Open vs. Divided Carb Spacer Comparison - Dual Plane - HP? Carb Tuning?

I was wondering about the effect of an open vs. divided carburetor spacer on a dual plane intake manifold. In my case, I have a dual plane intake manifold where the intake runners left/right are separated. I now have 1" open carburetor spacers, so the left/right side of intake is now "mixed between right/left" just under the carburetor.

1. Which type of spacer would theoretically make more horsepower/torque?
2. How would this affect carburetor tuning?

Just curious....any thoughts are appreciated.

You already have an open spacer, I'd get a cheap four-hole and try it. There are no definitive "this spacer will work like this" answers, it all depends on YOUR combination.
A two inch open spacer worked like gangbusters on a single plane small block, turned a dual plane big block into a turd. Just my experience, on my stuff.

I do not bother fitting any spacers to the Edelbrock Performer or the RPM style manifolds.
I have found both manifolds slightly better for different reasons with the carb directly on the manifold. The Performer was on a mild 350 with a 2 inch open spacer and we experienced fuel distribution issues. The Performer RPM was on a fairly wild 363 Ford with half inch open spacer and it was noticeably better idle and low down manners.
No dyno involved just all about the way they drove on the street.
Other brands of manifold I cannot comment on.

Thanks guys. My specific situation is a factory 6-pack (dual plane), but it has "open" nitrous spacers...so basically I'm stuck with open spacers. I was just thinking that since the spacers are open, the "carburetor signal" from left-to-right is equalized (for better or worse) and the idle mixture screws "should" be set about the same left/right. Does that make any sense? I do have an option of putting a "divided" spacer on the center carburetor. Just sort of wondering....

Do you still have metering plates on the end carbs?
Have you driven the car both with and without the nitrous plates? If so , how did it drive different?
If I were to run a sixpack with nitrous, I would surely use an O2 sensor in each side, an a data recorder.

I recently did some testing (seat o' the pants only, no dyno or track..)between open and four-hole spacers, on my particular setup. Edelbrock RPM intake and heads(not ported) on a .040 over 8.9 to 1 440 with 850 mech. secondary carb, the older Comp 274XE cam and 1-7/8 headers. Tried both style spacers in a one-inch height and also a four hole in half-inch height. I feel that all the spacers I tried (regular phenolic mr. gasket and edelbrock pieces) added something over no spacer. So, the one-inch tall pieces felt like they both made a noticeable (better) difference than the shorter spacer. The four-hole (1-inch) spacer made a noticeable difference in the lower RPM range, from about 3000-3500 and up it seemed to perform the same as it did with no spacer. Tuning was the same exept an extra 1/16 of a turn out on the 4 corners(that could of just been the temp change from when I started though...Demon carb, so, touchy touchy..) The open spacer worked very well, contrary to the opinions of many, with the RPM intake. I felt there was a very slight loss of torque off-idle, but noticeably more starting about 3000 and could feel extra pull at the top end, where it would flatten out before. It's worth noting that my big-block(I've discovered) wasn't in the healthiest state, but it still ran pretty hard considering blow-by getting oil in the chambers. With the open spacer I lost one full inch of vacuum at idle and had to re-tune for that. Timing remained the same. Jetting was adjusted prior with the four-hole and left untouched for the open unit for a more apples-to-apples comparison..I have a feeling jets could be maximized with some further tweaking for the open spacer. I also feel the side-to-side carb adjustments worked best when tuned equally regardless of spacer style. All this aside, it does come down to your specific setup so get both and give 'em a shot as it's not too big of a deal to swap them. And even with the slight shift of power away from the off-idle and low RPMs with the open spacer, it still smoked the tires on command...still more torque available than useable on the street..they're still big-blocks afterall! Hope my long ramble helps..good luck!

PurpleBeeper said:

My specific situation is a factory 6-pack (dual plane), but it has "open" nitrous spacers...so basically I'm stuck with open spacers.

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33 IMP said:

Do you still have metering plates on the end carbs?
Have you driven the car both with and without the nitrous plates? If so , how did it drive different?
If I were to run a sixpack with nitrous, I would surely use an O2 sensor in each side, an a data recorder.

Click to expand...

