 Manifold vacuum under real world driving and decel, Anyone have data on this? |
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| emerygt350 |
 May 27 2022, 06:46 AM
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#1
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Advanced Member  Group: Members Posts: 2,883 Joined: 20-July 21 From: Upstate, NY Member No.: 25,740 Region Association: North East States  |
After pbanders alerted me to the error in the decel diagrams, and I hooked the darn thing up correctly, I have been wondering what the actual vacuum while driving looks like for others.
I have a glowshift vacuum gauge on the dash (from tuning the MPS) and have been watching the max vacuum on decel, idle, cruise etc. On a hot engine at 900 rpm idle I pull 17ish pounds. On decel 23ish pounds. Hooked up my vacuum pump to the decel valve while idling and gave it some vacuum. No change in idle till 20ish, then increasing RPM from 20 to 28 pounds. Bench testing the decel showed opening at 18 lbs. I can't imagine many of you have a vacuum gauge on the dash but perhaps the bluetooth 123 records that kind of stuff? 73 2.0 djet. |
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| Van B |
May 27 2022, 03:58 PM
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#2
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Senior Member Group: Members Posts: 1,621 Joined: 20-October 21 From: WR, GA Member No.: 26,011 Region Association: None 
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Decel valve has a bit of a progression to it. That progressive opening is over a shorter span as you tighten it up more due to an aging valve spring.
I have a NOS and my original and the difference between starting to open and full open is 2 in-hg on the old one and 3 in-hg on the new one with both set to start opening at 20 in-hg. It’s also easy to predict the difference because the old one needs more turns in to resist until 20”. I would say your valve is probably set to start opening at 18 and is fully open by 20. |
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| mgphoto |
May 27 2022, 05:02 PM
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#3
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"If there is a mistake it will find me" Group: Members Posts: 1,375 Joined: 1-April 09 From: Los Angeles, CA Member No.: 10,225 Region Association: Southern California |
My 123 has the vacuum hose hooked up but I’m not changing the advance or retard.
0 in Hg at part throttle to WOT I have a consistent 19 in Hg at idle |
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| mgphoto |
May 28 2022, 03:59 PM
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#4
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"If there is a mistake it will find me" Group: Members Posts: 1,375 Joined: 1-April 09 From: Los Angeles, CA Member No.: 10,225 Region Association: Southern California |
Spent some time today watching the vacuum gauge during a long ride.
Idle hot is 15in Hg part load is 4in Hg full load to WOT 0in Hg and long throttle braking (down hill 1/4 mile) 25in Hg. D-Jet about 30k on a 10 year old build. |
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| emerygt350 |
May 28 2022, 05:05 PM
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#5
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Advanced Member Group: Members Posts: 2,883 Joined: 20-July 21 From: Upstate, NY Member No.: 25,740 Region Association: North East States |
Interesting! I assume you have a decel valve?
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| mgphoto |
May 28 2022, 05:28 PM
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#6
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"If there is a mistake it will find me" Group: Members Posts: 1,375 Joined: 1-April 09 From: Los Angeles, CA Member No.: 10,225 Region Association: Southern California |
QUOTE(emerygt350 @ May 28 2022, 04:05 PM)  Interesting! I assume you have a decel valve? No removed long ago. |
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| emerygt350 |
May 29 2022, 03:54 AM
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#7
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Advanced Member Group: Members Posts: 2,883 Joined: 20-July 21 From: Upstate, NY Member No.: 25,740 Region Association: North East States |
Ahh, and where is that vacuum line hooked up? Tb or manifold?
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| GregAmy |
May 29 2022, 05:34 AM
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#8
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Advanced Member Group: Members Posts: 2,490 Joined: 22-February 13 From: Middletown CT Member No.: 15,565 Region Association: North East States
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Per my logs, 45.3 kPA typical at idle.
A brief scan for overrun situations shows 18.4-20.0 kPA. 3.5 to 5.1 degrees total ignition timing, adjusting to manage the idle at 900 RPM. 2056cc with FAT 440 "EFI" cam, D-Jet induction system managed by Microsquirt. |
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| mgphoto |
May 29 2022, 08:21 PM
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#9
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"If there is a mistake it will find me" Group: Members Posts: 1,375 Joined: 1-April 09 From: Los Angeles, CA Member No.: 10,225 Region Association: Southern California |
QUOTE(emerygt350 @ May 29 2022, 02:54 AM) Ahh, and where is that vacuum line hooked up? Tb or manifold? Throttle body |
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