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Normal Vacuum Reading at Idle?

 
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xpernet  



Joined: 07 Apr 2004
Posts: 414
Location: Kitchener, Ontario Canada

PostPosted: Sun Feb 22, 2009 4:35 am    Post subject: Normal Vacuum Reading at Idle? Reply with quote

Im getting ~45KPa at 800rpm, ive heard that it has to be less than that. Whats my target reading?
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ideola  



Joined: 01 Oct 2004
Posts: 15419
Location: Woodstock IL

PostPosted: Sun Feb 22, 2009 5:29 am    Post subject: Reply with quote

At idle, vacuum should be in the range of 17-20 HG on a healthy, no-vac-leak, fully warmed-up NA car. If I'm doing the conversion correctly, at 45 kPa, you're only pulling 13 HG, which is too low, and indicates that the idle is either too low, the ignition is retarded too much, or most likely that you have a vac leak somewhere.
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Rasta Monsta  



Joined: 12 Jul 2006
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PostPosted: Sun Feb 22, 2009 6:18 am    Post subject: Reply with quote

Could also be crappy oil, worn rings.

Great vacuum gauge guide RIGHT HERE.
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RC  



Joined: 25 Mar 2007
Posts: 2634
Location: Australia

PostPosted: Sun Feb 22, 2009 3:27 pm    Post subject: Re: Normal Vacuum Reading at Idle? Reply with quote

xpernet wrote:
Im getting ~45KPa at 800rpm, ive heard that it has to be less than that. Whats my target reading?


Na. Thats about right.
Consider idle is now down to 800RPM Vs stock 950 and being a 77 most likely have the low comp, small valve, poor cam, 95Hp engine.

You can run 20° ignition advance at idle safely, even up to 100Kpa MAP. That should lower idle MAP slightly and give quicker acceleration. Probably wont get it much lower than 40Kpa anyway by increasing/decreasing VE bins by 1 or 2 units at a time. Arrange bins so that idle falls right in centre, ie. 800RPM @ 45 or say 40 Kpa, whatever. Compromise between smooth idle, low MAP, and AFR. Should be able to run 14:1 at idle and even up to 14.7 if accurate. Each engine is different though and individual tunes will vary, whatever gives you the best compromise for your case.

BTW 45Kpa is nearly 17"Hg, so dont worry.
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Smoothie  



Joined: 01 Jan 2003
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PostPosted: Sun Feb 22, 2009 10:07 pm    Post subject: Reply with quote

? I get 45kPa = roughly 13"hg.
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Sun Feb 22, 2009 10:17 pm    Post subject: Reply with quote

Smoothie wrote:
? I get 45kPa = roughly 13"hg.

So do I, using a calculator program on my Palm...

13 HG is low, even at 800 RPM, at least on a CIS car.
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xpernet  



Joined: 07 Apr 2004
Posts: 414
Location: Kitchener, Ontario Canada

PostPosted: Mon Feb 23, 2009 12:51 am    Post subject: Reply with quote

my bad guys, i meant 65Kpa ... So if im not mistaken, i should be around 17-20HG @ 950RPM... that means i got some tweaking to do after im done my midterms lol

Yeah i have the 95hp engine... so i can run 20° advance eh? Good to know
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Martijnus  



Joined: 29 Dec 2006
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PostPosted: Mon Feb 23, 2009 1:34 am    Post subject: Reply with quote

I'm reading about 50-55 kpa at idle with ITB's.

My afr at idle is around 13 though... at that it runs best but to be honest I haven't tried to go to 14.7 yet.
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Raceboy  



Joined: 01 Mar 2004
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PostPosted: Mon Feb 23, 2009 1:53 am    Post subject: Reply with quote

40-45 kPA is normal for 924 engine. I have installed VEMS on three 924-s and all of them had about the same vacuum readings at idle. All of them were Euro 125hp engines.
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Martijnus  



Joined: 29 Dec 2006
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PostPosted: Mon Feb 23, 2009 1:59 am    Post subject: Reply with quote

I'm quite interested in the correlation between TPS and MAP during driving on a stock 924 engine (with EFI...doesn't matter..at least no intake mods)...

would be a great base to see how that corresponds to readings with ITB's (mostly for ignition purposes)
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"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment
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RC  



Joined: 25 Mar 2007
Posts: 2634
Location: Australia

PostPosted: Mon Feb 23, 2009 2:27 am    Post subject: Reply with quote

ideola wrote:
Smoothie wrote:
? I get 45kPa = roughly 13"hg.

So do I, using a calculator program on my Palm...

13 HG is low, even at 800 RPM, at least on a CIS car.


Ha ha, can see you both fell in the same trap guys.
Pressure calculators are useful tools but they dont think for you if you are unsure of the units or application.

We are not measuring pressure but vacuum here. 45 Kpa is absolute which is around 13" Hg absolute. However since vacuum gauges are calibrated to measure vacuum (negative pressure) the 0 baseline is atmospheric pressure at sea level. Although it varies due to prevailing atmospheric conditions it is generally accepted at 101.295 Kpa. and in absolute terms this equates to 29.9213 inches mercury.

No pressure, ie. 0Kpa will read 29.9 (practically 30) on a vacuum gauge. 30 - 13 = 17
So for all automotive applications 45Kpa is actually 16.63"Hg.

Anyway, 65 Kpa is a different matter. It is too high, or low if we are talking vacuum.
May need some more tuning and a good check for leaks. Try the advance too, will also help low end power and acceleration.
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