Engine Vacuum?

[Kieran]

No, not the type that sucks muck off your carpets..../pan

Can anyone explain the relationship between engine vacuum and the overall state of one's engine?

Reason I ask is I've seen many VR-4s with boost gauges and many different levels of Vacuum. The Jap service manuals suggest a minimum of -420mm/hg at idle. Ariadne has always read high vaccuum at idle, whereas I've seen other cars with low readings.

Can anyone explain it?

[Nick Mann]

Nope. I hover around 400, so I don't meet the spec!

[peter thomson]

around 500 on my one

[bradc]

it would be related to at least three things, how much air is being sucked into the engine (rpm, temperature, etc), as well as how healthy your turbos are, if they aren't spinning too well then there would be more vacuum for example. Lastly, where you've mounted the sensor and the type of gauge you have could influence it as well.

[Beastlee]

Approx 487 for me but I always work in bar so I normally see -0.65 to -0.75 bar at idle.

[psbarham]

the air filter will play a big part in the vacuum levels, if its clogged up then the reading will be higher as the engine can't suck so easily, also the position of the throttle butterfly will affect the vacumm. another thing that will affect is is piston rings, if they are worn then the engine will draw a certain amount of air through the rings and not the maf

[Beastlee]

Ok so what would be defined as higher and lower? If the expected value is -0.6bar for example would you say -0.55bar or -0.65 bar is higher?

[White Lightning]

I thought one of the biggest factors was the compression of the engine? The better the compression, the higher the vacuum?

Or maybe I am talking rubbish ... :inquisiti

[psbarham]

-0.65 is higher, higher vacuum, that is not higher pressure, a higher pressuer would be -0.55

[Paul C]

The Map ecu2 has it around -19inhg whch is around 480mmhg approx

[Beastlee]

Which rings true with my conversion, it's all good.

[Kieran]

....as well as how healthy your turbos are, if they aren't spinning too well then there would be more vacuum for example....

Eh?! I don't understand that - Can you repeat with more explanation?

Filter - I have a nearly new HKS Super power flow so that's not holding things up. I show -650mm/hg at idle in 'N'. On over-run I've seen up to -800!!

Interestingly, since I've been fixing boost leaks, my vacuum levels have improved.... Before they were around -580mm/hg.

[miller]

i need a booooost gauge...theres soooo much else going on under there i need to learn about :speechles :speechles


Mike

[Dan_G]

Kieran,

Just out of curiousity how are you finding your boost leaks?

I'm beggining to wonder if i have a leak somewhere due to my sheeyite lumpy idle / stall often type situation.

changed maf... no difference...
cleaned idle stepper... no difference...

[Nick Mann]

He's got something a bit like this.....

http://cgi.ebay.com/ebaymotors/Turbo-Intercooler-Intake-Leak-Pressure-Tester-All-Sizes_W0QQcmdZViewItemQQ_trksidZp1638Q2em122QQcate goryZ33742QQihZ009QQitemZ190199547801QQrdZ1QQsspag enameZWD2V

[bradc]

K - what I mean is that if the turbos are screwed and aren't spinning very well then that would be a restriction on the intake, and in turn that would affect the vacuum reading

[Turbo_Steve]

Also, don't forget that boost gauges don't tend to be all that accurate on the vacuum side, as the diaphragm is mainly calibrated for positive pressure, so does not deform as evenly under vacuum.

You should see heavy vacuum on overrun as the throttle butterfly is closed and the idle controller is barely operating. Engine doesn't need to pump any air and doesn't use any fuel.

Airfilter, inlet shape, idle speed and air temperature will all affect vaccum levels, as well as load (how charged is battery? is aircon on? how thin is oil? how hot is water?) and any exhaust restriction.

