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Carb spacer... |
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RamblinMan
AMC Addicted Joined: Sep/26/2007 Location: Port Charlotte Status: Offline Points: 1237 |
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Posted: Apr/15/2013 at 2:21pm |
There is no set rule.
An open spacer makes for a larger plenum volume, which is *generally* advantageous for top end performance.
A 4-hole spacer helps build velocity, which is *generally* advantageous for bottome end torque.
That being said, the combo dictates the need. There is a sweet spot for every combo. Example: You have a combo that calls for an open plenum single plane manifold. The plenum volume will either be too big or too small for the combo in most cases. If it's too big it will be lazy on the bottom end due to poor velocity. A 4-hole spacer will help increase velocity and will help with the lazy bottom end. If the plenum is to small, top end power will be hampered and an open spacer will open it up.
In addition, your carb will usually either be a little too big (lazy bottom end) or a little too small (runs out of breath at the at the top end). A 4-hole spacer will boost the signal to the venturis to an oversized carb, improving bottom end drivability. An open spacer will not usually have much effect on a too-small carb have more top end because the carb venturis are the choke point and nothing will make them bigger. What an open spacer WILL do in that situation will allow cross-signal to the venturis that will have a very small effect, usuually increasing the top end by a little, but it is usually not more than a couple of hundred RMPS.
Then theres the height of the spacer. Bigger is NOT always better. a four hole spacer that is 2" is not necessarily better that the same spacer in a 1" variety or even a 1/2" variety.
They ONLY practical way to tune with spacers is on the dyno. Frankly, they're cheap to buy if you are spending the money on dyno time anyway. When dynoing a new motor, I take three carbs in 100 CFM increments and 6 spacers, three 4-holes in 1/2", 1", and 2" and the same three open spacers.
When I dynoed my 434 SBC, I had 750 (self built), 850 (AED brand), and 950 (self built HP) Holley carbs, and all the above mentioned spacers. It made the best overall power with the 850 AED on a 4-hole spacer on the victor junior intake.
All of this is to say, there's a right answer for your combination, but no one here can give it to you. It's all trial and error. You can aim, and get it in the ballpark if you are a pretty experienced tuner, but the odds of the first setup being the perfect setup is highly unlikely, unless you're real lucky.
Bear in mind however, we are talking about variances in the <20hp realm, so don't loose a ton of sleep over your carb spacer. My current 401 is running a 950 Pro Systems carb on 1" open phenolic spacer on a Torker intake. It's purely an educated guess based on the smallish plenum of the Torker and the large carb. I'm not gonna dyno the motor, but for the street, it will be more power than the car can plant on the pavement.
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The Right Reverend of Blessed Acceleration
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304-dude
AMC Addicted Joined: Sep/29/2008 Location: Central Illinoi Status: Offline Points: 9081 |
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As questioned above, you can only use a spacer that will conform to the intakes flange, a 4v spacer will not fit a 2v intake or visa versa.
There are two sides to a spacer, both were mentioned by good information. To put it simple, a spacer adds two benefits, as it does keep the carburetor cooler and tunes airflow, depending on the spacers thickness and materials from what it is made. When I had added a Offy 360 with spread bore pattern with a 650cfm Holley ( from stock 2v intake), I wanted to keep low end just as responsive as before by adding a spread bore multi plate spacer. It was adjustable to 1"... at the time I was told I had way too much carb and the intake was useless for my stock 304. Having the spacer allowed such a combo to operate well without major work with tuning jets and leaning out. I was told from a racer, rising the floor by adding a spacer will allow for over carbed / intake combination. It is safer to under carb, but I had a good deal for the combo, I could not pass up. |
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Mr. Ed
Supporter of TheAMCForum Joined: Nov/12/2010 Location: Sewell, NJ Status: Offline Points: 4916 |
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My 2 barrel had a spacer under it. So should I get a spacer for the 4 barrel?
