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vacuum at intake

 
Old 04-23-2016, 03:16 PM
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Swiss964
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Hi All, I measure a steady vacuum value of 13 Inch Hg at the intake using the fuel vapour recycling port. I do find this value really low, Theoretically it should be 17 not less for a std engine, but the flat 6 is not a std engine...What's your experience on this ?
My vacuum tester gives me 19 inch HG on another car (30 year old audi 100 with 250 Kmiles), so it works ...
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Old 04-24-2016, 08:10 AM
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-nick
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Are you using the stock chip? How many miles on the motor? I recorded 15" at idle on mine using a Wong chip and after a top end + rings rebuild that was broken in. Wong advances idle timing a bit, which should result in a little more vacuum. So your 13" sounds in the ball park. I'm not a cam expert, but lobe separation is supposed to be a large factor in idle vacuum. Stock cams must be a bit different than most production motors I guess.
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Old 04-24-2016, 08:38 AM
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Swiss964
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Thanks, I am using a wong chip, but the car came originally with a RUF chip, I did measure the vacuum before and after and there was no change
Car has 220 000 KM and top end was done at 160 000KM (rings, resurfacing, valve guides...) car does not smoke blue at all and use very very little oil, 1l every 5000KM
The 13 inch HG i get is very steady, so no valves issues
I know that the Wong chip has 4░ ignition advance at idle, I had 2.5░ with the RUf Chip and I know that the standard one gives below 1░

I still would like to understand if the 15 inch HG is what we should be targeting or a common value for old 964 that maybe all have a common problem

thanks
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Old 04-26-2016, 01:37 AM
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What elevation are you at? Higher elevations will cause lower vacuum readings.
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Old 04-26-2016, 05:31 AM
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Swiss964
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Thanks, I am at 500 meter above see level, that's~ 1600ft, I found that correction is 1.7inHG
But my point was the following, should 964 flat6s read 15 in Hg or should they be closer to 18-19,
As said previously in this thread, I have tested a very old car in the same conditions and could read 18-19
What is the target value then for our flat 6?
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