supercharged 350, Tremec TKO, and 3.73 12-bolt

See my website updated 8/13/00 at: (

Aug 29th, 00, 07:36 AM
Ignition Man,

Yes I do have an unused vacuum port on the back of my carb. When I asked about using this port in the past, a couple of people said it is for a PCV hookup and shouldn't be used for the PB booster. It would solve my problem though.

This gets into another post I did asking about crankcase ventilation. Some suggested to go ahead and hook up a PCV with the right valve cover hooked to the air cleaner base and a PCV in the left valve cover.

I'm tempted to just use the carb vacuum port for the brake PB, hook the right valve cover breather to the air cleaner base as I need to plug the air cleaner base hole anyway, and just put a regular breather cap in the left valve cover.

Mark M

Aug 29th, 00, 10:37 AM
How about drilling and tapping another hole, perhaps on the same vertical surface/distance from the carb on the same runner?

Is there a min/max distance from the carb that this vacuum tap should be held to?

Can you tie both the PCV and PB booster into the same vacuum tap on the back of the carb?

Click here to see see my car and hear 5-speeds.

Aug 29th, 00, 11:47 AM
Tapping a new hole might have been a possibility when the intake was off the car but I don't want to touch it now.

Distance from the carb is an interesting question. I would think the amount of vacuum would be more constant the closer the tap is to the carb.

The split is a possible way to go. Many times the fittings that go in the manifold have 1, 2 or even 3 ports. The question here would be where might I find a Y piece of hardware to use. I'll have to think about that one...

Mark M.

Aug 29th, 00, 02:43 PM
I don't think I phrased my questions very well. I have no clue what the correct answers should be.

I think (very scary) that the PCV is located on the carb base so that the crankcase gasses are better distributed and not burned in only one cylinder. What effect adding additional vacuum accesories to this connection would have is what I'm wondering about. I don't want to screw up the power brakes!

Click here to see see my car and hear 5-speeds.

Aug 30th, 00, 02:42 AM
I took a hard look at the port on the back of the carb and it is a nice big 3/8" line so I think it would be fine for the PB booster.

I still have some concerns about hooking the PB booster and a PCV to the same port. But then I don't really know how much air flows through a PCV. My thinking goes like this....

I suspect that the way a PB booster works is by having negative pressure in front of a diaphram in the booster which in effect helps the driver "push" the pedal. If you hook a PCV valve to the same line, this negative pressure would be lower, but by how much would be determined by the air flow through the PCV. The more air flow through the PCV, the less negative pressure at the booster. But would there be enough negative pressure at the booster anyway? How much vacuum the engine is generating would be a key question here.

In order to get the engine going, I'm just going to hook the port on the back of the carb to the booster and see how it works. I may then do some experimentation with a splitter and a vacuum gauge and see if I can hook up a PCV also.

Mark M.

Aug 31st, 00, 10:57 AM
Edelbrock makes a special low-profile fitting just to remedy this problem.

Aug 31st, 00, 12:23 PM
About running a pcv valve on the same line as the brake booster, don't do it.
You won't have enough vacuum to the booster and your power brakes won't work properly.

Sep 1st, 00, 11:28 AM

I think you provided the final answer to my engine hookup riddle. Thanks for the input. I found the part you describe on Edelbrock's web page. Part # 8096.


Mark M.