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Thread: Dual-Plane manifolds & open spacers.

  1. #1

    Default Dual-Plane manifolds & open spacers.

    Earlier in this forum, there was some discussion about the negative effects of using a dual-plane manifold with a Holley TBI, particularly with a Terminator system, because it had just a single WBO2 sensor. The discussion got sidetracked and I did not get a clear resolution from it. I'm using an Edelbrock RPM Air-Gap manifold on a SBF. Unlike many other manifolds, this one has a fairly large cutout (from the factory) in the partition between the two "planes". The cutout is about a 1/2" deep and almost the full length of the partition. Is this providing the same function/benefit as an open spacer for the TBI (except for the added volume)?

  2. #2
    Join Date
    Dec 2009


    This has been discussed before. It should be fine. You'll just have to try it & see.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, FRPP 3.31 gears, Moser 31 spline axles, Cobra Trac-Lok differential, 4-disc brakes/Cobra booster/MC, 5-lug Bullitt wheels & 245/45R17 tires.

  3. #3


    I have a SBF as well and a Weiand intake that did not have that section cut out. Before I installed the Terminator, I had a local machine shop cut the cut the groove in it the same way as the Air-Gap. I drove it like that for about a year, and I had a few of the anomalies associated with a dual-plane manifold from time to time, but nothing that caused major concern. I installed a 1 inch spacer a couple months back and it's much smoother now. So I would suggest a spacer with that manifold, over to you of course.

  4. #4


    Thanks. I'll go with no spacer for a while because of hood clearance (< 1/4"). However, I hadn't thought of the idea of enlarging the groove, if I find I have any problems. By the way, what should I watch for?

  5. #5


    My AFR always had a hard time being very steady, particularly at idle until I put the spacer on. It would also dip below idle no matter what I tried to do, even with 22° of timing at idle. The throttle response has also been quite a bit more crisp. All in all it just smooths everything out, nothing ridiculous, just subtle enough to make me happy.

  6. #6


    I have the GM Performance dual-plane manifold (probably same thing as the Edelbrock), and it has the divider wall machined down about 1/2"-3/4" inch as well. I went with the 1" open spacer right off the bat, because I didn't want to have any issues. I have no clearance issues, so why not. Ran great right off the bat. If you can fit it, do it I say.

  7. #7


    Hmm...I wonder if machining a larger opening in the existing divider would help, or if I should be shopping for a lower profile manifold. (Cutting a hole in the hood is not an option in my case.) Do you think it's the crossflow between the "planes" or the added volume (and height) that improves the idle?

  8. #8


    The added height & volume is an old school carburetor thing that was proven to make a little more top end. For fuel injection where the injector spray is under vacuum with properly tuned manifolds, I don't know how much more it really helps. Like I said I just did it to rule out a potential issue. But obviously you want the fuel injector spray from the left side & right side mixing as the pulses are probably at different times. And if you have an injector failure, you don't want the runners below that one bad injector to run out of fuel and boom, run those cylinders lean. Machining down the divider a lot more would probably suffice just fine, and I know a lot of people run the Holley EFI TBI with no spacer.

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