High vacumn friend or foe

P

Phantom Menace

Registered User
Sitting at idle my vacumn is at 20. I've often heard it should be around 10. Is 20 to high? Should I be concerned? If it is too high, what can be the cause?

Thanks
 
When I had this same motor in my XR-7, I was consistantly holding steady at 10. I put the engine into a Tbird. When we put the supercharger and tubes in we used the plastic gaskets that Tbird88 sells. I wonder if THEY made that much of a differance. The system sure does seem tight. Anyway, if 20 is good, than I'm sitting pretty.
 
yeah 20 is perfect, when the vac hose fell of my inlet plenum and the idle was ******, the vac was at 10.
 
My thought is 20

I had a really bad Idle and It sat right at 10. It did make the car sound like it had a huge V8 but made it run like crap, so I'm going to go with 20.
 
If 20 is good...

Then would 30 be better? How can we make it run even higher?

Only time I ever got close to 30 was when I was cruising at like 75MPH and dropped it in 2nd. Got it pretty close to 30 then. Don't try it at home, had to massage the clutch.
 
Since we're on the subject of vacume, with my mods (MP 3.5" cold air, 87mm MAF, 85mm TB, MP II, MP inlet plenum, raised top) will the vacume be affected? Right now I pull about 10-12.
 
Vaccum shouldn't be affected by intake mods. That doesn't change the amount of pressure drop after your blower. Unless you have a huge arse cam, you should be right around 20 hg/in of vaccum, at idle, in park.

Thomas
 
A longer duration camshaft will increase the overlap period where both intake and exhaust valves are open. This will bleed off some of your vacuum at idle and allow some of the exhaust back into the intake manifold. This is why a cammed car runs rough at idle, because there is exhaust making it's way back into the intake manifold to be re-burned.

Idle vaccum on our cars is normal at about 18-20. Anything higher is an inaccurate guage. Vaccum is actually bad in the sense that it requires work for the motor to overcome and hence reduces efficiency. It also reduces the cylinder charge which has the same effect as reducing compression. However, running the motor at maximum effiency means WOT at torque peak which is 2600rpm. Somehow I don't think that is very practical! ;)
 
I've got one of those cams Dave is talking about. It idles so rough it feels like two plug wires are off, and I only get 7-8 inches of vacuum in gear (800 rpms) and 10-11 inches in park (1100 rpms).

However with a little gas pedal, it smooths right out.

David
 
Also, 30" is typically not possible to achieve with only atmospheric pressure to draw from. IF you did get to 30, then there could be no particals (including air) going through the motor. 29.5 is considered a total vacuum for most industrial purposes, and no engine is that tight inside. If your gauge ever read 30, I'd say it was wrong. :(
 
vacumn readings

look in a good motor manual under vacumn readings, you gain alot of dianostic skills. gives info on bad rings, broken vavle springs, late timing, etc in fact in the 50,s we could set or timing with the gauge only!!!! all the best .............FAST FREDDIE
 
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