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Thread: 16V Bigport+20V Quads. Low Manifold pressure issues

  1. #1
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    Default 16V Bigport+20V Quads. Low Manifold pressure issues

    I am hoping someone can shed some light on an issue I’ve been having, namely low Manifold vacuum.

    I am running S/T Quads on a modified 16V Bigport (10.5:1 Compression, Tomei 264 Cams). AE92

    Car has A/C & P/S; closed air Intake; i.e. modified S/T Plenum running to AE92 Airbox.

    Basically, according to AK (ECU+Harness supplier); my Manifold Vacuum should be around 45-50kPa but its reading around 90-95kPa i.e bugger all vacuum, at idle.

    NOTE: AK said Engine has plenty of Vacuum

    I have checked all hoses & connections - there are no leaks.

    I also took off the T3 Manifold, enlarged the fittings (T3 Manifold came with 1/16 NPT to 2.5mm I.D. Barb, with only ~6-7mm of metal for hose to attach to so I drilled out holes and replaced with 1/8BSP 90 Elbows with a long 4.5mm I.D. Barb to ensure hose was properly retained.

    I put it all back together and. . . there was no change in the Manifold readings at all.

    AK quite rightly did say he doubted changing the fittings would fix the issue as he thought it was more due to the VAC layout so now I’m pondering the vacuum layout itself.

    The Vacuum Collector Tank is bolted to Passenger side of diagonal strut on OEM brace, and originally had 7 Vac lines running to it;

     1 x ½” I.D. VAC line running from 20V ISCV outlet to larg(er) end union on Vacuum Collector Tank
     4 x 4mm VAC lines running from u/side of T3 Manifold (adj. to ITBs) to 1/8 NPT unions on u/s of Collector Tank *
     1 x 4mm VAC line running from FPR to 1/8 NPT union on u/s of Collector Tank.
     1 x 4mm VAC line running from ECU Map union to 1/8 NPT union on u/s of Collector Tank.

    * There’s an average 500mm vac line run from ITBs to VAC collector.

     There are also two 1/8 BSP outlets on the top of the T3 Manifold in line with ITB#1 & ITB#4, running to Brake Booster & P/Steer union respectively.

    The Manifold VAC gets closer to the figure (45-50kPa) recommended by Harness/ ECU supplier* when I disconnect ISCV; i.e. drops by about 25-30kPa to 65-70kPa. (but still varies a fair bit, with low / high readings = 57 / 72 kPa).

    Disconnecting the FPR line has negligible effect on kPa readings. . . .

    The car also runs bloody rich. . . . afraid to take it for a run (to test Brake / Steering operation) in case I stuff the Catalyst.

    AK suggested I tap another 4 holes and run lines to a separate VAC Tank; i.e. drive just the ISCV off one, and the MAP & FPR off the other. He* said this was a “desperation” measure, as the Manifold vacuum should not be this low, but that doing this might help me isolate / identify the problem.

    I’m pretty reluctant to do this, as it would start to make things really messy, with a whole spaghetti load of vac lines cluttering the bay, so what I proposed to do was run larger 6mm Vac hose for 20-30mm from ITBs then split each runner into two via a “Y” fitting & merge the other 4 lines into 2, then 1, then run that line via a Collector / Valve (@ OEM layout in AE111 Levin).

    I was intending to try this out 2morrow but what I wanted to ask first was this;

    Q: Could the location of the VAC collector (i.e. the 500mm run from ITB-to-Tank) be part of the problem? After all the Collector on the AE101 / AE111’s sits right on top of the Manifold. Would it be worth moving the Collector closer to the ITBs (i.e. to the bulkhead above the steering rack) or am I barking up the wrong tree?

    PS: CSV files from ECU can be attached for Reference (if requested).
    Last edited by GeeEss; 18th November 2011 at 08:27 PM.

  2. #2
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    Default

    Have you timed in the cams correctly ?

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    Default

    with bigger cams you will always have a lower vaccum at idle all you can do is try and improve it i dont think you will get 45- 50 kpa at idle with big cams

  4. #4
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    Default

    Well; the static timing is OK; and according to the manufacturer these cams (Tomei PonCams) are P+P / don't need to be dialled in.


    Comment Added: 20/11/11

    When the Map vacuum line is disconnected from the Collector and run straight to the VAC. port on TB#4 the MAP Vacuum then reads down to 32kPa (but readings of course vary considerably as it's only connected to 1 cylinder. Because of this AK (Harness/ECU guy) reckons the problem is with my Vac. set up (and not Cam timing etc)

    As discussed I will (hopefully) split the 4 TB vac lines into two then merge via a small collector before running to ECU MAP port. Be interesting to see what Vac readings I get then. . .
    Last edited by GeeEss; 20th November 2011 at 10:34 AM.

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