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?

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?

Erik

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

KompressorLOgic

iTrader: (6)

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.

HHTurboVert

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?

tIIsleeper

Vaccum is so low that you cant get an accurate reading in psi. Psi is to large of an increment. mmHg is a small amount so you can get a more accurate reading.

NZConvertible

You mean inHg, not mmHg. 12inHg is 5.9psi while 12mmHg is only 0.23psi (i.e teeny tiny). 12inHg is quite low vac for idle, and means either a post-throttle vac leak or low compression.

Damn good question. If you look at the stock boost gauge you'll see both sides of the scale are in mmHg, but for some reason with aftermarket gauges (particularly non-metric ones) there is a convention to use different scales for vac and boost. Non-metric is almost always inHg and psi; metric gauges are often mmHg and bar but there are lots of variations. There's no technical reason why they have to be different, and personally I'd rather have the same units with a minus on one side and a plus on the other (like the stock gauge). My personal preference would be kPa. 100kPa = ~1atm, so it's easy.

RETed

I was going to say...but NZConvertible beat me to it.
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.


It's just how manufacturers make them.
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

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.

KillerRx4

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?

RETed

No, you're right.
They do NOT teach you that in school.
I've just seen those markings on "boost gauges - many of them were very old.


-Ted

RETed

It's almost all intake side.
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.


Either or, but it's typically slightly retarded ignition timing that's most common.


-Ted

KillerRx4

So would need to be bigger than these?

RETed

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

Maxthe7man

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

KillerRx4

ok so exhaust port like this will drop vac at idle?

RETed

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

KillerRx4

Ted, this is a turbo engine

RETed

Turbo engine?
Shoot, then you're hiding a lot of stuff we don't know about.


-Ted

KillerRx4

Hahaha hiding stuff? Not hiding anything mate, what makes you say that?
If i was hiding something I wouldnt be putting internal engine pics on the net.


reference stock exhaust pic almost the same angle.

RETed

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