carb spacer - should i get an open or dual
- Thread starter duster360
- Start date
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pyrojim
Well-Known Member
When I went through it, I was told open for better street manners, closed is more for the strip. I went open in a 360 with an eddy air gap and a Holley 650. Runs great!
Locomotion
Well-Known Member
With all due respect....you got that backwards. An open spacer will basically create a larger plenum under the carb, which will decrease the vacuum signal to the carb. It's more suited for higher rpm's.
A 4 hole or 2 "oval" hole spacer helps throttle response and is better suited for street and milder applications.
An old trick with the LD340 was to mill down the center divider for performance/race applications. But my 2 cents would recommend the more conservative approach on the street. Not saying an open spacer wouldn't work, just not the better option. One should also consider a phenolic/wood spacer over aluminum on the street. It's a better insulator to help keep heat away from the carb.
I was thinking the same thing Myron. Just wanted to be sure. Good tip on the phenolic spacer.
Fisher
Old Guy with a Cool car.
You will never know what the motor wants unless you dyno it. We tried carb spacers on mine, and the motor didnt like it. Got best results with out one. Each motor is different, my motor didnt like lots of timing either.
pyrojim
Well-Known Member
Locomotion
Well-Known Member
Yes, each motor is different. As a rule of thumb, when you change anything on an engine, it often changes the optimum settings that other parts require - i.e.: bigger cam, open spacer, bigger carb, etc. can require bigger jets and/or pump squirters IF the combo was optimized before the change. Otherwise, the engine may be too lean to see a benefit. Gotta experiment to see just how much jets and squirter is required because one can "mask" the need of the other to an extent. Jet for MPH and squirters/pump cams for 60' times. It all boils down to how anal/detailed someone wants to be. Patience, time, place and knowledge to interpret are required for useful data/results.
I've seen parts get changed, like to an open spacer, for example, and there was an obvious gain. But that could indicate that the original combo was on the rich side and the open spacer leaned it out towards where it needed to be. "Optimized" carb with a 4 hole spacer may or may not be better.
Not sure if it would apply to all dual planes, but I would think so. I tried a Weiand Stealth dual plane and ran it for a year or so while playing with carb settings trying to see if it would run better than the single plane I had (It didn't on MY combo.). Best jetting may have been leaner but even more interesting was that one day I noticed that some plugs were a little lighter in color than others. I had them all together, in order, in a little rack and realized that the ones fed by the lower plenum were the lighter ones! Back-to-back testing on the same day showed that my car was about .05 faster when jetting was 2 or 3 numbers different between the sides with the lower plenum side being richer! Plugs were now more evenly colored. I've always run a 1" 4 hole TAPERED spacer on both, the single plane & dual plane. I figured it was a good compromise between open & 4 hole while staying on the conservative side. It's fun to test, but only if you have the time & money. I'm limited.
Moral: Bigger plenum tends to reduce vacuum signal and requires more fuel.
I didn't have a way of getting more detailed distribution info. Some modern dual plane intake designs may or may not be designed to compensate for that. Each combo is different.
Even if the engines are the same, the rest of the drivetrain as well as where you live (weather, altitude, etc.) can be a variable for carb setups.
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