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Vacuum Reading at idle

Hi guys

What should our vacuum reading be at idle? Trying to make sure I don't have Vacuum leak. Currently fluctuating between 15 & 17 in-hg.

Thanks in advance

John

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1988 Porsche 930 Targa - 3.4L Twin Plug Motor, DC 13 Cams, Supertec Head Studs, ARP Rod Bolts, Port and Polished Heads, Garretson Andial Intercooler, GSX 61 BB Stage 5Turbo, Tial F46 WG .8 Bar Spring, PowerHaus Headers, Rarly8 Dual Tip Muffler, MS3 Pro EFI conversion, 57lbs. Cobra Injectors, Center Force Stage 2 Clutch , Rebuilt 4 Speed with LSD.
Old 12-27-2014, 02:09 PM
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In my lowered compression SC (just over 8:1), I get about 15 in Hg vacuum. When you say it fluctuates, are you saying that it oscillates between these values or does it hold steady sometimes at one and sometimes at another value? It should be steady.
Old 12-27-2014, 07:15 PM
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I am reading it through SDS EFI handheld programmer. It's a electronic readout
Reading map sensor. It is mostly steady at 15 in Hg., but will jump to 17 in Hg. on occasion.
My ignition may have been set to low. Had it set at 5 deg for emissions.
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1988 Porsche 930 Targa - 3.4L Twin Plug Motor, DC 13 Cams, Supertec Head Studs, ARP Rod Bolts, Port and Polished Heads, Garretson Andial Intercooler, GSX 61 BB Stage 5Turbo, Tial F46 WG .8 Bar Spring, PowerHaus Headers, Rarly8 Dual Tip Muffler, MS3 Pro EFI conversion, 57lbs. Cobra Injectors, Center Force Stage 2 Clutch , Rebuilt 4 Speed with LSD.

Last edited by jpilcher; 12-27-2014 at 07:47 PM..
Old 12-27-2014, 07:31 PM
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If your engine is anything but stock the reading may vary. A healthy stock 930 will read as you are seeing, between 15"-17". It should be steady on a vac gage.
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Old 12-28-2014, 08:27 AM
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Pulsating vacuum can mean a really loose intake rocker.
Old 12-28-2014, 11:24 AM
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Engine fully warmed up reading 15.7 in Hg steady. It fluctuates as the the engine is warming up.
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1988 Porsche 930 Targa - 3.4L Twin Plug Motor, DC 13 Cams, Supertec Head Studs, ARP Rod Bolts, Port and Polished Heads, Garretson Andial Intercooler, GSX 61 BB Stage 5Turbo, Tial F46 WG .8 Bar Spring, PowerHaus Headers, Rarly8 Dual Tip Muffler, MS3 Pro EFI conversion, 57lbs. Cobra Injectors, Center Force Stage 2 Clutch , Rebuilt 4 Speed with LSD.
Old 12-28-2014, 11:48 AM
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The hotter the cam profile results in less idle vacuum observed.

Due to your intake manifold acting as a plenum it's unlikely you will be able to identify a vacuum leak unless you spray around all joints to see if your idle speed changes.

ecu doesn't know the difference between air coming in via a vacuum leak, or going past the butterfly as intended since the air is metered by MAP sensor and crank rpm, CIS by comparison meters the air at the filter and downstream leaks upset the fuel calibration.

Slight wandering vacuum is no issue, especially under warm up. Also, when the engine is cold you will see less vacuum since the throttle needs to be cracked open wider to make up for friction loss due to cold oil. As you refine your engine temp values you'll see less vacuum fluctuation but I wouldn't expect to get it dead steady.
Old 12-28-2014, 02:11 PM
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Thanks Jim

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1988 Porsche 930 Targa - 3.4L Twin Plug Motor, DC 13 Cams, Supertec Head Studs, ARP Rod Bolts, Port and Polished Heads, Garretson Andial Intercooler, GSX 61 BB Stage 5Turbo, Tial F46 WG .8 Bar Spring, PowerHaus Headers, Rarly8 Dual Tip Muffler, MS3 Pro EFI conversion, 57lbs. Cobra Injectors, Center Force Stage 2 Clutch , Rebuilt 4 Speed with LSD.
Old 12-28-2014, 03:49 PM
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