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Carb Spacers Which Style is Best?

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I am doing some clean up on my engine compartment. Just had my intake manifold coated and am in the process of reinstalling. The intake is an Edelbrock RPM Air-Gap. I have always used carb spacers/insulators to help prevent vapor lock and hard starting because of fuel evaporation due to heat.
Today I was looking at my manifold and thinking I needed to be sure to put a spacer on it. I started looking at spacers and realized that in the past I had used multiple styled usually based on what was available at the local parts store. However today was one of those days when you begin to question stuff. I looked at the Summit site and basically there are three different styles, 4-Hole, Full Open and Divided Plenum. SO the question is what are the operating differences and what impact does it have on drivability / tuneability? I am using an Edelbrock carb car is a regular driver.
 
I would most likely use what matches my intake flange opening. I doubt there is any gain or loss no mater which one you chose.
 
Wondering if there is any effect on vacuum and idle circuit.
 
There is no hard and fast rule about what works. It's entirely dependant on combination. What helps on mine might merely lighten your wallet.
That said, the few actual dyno tests I have seen suggest an open spacer on a dual plane, a four hole on a single plane.
Frankly, unless I i needed a wood or phenolic spacer for heat reasons.... I would try the $40 speedmaster version of the $130 HV super sucker or Wilson tapered spacer.
The only way to know what works for you is to try an assortment. (I used an open 2" on a victor. Don't know if it helped or not, I used it to help a fuel line connection.) I have seen a $15 spacer make 10hp. Cheapest power you will ever find, if you find what works.
 
The RPM is a dual plane intake. The planes are separated for good reason. To preserve the operating characteristics of a DP intake [ better carb signal, low end tq ], use a divided spacer,
either a two or four hole.
 
Edelbrock makes a lot of different carbs. Which model do you have ? Has it ever been on this manifold ? AFB-Performer series, AVS-Thunder series will work.
Thermo-Quad will not. It is a " spread-bore"type carb.
RPM Air-Gap is a fully divided plenum with a square bore. Should be an AFB type carb with a divided, phenolic spacer. And obviously longer carb studs.

This info is for any one thinking of going to this manifold and wondering what fits.


rpm airgap small mopar.jpg
 
Maybe something like this one for a divided , not notched plenum. Probably other manufacturers out there with similar spacers perhaps cheaper, idk. Open or tapered depends on the combo.

8E33551F-88EF-415F-85D6-52D9B50DA138.jpeg
 
I have used a wood spacer for years for the insulating qualities. I don't race and rarely drive hard so it doesn't much matter about the engineering on which one to use. What does matter is the carb getting hot and this spacer works well. I have used others but like this the best
 
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