Reading boost from before the throttle versus after.

[Jesus-Ninja]

So, here's a thing. Normally, if I were setting up a boost gauge and EBC, I'd feed them both from the plenum for their readings. Gives you boost and vac, right? ie what's really going on in the engine.

Well, I was wading through the spaghetti of vac lines at the weekend, and I realised that my EBC and gauge are reading from a T piece, where one tube runs to the plenum, and the other runs to the black hard pipe before the butterfly.

Now, this must have been how it was as stock, but is that right?

[Nick Mann]

Nope? The small pipe at the back of the plenum feeds the dump valve "signal" connection. The small pipe facing forwards close to the butterfly feeds the stock FPR solenoid. The two feeds on the plastic elbow are for the vacuum advance on the power steering and the feed for the T piece that feeds the wastegate actuators and the boost solenoid.

[Davezj]

The boost control is off the black plastic elbow before the throttle body and there is no boost gauge as standard.


Yes, Nick is right for the full story.

[Jesus-Ninja]

Ah, just realised I was being an idiot, and not for the first time. There was a loose end and it looked like it went to the black plastic elbow, but now I found the one that is supposed to go there, and the one I mistakenly thought was it, is actually the hose that goes to the hard narrow pipe that runs down to the dump valve.

Still, I'm still running my EBC off plenum pressure, and thinking about it, that might not be right, as I suppose in theory the tubs could still be boosting with the butterfly closed (unlikely as there's no fuelling when it is, and the DV will vent anyway)

You reckon I should move it to read from before the throttle?

[SEAN-NZ]

wouldnt that give innacurate readings? as with partial throttle you will still get some boost? but the pressure will be higher before the throttle body than after due to the buterfly only being half open, so the ebc will think you have more boost than you do

[Nick Mann]

I'd say that control should be done before the butterfly, monitoring after. Assuming your EBC has a control and a monitor, then I'd plumb them in to that observation. I can't say with certainty that I am right, but it makes sense in my head!

[Davezj]

I have always plumbed the boost control in where the original factory boost solenoid goes (before the Throttle body) and used the Plenum as the monitor (after the throttle body).

but you have got me thinking............

i thought i had a good reason for the control being before the throttle body but i don't really, or do i.

mulls it over some more....

yes i do or i think i do, mull it over some more..... not sure it makes any difference now.

i am confused!

i think more testing would be needed to see what happens to the boost pressure in the rest of the boost pipework when the throttle body closes. the dump valve should open at this point and any unused boost pressure gets vented to try and keep the turbo's spinning as fast as possible until boost pressure is required again.

does that sound right.