[CANDEE]

I have between -20 and -21inHg - about 500-540mmHg :)

[amsoil]

Most I've ever seen is 23"hg . We used to work on 21 + was good under 18 was worth the effort of finding out why. Uslually ended up as an inlet gasket split or just leaking.
The Mighty Vac tuning tool was brill for all of this and the booklet that came with it would explain how to solve most of the woes of the world just with a vac gauge connected to the correct place. Bit of a lost art me thinks.

[psbarham]

idle on mine is -800mbar 23in mercury and on over run its about -850mbar which is -25in mercury

[amsoil]

idle on mine is -800mbar 23in mercury and on over run its about -850mbar which is -25in mercury

Sounds very healthy but the over run bit doesn't count!/pan :scholar:

[Beastlee]

So mine works out to -19.19in which is quite low in reality. I know most of my vacuum piping is OK as I just fitted the Profec and therefore has freshly installed and checked hoses. I'm on new Amsoil and there's no sign of any other problems but I suppose the air-filter may need changing even though it is approx 6 months old from the last Mitsi service.
Does the BOV make any difference at idle? I wouldn't have thougth so but if it;s leaking surely that has an effect even at idle. Mine is still the original so I'd like to get it replaced anyway, perhaps with an Evo one but more prefferable with an HQS SSQV.

[psbarham]

Sounds very healthy but the over run bit doesn't count!/pan :scholar:

yeh not bad considering its age and mileage, if I'm cruel to it and shut the throttle in second or third at 6k (private track of course because the gearing on mine means both these revs are completely illegal for the queens highway /walks away whistling innocently) then -1000 mbar is easily done

[Davezj]

my vacuum is -500mm/hg at idle and -650mm/hg after bip of throttle.

[pitslayer]

sorry bringing this thread back up, because i have a suspiscion my car isnt running full power. ive now grown acustom to the cars noises, now.
and i can no longer hear my BOV going off, and its either me being really paranoid, but the rear turbo seems to be very loud when its sucking in air, and has been for a while now. its not a bad noise, but i can hear it over my music, when using little throttle.
so i fitted a boost gauge. now boost is going no more than 6psi, which should be correct for the car.
but this inhg.....what is it, i know its the engine vacuum pressure. after searching come across this post, now my boost gauge only goes upto 30inhg.
which im guessing needs to be times`d by ten, to get the real reading? as you all have 400 inhg etc.....

so my query is. if inhg is the engine vacuum, and at idle im only reading 25inhg, which i guess to be 250inhg? whats up with that?

i just feel if the car has lost, a lot of power, i struggled to leave a bog standard astra behind yesterday. and im only getting 140miles on average to a tank.
any ideas?

[Kieran]

Kyle - you've got your conversion factors wrong. We're talking Milimeteres of mercury, your gauge is reading inches of mercury.

Assuming your gauge is reading correctly, 25inhg equals 635mmhg.

You're well within spec. The minimum vaccuum is -420mmhg.:scholar:

[bradc]

if you can hear the car sucking air in and are getting terrible fuel economy, an air leak would be the obvious thing to look for.

[pitslayer]

that might be fun, will have a look all around the pipes and see for an air leak or hear.

cheers kieran, for clearing that up

[kc427]

mine is about -475mmhg when in idle

[Kieran]

Bumping an old thread here, but I am slightly puzzled.

I've just picked Alex up from work and Ariadne was going like bloody hell - So well in fact I wondered if I'd accidentally knocked my boost control to the 'High' [0.9Bar] setting. I escribed this to the fact that it's much colder tonight.

However, this evening when I reversed neatly onto the drive (No garage doors were harmed in this parking operation, I'll say it before one of you commedians do! /grr) I noticed that at idle my Vacuum reading's increased to -700mmhg! :inquisiti

Aircon on or off doesn't make a difference and as I said, the car felt quite a bit more eager than standard, so I don't think it's a dirty filter or anything like that.... Any ideas, or is increased engine vacuum something to be expected in cold weather?

[elnevio]

I was thinking that mine was going rather well, somewhat more effortlessly, this afternoon. Maybe it is the weather. Didn't note the vacuum readings though...

