Hi,

I've got a boost gauge which is attached to the manifold. At maximum boost it reads 15 psi and then holds at 14 psi - same figures quoted at my last dyno run so sounds fairly accurate.

My question is why does it idle at -9 is this normal? How come zero on the gauge seems to have no relevance to the status of the turbo (boost)? I assumed that gauge would read zero until turbo starts spinning up and then drop back to zero as it spins down. How come I have a negative reading at idle or even when just trundling along?

So the operating range appears to be -9 to 15 psi

Is this normal or am I just thicker than usual?

Cheers,
Rich.

 

It runs a vacuum when not boosting thats why you get - readings.

 

Rich, this is normal as until the turbo is spinning fast enough to produce positive pressure there will be negative pressure in the inlet manifold, just as there would be in a naturally aspirated car.

Hope this helps.

D.

Bugger - beaten again!

[This message has been edited by Diablo (edited 14-04-2000).]

 

On a normally aspirated engine without a Turbo the pressure in the inlet manifold varies between almost zero and minus 10 to 15psi.

When the pistons move down the cylinders this lowers the pressure and so you have negative pressure within the manifold, as the throttle is opened there is less restriction to air filling the manifold at amospheric pressure, thus the gauge reads 0 pressure at WOT.

On a turbo you have positive pressure if the turbo is spinning and compressing the air into the manifold


Thus your readings are perfectly normal and you would have a problem if you were not reading -ve pressure at idle and coasting


Looks like we all anwered at the same time

Paul

[This message has been edited by pnebbs (edited 14-04-2000).]

 

Cheers guys - I'm now off to enroll on a physics for beginners course. I'll just get me coat

Rich.

[This message has been edited by Rich_R (edited 14-04-2000).]

 

Dude,

You can use the boost gauge as a kind of power/economy meter. The higher the boost (+), the more mix going in, the more power, the higher the vaccuum reading(-), the less mix going in, so the more economy(ish!).

So you can see, unless I am very much mistaken, that cruising along at a given roadspeed with, lets say, 2000rpm gives closer to 0 (less vaccuum) than cruising along in a lower gear at the same roadspeed (higher revs)... I believe this would indicate that the engine is more working more efficiently at the slightly higher revs, thus giving better economy...

Moray
(Preparing to be be wrong...)

[This message has been edited by MorayMackenzie (edited 14-04-2000).]

 

Hello,
To be precise (pedantic!) a normally aspirated engine will have a slightly higher pressure than atmospheric at WOT due to the fact that most engines have a volumetric efficiency of greater than 100% It will also depend on where in the manifold the pressure is read.
This means the cylinders fill with more charge than their static capacity would indicate. This is due to inertial air flow (ramming) - and can be as high as 120% So a 2.0L N/A engine can actually fill with 2.4L of charge.

Best regards

Guy

 

Cheers for all the info guys

I've got another question:

As I mentioned before it peaks at 15 psi and holds at 14 except in second gear where it drops off steadily from 14 to about 11

Second gear is quite short but does this sound normal?

Cheers,
Rich.