Is 12mm Hg vaccum at idle normal on a TII?
#1
Is 12mm Hg vaccum at idle normal on a TII?
1.My TII vert has never shown more than 12 mm Hg vacuum at idle (750rpm), is this normal? All the rest of my pals with turbo cars( not 7's) have about -20mm Hg at idle. Why the difference?
2. This might seem random, but I was wondering if anybody knows why boost gauges read vacuum in mm Hg, but boost in Psi or Bar? Why not just choose one scale for vacuum and boost?
3. I'm having trouble finding the cause of a current leak in my car. My battery drains over the course of about three days if not driven. I figure I can use a muliti meter to find the cause somehow, but I'm not exactly sure how to. Would anyone electrical inclined care to elaborate?
2. This might seem random, but I was wondering if anybody knows why boost gauges read vacuum in mm Hg, but boost in Psi or Bar? Why not just choose one scale for vacuum and boost?
3. I'm having trouble finding the cause of a current leak in my car. My battery drains over the course of about three days if not driven. I figure I can use a muliti meter to find the cause somehow, but I'm not exactly sure how to. Would anyone electrical inclined care to elaborate?
#2
what about at startup...when the idle is higher? Does it read more than 12mm/hg then?
From my experience, A stock/relatively stock ported motor should pull about 16-20 @ idle.
For reference, my 91 T2 stock motor is at 18mm/hg
From my experience, A stock/relatively stock ported motor should pull about 16-20 @ idle.
For reference, my 91 T2 stock motor is at 18mm/hg
#3
that is low, either your motor has kinda low compression( aged motor), or u got pin hole vacume leaks.... my mothr with 160K on it had vacume of about 12 , where as new motors ive seen have closer to 20.
#4
My car doesn't idle higher at start up, I removed the AWS. I may have some small vac leaks I'll do my best to check. Hopefully I don't have low compression, I'm going to do a comp. test as soon as I figure out how to rig my comp tester to work on my rotary.
Any takers on my 2nd and 3rd questions?
Any takers on my 2nd and 3rd questions?
#6
Originally Posted by HHTurboVert
1.My TII vert has never shown more than 12 mm Hg vacuum at idle (750rpm), is this normal? All the rest of my pals with turbo cars( not 7's) have about -20mm Hg at idle. Why the difference?
I was wondering if anybody knows why boost gauges read vacuum in mm Hg, but boost in Psi or Bar? Why not just choose one scale for vacuum and boost?
#7
Originally Posted by HHTurboVert
1.My TII vert has never shown more than 12 mm Hg vacuum at idle (750rpm), is this normal? All the rest of my pals with turbo cars( not 7's) have about -20mm Hg at idle. Why the difference?
Can you confirm it's "inHg" - inches mercury - and not "mmHg"???
If you are seeing 12 inHg, then you either got an intake leak (most common), a big ported motor (do you?), low compression, or your ignition timing is off...or all of the above.
Healthy stock port 13BT's will usually pull about 18inHg of vacuum at idle @ 750RPM.
2. This might seem random, but I was wondering if anybody knows why boost gauges read vacuum in mm Hg, but boost in Psi or Bar? Why not just choose one scale for vacuum and boost?
There are some "odd" boost gauges that do show "- psi vacuum".
If you think about it...boost (or positive) pressure is NOT vacuum.
PSI = pounds per square inch, and is a (positive) pressure unit.
It's the same thing with "bar" - it's a pressure measurement.
Since pressure <> vacuum, why use a pressure measurement for vacuum?
Vacuum units have been traditionally used some type of liquid and how high in a calibrated "column" the vacuum force can suck it up to.
Thus, the mercury designation.
There is also inches - water, which is another common vacuum unit designation.
If you've ever taken a physics class, you would've covered all of this.
-Ted
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#8
There is no such thing as negative pressure, it's physically impossible. The plus and minus, or boost and vacuum, refer only the pressure relative to atmospheric pressure. So it's no less correct to use psi for vacuum than it is for boost, because it reality all pressures are positive. Since atmospheric pressure is 14.7psi, 6psi of vaccum is an absolute pressure of 8.7psi. Same goes for using inHg for pressure higher than atmospheric, the concept is exactly the same. I hope they're not teaching that vacuum and boost need different units because they're fundamentally different from each other, coz that's just plain wrong.
#9
Junior Member
Joined: Jul 2003
Posts: 48
Likes: 0
From: Sydney, Australia
Ted,
How big much bigger than stock ports would pull 12inhg in an otherwiuse healthy engine with no vac leaks. Are we talking intake or exhaust or combination?
Also what timing would affect it too much or too little advance?
How big much bigger than stock ports would pull 12inhg in an otherwiuse healthy engine with no vac leaks. Are we talking intake or exhaust or combination?
Also what timing would affect it too much or too little advance?
#10
Originally Posted by NZConvertible
There is no such thing as negative pressure, it's physically impossible. The plus and minus, or boost and vacuum, refer only the pressure relative to atmospheric pressure. So it's no less correct to use psi for vacuum than it is for boost, because it reality all pressures are positive. Since atmospheric pressure is 14.7psi, 6psi of vaccum is an absolute pressure of 8.7psi. Same goes for using inHg for pressure higher than atmospheric, the concept is exactly the same. I hope they're not teaching that vacuum and boost need different units because they're fundamentally different from each other, coz that's just plain wrong.
They do NOT teach you that in school.
I've just seen those markings on "boost gauges - many of them were very old.
-Ted
#11
Originally Posted by KillerRx4
How big much bigger than stock ports would pull 12inhg in an otherwiuse healthy engine with no vac leaks. Are we talking intake or exhaust or combination?
The ports have to be very extreme, especially if it's a street port.
A bridge port would easily do 12inHg, but you'd notice the idle is very weird.
Also what timing would affect it too much or too little advance?
-Ted
#13
Yeah.
Increasing the intake port *opening* timing will kill idle vacuum.
Those pics show a little porting on the "outside", but I've seen more extreme versions - see Judge Ito's huge street port that hangs like only 1/3rd of the corner seal!
-Ted
Increasing the intake port *opening* timing will kill idle vacuum.
Those pics show a little porting on the "outside", but I've seen more extreme versions - see Judge Ito's huge street port that hangs like only 1/3rd of the corner seal!
-Ted
#14
Exhaust closing time will effect your vacuum reading in conjunction with port opening time..Depending on the severity of the bridge, the idle speed and the altitude the guage reading will be in the neighbourhood of 6-10 inhg, if it's a bridgeport pulling more than 12, its not much of a bridgeport and probably isnt accomplishing much as a bridgeport.....Max
#16
It's hard to get a good reference versus stock, but it looks to have very late closing of the exhaust port.
Most porting for N / A's don't go that big, unless you know what you're doing.
I think you know what you're doing.
Anything to increase overlap will affect idle vacuum.
-Ted
Most porting for N / A's don't go that big, unless you know what you're doing.
I think you know what you're doing.
Anything to increase overlap will affect idle vacuum.
-Ted
#19
Junior Member
Joined: Jul 2003
Posts: 48
Likes: 0
From: Sydney, Australia
#20
Just by eyeballing the pics, it looks to be at least 10mm upward?
That's a lot of overlap.
I'd say with a street port, it should kill the idle vacuum down close to 16 or even 15 inHg...if I were to hazard a guess...
-Ted
That's a lot of overlap.
I'd say with a street port, it should kill the idle vacuum down close to 16 or even 15 inHg...if I were to hazard a guess...
-Ted
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