[Nick Mann]

Thats a large amount of vacuum! I'd suggest a blockage of some sort, but that shouldn't be increasing your grunt?

[Kieran]

I'll see what it's like in the morning when I take Alex to work again. Hopefully it'll be warmer (The MMCS indicated 9 degrees tonight but it felt colder than that).

[OnBoost!]

Finally fitted my boost gauge.....:D


It's about 19 inhg,is that normal and means it's fairly healthy?

Max boost i think is about 8.5psi....seems a bit higher than the factory specs.......

[scott.mohekey]

I read about 19 - 20 inhg as well.. boost will peak at around 8.5 but usually sits at around 7.

[Fully]

it would be related to at least three things, how much air is being sucked into the engine (rpm, temperature, etc), as well as how healthy your turbos are, if they aren't spinning too well then there would be more vacuum for example. Lastly, where you've mounted the sensor and the type of gauge you have could influence it as well.

lol dude, turbos don't create vacuum - pistons do that :P

Our AP (atmospheric pressure) is 14.7 pounds (psi) at sea level. At higher elevations this pressure is less, of course, because there's that much less atmosphere stacked up over us. So 14.7 pounds is all that we have to urge a mixture to enter our engine. When the piston goes down and creates a vacuum, the atmosheric pressure is higher than the negative pressure of the vacuum, so stuff goes in.

If you have any turbo or supercharged that is making 14.7 psi of boost, then you have effectivly added another atmosphere of pressure. Twice that amount of boost and you're added two atmosphere and you command the bridge of a rocket ship!

In a typical vehicle in good condition, your engine may be ldling at 600-700rpm and vacuum is high, maybe 17-20 inches unless you have an aftermarket camshaft, which would lower your the idle vacuum. This vacuum will drop rapidly when the engine is accelerated quickly, but then smooth out at most speeds. When you're in top gear and crusing, your gauge may again read something like your idle vacuum, indicating that you are driving at good efficiency.

The engine compression ratio has an important effect on the engine boost level. The higher the static compression ratio, the less boost you can run without detonation.
Our engines (JDM) run a compression ratio of 10:1 or higher because of Japan's 100-octane gas and less stringent emissions laws.

Just as a guideline:
A engine with 5-7psi of boost your engine compression ratio of 9:1 or lower.
If you are rebuilding its a good idea to equip it with new forged pistons of 8:1 compression. This will allow (all other factors being equal) 8 - 12 psi of boost.

[bradc]

Our engines are actually 8.5:1 and the new EVO X engine, the 4B11T has a 9:1 compression ratio and peaks at over 20psi stock.

[Fully]

^ Did you even read my post?

My compression is much closer to 10:1 than 8.5:1 on my car!
Buts that a different matter, I was explaining how vacuum works.

[miller]

thats a good post fully, making me unnerstand boost and negative vacuum. So when im driving along and ease off the smiley pedal the boost pipes are all negative, then a slight acceleration brings it up to say -5psi ARE the turbos then spooling up even through the band of negative pressure?

If so then economical driving is keeping the needle under 0 all of the time?

Mike the boost learner!

[bradc]

I read the post, I was replying to the bottom two paragraphs, not to the bit about negative pressure. Have you already had your engine rebuilt with the new rods and pistons? I thought you were still waiting for them.

Mike - basically yes, the turbos will be spinning even at idle, but they aren't doing all that much work until the needle goes above 0.

[Fully]

Yes I have had my motor and gearbox/torque converter rebuilt and a head plane. About a year before I joined these forums.

I am going to rebuild another 6A13TT motor, for my project car. Instead of going on the race track. As it was costing about $20g a season.

Yer Miller might as well get a N/A engine if you were going to do that :p

[bradc]

Ahh didn't know that, so your current motor has stock pistons and rods but a little bit skimmed from the top of the head. How many mm approximately?

[Wodjno]

-490mm/hg here @ 640rpm .. /Hmmm