Yes & no. I ran the six pack a lot without the nitrous plates, but my carbs were very modified & not performing correctly. Now the carbs are all OK & using the Promax metering plates set to factory spec's. I can't really say plates vs. no plates??? I wish I had the cash for oxygen sensors & a data recorder! I was just wondering about all this.

PurpleBeeper said:

I was wondering about the effect of an open vs. divided carburetor spacer on a dual plane intake manifold. In my case, I have a dual plane intake manifold where the intake runners left/right are separated. I now have 1" open carburetor spacers, so the left/right side of intake is now "mixed between right/left" just under the carburetor.

1. Which type of spacer would theoretically make more horsepower/torque?
2. How would this affect carburetor tuning?

Just curious....any thoughts are appreciated.

Click to expand...

Motor specifics matter to even speculate. My gut tells me that a 6 pack set-up on a mild 440 would see little to no meaningful difference in power. This is because you have so much cfm already available. As you increase plenum volume, I suspect that you might need to open your idle mixture screws and possibly IFRs . But I think we're splittin hairs here.

You could be super sneaky and plumb some jets on the underneath of the manifold & get rid of the plates.....

69 GTX said:

You could be super sneaky and plumb some jets on the underneath of the manifold & get rid of the plates.....

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I have a six pack manifold where exactly that was tried, by a well-known custom nitrous installer. He gave up, said there wasn't enough room under the manifold with a factory valley plate, even less with a plate for aftermarket heads.
My manifold has two 1/8 pipe holes in each port, but I'm either going to use aluminum plugs or have it heliarced closed.I

69 GTX said:

You could be super sneaky and plumb some jets on the underneath of the manifold & get rid of the plates.....

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I thought about that YEARS ago....I could stuff some of that insulation crap I've seen in some late 70's big blocks (maybe other years?) to hide it too!

33 IMP said:

I have a six pack manifold where exactly that was tried, by a well-known custom nitrous installer. He gave up, said there wasn't enough room under the manifold with a factory valley plate, even less with a plate for aftermarket heads.
My manifold has two 1/8 pipe holes in each port, but I'm either going to use aluminum plugs or have it heliarced closed.I

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I didn't realize it was that tight....but the factory sheet metal valley pan could pretty easily be modified.... crap...you guys have me thinking about how to put THREE stages of nitrous on a car I want to change over to a long-distance cruiser. I'd better not. I've already had a couple nasty nitrous explosions with one stage on my Mustang.

@PurpleBeeper ,@69 GTX , my manifold was kinda hard to get to, but I ran across it this morning, thought I'd post a pic.
The heat crossover had been cut, and the first attempt on the low-side ports have been plugged and redrilled. I think the problem was poor access to the high-side ports, interference with the valley pan
For what it's worth, this manifold has horrible core shift.
A hidden setup might work on a single plane manifold.
Might.

33 IMP said:

@PurpleBeeper ,@69 GTX , my manifold was kinda hard to get to, but I ran across it this morning, thought I'd post a pic.
The heat crossover had been cut, and the first attempt on the low-side ports have been plugged and redrilled. I think the problem was poor access to the high-side ports, interference with the valley pan
For what it's worth, this manifold has horrible core shift.
A hidden setup might work on a single plane manifold.
Might.

View attachment 728386 View attachment 728387 View attachment 728388

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Very cool Imp. I certainly thought about the "under the manifold" systems & even above-the-manifold fogger systems. As I said before, it seems like it's pretty easy to modify the valley pan. I mean, there's a ton of room in the valley area of the block above the cam, so as long as you clear the heads, I think you're good.

Your current nozzle placement looks like it will work just fine. You should be able to notch the valley pan & modify it with some basic sheet metal (weld or even pop-rivet with some RTV). You could also stuff some of that factory late 70's fiberglass mat stuff up in there. Put your nitrous feed line inside a slightly larger black rubber hose & the system would be completely hidden.

I remember posting this pic on this forum when I started a thread about hidden NOS on a 6bbl intake:

I don’t think the big blocks have that luxury

rumblefish360 said:

I don’t think the big blocks have that luxury

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I wasn't paying attention.
I see the 6bbl intake I posted is not a big block Mopar.
Is it a small block Mopar 6bbl?

I think it is a small block manifold.

That hidden-under-the-manifold setup would work on a maxwedge crossram. Use an A&A , not an original.

That way you could say "i picked up 200 hp when i swapped to the crossram!"