I have a mildly modified SL6 with a Offenhauser manifold and Holley 4160 (390 cfm) it currently has a 1" thick 4 hole carb spacer. I want as much low end grunt as it can deliver. Do I need to leave the spacer in or loose it? See below for more engine info if necessary.

bwhitejr

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'72 Duster (Performance 360)
'83 Ramcharger (Performance 318)
'80 TrailDuster (360)
'80 D-150 Truck (See Below)
CompCams 252S, Holley 390cfm, Offy manifold
Ported, Polished and Gasket Matched
P4286813 Springs,0.040 Overbore,
0.090 Shaved Head

Don't think it matters much with a 4-hole spacer.

Is it aluminum or plastic (or wood)?

Aluminum will conduct heat to the carb, which you don't want............

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

My research indicates that a four holes will give better low RPM grunt. If you are worried about heat grab a heat shield off of a four barrel mid 80s Ford truck or van.

You could always make one out of Delrin.

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The Old Goat '83 D150 225-2

AKA Ice Titan

I got one of those carb spacers by motorvation that you can purchase
from summit racing equipment. I noticed an improvement in my low end
power and just a tiny bit more gas mileage. But as mentioned if you think
heat will be an issue get a heat shield

Back when I had a Weber DGEV mounted to a cast iron 2bbl. manifold (super six), I had to use an adaptor for a Datsun intake that I modified as an adaptor. This acted like a 1.5" spacer, and the car made good low end torque at the time (by seat-of-the-pants feel). When I switched to a Clifford manifold with proper adaptor, I think the car lost low end torque. It could've just been that the runners and plenum in the cast iron 2bbl. were better for low RPMs. Because it's so easy to try, I think a spacer is definitely worth exploring on any carburated vehicle.

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'68 Dodge Dart 4dr Sedan

Do you have to readjust the kickdown linkage when you install a carb spacer?

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

Yes, with a rod-type kickdown linkage you will have to figure something out so that it still works properly/won't bind.

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'68 Dodge Dart 4dr Sedan

most carb spacers I have seen are actually phenolic, which often gets confused with wood (in most any PAW catalog).

I will probably make my own spacer at work out of phenolic once I get a chance.

I have seen many claims to the gains, but no hard numbers from anyone yet. I am guessing its a mild tunnel ram type effect.