Thanks! Mr. Ed Edited by Mr. Ed - Apr/15/2013 at 8:35am |
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FuzzFace2
AMC Addicted Joined: Jul/05/2007 Location: Angier, N.C. Status: Offline Points: 10343 |
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Dave ----
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TSM = Technical Service Manual
75 Gremlin X v8 for sale 70 Javelin 360/auto drag car 70 Javelin 360/T5 Street car |
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Mr. Ed
Supporter of TheAMCForum Joined: Nov/12/2010 Location: Sewell, NJ Status: Offline Points: 4916 |
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I thought the spacer was primarily to insulate the carb from the heat of the manifold. Yes, no?
Thanks! Mr. Ed |
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2014 Jeep Grand Cherokee Laredo "Gwendolyn."
1978 Concord Sport coupe "Mr. Black". 1982 Concord wagon. The Admiral. FOR SALE! 1976 Sportabout X, 304, auto, air. The Bronze Goddess |
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FSJunkie
AMC Addicted Joined: Jan/09/2011 Location: Flagstaff, AZ Status: Offline Points: 4741 |
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Every carb I've ever seen was designed to have a gasket seal over all of the underside except the throttle bores themselves. Alot of the hollowed out spaces under a carb are designed to derive their vacuum from ports carved into the throttle bores, and not just from an open space. Plus, the rough underside of the carb will create extreme turbulance under the throttle plates and around the idle discharge ports. Most stock intake manifolds have a closed plenum with individual intake bores, though they do eventually combine or have cross-drilled passages between them. I am always very careful to make sure the spacer matched both the diameter of the intake manifold bores and the carburetor throttle bores to provide a streamlined path of entry. It's worked very well for me.
When faced with an open plenum, I would still install a closed spacer while matching the spacer bores to the throttle bores only.
Watch out on 327's with factory Holley 4bbls. They have two extra secondary idle discharge ports with their own holes drilled in the manifold and spacer.
Any spacer increases plenum volume, and has all the effects on engine performance that a larger plenum poses. Where I live, fuel percolation is the carb is paramount, so I tend to pile on at leat 3/4" of spacers. More than that and the cab is just being heated by convection.
The primary way a carburetor cools itself is by the intake air flowing through it. If your carb ever gets really hot while driving, downshifting and reving it up to increase airflow helps. That is also why I believe in cold air intakes when the weather is above 85 degrees. Not for power, but for drivability. I've seen carbs get so hot, the accelerator pump shot vaporizes before it leaves the discharge nozzles. ....Arizona problems.
Edited by FSJunkie - Apr/14/2013 at 11:25am |
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1955 Packard
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jarcher401
AMC Apprentice Joined: Mar/11/2012 Location: pa Status: Offline Points: 79 |
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No lie, but on the engine dyno I gained 10hp and 11tq from adding a 1 inch spacer.
600cfm carb 8.5:1 Shorty Headers Gasket matched heads performer intake lunati 64501 cam
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WCKAMX390
AMC Apprentice Joined: Dec/21/2012 Location: Fresno Status: Offline Points: 44 |
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Used 1" spacer on my 390 ci, 4 spd AMX. Noticed a little better top end. Can't prove it, but felt it. Had to take spacer off because hood would not close and did not want to run hoodless...
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1970AMX, 390ci, 4spd, RamAir, 3.54posi
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Red20
AMC Addicted Joined: Jan/29/2013 Location: Oceanside CA Status: Offline Points: 1556 |
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I use a 1", 4 hole spacer on a dual plane edelbrock performer intake on a 360 with an edelbrock 650 carb w/ mech secondaries. I noticed an increase in torque myself. No numbers to back it up, but my seat-o-the-pants dyno confirms it. One thing i did was chamfer the edges of the holes on the bottom side of the phenolic insert to help with air flow and reduce de-atomization of the fuel air mixture. I used a router and 1/8" roundover bit to do that. Don't know how much it helps, but I can't see it hurting.
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tsanchez
AMC Addicted Joined: Jul/09/2007 Location: Arizona Status: Offline Points: 4302 |
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Like NDA says, it depends on the engine, signal at carb and manifold entry. Most of what a spacer does is add plenum volume. I have had many time that it caused driveability issues, especially with air valve secondary carbs.
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